№ 9. Nervous
system. Brain.
The
central nervous system (CNS) represents the largest part of the nervous system,
including the brain and the spinal cord. Together with the peripheral nervous
system, it has a fundamental role in the control of behavior. The CNS is
contained within the dorsal cavity, with the brain within the cranial cavity,
and the spinal cord in the spinal cavity. The CNS is covered by the meninges.
The brain is also protected by the skull, and the spinal cord is also protected
by the vertebrae.
The nervous
system.
Function
Brain
Function
Since the strong theoretical influence of cybernetics
in the fifties, the CNS is conceived as a system devoted to information
processing, where an appropriate motor output is computed as a response to a
sensory input. Yet, many threads of research suggest that motor
activity exists well before the maturation of the sensory systems and then,
that the senses only influence behavior without dictating it. This has brought
the conception of the CNS as an autonomous system.
Neural
development
In
the developing fetus, the CNS originates from the neural plate, a specialised
region of the ectoderm, the most external of the three
embryonic layers. During embryonic development, the neural plate folds and
forms the neural tube. The internal cavity of the neural tube will give rise to
the ventricular system. The regions of the neural tube will differentiate
progressively into transversal systems. First, the whole neural tube will
differentiate into its two major subdivisions: brain (rostral/cephalic) and
spinal cord (caudal). Consecutively, the brain will differentiate into
prosencephalon and brainstem. Later, the prosencephalon will subdivide into
telencephalon and diencephalon, and the brainstem into mesencephalon and
rhombencephalon.
Neural development.
Neuroanatomy
The
telencephalon gives rise to the striatum (caudate nucleus and putamen), the
hippocampus and the neocortex, its cavity becomes the lateral ventricles (first
and second ventricles). The diencephalon give rise to
the subthalamus, hypothalamus, thalamus and epithalamus, its cavity to the
third ventricle. The mesencephalon gives rise to the tectum, pretectum,
cerebral peduncle and its cavity develops into the mesencephalic duct or
cerebral aqueduct. Finally, the rhombencephalon gives rise to the pons, the cerebellum
and the medulla oblongata, its cavity becomes the fourth ventricle.
|
Central |
Brain |
Prosencephalon |
Telencephalon |
Rhinencephalon,
Amygdala, Hippocampus, Neocortex, Lateral ventricles |
|
|
Diencephalon |
Epithalamus,
Thalamus, Hypothalamus, Subthalamus, Pituitary gland, Pineal gland, Third
ventricle |
||||
|
Brain stem |
Mesencephalon |
Tectum, Cerebral
peduncle, Pretectum, Mesencephalic duct |
|||
|
Rhombencephalon |
Metencephalon |
Pons, Cerebellum, |
|||
|
Myelencephalon |
Medulla oblongata |
||||
|
Spinal cord |
|||||
Evolution
The
basic pattern of the CNS is highly conserved throughout the different species
of vertebrates and during evolution. The major trend that can be observed is
towards a progressive telencephalisation: while in the reptilian brain that
region is only an appendix to the large olfactory bulb, it represent
most of the volume of the mammalian CNS. In the human brain, the telencephalon
covers most of the diencephalon and the mesencephalon. Indeed, the allometric
study of brain size among different species shows a striking continuity from
rats to whales, and allows us to complete the knowledge about the evolution of
the CNS obtained through cranial endocasts.
Human
brain
The
human brain controls the central nervous system (CNS), by way of the cranial
nerves and spinal cord, the peripheral nervous system (PNS) and regulates
virtually all human activity. Involuntary, or "lower," actions, such
as heart rate, respiration, and digestion, are unconsciously governed by the
brain, specifically through the autonomic nervous system. Complex, or
"higher," mental activity, such as thought, reason, and abstraction,
is consciously controlled.
The human
brain main areas.
Anatomically,
the brain can be divided into three parts: the forebrain, midbrain, and
hindbrain; the forebrain includes the several lobes of the cerebral cortex that
control higher functions, while the mid- and hindbrain are more involved with
unconscious, autonomic functions. During encephalization, human brain mass
increased beyond that of other species relative to body mass. This process was
especially pronounced in the neocortex, a section of the brain involved with
language and consciousness. The neocortex accounts for about 76% of the mass of
the human brain; with a neocortex much larger than other animals, humans enjoy
unique mental capacities despite having a neuroarchitecture similar to that of
more primitive species. Basic systems that alert humans to stimuli, sense
events in the environment, and maintain homeostasis are similar to those of
basic vertebrates. Human consciousness is founded upon the extended capacity of
the modern neocortex, as well as the greatly developed structures of the brain
stem.
The human
brain lobes.
The
central structures of the brain are the thalamus, hypothalamus, and pituitary
gland. The thalamus relays sensory information to the cerebrum; the
hypothalamus helps regulate body functions such as thirst and appetite, as well
as sleep, aggression, and sexual behavior; and the pituitary gland produces
hormones that play a role in growth, development, and various other
physiological variables. The pons, medulla, and midbrain are the three
structures that compose the brain stem. The ventricles are natural cavities
inside the brain filled with cerebrospinal fluid.
The brain
structures.
Neurophysiology
The
human brain is the source of the conscious, cognitive mind. The mind is the set
of cognitive processes related to perception, interpretation, imagination,
memories, and crucially language (cf. Broca's area) of which a person may or
may not be aware. Beyond cognitive functions, the brain regulates autonomic
processes related to essential body functions such as respiration and
heartbeat.The brain controls all movement from lifting a pencil to building a
superstructure.
Extended
neocortical capacity allows humans some control over emotional behavior, but
neural pathways between emotive centers of the brain stem and cerebral motor
control areas are shorter than those connecting complex cognitive areas in the
neocortex with incoming sensory information from the brain stem. Powerful
emotional pathways can modulate spontaneous emotive expression regardless of
attempts at cerebral self-control. Emotive stability in humans is associated
with planning, experience, and an environment that is both stable and stimulating.
The
19th century discovery of the primary motor cortex mapped to correspond with
regions of the body led to popular belief that the brain was organized around a
homunculus. A distorted figure drawn to represent the body's motor map in the
prefrontal cortex was popularly recognized as the brain's homunculus, but
function of the human brain is far more complex than this simple figure
suggests. A similar, "sensory homunculus" can be drawn in the
parietal lobe that parallels that in the frontal lobe.
The
human brain appears to have no localized center of conscious control. The brain
seems to derive consciousness from interaction among numerous systems within
the brain. Executive functions rely on cerebral activities, especially those of
the frontal lobes, but redundant and complementary processes within the brain
result in a diffuse assignment of executive control that can be difficult to
attribute to any single locale. Visual perception generally is processed in the
occipital lobe, wherease the primary auditory cortex resides in the temporal
lobe.
A sketch of the human brain by artist Priyan
Weerappuli superimposed upon the profile of Michelangelo's David.
Midbrain functions include routing,
selecting, mapping, and cataloging information, including information perceived
from the environment and information that is remembered and processed
throughout the cerebral cortex. Endocrine functions housed in the midbrain play
a leading role in modulating arousal of the cortex and of autonomic systems.
Nerves
from the brain stem complex where autonomic functions are modulated join nerves
routing messages to and from the cerebrum in a bundle that passes through the
spinal column to related parts of a body. Twelve pairs of cranial nerves,
including some that innervate parts of the head, follow pathways from the
medulla oblongata outside the spinal cord.
A
definite description of the biological basis for consciousness so far eludes
the best efforts of the current generation of researchers. But reasonable
assumptions based on observable behaviors and on related internal responses
have provided the basis for general classification of elements of consciousness
and of likely neural regions associated with those elements. Researchers know
people lose consciousness and regain it, they have identified partial losses of
consciousness associated with particular neuropathologies and they know that
certain conscious activities are impossible without particular neural
structures.
VIDEO
Structure and Physiology of
the Nervous System
Grey
matter, the thin layer of cells covering the cerebrum, was believed by most
scholars to be the primary center of cognitive and conscious processing. White
matter, the mass of glial cells that support the cerebral grey matter, was
assumed to primarily provide nourishment, physical support, and connective
pathways for the more functional cells on the cerebral surface. But research
fueled by the interest of Dr. Marian Diamond in the glial structure of Albert Einstein's
brain led to a line of research that offered strong evidence that glial cells
serve a computational role beyond merely transmitting processed signals between
more functional parts of the brain. In 2004, Scientific American published an
article suggesting scientists in the early 21st century are only beginning to
study the "other half of the brain."
For
many millennia the function of the brain was unknown. Ancient Egyptians threw
the brain away prior to the process of mummification. Ancient thinkers such as
Aristotle imagined that mental activity took place in the heart. Greek scholars
assumed correctly that the brain serves a role in cooling the body, but
incorrectly presumed the brain to function as a sort of radiator, rather than
as a thermostat as is now understood. The Alexandrian biologists Herophilos and
Erasistratus were among the first to conclude that the brain was the seat of
intelligence. Galen's theory that the brain's ventricles were
the sites of thought and emotion prevailed until the work of the Renaissance
anatomist.
Vesalius.
The
modern study of the brain and its functions is known as neuroscience.
Psychology is the scientific study of the mind and behavior. Neurophysiology is
the study of normal healthy brain activity, while neurology and psychiatry are
both medical approaches to the study of the mind and its disorders and
pathology or mental illness respectively.
The
brain is now thought to be the primary organ responsible for the phenomena of
consciousness and thought. It also integrates and controls (together with the
central nervous system) allostatic balance and autonomic functions in the body,
regulates as well as directly producing many hormones, and performs processing,
recognition, cognition and integration related to emotion. Studies of brain
damage resulting from accidents led to the identification of specialized areas
of the brain devoted to functions such as the processing of vision and
audition.
Neuroimaging
has allowed the function of the living brain to be studied in detail without
damaging the brain. New imaging techniques allowed blood flow within the brain
to be studied in detail during a wide range of psychological tests. Functional
neuroimaging such as functional magnetic resonance imaging and positron
emission tomography allows researchers to monitor activities of the brain as
they occur (see also history of neuroimaging).
Molecular
analysis of the brain has provided insight into some aspects of what the brain
does as an organ, but not how it functions in higher-level processes. Further,
the molecular and cell biological examination of brain pathology is hindered by
the scarcity of appropriate samples for study, the (usual) inability to biopsy
the brain from a living person suffering from a malady, and an incomplete description
of the brain's microanatomy. With respect to the normal brain, comparative
transcriptome analysis between the human and chimpanzee brain and between brain
and liver (a common molecular baseline organ) has revealed specific and
consistent differences in gene expression between human and chimpanzee brain
and a general increase in the gene expression of many genes in humans as
compared to chimpanzees. Furthermore, variations in gene
expression in the cerebral cortex between individuals in either species is
greater than between sub-regions of the cortex of a single individual.
In
addition to pathological and imaging studies, the study of computational
networks, largely in computer science, provided another means through which to
understand neural processes. A body of knowledge developed for the production
of electronic, mathematical computation of systems provided a basis for
researchers to develop and refine hypotheses about the computational function
of biological neural networks. The study of neural networks now involves study
of both biological and artificial neural networks.
A
new discipline of cognitive science has started to fuse the results of these
investigations with observations from psychology, philosophy, linguistics, and
computer science as expressed in On Intelligence.
Recently
the brain was used in bionics by several groups of researchers. In a particular
example, a joint team of United States Navy researchers and Russian scientists
from
The
first language area within the left hemisphere to be discovered is called
Broca's Area, after Paul Broca. The Broca's area doesn't just handle getting
language out in a motor sense, though. It seems to be more generally involved
in the ability to deal with grammar itself, at least the more complex aspects
of grammar. For example, it handles distinguishing a sentence in passive form
from a simpler subject-verb-object sentence. For instance, the sentence:
"The boy was hit by the girl." implies the girl hit the boy, not the
other way around. As a simple subject-verb-object interpretation it could mean:
"The boy was hit by the girl.", and therefore, the boy hit the girl.
The
second language area to be discovered is called Wernicke's Area,
after Carl Wernicke, a German neurologist. The problem of not understanding the
speech of others is known as Wernicke’s Aphasia. Wernicke's is not just about
speech comprehension. People with Wernicke's Aphasia also have difficulty
naming things, often responding with words that sound similar, or the names of
related things, as if they are having a very hard time with their mental
"dictionaries."
Brain
enhancement
Various
methods have been proposed to improve the cognitive performance of the human
brain including pharmacological methods (nootropic drugs), electric stimulation
(direct current polarization) and surgery. More advanced methods of brain
enhancement may be possible in the future, perhaps including direct
brain-computer interfaces. These proposed enhancements are a major focus of
Transhumanism.
Comparison
of the brain and a computer
Much
interest has been focused on comparing the brain with computers. A variety of
obvious analogies exist: for example, individual neurons can be compared with a
microchip, and the specialised parts of the brain can be compared with graphics
cards and other system components. However, such comparisons are fraught with
difficulties. Perhaps the most fundamental difference between brains and
computers is that today's computers operate by performing often sequential
instructions from an input program, while no clear analogy of a program appears
in human brains. The closest equivalent would be the idea of a logical process,
but the nature and existence of such entities are subjects of philosophical
debate. Given Turing's model of computation, the Turing machine, this may be a
functional, not fundamental, distinction. However, Maass and Markram have
recently argued that "in contrast to Turing machines, generic computations
by neural circuits are not digital, and are not carried out on static inputs,
but rather on functions of time" (the Turing machine computes computable
functions). Ultimately, computers were not designed to be models of the brain,
though subjects like neural networks attempt to abstract the behavior of the
brain in a way that can be simulated computationally.
In
addition to the technical differences, other key differences exist. The brain
is massively parallel and interwoven, whereas
programming of this kind is extremely difficult for computer software writers
(most parallel systems run semi-independently, for example each working on a
small separate 'chunk' of a problem). The human brain is also mediated by
chemicals and analog processes, many of which are only understood at a basic
level and others of which may not yet have been discovered, so that a full
description is not yet available in science. Finally, and perhaps most
significantly, the human brain appears hard-wired with certain abilities, such
as the ability to learn language (cf. Broca's area), to interact with
experience and unchosen emotions, and usually develops within a culture. This
is different from a computer in that a computer needs software to perform many
of its functions beyond its basic computational capabilities.
Nevertheless,
there have been numerous attempts to quantify differences in capability between
the human brain and computers. According to Hans Moravec, by extrapolating from
known capabilities of the retina to process image inputs, a brain has a
processing capacity of 100 trillion instructions per second (100 million MIPS),
and is likely to be surpassed by computers by 2030.
The
computational power of the human brain is difficult to ascertain, as the human
brain is not easily paralleled to the binary number processing of today's
computers. For instance, multiplying two large numbers can be accomplished in a
fraction of a second with a typical calculator or desktop computer, while the
average human may require a pen-and-paper approach to keep track of each stage
of the calculation over a period of five or more seconds. Yet, while the human
brain is calculating a math problem in an attentive state, it is subconsciously
processing data from millions of nerve cells that handle the visual input of
the paper and surrounding area, the aural input from both ears, and the sensory
input of millions of cells throughout the body. The brain is regulating the
heartbeat, monitoring oxygen levels, hunger and thirst requirements, breathing
patterns and hundreds of other essential factors throughout the body. It is
simultaneously comparing data from the eyes and the sensory cells in the arms
and hands to keep track of the position of the pen and paper as the calculation
is being performed. It quickly traverses a vast, interconnected network of
cells for relevant information on how to solve the problem it is presented,
what symbols to write and what their functions are, as it graphs their shape
and communicates to the hand how to make accurate and controlled strokes to
draw recognizable shapes and numbers onto a page.
The
nervous system is the part of an animal's body that coordinates the actions of
the animal and transmits signals between different parts of its body. In most
types of animals it consists of two main parts, the central nervous system
(CNS) and the peripheral nervous system (PNS). The CNS contains the brain and
spinal cord. The PNS consists mainly of nerves, which are long fibers that
connect the CNS to every other part of the body. The PNS includes motor
neurons, mediating voluntary movement, the autonomic nervous system, comprising
the sympathetic nervous system and the parasympathetic nervous system and
regulating involuntary functions, and the enteric nervous system, a
semi-independent part of the nervous system whose function is to control the
gastrointestinal system.
The
central nervous system (CNS) is the part of the nervous system that integrates
the information that it receives from, and coordinates the activity of, all
parts of the bodies of bilaterian animals—that is, all multicellular animals
except radially symmetric animals such as sponges and jellyfish. It contains
the majority of the nervous system and consists of the brain and the spinal
cord. Some classifications also include the retina and the cranial nerves in
the CNS. Together with the peripheral nervous system, it has a fundamental role
in the control of behavior. The CNS is contained within the dorsal cavity, with
the brain in the cranial cavity and the spinal cord in the spinal cavity. In
vertebrates, the brain is protected by the skull, while the spinal cord is
protected by the vertebrae, and both are enclosed in the meninges.
Neural
development
Neural
development refers to the processes that generate, shape, and reshape the
nervous system, from the earliest stages of embryogenesis to the final years of
life. The study of neural development aims to describe the cellular basis of brain
development and to address the underlying mechanisms. The field draws on both
neuroscience and developmental biology to provide insight into the cellular and
molecular mechanisms by which complex nervous systems develop. Defects in
neural development can lead to cognitive, motor, and intellectual disability,
as well as neurological disorders such as autism, Rett syndrome, and mental
retardation.
Overview
of brain development
The
nervous system is derived from the ectoderm – the outermost tissue layer – of
the embryo. In the third week of development the neuroectoderm appears and
forms the neural plate along the dorsal side of the embryo. This neural plate
is the source of the majority of neurons and glial cells in the mature human. A
groove forms in the neural plate and, by week four of development, the neural
plate wraps in on itself to make a hollow neural tube. Because this neural tube
later gives rise to the brain and spinal cord any mutations at this stage in
development can lead to lethal deformities like anencephaly or lifelong
disabilities like spina bifida. The most anterior part of the neural tube is
called the telencephalon, which expands rapidly due to cell proliferation, and
eventually gives rise to the brain. Gradually some of the cells stop dividing
and differentiate into neurons and glial cells, which are the main cellular
components of the brain. The newly generated neurons migrate to different parts
of the developing brain to self-organize into different brain structures. Once
the neurons have reached their regional positions, they extend axons and
dendrites, which allow them to communicate with other neurons via synapses.
Synaptic communication between neurons leads to the establishment of functional
neural circuits that mediate sensory and motor processing, and underlie
behavior. The human brain does most of its development within the first 20
years of life.
Aspects
of neural development
Some
landmarks of neural development include the birth and differentiation of
neurons from stem cell precursors, the migration of immature neurons from their
birthplaces in the embryo to their final positions, outgrowth of axons and
dendrites from neurons, guidance of the motile growth cone through the embryo
towards postsynaptic partners, the generation of synapses between these axons
and their postsynaptic partners, and finally the lifelong changes in synapses,
which are thought to underlie learning and memory.
Typically,
these neurodevelopmental processes can be broadly divided into two classes:
activity-independent mechanisms and activity-dependent mechanisms.
Activity-independent mechanisms are generally believed to occur as hardwired
processes determined by genetic programs played out within individual neurons.
These include differentiation, migration and axon guidance to their initial
target areas. These processes are thought of as being independent of neural
activity and sensory experience. Once axons reach their target areas,
activity-dependent mechanisms come into play. Although synapse formation is an
activity-independent event, modification of synapses and synapse elimination
requires neural activity.
Developmental
neuroscience uses a variety of animal models including mice Mus musculus, the
fruit fly Drosophila melanogaster, the zebrafish Danio rerio, Xenopus laevis
tadpoles and the worm Caenorhabditis elegans, among others.
Neural
induction
During
early embryonic development the ectoderm becomes specified to give rise to the
epidermis (skin) and the neural plate. The conversion of undifferentiated
ectoderm to neuro-ectoderm requires signals from the mesoderm. At the onset of
gastrulation presumptive mesodermal cells move through the dorsal blastopore
lip and form a layer in between the endoderm and the ectoderm. These mesodermal
cells that migrate along the dorsal midline give rise to a structure called the
notochord. Ectodermal cells overlying the notochord develop into the neural
plate in response to a diffusible signal produced by the notochord. The
remainder of the ectoderm gives rise to the epidermis (skin). The ability of
the mesoderm to convert the overlying ectoderm into neural tissue is called
neural induction.
The
neural plate folds outwards during the third week of gestation to form the
neural groove. Beginning in the future neck region, the neural folds of this
groove close to create the neural tube. The formation of the neural tube from
the ectoderm is called neurulation. The ventral part of the neural tube is
called the basal plate; the dorsal part is called the alar plate. The hollow
interior is called the neural canal. By the end of the fourth week of
gestation, the open ends of the neural tube, called the neuropores, close off.
A
transplanted blastopore lip can convert ectoderm into neural tissue and is said
to have an inductive effect. Neural inducers are molecules that can induce the
expression of neural genes in ectoderm explants without inducing mesodermal
genes as well. Neural induction is often studied in xenopus embryos since they
have a simple body pattern and there are good markers to distinguish between
neural and non-neural tissue. Examples of neural inducers are the molecules
noggin and chordin.
When
embryonic ectodermal cells are cultured at low density in the absence of
mesodermal cells they undergo neural differentiation (express neural genes),
suggesting that neural differentiation is the default fate of ectodermal cells.
In explant cultures (which allow direct cell-cell interactions) the same cells
differentiate into epidermis. This is due to the action of BMP4 (a TGF-β
family protein) that induces ectodermal cultures to differentiate into
epidermis. During neural induction, noggin and chordin are produced by the
dorsal mesoderm (notochord) and diffuse into the overlying ectoderm to inhibit
the activity of BMP4. This inhibition of BMP4 causes the cells to differentiate
into neural cells. Inhibition of TGF-β and BMP signaling can efficiently
induce neural tissue from human pluripotent stem cells, a model of early human
development.
Regionalization
Late
in the fourth week, the superior part of the neural tube flexes at the level of
the future midbrain—the mesencephalon. Above the mesencephalon is the
prosencephalon (future forebrain) and beneath it is the rhombencephalon (future
hindbrain).
The
optical vesicle (which eventually become the optic
nerve, retina and iris) forms at the basal plate of the prosencephalon. The
alar plate of the prosencephalon expands to form the cerebral hemispheres (the
telencephalon) whilst its basal plate becomes the diencephalon. Finally, the
optic vesicle grows to form an optic outgrowth.
Patterning
of the nervous system
In
chordates, dorsal ectoderm forms all neural tissue and the nervous system.
Patterning occurs due to specific environmental conditions - different concentrations
of signaling molecules
Dorsoventral
axis
The
ventral half of the neural plate is controlled by the notochord, which acts as
the 'organiser'. The dorsal half is controlled by the ectoderm plate, which
flanks either side of the neural plate.
Ectoderm
follows a default pathway to become neural tissue. Evidence for this comes from
single, cultured cells of ectoderm, which go on to form neural tissue. This is
postulated to be because of a lack of BMPs, which are blocked by the organiser.
The organiser may produce molecules such as follistatin,
noggin and chordin that inhibit BMPs.
The
ventral neural tube is patterned by Sonic Hedgehog (Shh) from the notochord,
which acts as the inducing tissue. Notochord-derived Shh signals to the floor
plate, and induces Shh expression in the floor plate. Floor plate-derived Shh
subsequently signals to other cells in the neural tube, and is essential for
proper specification of ventral neuron progenitor domains. Loss of Shh from the
notochord and/or floor plate prevents proper specification of these progenitor
domains. Shh binds Patched1, relieving Patched-mediated inhibition of
Smoothened, leading to activation of Gli family of transcription factors (Gli1,
Gli2, and Gli3) transcription factors.
In
this context Shh acts as a morphogen - it induces cell differentiation
dependent on its concentration. At low concentrations it forms ventral
interneurones, at higher concentrations it induces motor neuron development,
and at highest concentrations it induces floor plate differentiation. Failure
of Shh-modulated differentiation causes holoprosencephaly.
The
dorsal neural tube is patterned by BMPs from the epidermal ectoderm flanking
the neural plate. These induce sensory interneurones by activating Sr/Thr
kinases and altering SMAD transcription factor levels.
Rostrocaudal
(Anteroposterior) axis
Signals
that control anteroposterior neural development include FGF and retinoic acid,
which act in the hindbrain and spinal cord.[6] The
hindbrain, for example, is patterned by Hox genes, which are expressed in
overlapping domains along the anteroposterior axis under the control of
retinoic acid. The 3' genes in the Hox cluster are induced by retinoic acid in
the hindbrain, whereas the 5' Hox genes are not induced by retinoic acid and
are expressed more posteriorly in the spinal cord. Hoxb-1 is expressed in
rhombomere 4 and gives rise to the facial nerve. Without this Hoxb-1
expression, a nerve similar to the trigeminal nerve arises.
Neuronal
migration
Corticogenesis: younger neurons
migrate past older ones using radial glia as a scaffolding.
Cajal-Retzius cells (red) release reelin (orange).
Neuronal
migration is the method by which neurons travel from their origin or birthplace
to their final position in the brain. There are several ways they can do this,
e.g. by radial migration or tangential migration. This time lapse displays
sequences of radial migration (also known as glial guidance) and somal
translocation.
Tangential
migration of interneurons from ganglionic eminence. Radial migration Neuronal
precursor cells proliferate in the ventricular zone of the developing
neocortex. The first postmitotic cells to migrate form the preplate, which are
destined to become Cajal-Retzius cells and subplate neurons. These cells do so
by somal translocation. Neurons migrating with this mode of locomotion are
bipolar and attach the leading edge of the process to the pia. The soma is then
transported to the pial surface by nucleokinesis, a process by which a
microtubule "cage" around the nucleus elongates and contracts in
association with the centrosome to guide the nucleus to its final destination.
Radial glia, whose fibers serve as a scaffolding for migrating cells, can
itself divide or translocate to the cortical plate and differentiate either
into astrocytes or neurons. Somal translocation can occur at any time during
development.
Subsequent
waves of neurons split the preplate by migrating along radial glial fibres to
form the cortical plate. Each wave of migrating cells travel past their
predecessors forming layers in an inside-out manner, meaning that the youngest
neurons are the closest to the surface. It is estimated that glial guided
migration represents 90% of migrating neurons in human and about 75% in
rodents.
Tangential
migration Most interneurons migrate tangentially through multiple modes of
migration to reach their appropriate location in the cortex. An example of
tangential migration is the movement of interneurons from the ganglionic
eminence to the cerebral cortex. One example of ongoing tangential migration in
a mature organism, observed in some animals, is the rostral migratory stream
connecting subventricular zone and olfactory bulb.
Others
modes of migration There is also a method of neuronal migration called
multipolar migration. This is seen in multipolar cells, which are abundantly
present in the cortical intermediate zone. They do not resemble the cells
migrating by locomotion or somal translocation. Instead these multipolar cells
express neuronal markers and extend multiple thin processes in various
directions independently of the radial glial fibers.
Neurotrophic
factors
The
survival of neurons is regulated by survival factors, called trophic factors.
The neurotrophic hypothesis was formulated by Victor Hamburger and Rita Levi
Montalcini based on studies of the developing nervous system. Victor Hamburger
discovered that implanting an extra limb in the developing chick led to an
increase in the number of spinal motor neurons. Initially he thought that the
extra limb was inducing proliferation of motor neurons, but he and his
colleagues later showed that there was a great deal of motor neuron death
during normal development, and the extra limb prevented this cell death.
According to the neurotrophic hypothesis, growing axons compete for limiting
amounts of target-derived trophic factors and axons that neurons that fail to
receive sufficient trophic support die by apoptosis. It is now clear that
factors produced by a number of sources contribute to neuronal survival.
Nerve
Growth Factor (NGF): Rita Levi Montalcini and Stanley Cohen purified the first
trophic factor, Nerve Growth Factor (NGF), for which they received the Nobel
Prize. There are three NGF-related trophic factors: BDNF, NT3, and NT4, which
regulate survival of various neuronal populations. The Trk proteins act as
receptors for NGF and related factors. Trk is a receptor tyrosine kinase. Trk
dimerization and phosphorylation leads to activation of various intracellular
signaling pathways including the MAP kinase, Akt, and PKC pathways.
CNTF:
Ciliary neurotrophic factor is another protein that acts as a survival factor
for motor neurons. CNTF acts via a receptor complex that includes CNTFRα,
GP130, and LIFRβ. Activation of the receptor leads to phosphorylation and
recruitment of the JAK kinase, which in turn phosphorylates LIFRβ.
LIFRβ acts as a docking site for the STAT transcription factors. JAK
kinase phosphorylates STAT proteins, which dissociate from the receptor and
translocate to the nucleus to regulate gene expression.
GDNF:
Glial derived neurotrophic factor is a member of the TGFb family of proteins,
and is a potent trophic factor for striatal neurons. The functional receptor is
a heterodimer, composed of type 1 and type 2 receptors. Activation of the type
1 receptor leads to phosphorylation of Smad proteins, which translocate to the
nucleus to activate gene expression.
Synapse
formation
Neuromuscular
junction Much of our understanding of synapse formation comes from studies at
the neuromuscular junction. The transmitter at this synapse is acetylcholine.
The acetylcholine receptor (AchR) is present at the surface of muscle cells
before synapse formation. The arrival of the nerve induces clustering of the
receptors at the synapse. McMahan and Sanes showed that the synaptogenic signal
is concentrated at the basal lamina. They also showed that the synaptogenic
signal is produced by the nerve, and they identified the factor as Agrin. Agrin
induces clustering of AchRs on the muscle surface and synapse formation is
disrupted in agrin knockout mice. Agrin transduces the signal via MuSK receptor
to rapsyn. Fischbach and colleagues showed that receptor subunits are
selectively transcribed from nuclei next to the synaptic site. This is mediated
by neuregulins.
In
the mature synapse each muscle fiber is innervated by one motor neuron.
However, during development many of the fibers are innervated by multiple
axons. Lichtman and colleagues have studied the process of synapses
elimination. This is an activity-dependent event. Partial blockage of the
receptor leads to retraction of corresponding presynaptic terminals.
CNS
synapses Agrin appears not to be a central mediator of CNS synapse formation
and there is active interest in identifying signals that mediate CNS
synaptogenesis. Neurons in culture develop synapses that are similar to those
that form in vivo, suggesting that synaptogenic signals can function properly
in vitro. CNS synaptogenesis studies have focused mainly on glutamatergic
synapses. Imaging experiments show that dendrites are highly dynamic during
development and often initiate contact with axons. This is followed by
recruitment of postsynaptic proteins to the site of contact. Stephen Smith and
colleagues have shown that contact initiated by dendritic filopodia can develop
into synapses.
Induction
of synapse formation by glial factors: Barres and colleagues made the
observation that factors in glial conditioned media induce synapse formation in
retinal ganglion cell cultures. Synapse formation in the CNS is correlated with
astrocyte differentiation suggesting that astrocytes might provide a
synaptogenic factor. The identity of the astrocytic factors is not yet known.
Neuroligins
and SynCAM as synaptogenic signals: Sudhof, Serafini, Scheiffele and colleagues
have shown that neuroligins and SynCAM can act as factors that induce
presynaptic differentiation. Neuroligins are concentrated at the postsynaptic
site and act via neurexins concentrated in the presynaptic axons. SynCAM is a
cell adhesion molecule that is present in both pre- and post-synaptic
membranes.
Synapse
elimination
Several
motorneurones compete for each neuromuscular junction, but only one survives
until adulthood. Competition in vitro has been shown to involve a limited
neurotrophic substance that is released, or that neural activity infers
advantage to strong post-synaptic connections by giving resistance to a toxin
also released upon nerve stimulation. In vivo it is suggested that muscle
fibres select the strongest neuron through a retrograde signal.
Tangential
migration of interneurons from ganglionic eminence.
Radial
migration Neuronal precursor cells proliferate in the ventricular zone of the
developing neocortex. The first postmitotic cells to migrate form the preplate,
which are destined to become Cajal-Retzius cells and subplate neurons. These
cells do so by somal translocation. Neurons migrating with this mode of
locomotion are bipolar and attach the leading edge of the process to the pia.
The soma is then transported to the pial surface by nucleokinesis, a process by
which a microtubule "cage" around the nucleus elongates and contracts
in association with the centrosome to guide the nucleus to its final
destination. Radial glia, whose fibers serve as a scaffolding for migrating
cells, can itself divide or translocate to the cortical plate and differentiate
either into astrocytes or neurons. Somal translocation can occur at any time
during development.
Subsequent
waves of neurons split the preplate by migrating along radial glial fibres to
form the cortical plate. Each wave of migrating cells travel past their
predecessors forming layers in an inside-out manner, meaning that the youngest
neurons are the closest to the surface. It is estimated that glial guided
migration represents 90% of migrating neurons in human and about 75% in
rodents.
Tangential
migration Most interneurons migrate tangentially through multiple modes of
migration to reach their appropriate location in the cortex. An example of
tangential migration is the movement of interneurons from the ganglionic
eminence to the cerebral cortex. One example of ongoing tangential migration in
a mature organism, observed in some animals, is the rostral migratory stream
connecting subventricular zone and olfactory bulb.
Others
modes of migration There is also a method of neuronal migration called
multipolar migration. This is seen in multipolar cells, which are abundantly
present in the cortical intermediate zone. They do not resemble the cells
migrating by locomotion or somal translocation. Instead these multipolar cells
express neuronal markers and extend multiple thin processes in various
directions independently of the radial glial fibers.
Neurotrophic
factors
The
survival of neurons is regulated by survival factors, called trophic factors.
The neurotrophic hypothesis was formulated by Victor Hamburger and Rita Levi
Montalcini based on studies of the developing nervous system. Victor Hamburger
discovered that implanting an extra limb in the developing chick led to an
increase in the number of spinal motor neurons. Initially he thought that the
extra limb was inducing proliferation of motor neurons, but he and his
colleagues later showed that there was a great deal of motor neuron death
during normal development, and the extra limb prevented this cell death.
According to the neurotrophic hypothesis, growing axons compete for limiting amounts
of target-derived trophic factors and axons that neurons that fail to receive
sufficient trophic support die by apoptosis. It is now clear that factors
produced by a number of sources contribute to neuronal survival.
Nerve
Growth Factor (NGF): Rita Levi Montalcini and Stanley Cohen purified the first
trophic factor, Nerve Growth Factor (NGF), for which they received the Nobel
Prize. There are three NGF-related trophic factors: BDNF, NT3, and NT4, which
regulate survival of various neuronal populations. The Trk proteins act as
receptors for NGF and related factors. Trk is a receptor tyrosine kinase. Trk
dimerization and phosphorylation leads to activation of various intracellular
signaling pathways including the MAP kinase, Akt, and PKC pathways.
CNTF:
Ciliary neurotrophic factor is another protein that acts as a survival factor
for motor neurons. CNTF acts via a receptor complex that includes CNTFRα,
GP130, and LIFRβ. Activation of the receptor leads to phosphorylation and
recruitment of the JAK kinase, which in turn phosphorylates LIFRβ.
LIFRβ acts as a docking site for the STAT transcription factors. JAK
kinase phosphorylates STAT proteins, which dissociate from the receptor and
translocate to the nucleus to regulate gene expression.
GDNF:
Glial derived neurotrophic factor is a member of the TGFb family of proteins,
and is a potent trophic factor for striatal neurons. The functional receptor is
a heterodimer, composed of type 1 and type 2 receptors. Activation of the type
1 receptor leads to phosphorylation of Smad proteins, which translocate to the
nucleus to activate gene expression.
Synapse
formation
Neuromuscular
junction Much of our understanding of synapse formation comes from studies at
the neuromuscular junction. The transmitter at this synapse is acetylcholine.
The acetylcholine receptor (AchR) is present at the surface of muscle cells
before synapse formation. The arrival of the nerve induces clustering of the
receptors at the synapse. McMahan and Sanes showed that the synaptogenic signal
is concentrated at the basal lamina. They also showed that the synaptogenic
signal is produced by the nerve, and they identified the factor as Agrin. Agrin
induces clustering of AchRs on the muscle surface and synapse formation is
disrupted in agrin knockout mice. Agrin transduces the signal via MuSK receptor
to rapsyn. Fischbach and colleagues showed that receptor subunits are
selectively transcribed from nuclei next to the synaptic site. This is mediated
by neuregulins.
In
the mature synapse each muscle fiber is innervated by one motor neuron.
However, during development many of the fibers are innervated by multiple
axons. Lichtman and colleagues have studied the process of synapses
elimination. This is an activity-dependent event. Partial blockage of the
receptor leads to retraction of corresponding presynaptic terminals.
CNS
synapses Agrin appears not to be a central mediator of CNS synapse formation
and there is active interest in identifying signals that mediate CNS synaptogenesis.
Neurons in culture develop synapses that are similar to those that form in
vivo, suggesting that synaptogenic signals can function properly in vitro. CNS
synaptogenesis studies have focused mainly on glutamatergic synapses. Imaging
experiments show that dendrites are highly dynamic during development and often
initiate contact with axons. This is followed by recruitment of postsynaptic
proteins to the site of contact. Stephen Smith and colleagues have shown that
contact initiated by dendritic filopodia can develop into synapses.
Induction
of synapse formation by glial factors: Barres and colleagues made the
observation that factors in glial conditioned media induce synapse formation in
retinal ganglion cell cultures. Synapse formation in the CNS is correlated with
astrocyte differentiation suggesting that astrocytes might provide a
synaptogenic factor. The identity of the astrocytic factors is not yet known.
Neuroligins
and SynCAM as synaptogenic signals: Sudhof, Serafini, Scheiffele and colleagues
have shown that neuroligins and SynCAM can act as factors that induce
presynaptic differentiation. Neuroligins are concentrated at the postsynaptic
site and act via neurexins concentrated in the presynaptic axons. SynCAM is a
cell adhesion molecule that is present in both pre- and post-synaptic
membranes.
Synapse
elimination
Several
motorneurones compete for each neuromuscular junction, but only one survives
until adulthood. Competition in vitro has been shown to involve a limited
neurotrophic substance that is released, or that neural activity infers
advantage to strong post-synaptic connections by giving resistance to a toxin
also released upon nerve stimulation. In vivo it is suggested that muscle
fibres select the strongest neuron through a retrograde signal.
At
the cellular level, the nervous system is defined by the presence of a special
type of cell, called the neuron, also known as a "nerve cell".
Neurons have special structures that allow them to send signals rapidly and
precisely to other cells. They send these signals in the form of
electrochemical waves traveling along thin fibers called axons, which cause
chemicals called neurotransmitters to be released at junctions called synapses.
A cell that receives a synaptic signal from a neuron may be excited, inhibited,
or otherwise modulated. The connections between neurons form neural circuits
that generate an organism's perception of the world and determine its behavior.
Along with neurons, the nervous system contains other specialized cells called
glial cells (or simply glia), which provide structural and metabolic support.
The
peripheral nervous system (PNS, or occasionally PeNS) consists of the nerves
and ganglia outside of the brain and spinal cord. The main function of the PNS
is to connect the central nervous system (CNS) to the limbs and organs. Unlike
the CNS, the PNS is not protected by the bone of spine and skull, or by the
blood–brain barrier, leaving it exposed to toxins and mechanical injuries. The
peripheral nervous system is divided into the somatic nervous system and the
autonomic nervous system; some textbooks also include sensory systems. It is
also a part of the nervous system.
The
cranial nerves are part of the PNS with the exception of cranial nerve II, the
optic nerve, along with the retina. The second cranial nerve is not a true
peripheral nerve but a tract of the diencephalon. Cranial nerve ganglia
originate in the CNS. However, the remaining eleven cranial nerve axons extend
beyond the brain and are therefore considered part of the PNS.
Specific
nerves and plexi
Ten
out of the twelve cranial nerves originate from the brainstem, and mainly
control the functions of the anatomic structures of the head with some
exceptions. The nuclei of cranial nerves I and II lie in the forebrain and
thalamus, respectively, and are thus not considered to be true cranial nerves.
CN X (10) receives visceral sensory information from the thorax and abdomen,
and CN XI (11) is responsible for innervating the sternocleidomastoid and
trapezius muscles, neither of which is exclusively in the head.
Spinal
nerves take their origins from the spinal cord. They control the functions of
the rest of the body. In humans, there are 31 pairs of spinal nerves: 8
cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. In the cervical
region, the spinal nerve roots come out above the corresponding vertebrae (i.e.
nerve root between the skull and 1st cervical vertebrae is called spinal nerve
C1). From the thoracic region to the coccygeal region, the spinal nerve roots
come out below the corresponding vertebrae. It is important to note that this
method creates a problem when naming the spinal nerve root between C7 and T1
(so it is called spinal nerve root C8). In the lumbar and sacral region, the
spinal nerve roots travel within the dural sac and they travel below the level
of L2 as the cauda equina.
Cervical
spinal nerves (C1–C4)
The
cervical plexus is a plexus of the ventral rami of the first four cervical
spinal nerves which are located from C1 to C4 cervical segment in the neck.
They are located laterally to the transverse processes between prevertebral
muscles from the medial side and vertebral (m.scalenus, m.levator scapulae,
m.splenius cervicis) from lateral side. There is anastomosis with accessory
nerve, hypoglossal nerve and sympathetic trunk.
It
is located in the neck, deep to sternocleidomastoid. Nerves formed from the
cervical plexus innervate the back of the head, as well as some neck muscles.
The branches of the cervical plexus emerge from the posterior triangle at the
nerve point, a point which lies midway on the posterior border of the
Sternocleidomastoid.
The
first 4 cervical spinal nerves, C1 through C4, split and recombine to produce a
variety of nerves that subserve the neck and back of head.
Spinal
nerve C1 is called the suboccipital nerve which provides motor innervation to
muscles at the base of the skull. C2 and C3 form many of the nerves of the
neck, providing both sensory and motor control. These include the greater
occipital nerve which provides sensation to the back of the head, the lesser
occipital nerve which provides sensation to the area behind the ears, the
greater auricular nerve and the lesser auricular nerve. See occipital
neuralgia. The phrenic nerve arises from nerve roots C3, C4 and C5. It
innervates the diaphragm, enabling breathing. If the spinal cord is transected
above C3, then spontaneous breathing is not possible. See myelopathy
Brachial
plexus (C5–T1)
The
last four cervical spinal nerves, C5 through C8, and the first thoracic spinal
nerve, T1,combine to form the brachial plexus, or plexus brachialis, a tangled
array of nerves, splitting, combining and recombining, to form the nerves that
subserve the upper-limb and upper back. Although the brachial plexus may appear
tangled, it is highly organized and predictable, with little variation between
people. See brachial plexus injuries.
Lumbosacral
plexus (L1–S4)
The
anterior divisions of the lumbar nerves, sacral nerves, and coccygeal nerve
form the lumbosacral plexus, the first lumbar nerve being frequently joined by
a branch from the twelfth thoracic. For descriptive purposes this plexus is
usually divided into three parts:
lumbar
plexus, sacral plexus, and pudendal plexus.
Neurotransmitters
The
main neurotransmitters of the peripheral nervous system are acetylcholine and
noradrenaline. However, there are several other neurotransmitters as well,
jointly labeled Non-noradrenergic, non-cholinergic (NANC) transmitters.
Examples of such transmitters include non-peptides: ATP, GABA, dopamine, NO,
and peptides: neuropeptide Y, VIP, GnRH, Substance P and CGRP.
Nervous
systems are found in most multicellular animals, but vary greatly in
complexity. The only multicellular animals that have no nervous system at all
are sponges, placozoans and mesozoans, which have very simple body plans. The
nervous systems of ctenophores (comb jellies) and cnidarians (e.g., anemones,
hydras, corals and jellyfishes) consist of a diffuse nerve net. All other types
of animals, with the exception of a few types of worms, have a nervous system
containing a brain, a central cord (or two cords running in parallel), and
nerves radiating from the brain and central cord. The size of the nervous system
ranges from a few hundred cells in the simplest worms, to on the order of 100
billion cells in humans.
At
the most basic level, the function of the nervous system is to send signals
from one cell to others, or from one part of the body to others. The nervous
system is susceptible to malfunction in a wide variety of ways, as a result of
genetic defects, physical damage due to trauma or poison, infection, or simply
aging. The medical specialty of neurology studies the causes of nervous system
malfunction, and looks for interventions that can prevent it or treat it. In
the peripheral nervous system, the most commonly occurring type of problem is
failure of nerve conduction, which can have a variety of causes including
diabetic neuropathy and demyelinating disorders such as multiple sclerosis and
amyotrophic lateral sclerosis.
Neuroscience
is the field of science that focuses on the study of the nervous system.
Structure
The
nervous system derives its name from nerves, which are cylindrical bundles of
fibers that emanate from the brain and central cord, and branch repeatedly to
innervate every part of the body. Nerves are large enough to have been
recognized by the ancient Egyptians, Greeks, and Romans, but their internal
structure was not understood until it became possible to examine them using a
microscope. A microscopic examination shows that nerves consist primarily of
the axons of neurons, along with a variety of membranes that wrap around them
and segregate them into fascicles. The neurons that give rise to nerves do not
lie entirely within the nerves themselves—their cell bodies reside within the
brain, central cord, or peripheral ganglia.
All
animals more advanced than sponges have nervous systems. However, even sponges,
unicellular animals, and non-animals such as slime molds have cell-to-cell
signalling mechanisms that are precursors to those of neurons. In radially
symmetric animals such as the jellyfish and hydra, the nervous system consists
of a diffuse network of isolated cells. In bilaterian animals, which make up
the great majority of existing species, the nervous system has a common
structure that originated early in the Cambrian period, over 500 million years
ago.
Cells
The
nervous system contains two main categories or types of cells: neurons and
glial cells.
Neurons
The
nervous system is defined by the presence of a special type of cell—the neuron
(sometimes called "neurone" or "nerve cell"). Neurons can
be distinguished from other cells in a number of ways, but their most
fundamental property is that they communicate with other cells via synapses,
which are membrane-to-membrane junctions containing molecular machinery that
allows rapid transmission of signals, either electrical or chemical. Many types
of neuron possess an axon, a protoplasmic protrusion that can extend to distant
parts of the body and make thousands of synaptic contacts. Axons frequently
travel through the body in bundles called nerves.
Even
in the nervous system of a single species such as humans, hundreds of different
types of neurons exist, with a wide variety of morphologies and functions.
These include sensory neurons that transmute physical stimuli such as light and
sound into neural signals, and motor neurons that transmute neural signals into
activation of muscles or glands; however in many species the great majority of
neurons receive all of their input from other neurons and send their output to
other neurons.
Glial
cells
Glial
cells (named from the Greek for "glue") are non-neuronal cells that provide
support and nutrition, maintain homeostasis, form myelin, and participate in
signal transmission in the nervous system. In the human brain, it is estimated
that the total number of glia roughly equals the number of neurons, although
the proportions vary in different brain areas. Among the most important
functions of glial cells are to support neurons and hold them in place; to
supply nutrients to neurons; to insulate neurons electrically; to destroy
pathogens and remove dead neurons; and to provide guidance cues directing the
axons of neurons to their targets. A very important type of glial cell
(oligodendrocytes in the central nervous system, and Schwann cells in the
peripheral nervous system) generates layers of a fatty substance called myelin
that wraps around axons and provides electrical insulation which allows them to
transmit action potentials much more rapidly and efficiently.
Anatomy
in vertebrates
The
nervous system of vertebrate animals (including humans) is divided into the
central nervous system (CNS) and peripheral nervous system (PNS).
The
central nervous system (CNS) is the largest part, and includes the brain and
spinal cord. The spinal cavity contains the spinal cord, while the head
contains the brain. The CNS is enclosed and protected by meninges, a
three-layered system of membranes, including a tough, leathery outer layer
called the dura mater. The brain is also protected by the skull, and the spinal
cord by the vertebrae.
The
peripheral nervous system (PNS) is a collective term for the nervous system
structures that do not lie within the CNS. The large majority of the axon
bundles called nerves are considered to belong to the PNS, even when the cell
bodies of the neurons to which they belong reside within the brain or spinal
cord. The PNS is divided into somatic and visceral parts. The somatic part
consists of the nerves that innervate the skin, joints, and muscles. The cell
bodies of somatic sensory neurons lie in dorsal root ganglia of the spinal
cord. The visceral part, also known as the autonomic nervous system, contains
neurons that innervate the internal organs, blood vessels, and glands. The
autonomic nervous system itself consists of two parts: the sympathetic nervous
system and the parasympathetic nervous system. Some authors also include
sensory neurons whose cell bodies lie in the periphery (for senses such as
hearing) as part of the PNS; others, however, omit them.
Diagram
showing the major divisions of the vertebrate nervous system.
The
vertebrate nervous system can also be divided into areas called grey matter
("gray matter" in American spelling) and white matter. Grey matter
(which is only grey in preserved tissue, and is better described as pink or
light brown in living tissue) contains a high proportion of cell bodies of
neurons. White matter is composed mainly of myelinated axons, and takes its
color from the myelin. White matter includes all of the nerves, and much of the
interior of the brain and spinal cord. Grey matter is found in clusters of
neurons in the brain and spinal cord, and in cortical layers that line their
surfaces. There is an anatomical convention that a cluster of neurons in the
brain or spinal cord is called a nucleus, whereas a cluster of neurons in the
periphery is called a ganglion. There are, however, a few exceptions to this
rule, notably including the part of the forebrain called the basal ganglia.
Comparative
anatomy and evolution
Neural
precursors in sponges
Sponges
have no cells connected to each other by synaptic junctions, that is, no
neurons, and therefore no nervous system. They do, however, have homologs of
many genes that play key roles in synaptic function. Recent studies have shown
that sponge cells express a group of proteins that cluster together to form a
structure resembling a postsynaptic density (the signal-receiving part of a
synapse). However, the function of this structure is currently unclear.
Although sponge cells do not show synaptic transmission, they do communicate
with each other via calcium waves and other impulses, which mediate some simple
actions such as whole-body contraction.
Radiata
Jellyfish,
comb jellies, and related animals have diffuse nerve nets rather than a central
nervous system. In most jellyfish the nerve net is spread more or less evenly
across the body; in comb jellies it is concentrated near the mouth. The nerve
nets consist of sensory neurons that pick up chemical, tactile, and visual
signals, motor neurons that can activate contractions of the body wall, and
intermediate neurons that detect patterns of activity in the sensory neurons
and send signals to groups of motor neurons as a result. In some cases groups
of intermediate neurons are clustered into discrete ganglia.
The
development of the nervous system in radiata is relatively unstructured. Unlike
bilaterians, radiata only have two primordial cell layers, endoderm and
ectoderm. Neurons are generated from a special set of ectodermal precursor
cells, which also serve as precursors for every other ectodermal cell type.
Bilateria
The
vast majority of existing animals are bilaterians, meaning animals with left
and right sides that are approximate mirror images of each other. All bilateria
are thought to have descended from a common wormlike ancestor that appeared in
the Cambrian period, 550–600 million years ago. The fundamental bilaterian body
form is a tube with a hollow gut cavity running from mouth to anus, and a nerve
cord with an enlargement (a "ganglion") for each body segment, with
an especially large ganglion at the front, called the "brain".
Nervous
system of a bilaterian animal, in the form of a nerve cord with segmental
enlargements, and a "brain" at the front.
Worms
are the simplest bilaterian animals, and reveal the basic structure of the bilaterian
nervous system in the most straightforward way. As an example, earthworms have
dual nerve cords running along the length of the body and merging at the tail
and the mouth. These nerve cords are connected by transverse nerves like the
rungs of a ladder. These transverse nerves help coordinate the two sides of the
animal. Two ganglia at the head end function similar to a simple brain.
Photoreceptors on the animal's eyespots provide sensory information on light
and dark.
The
nervous system of one very small worm, the roundworm Caenorhabditis elegans,
has been mapped out down to the synaptic level. Every neuron and its cellular
lineage has been recorded and most, if not all, of the neural connections are
known. In this species, the nervous system is sexually dimorphic; the nervous
systems of the two sexes, males and hermaphrodites, have different numbers of
neurons and groups of neurons that perform sex-specific functions. In C.
elegans, males have exactly 383 neurons, while hermaphrodites have exactly 302
neurons.
Earthworm
nervous system.
Top: side view of the front of the worm. Bottom: nervous system in isolation,
viewed from above.
Arthropods
Internal
anatomy of a spider, showing the nervous system in blue.
Arthropods,
such as insects and crustaceans, have a nervous system made up of a series of
ganglia, connected by a ventral nerve cord made up of two parallel connectives
running along the length of the belly. Typically, each body segment has one
ganglion on each side, though some ganglia are fused to form the brain and
other large ganglia. The head segment contains the brain, also known as the
supraesophageal ganglion. In the insect nervous system, the brain is
anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum.
Immediately behind the brain is the subesophageal ganglion, which is composed
of three pairs of fused ganglia. It controls the mouthparts, the salivary
glands and certain muscles. Many arthropods have well-developed sensory organs,
including compound eyes for vision and antennae for olfaction and pheromone
sensation. The sensory information from these organs is processed by the brain.
In
insects, many neurons have cell bodies that are positioned at the edge of the
brain and are electrically passive—the cell bodies serve only to provide
metabolic support and do not participate in signalling. A protoplasmic fiber
runs from the cell body and branches profusely, with some parts transmitting
signals and other parts receiving signals. Thus, most parts of the insect brain
have passive cell bodies arranged around the periphery, while the neural signal
processing takes place in a tangle of protoplasmic fibers called neuropil, in
the interior.
"Identified"
neurons
A
neuron is called identified if it has properties that distinguish it from every
other neuron in the same animal—properties such as location, neurotransmitter,
gene expression pattern, and connectivity—and if every individual organism
belonging to the same species has one and only one neuron with the same set of properties.
In vertebrate nervous systems very few neurons are "identified" in
this sense—in humans, there are believed to be none—but in simpler nervous
systems, some or all neurons may be thus unique. In the roundworm C. elegans,
whose nervous system is the most thoroughly described of any animal's, every
neuron in the body is uniquely identifiable, with the same location and the
same connections in every individual worm. One notable consequence of this fact
is that the form of the C. elegans nervous system is completely specified by
the genome, with no experience-dependent plasticity.
The
brains of many molluscs and insects also contain substantial numbers of
identified neurons. In vertebrates, the best known identified neurons are the
gigantic Mauthner cells of fish. Every fish has two Mauthner cells, located in
the bottom part of the brainstem, one on the left side and one on the right.
Each Mauthner cell has an axon that crosses over, innervating neurons at the
same brain level and then travelling down through the spinal cord, making
numerous connections as it goes. The synapses generated by a Mauthner cell are
so powerful that a single action potential gives rise to a major behavioral
response: within milliseconds the fish curves its body into a C-shape, then
straightens, thereby propelling itself rapidly forward. Functionally this is a
fast escape response, triggered most easily by a strong sound wave or pressure
wave impinging on the lateral line organ of the fish. Mauthner cells are not
the only identified neurons in fish—there are about 20 more types, including
pairs of "Mauthner cell analogs" in each spinal segmental nucleus.
Although a Mauthner cell is capable of bringing about an escape response
individually, in the context of ordinary behavior other types of cells usually
contribute to shaping the amplitude and direction of the response.
Mauthner
cells have been described as command neurons. A command neuron is a special
type of identified neuron, defined as a neuron that is capable of driving a specific
behavior individually. Such neurons appear most commonly in the fast escape
systems of various species—the squid giant axon and squid giant synapse, used
for pioneering experiments in neurophysiology because of their enormous size,
both participate in the fast escape circuit of the squid. The concept of a
command neuron has, however, become controversial, because of studies showing
that some neurons that initially appeared to fit the description were really
only capable of evoking a response in a limited set of circumstances.
Function
At
the most basic level, the function of the nervous system is to send signals
from one cell to others, or from one part of the body to others. There are
multiple ways that a cell can send signals to other cells. One is by releasing
chemicals called hormones into the internal circulation, so that they can
diffuse to distant sites. In contrast to this "broadcast" mode of
signaling, the nervous system provides "point-to-point"
signals—neurons project their axons to specific target areas and make synaptic
connections with specific target cells. Thus, neural signaling is capable of a
much higher level of specificity than hormonal signaling. It is also much
faster: the fastest nerve signals travel at speeds that exceed
At
a more integrative level, the primary function of the nervous system is to
control the body. It does this by extracting information from the environment
using sensory receptors, sending signals that encode this information into the central
nervous system, processing the information to determine an appropriate
response, and sending output signals to muscles or glands to activate the
response. The evolution of a complex nervous system has made it possible for
various animal species to have advanced perception abilities such as vision,
complex social interactions, rapid coordination of organ systems, and
integrated processing of concurrent signals. In humans, the sophistication of
the nervous system makes it possible to have language, abstract representation
of concepts, transmission of culture, and many other features of human society
that would not exist without the human brain.
Neurons
and synapses
Major
elements in synaptic transmission. An electrochemical wave called an action potential travels along the
axon of a neuron. When the wave reaches a synapse, it provokes release of a
small amount of neurotransmitter molecules, which bind to chemical receptor
molecules located in the membrane of the target cell.
Most
neurons send signals via their axons, although some types are capable of
dendrite-to-dendrite communication. (In fact, the types of neurons called
amacrine cells have no axons, and communicate only via their dendrites.) Neural
signals propagate along an axon in the form of electrochemical waves called
action potentials, which produce cell-to-cell signals at points where axon
terminals make synaptic contact with other cells.
Synapses
may be electrical or chemical. Electrical synapses make direct electrical
connections between neurons, but chemical synapses are much more common, and
much more diverse in function. At a chemical synapse, the cell that sends
signals is called presynaptic, and the cell that receives signals is called
postsynaptic. Both the presynaptic and postsynaptic areas are full of molecular
machinery that carries out the signalling process. The presynaptic area
contains large numbers of tiny spherical vessels called synaptic vesicles,
packed with neurotransmitter chemicals. When the presynaptic terminal is electrically
stimulated, an array of molecules embedded in the membrane are activated, and
cause the contents of the vesicles to be released into the narrow space between
the presynaptic and postsynaptic membranes, called the synaptic cleft. The
neurotransmitter then binds to receptors embedded in the postsynaptic membrane,
causing them to enter an activated state. Depending on the type of receptor,
the resulting effect on the postsynaptic cell may be excitatory, inhibitory, or
modulatory in more complex ways. For example, release of the neurotransmitter
acetylcholine at a synaptic contact between a motor neuron and a muscle cell
induces rapid contraction of the muscle cell. The entire synaptic transmission
process takes only a fraction of a millisecond, although the effects on the
postsynaptic cell may last much longer (even indefinitely, in cases where the
synaptic signal leads to the formation of a memory trace). Structure of a
typical chemical synapse
Postsynaptic
There are literally hundreds of different types
of synapses. In fact, there are over a hundred known neurotransmitters, and
many of them have multiple types of receptors. Many synapses use more than one
neurotransmitter—a common arrangement is for a synapse to use one fast-acting
small-molecule neurotransmitter such as glutamate or GABA, along with one or
more peptide neurotransmitters that play slower-acting modulatory roles.
Molecular neuroscientists generally divide receptors into two broad groups:
chemically gated ion channels and second messenger systems. When a chemically
gated ion channel is activated, it forms a passage that allow specific types of
ion to flow across the membrane. Depending on the type of ion, the effect on
the target cell may be excitatory or inhibitory. When a second messenger system
is activated, it starts a cascade of molecular interactions inside the target
cell, which may ultimately produce a wide variety of complex effects, such as
increasing or decreasing the sensitivity of the cell to stimuli, or even
altering gene transcription.
According
to a rule called Dale's principle, which has only a few known exceptions, a
neuron releases the same neurotransmitters at all of its synapses. This does
not mean, though, that a neuron exerts the same effect on all of its targets, because
the effect of a synapse depends not on the neurotransmitter, but on the
receptors that it activates. Because different targets can (and frequently do)
use different types of receptors, it is possible for a neuron to have
excitatory effects on one set of target cells, inhibitory effects on others,
and complex modulatory effects on others still. Nevertheless, it happens that
the two most widely used neurotransmitters, glutamate and GABA, each have
largely consistent effects. Glutamate has several widely occurring types of
receptors, but all of them are excitatory or modulatory. Similarly, GABA has
several widely occurring receptor types, but all of them are inhibitory.
Because of this consistency, glutamatergic cells are frequently referred to as "excitatory
neurons", and GABAergic cells as "inhibitory neurons". Strictly
speaking this is an abuse of terminology—it is the receptors that are
excitatory and inhibitory, not the neurons—but it is commonly seen even in
scholarly publications.
One
very important subset of synapses are capable of forming memory traces by means
of long-lasting activity-dependent changes in synaptic strength. The best-known
form of neural memory is a process called long-term potentiation (abbreviated
LTP), which operates at synapses that use the neurotransmitter glutamate acting
on a special type of receptor known as the NMDA receptor. The NMDA receptor has
an "associative" property: if the two cells involved in the synapse
are both activated at approximately the same time, a channel opens that permits
calcium to flow into the target cell. The calcium entry initiates a second
messenger cascade that ultimately leads to an increase in the number of
glutamate receptors in the target cell, thereby increasing the effective
strength of the synapse. This change in strength can last for weeks or longer.
Since the discovery of LTP in 1973, many other types of synaptic memory traces
have been found, involving increases or decreases in synaptic strength that are
induced by varying conditions, and last for variable periods of time. Reward
learning, for example, depends on a variant form of LTP that is conditioned on
an extra input coming from a reward-signalling pathway that uses dopamine as
neurotransmitter. All these forms of synaptic modifiability, taken
collectively, give rise to neural plasticity, that is, to a capability for the
nervous system to adapt itself to variations in the environment.
he
autonomic nervous system (ANS or visceral nervous system or involuntary nervous
system) is the part of the peripheral nervous system that acts as a control
system functioning largely below the level of consciousness, and controls
visceral functions. The ANS affects heart rate, digestion, respiratory rate,
salivation, perspiration, pupillary dilation, micturition (urination), and
sexual arousal. Most autonomous functions are involuntary but a number of ANS
actions can work alongside some degree of conscious control. Everyday examples
include breathing, swallowing, and sexual arousal, and in some cases functions
such as heart rate.
Within
the brain, the ANS is located in the medulla oblongata in the lower brainstem.
The medulla's major ANS functions include respiration (the respiratory control
centre, or "rcc"), cardiac regulation (the cardiac control centre, or
"ccc"), vasomotor activity (the vasomotor centre or "vmc"),
and certain reflex actions (such as coughing, sneezing, vomiting and
swallowing). These then subdivide into other areas and are also linked to ANS
subsystems and nervous systems external to the brain. The hypothalamus, just
above the brain stem, acts as an integrator for autonomic functions, receiving
ANS regulatory input from the limbic system to do so.
The
ANS is classically divided into two subsystems: the parasympathetic nervous
system (PSNS) and sympathetic nervous system (SNS) which operate independently
in some functions and interact co-operatively in others. In many cases the two
have "opposite" actions where one activates a physiological response
and the other inhibits it. An older simplification of the sympathetic and
parasympathetic nervous systems as "excitory" and
"inhibitory" was overturned due to the many exceptions found. A more
modern characterisation is that the sympathetic nervous system is a "quick
response mobilising system" and the parasympathetic is a "more slowly
activated dampening system", but even this has exceptions, such as in
sexual arousal and orgasm where both play a role. The enteric nervous system is
also sometimes considered part of the autonomic nervous system, and sometimes
considered an independent system.
ANS
functions can generally be divided into sensory (afferent) and motor (efferent)
subsystems. Within both there are inhibitory and excitatory synapses between
neurons. Relatively recently, a third subsystem of neurons that have been named
'non-adrenergic and non-cholinergic' neurons (because they use nitric oxide as
a neurotransmitter) have been described and found to be integral in autonomic
function, particularly in the gut and the lungs.
Anatomy
ANS
innervation is divided into sympathetic nervous system and parasympathetic
nervous system divisions. The sympathetic division has thoracolumbar “outflow”,
meaning that the neurons begin at the thoracic and lumbar (T1-L2) portions of
the spinal cord. The parasympathetic division has craniosacral “outflow”,
meaning that the neurons begin at the cranial nerves (CN 3, CN7, CN 9, CN10)
and sacral (S2-S4) spinal cord.
The
ANS is unique in that it requires a sequential two-neuron efferent pathway; the
preganglionic neuron must first synapse onto a postganglionic neuron before
innervating the target organ. The preganglionic, or first, neuron will begin at
the “outflow” and will synapse at the postganglionic, or second, neuron’s cell
body. The postganglionic neuron will then synapse at the target organ.
Sympathetic
division
The
sympathetic division (thoracolumbar outflow) consists of cell bodies in the
lateral horn of spinal cord (intermediolateral cell columns) from T1 to L2. These
cell bodies are GVE (general visceral efferent) neurons and are the
preganglionic neurons. There are several locations upon which preganglionic
neurons can synapse for their postganglionic neurons:
·
Paravertebral ganglia (3) of the
sympathetic chain (these run on either side of the vertebral bodies)
·
Cervical Ganglia (3)
·
Thoracic Ganglia (12) and Rostral
Lumbar Ganglia (2 or 3)
·
Caudal Lumbar Ganglia and Pelvic
Ganglia
·
Prevertebral ganglia (celiac
ganglion, aorticorenal ganglion, superior mesenteric ganglion, inferior
mesenteric ganglion)
·
Chromaffin cells of [adrenal medulla]
(this is the one exception to the two-neuron pathway rule: synapse is direct
onto the target cell bodies)
These
ganglia provide the postganglionic neurons from which innervation of target
organs follows. Examples of splanchnic (visceral) nerves are:
·
Cervical cardiac nerves &
thoracic visceral nerves which synapse in the sympathetic chain
·
Thoracic splanchnic nerves (greater,
lesser, least) which synapse in the prevertebral ganglion
·
Lumbar splanchnic nerves which
synapse in the prevertebral ganglion
·
Sacral splanchnic nerves which
synapse in the inferior hypogastric plexus
These
all contain afferent (sensory) nerves as well, known as GVA (general visceral
afferent) neurons.
Parasympathetic
division
The
parasympathetic division (craniosacral outflow) consists of cell bodies from
one of two locations: brainstem (Cranial Nerves III, VII, IX, X) or sacral
spinal cord (S2, S3, S4). These are the preganglionic neurons, which synapse
with postganglionic neurons in these locations:
Parasympathetic
ganglia of the head (Ciliary (CN III), Submandibular (CN VII), Pterygopalatine
(CN VII), Otic (CN IX))
In
or near wall of organ innervated by Vagus (CN X), Sacral nerves (S2, S3, S4))
These
ganglia provide the postganglionic neurons from which innervations of target
organs follows. Examples are:
The
preganglionic parasympathetic splanchnic (visceral) nerves
Vagus
nerve, which wanders through the thorax and abdominal regions innervating,
among other organs, the heart, lungs, liver and stomach.
Functions
of the brain
The
brain is made up of several parts. Each part has a certain function:
Cerebral
Cortex
Thought
, voluntary movement , language, reasoning and perception are the major functions
of the cerebral cortex.
Cortex literally means "bark" (of a
tree) in latin and is so termed because it is a sheet of tissue that makes up
the outer layer of the brain.
The thickness of the cerebral cortex is
between 2 to
The cortex has numerous grooves and bumps to
increase its surface area. A bump or bulge on the cortex is called a gyrus (the
plural of the word gyrus is "gyri") and a groove is called a sulcus
(the plural of the word sulcus is "sulci").
Cerebellum
The
major functions of the cerebellum are maintenance of movement, balance and
posture. The word "cerebellum" comes from the Latin word for
"little brain." It is divided into two parts or hemispheres and has a
cortex that covers the hemispheres.
Hypothalamus
The
hypothalamus regulates the body temperatures, emotions and hunger, thirst and
controls the circadian rhythms.
This pea sized organ is in control of body
temperature. It acts like a "thermostat" by sensing changes in body
temperature and sends out signals to adjust the temperature.
Brain
stem or Medulla oblongata
This
area is vital for life as it controls breathing, heart rate and blood pressure.
The brain stem comprises of the medulla, pons, tectum, reticular formation and
tegmentum.
Thalamus
Works
by integrating sensory information and motor information. The thalamus receives
sensory information and relays this information to the cerebral cortex.
The cerebral cortex also sends information to
the thalamus which then transmits this information to other areas of the brain
and spinal cord.
Limbic
System
This
part of the brain includes amygdala, the hippocampus, mammillary bodies and
cingulate gyrus. These help in controlling the emotional response. The
hippocampus is also important for learning and memory.
Basal
Ganglia
This
part works in maintaining balance and movements. It includes structures like
the globus pallidus, caudate nucleus, subthalamic nucleus, putamen and
substantia nigra.
Midbrain
This
part of the brain has sites controlling vision, hearing, eye movement and
general body movement. The structures that are part of the midbrain are
superior and inferior colliculi and red nucleus.
Functions
of the Cerebrospinal nervous system
This
system has 12 pairs of cranial nerves.
These are attached to the brain and have specific functions. Each
cranial nerve leaves the skull through an opening at its base.
The nerves and their functions include:
·
Olfactory – for smell
·
Optic - Sight
·
Oculomotor - Movement of the eyeball,
lens, and pupils
·
Trochlear – Movement of the Superior
oblique muscle of the eye
·
rigeminal – Innervates the eyes,
cheeks and the jaw areas and controls chewing
·
Abducens - Moves the eye outward
·
Facial - Controls muscles of the
face, scalp, ears; controls salivary glands; receives taste sensation from the
anterior two-thirds of the tongue
·
Acoustic – Hearing and maintenance of
balance
·
Glossopharyngeal – Taste sensation
from the back of the tongue and throat
·
Vagus – Innervates the chest and
abdominal organs
·
Spinal Accessory - movement of head
and shoulders
·
Hypoglossal - Controls muscles of tongue
Functions
of the Autonomic nervous system
The
autonomic nervous system is divided into sympathetic and parasympathetic
nervous systems. These two systems have opposite effects on the same set of
organs.
The sympathetic nervous system is important during
an emergency and is associated with “fight or flight reaction”. The energy is
directed away from digestion, there is dilation of pupils, increased heart
rate, increased perspiration and salivation, increased breathing etc.
The parasympathetic nervous system is
associated with a relaxed state. The pupils contract, energy is diverted for
digestion of food, heart rate slows etc.
Nervous System Disorders Glossary
|
A |
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acetylcholine
- |
a
chemical in the brain that acts as a neurotransmitter. |
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action
tremor |
a
tremor that increases when the hand is moving voluntarily. |
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activities
of daily living (ADLs) |
personal
care activities necessary for everyday living, such as eating, bathing, grooming,
dressing, and toileting; a term often used by healthcare professionals to
assess the need and/or type of care a person may require. |
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advance
directives |
documents
(such as a Living Will) completed and signed by a person who is legally competent
to explain his/her wishes for medical care should he/she become unable to
make those decisions at a later time. |
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agitation
|
a
non-specific symptom of one or more physical, or psychological processes in which
vocal or motor behavior (screaming, shouting, complaining, moaning, cursing,
pacing, fidgeting, wandering) pose risk or discomfort, become disruptive or
unsafe, or interfere with the delivery of care in a particular environment. |
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agonist
|
a
drug that increases neurotransmitter activity by stimulating the receptors of
a neurotransmitter directly. |
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akinesia
|
no
movement. |
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Alzheimer's
disease |
a
progressive, degenerative disease that occurs in the brain and results in impaired
memory, thinking, and behavior. |
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arteriogram
(also called angiogram) |
a
procedure that provides a scan of arteries and/or veins going to and through
the brain. |
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ataxia
|
loss
of balance. |
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amyotrophic
lateral sclerosis (ALS) |
a
terminal neurological disorder characterized by progressive degeneration of
motor cells in the spinal cord and brain. It is often referred to as
"Lou Gehrig's disease." |
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athetosis
|
slow,
involuntary movements of the hands and feet. |
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Atrophy |
wasting,
shrinkage of muscle tissue or nerve tissue. |
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Axon |
the
long, hair-like extension of a nerve cell that carries a message to the next
nerve cell. |
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|
B |
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basal
ganglia |
several
large clusters of nerve cells, including the putamen and globus pallidus,
deep in the brain below the cerebral hemispheres. |
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Bell's
palsy |
an
unexplained episode of facial muscle weakness or paralysis that begins
suddenly and steadily worsens. |
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blink
rate |
the
number of times per minute that the eyelid automatically closes - normally 10
to 20 per minute. |
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blood-brain
barrier |
the
protective membrane that separates circulating blood from brain cells. |
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Bradykinesia |
slowness
of movement. |
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bradyphrenia
|
slowness
of thought processes. |
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brain
attack (also called stroke) - |
happens
when brain cells die because of inadequate blood flow to the brain or when
function of a part of the brain is suddenly lost because of the rupture of a blood
vessel in the brain. |
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|
C |
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central
nervous system |
the
brain and the spinal cord. |
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|
cerebellum
|
a
large structure consisting of two halves (hemispheres) located in the lower part
of the brain; responsible for the coordination of movement and balance. |
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cerebral
embolism |
a
brain attack that occurs when a blood clot (embolus) or some other particle
forms in a blood vessel and travels to a blood vessel in the brain to the point
where it blocks blood flow in the vessel; often the clot forms away from the
brain, usually in the heart. |
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cerebral
hemorrhage |
a
type of stroke occurs when a defective artery in the brain bursts, flooding
the surrounding tissue with blood. |
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|
cerebral
spinal fluid analysis (also called spinal tap or lumbar puncture) |
a
procedure used to make an evaluation or diagnosis by examining the fluid
withdrawn from the spinal column. |
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|
cerebral
thrombosis |
the
most common type of brain attack; occurs when a blood clot (thrombus) forms
and blocks blood flow in an artery bringing blood to part of the brain. |
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|
Cerebrum |
consists
of two parts (lobes), left and right, which form the largest and most
developed part of the brain; initiation and coordination of all voluntary
movement take place within the cerebrum. The basal ganglia are located
immediately below the cerebrum. |
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Chorea |
rapid,
jerky, dance-like movement of the body. |
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|
computed
tomography scan (also called a CT or CAT scan) |
a
diagnostic imaging procedure that uses a combination of x-rays and computer
technology to produce cross-sectional images (often called slices), both
horizontally and vertically, of the body. A CT scan shows detailed images of
any part of the body, including the bones, muscles, fat, and organs. CT scans
are more detailed than general x-rays. |
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|
cortex
|
the
outer layer of the cerebrum, densely packed with nerve cells. |
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|
cryothalamotomy
|
a
surgical procedure in which a super-cooled probe is inserted into a part of
the brain called the thalamus in order to stop tremors. |
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|
D |
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|
delusions
|
a
condition in which the patient has lost touch with reality and experiences hallucinations
and misperceptions. |
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|
dementia
|
not
a disease itself, but group of symptoms that characterize diseases and
conditions; it is commonly defined as a decline in intellectual functioning
that is severe enough to interfere with the ability to perform routine
activities |
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|
Dendrite |
a
threadlike extension from a nerve cell that serves as an antenna to receive
messages from the axons of other nerve cells. |
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|
dopa
decarboxylase |
an
enzyme present in the body that converts levodopa to dopamine. |
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|
dopamine
|
a
chemical substance, a neurotransmitter, found in the brain that regulates
movement, balance, and walking. |
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|
dyskinesia
|
an
involuntary movement including athetosis and chorea. |
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|
Dysphagia |
difficulty
in swallowing. |
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|
dystonia |
a
slow movement or extended spasm in a group of muscles. |
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|
dystrophin
|
a
protein; a chemical substance made by muscle fibers |
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