NERVE SYSTEM
Usinglectures(on
theweb-page of the departmentpostedthe presentationtextandlectures), books,additional literatureand other sources, students must to
preparethe following theoreticalquestions:
1.
Spinal
ganglion origin, development, general structure and functional meaning.
2.
Morphological
and functional peculiarities of spinal node sensory neurons and neuralgia
compounds.
3.
Peripheral
nerve structure and meaning of connective tissue tunics.
4.
Degeneration
and regeneration of the nerve after the damage.
5.
Simple
and complex somatic reflex arch principal compounds.
6.
Basic
steps in the development of the nervous system.
7.
The
structural and functional characteristics of the spinal cord.
8.
Comparison
of gray matter and white matter in terms of: their location in the spinal cord
and their predominant neuronal components (cell bodies, axons, dendrites).
9.
Comparison
of dura mater, arachnoid, and pia mater in terms of their tissue type and the
presence of blood vessels.
10.
Description
of the blood-brain barrier in terms of its structural correlates and its
function.
11.
Description
of the source, composition and circulation of the cerebrospinal fluid.
Nerve
system anatomically is divided into the central nervous system (CNS),
comprising the brain and spinal cord, and the peripheral nervous system (PNS),
which constitutes all nervous tissue outside the CNS (peripheral nerves,
ganglia, plexus and nervous endings).
The
peripheral nervous system, or PNS, is part of the nervous system, and
consists of the nerves and neurons that reside or extend outside the central
nervous system (the brain and spinal cord) to serve the limbs and organs, for example. Unlike
the central nervous system, however, the PNS is not protected by bone or 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.
Histologically,
however, the entire nervous system merely consists of variations in the
arrangement of neurons and their supporting structures.
NERVOUS SYSTEM ORIGIN
Nerve system originate of nerve tube and ganglious
lamella. The brain and sense organs are developing from the cranial portion of
nerve tube. Middle part of nerve tube and ganglionic plate give rise to spinal
cord, dorsal-root ganglia (spinal ganglia), autonomic ganglia and chromaffin
tissue of human body. The
cells mass especially quiqly increases in the lateral part of nerve tube,
whereas dorsal and ventral portions are not inlarged and have ependymal
features. At this stage three zones can be
recognized in the wall of nerve tube: ependyma, which covers the spinal channel, mantial layer and marginal zone. The innermost layer
contains the precursors of glial ependymal cells. Population of middle zone
includes two type of cells: neuroblasts and spongyoblasts, whichgive rise
accordingly to nerve cells and glyal ones (astrocytes and oligodendrocytes). Later
grey matter of spinal cord is developing from mantial zone and white matter
from outer one.
The
development of dorsal-root ganglia and peripheral autonomic ganglia begins at
the same time. Ganglionic plate gives the
sources for their origin – neuroblasts and glioblasts, which differentiate into
neurons and mantial cells of spinal ganglia. Some cells migrate peripherally
and there produce autonomic ganglia and chromaffin tissue.
SPINAL CORD
Spinal cord is long cylindrical structure, which
elongates from the brain down to the lumbar part, in which it is continuing
into caudaequina.
In cross sections of the spinal
cord, white matter is peripheral and gray matter is central, assuming the shape
of an H. In the horizontal bar
of this H is an opening, the central canal, which is a remnant of the lumen of
the embryonic neural tube. It is lined by ependymal cells. The gray matter of
the legs of the H forms the anterior horns. These contain motor neurons,
arranged in 5 nuclei, whose axons make up the ventral roots of the spinal
nerves. Gray matter also forms the posterior horns (the arms of the H), which
receive sensory fibers from neurons in the spinal ganglia (dorsal roots). Here
are two nuclei: nucleus thoracicus and nucleus proprius.
The intermediate horns are better prominent in the
thorathic and lumbar part of spinal cord and contain lateral and medial nuclei.
The first ones belong to the sympathetic nerve system: they are the centers of
it.
Cross
section through the spinal cord.
Spinal cord neurons mainly are large and multipolar,
especially in the anterior horns, where large motor neurons are found.
Nerve cells of spinal cord are
classifying in special way: radicular, funicular and inner cells. Radicular
cells are found in the anterior horns nuclei and intermediate lateral one.
Axons of these cells move outside from spinal cord forming the anterior radix.
Funicular cells of other nuclei pass their axons to the gray matter in which
they form the ascendant funiculi of white matter (passways of the spinal cord).
Inner cells are present in all the nuclei, their axons connect the neurons in
gray matter.
Cross
section of the spinal cord in the transition between gray matter (below) and
white matter (above). Note the neuronal bodies and abundant cell processes in
the gray matter, whereas the white matter consists mainly of nerve fibers whose
myelin sheath was dissolved by the histologic procedure. PT stain. Medium
magnification.
A group of large multipolar neurons, as found in the
gray matter of the anterior horn. Cell nuclei are pale (or vesicular) and round
and contain a large amount of Nissl substance (RER). The smallest nuclei in the
field belong to glial cells. In an area like this, glia play a supportive and
nutritive role. They take the place of connective tissue within the central
nervous system (i.e., the brain and spinal cord).
A large, multipolar, motor neuron of the anterior
horn, seen whole, with all its processes stretched out in a spinal cord smear.
Notice the dark clumps of Nissl substance in the cytoplasm. The axon cannot be
identified with certainty in this particular view. Neuroglial nuclei surround
the neuron. Of these small nuclei, the lightest ones, showing small clumps of
chromatin, belong to astrocytes; any dark, round ones (such as the one in the
upper right corner) belong to oligodendroglia; and any dark, thin, cigar-shaped
ones to microglia (see possible one just to right of the neuron).
Section
of the gray matter of the spinal cord showing several motor neurons with their
basophilic bodies (Nissl bodies). Nucleoli are seen in some nuclei. The neurons
are surrounded by a mesh of neuronal and glial processes. PT stain. Medium
magnification.
Section
of spinal cord gray matter. The meshwork of cell neuron and glial processes
appears distinctly. The small nuclei are from glia cells. Note that these cells
are more numerous than neurons. H&E stain. Medium magnification.
Glial nuclei seen
in white matter of the cord, cut so that nerve processes are seen running
longitudinally. Most of these are round, dark oligodendroglial nuclei; these
are the cells responsible for the myelin wrapping of axons of the central
nervous system.
Silvered
preparation of astrocytes, showing their many fine cytoplasmic processes. Note
their close relationship to capillaries, the heavy black structures. Since
astrocytes touch both capillaries and neurons, they are thought to play an
important intermediary role in the nutrition and metabolism of neurons.
The spinal cord is a long, thin, tubular bundle
of nervous
tissue and support cells that extends from the brain (the medulla oblongata specifically). The brain and spinal cord together
make up the central
nervous system (CNS).
The spinal cord begins at the occipital bone and extends down to the space between the first and
second lumbar vertebrae; it does not extend the entire length of the vertebral column. It is around
The spinal cord is the main pathway for information
connecting the brain and peripheral nervous system. The length of the spinal
cord is much shorter than the length of the bony spinal column. The human
spinal cord extends from the foramen magnum and continues through to the conusmedullaris near the second lumbar vertebra, terminating in a fibrous extension known as the filumterminale.
It is about
The human spinal cord is divided into 31 different
segments. At every segment, right and left pairs of spinal nerves (mixed; sensory
and motor) form. Six to eight motor nerve rootlets branch out of right and left
ventro lateral sulci in a very orderly manner. Nerve rootlets combine to form
nerve roots. Likewise, sensory nerve rootlets form off right and left dorsal
lateral sulci and form sensory nerve roots. The ventral (motor) and dorsal
(sensory) roots combine to form spinal nerves (mixed; motor and sensory), one on each side of the
spinal cord. Spinal nerves, with the exception of C1 and C2, form inside intervertebral
foramen (IVF). Note that
at each spinal segment, the border between the central and peripheral nervous
system can be observed. Rootlets are a part of the peripheral nervous system.
In the upper part of the vertebral column, spinal
nerves exit directly from the spinal cord, whereas in the lower part of the
vertebral column nerves pass further down the column before exiting. The
terminal portion of the spinal cord is called the conusmedullaris. The pia mater continues as an extension called the filumterminale, which anchors the spinal cord to the coccyx. The caudaequina (“horse’s tail”) is the name for the collection of
nerves in the vertebral column that continue to travel through the vertebral
column below the conusmedullaris. The caudaequina forms as a result of the fact that
the spinal cord stops growing in length at about age four, even though the
vertebral column continues to lengthen until adulthood. This results in the
fact that sacral spinal nerves actually originate in the upper lumbar region.
The spinal cord can be anatomically divided into 31 spinal segments based on
the origins of the spinal nerves.
Each segment of the spinal cord is associated with a
pair of ganglia, called dorsal
root ganglia, which
are situated just outside of the spinal cord. These ganglia contain cell bodies
of sensory neurons. Axons of these sensory neurons travel into the spinal cord
via the dorsal roots.
Ventral roots consist of axons from motor neurons,
which bring information to the periphery from cell bodies within the CNS.
Dorsal roots and ventral roots come together and exit the intervertebral
foramina as they become spinal nerves.
The gray matter, in the center of the cord, is shaped
like a butterfly and consists of cell bodies of interneurons and motor neurons. It also consists of neuroglia cells and unmyelinated axons. Projections of the gray matter (the “wings”)
are called horns. Together, the gray horns and the gray commissure form the “gray H.”
The white matter is located outside of the gray matter
and consists almost totally of myelinated motor and sensory axons. “Columns” of white matter
carry information either up or down the spinal cord.
Within the CNS, nerve cell bodies are generally
organized into functional clusters, called nuclei. Axons within the CNS are
grouped into tracts.
There
are 33 spinal cord nerve segments in a human spinal cord:
- 8 cervical segments
forming 8 pairs of cervical nerves (C1 spinal nerves exit spinal column between
occiput and C1 vertebra; C2 nerves exit between posterior arch of C1
vertebra and lamina of C2 vertebra; C3-C8 spinal nerves through IVF above
corresponding cervica vertebra, with the exception of C8 pair which exit
via IVF between C7 and T1 vertebra)
- 12 thoracic
segments forming 12 pairs of thoracic nerves (exit spinal column through IVF below
corresponding vertebra T1-T12)
- 5 lumbar segments forming
5 pairs of lumbar nerves (exit spinal column through IVF, below
corresponding vertebra L1-L5)
- 5 sacral segments
forming 5 pairs of sacral nerves (exit spinal column through IVF, below
corresponding vertebra S1-S5)
- 3 coccygeal
segments joined up becoming a single segment forming 1 pair of coccygeal
nerves
(exit spinal column through the sacral hiatus).
The spinal cord is made from part of the neural tube during development. As the neural tube begins to
develop, the notochord begins to secrete a factor known as Sonic hedgehog or SHH. As a result, the floor plate then also begins to secrete SHH, and this will induce
the basal plate to develop motor neurons. Meanwhile, the overlying ectoderm secretes bone morphogenetic protein (BMP). This induces the roof plate to begin to secrete BMP, which will induce the alar plate to develop sensory neurons. The alar plate and the basal plate are separated by
the sulcus
limitans.
Medulla
spinalis of 8-week-old human embryo
Additionally, the floor plate also secretes netrins. The netrins act as chemoattractants to decussation of pain and temperature sensory neurons in the alar
plate across the anterior white commissure, where they then ascend towards the thalamus.
Lastly, it is important to note that the past studies
of Viktor Hamburger and Rita Levi-Montalcini in the chick embryo have been
further proven by more recent studies which demonstrated that the elimination
of neuronal cells by programmed
cell death (PCD)
is necessary for the correct assembly of the nervous system.
Overall, spontaneous embryonic activity has been shown
to play a role in neuron and muscle development but is probably not involved in
the initial formation of connections between spinal neurons.
Somatosensory organization is divided into the dorsal column-medial lemniscus tract (the touch/proprioception/vibration sensory pathway) and the anterolateral
system, or ALS (the
pain/temperature sensory pathway). Both sensory pathways use three different
neurons to get information from sensory receptors at the periphery to the cerebral cortex. These neurons are designated primary, secondary and
tertiary sensory neurons. In both pathways, primary sensory neuron cell bodies
are found in the dorsal
root ganglia, and
their central axons project into the spinal cord.
In the dorsal column-medial leminiscus tract, a
primary neuron's axon enters the spinal cord and then enters the dorsal column.
If the primary axon enters below spinal level T6, the axon travels in the fasciculus
gracilis, the medial part
of the column. If the axon enters above level T6, then it travels in the fasciculus
cuneatus, which is lateral
to the fasiculusgracilis. Either way, the primary axon ascends to the lower medulla, where it leaves its fasiculus and synapses with a
secondary neuron in one of the dorsal column nuclei: either the nucleus gracilis or the nucleus cuneatus, depending on the pathway it took. At this point, the
secondary axon leaves its nucleus and passes anteriorly and medially. The
collection of secondary axons that do this are known as internal
arcuate fibers. The
internal arcuate fibers decussate and continue ascending as the contralateral medial lemniscus. Secondary axons from the medial lemniscus finally
terminate in the ventral posterolateral nucleus (VPL) of the thalamus, where they synapse with tertiary neurons. From
there, tertiary neurons ascend via the posterior limb of the internal capsule and end in the primary
sensory cortex.
The proprioception of the lower limbs differs from the
upper limbs and upper trunk. There is a four-neuron pathway for lower limb
proprioception. This pathway initially follows the dorsal spino-cerebellar
pathway. It is arranged as follows: proprioceptive receptors of lower limb
-> peripheral process -> dorsal root ganglion -> central process ->Clarke's column -> 2nd order neuron -> medulla oblogata(Caudate nucleus) -> 3rd order neuron -> VPL of thalamus ->
4th order neuron -> posterior limb of internal capsule -> corona radiata
-> sensory area of cerebrum.
The anterolateral system works somewhat differently.
Its primary neurons axons enter the spinal cord and then ascend one to two
levels before synapsing in the substantiagelatinosa.
The tract that ascends before synapsing is known as Lissauer's tract. After synapsing, secondary axons decussate and
ascend in the anterior lateral portion of the spinal cord as the spinothalamic
tract. This tract
ascends all the way to the VPL, where it synapses on tertiary neurons. Tertiary
neuronal axons then travel to the primary sensory cortex via the posterior limb
of the internal capsule.
It should be noted that some of the "pain
fibers" in the ALS deviate from their pathway towards the VPL. In one such
deviation, axons travel towards the reticular
formation in the midbrain.
The reticular formation then projects to a number of places including the hippocampus (to create memories about the pain), the centromedian
nucleus (to cause diffuse,
non-specific pain) and various parts of the cortex. Additionally, some ALS
axons project to the periaqueductal
gray in the pons, and
the axons forming the periaqueductal gray then project to the nucleus
raphes magnus, which
projects back down to where the pain signal is coming from and inhibits it.
This helps control the sensation of pain to some degree.
Motor organization
The corticospinal
tract serves as the
motor pathway for upper motor neuronal signals coming from the cerebral cortex
and from primitive brainstem motor nuclei.
Cortical
upper motor neurons originate from Brodmann areas 1, 2, 3, 4, and 6 and then descend in the posterior
limb of the internal capsule, through the crus cerebri, down through the pons, and to the medullary
pyramids, where about 90%
of the axons cross to the contralateral side at the decussation of the
pyramids. They then descend as the lateral corticospinal tract. These axons
synapse with lower motor neurons in the ventral horns of all levels of the spinal cord. The remaining 10%
of axons descend on the ipsilateral side as the ventral corticospinal tract.
These axons also synapse with lower motor neurons in the ventral horns. Most of
them will cross to the contralateral side of the cord (via the anterior white commissure) right before synapsing.
The midbrain nuclei include four motor tracts that
send upper motor neuronal axons down the spinal cord to lower motor neurons.
These are the rubrospinal tract, the vestibulospinal
tract, the tectospinal tract and the reticulospinal
tract. The rubrospinal
tract descends with the lateral corticospinal tract, and the remaining three
descend with the anterior corticospinal tract.
The function of lower motor neurons can be divided into
two different groups: the lateral corticospinal tract and the anterior cortical
spinal tract. The lateral tract contains upper motor neuronal axons which synapse on dorsal lateral (DL) lower motor
neurons. The DL neurons are involved in distal limb control. Therefore, these DL neurons are found
specifically only in the cervical and lumbosacral enlargements within the
spinal cord. There is no decussation in the lateral corticospinal tract after
the decussation at the medullary pyramids.
The anterior corticospinal tract descends ipsilaterally in the anterior column, where the axons emerge and
either synapse on lower ventromedial (VM) motor neurons in the ventral horn
ipsilaterally or descussate at the anterior white commissure where they synapse on VM lower motor neurons contralaterally . The tectospinal, vestibulospinal and reticulospinal
descend ipsilaterally in the anterior column but do not synapse across the
anterior white commissure. Rather, they only synapse on VM lower motor neurons
ipsilaterally. The VM lower motor neurons control the large, postural muscles
of the axial skeleton. These lower motor neurons, unlike those of the DL,
are located in the ventral horn all the way throughout the spinal cord.
Spinal cord injuries can be caused by trauma to the
spinal column (stretching, bruising, applying pressure, severing, laceration,
etc.). The vertebral bones or intervertebral
disks can shatter,
causing the spinal cord to be punctured by a sharp fragment of bone. Usually, victims of spinal cord injuries will suffer
loss of feeling in certain parts of their body. In milder cases, a victim might
only suffer loss of hand or foot function. More severe injuries may result in paraplegia, tetraplegia (also known as quadriplegia), or full body paralysis below the site of injury to the spinal cord.
Damage to upper motor neuron axons in the spinal cord
results in a characteristic pattern of ipsilateral deficits. These include hyperreflexia, hypertonia and muscle weakness. Lower motor neuronal damage
results in its own characteristic pattern of deficits. Rather than an entire
side of deficits, there is a pattern relating to the myotome affected by the damage. Additionally, lower motor
neurons are characterized by muscle weakness, hypotonia, hyporeflexia and muscle atrophy.
Spinal shock and neurogenic shock can occur from a
spinal injury. Spinal shock is usually temporary, lasting only for 24–48 hours,
and is a temporary absence of sensory and motor functions. Neurogenic shock
lasts for weeks and can lead to a loss of muscle tone due to disuse of the
muscles below the injured site.
The two areas of the spinal cord most commonly injured
are the cervical spine (C1-C7) and the lumbar spine (L1-L5). (The notation C1, C7, L1, L5 refer to the
location of a specific vertebra in either the cervical, thoracic, or lumbar region of
the spine.)
Spinal
cord injury can also be non traumatic and caused by disease (transverse
myelitis, polio, spina bifida, Friedreich's
ataxia, spinal cord tumor, spinal stenosis etc.)
MENINGES
The central nervous system is protected by the skull
and the vertebral column. It is also encased in membranes of connective tissue
called the meninges. Starting with the outermost layer, the meninges are the
dura mater, arachnoid, and pia mater. The arachnoid and the pia mater are
linked together and are often considered a single membrane called the
pia-arachnoid.
Dura Mater
The dura mater is the external layer (meninx) and is
composed of dense connective tissue. The dura mater that envelops the spinal
cord is separated from the periosteum of the vertebrae by the epidural space,
which contains thin-walled veins, loose connective tissue, and adipose tissue.
The dura mater is always separated from the arachnoid
by the thin subdural space. The internal surface of all dura mater, as well as
its external surface in the spinal cord, is covered by simple squamous
epithelium of mesenchymal origin.
Arachnoid
The arachnoid (Gr. arachnoeides, cobweb-like) has two
components: a layer in contact with the dura mater, and a system of trabeculae
connecting the layer with the pia mater. The cavities between the trabeculae
form the subarachnoid space, which is filled with cerebrospinal fluid and is
completely separated from the subdural space. This space forms a hydraulic
cushion that protects the central nervous system from trauma. The subarachnoid
space communicates with the ventricles of the brain.
The arachnoid is composed of
connective tissue devoid of blood vessels. Its surfaces are covered by the same type of simple
squamous epithelium that covers the dura mater. Since the arachnoid has fewer
trabeculae in the spinal cord, it can be more cleanly distinguished from the
pia mater in that area.
In some areas, the arachnoid perforates the dura
mater, forming protrusions that terminate in venous sinuses in the dura mater.
These protrusions, which are covered by endothelial cells of the veins, are
called arachnoid villi. Their function is to reabsorb cerebrospinal fluid into
the blood of the venous sinuses.
Pia Mater
The pia mater is a loose connective
tissue containing many blood vessels. Although it is located
quite close to the nerve tissue, it is not in contact with nerve cells or
fibers. Between the pia mater and the neural elements is a thin layer of
neuroglial processes, adhering firmly to the pia mater and forming a physical
barrier at the periphery of the central nervous system. This barrier separates
the central nervous system from the cerebrospinal fluid.
The pia mater follows all the irregularities of the
surface of the central nervous system and penetrates it to some extent along with
the blood vessels. Pia mater is covered by squamous cells of mesenchymal
origin.
Blood vessels penetrate the central nervous system
through tunnels covered by pia mater – the perivascular spaces. The pia mater
disappears before the blood vessels are transformed into capillaries. In the
central nervous system, the blood capillaries are completely covered by
expansions of the neuroglial cell processes.
The structure of the meninges, with the superposition
of pia mater, arachnoid, and dura mater. Astrocytes form a 3-dimensional net
around the neurons (not shown). Note that the footlike processes of the
astrocytes form a continuous layer that involves the blood vessels that
contribute to the blood-brain barrier.
Blood-Brain
Barrier
The blood-brain barrier is a
functional barrier that prevents the passage of some substances, such as antibiotics
and chemical and bacterial toxic matter, from the blood to nerve tissue. It is
present in central nerve system.
The blood-brain barrier results from the reduced permeability
that is a property of blood capillaries of nerve tissue. Occluding junctions,
who provide continuity between the endothelial cells of these capillaries,
represent the main structural component of the barrier. The cytoplasm of these
endothelial cells does not have the fenestrations found in many other
locations, and very few pinocytotic vesicles are observed. The expansions of
neuroglial cell processes that envelop the capillaries are partly responsible
for their low permeability.
Blood-brain barrier of
spinal cord consists of:
Capillary wall
1. Continuous endothelium
2. Continuous basement membrane
3. Glial sheath
(foot processes of astrocytes)
HOROID PLEXUS & CEREBROSPINAL FLUID
The choroid plexus consists of invaginated folds of
pia mater that penetrate the interior of the ventricles. It is found in the
roofs of the third and fourth ventricles and in part in the walls of the
lateral ventricles. It is a vascular structure made up of dilated fenestrated
capillaries.
The choroid plexus is composed of loose connective
tissue of the pia mater, covered by a simple cuboidal or low columnar
epithelium that has the cytologic characteristics of ion-transporting cells.
The main function of the choroid plexus is to elaborate
cerebrospinal fluid, which contains only a small amount of solids and
completely fills the ventricles, central canal of the spinal cord, subarachnoid
space, and perivascular space. Cerebrospinal fluid is important for the
metabolism of the central nervous system and acts as a protective device.
Cerebrospinal fluid is very low in protein content. A
few desquamated cells and two to five lymphocytes per milliliter are also
present. Cerebrospinal fluid circulates through the ventricles, from which it
passes into the subarachnoid space. There, arachnoid villi provide the main
pathway for absorption of cerebrospinal fluid into the venous circulation.
There are no lymphatic vessels in nerve tissue.
A decrease in the absorption of cerebrospinal fluid or
a blockage of outflow from the ventricles results in the condition known as
hydrocephalus (Or. hydro, water, + kephale, head), which promotes a progressive
enlargement of the head followed by mental impairment and muscular weakness.
Photomicrograph
of choroid plexus section. The choroid plexus presents a core of loose
connective tissue rich in blood capillaries (BC) covered by a simple cubic
epithelium (arrowhead). H&E stain. Medium magnification.
PERIPHERAL NERVOUS SYSTEM
The main components of the peripheral nervous system
are the nerves, ganglia, plexus and nerve endings. Nerves are bundles of nerve
fibers surrounded by a series of connective tissue sheaths.
NERVE FIBERS
Nerve fibers consist of axons
enveloped by a special sheath derived from cells of ectodermal origin. Groups of
nerve fibers constitute the tracts of the brain, spinal cord, and peripheral
nerves. Nerve fibers exhibit
differences in their enveloping sheaths, related to whether the fibers are part
of the central or the peripheral nervous system.
Most axons in adult nerve tissue are covered by single
or multiple folds of a sheath cell. In peripheral nerve fibers, the sheath cell
is the Schwann cell, and in central nerve fibers it is the oligodcndrocyte. Axons
of small diameter are usually unmyelinated nerve fibers. Progressively thicker
axons are generally sheathed by increasingly numerous concentric wrappings of
the enveloping cell, forming the myelin sheaths. These fibers are known as
myelinated nerve fibers.
Myelinated Fibers
In myelinated fibers of the peripheral
nervous system, the plasmalemma of the covering Schwann cell winds and wraps
around the axon. The layers of
membranes of the sheath cell unite and form myelin, a lipoprotein complex whose
lipid component can be partly removed by standard histologic procedures.
Myelin consists of many layers of modified cell
membranes. These membranes have a higher proportion of lipids than do other
cell membranes. Central nervous system myelin contains two major proteins:
myelin basic protein and proteolipid protein. Several human demyelinating
diseases are due to an insufficiency or lack of one or both of these proteins.
Each axon is surrounded by myelin formed by a
sequential series of Schwann cells. The myelin sheath shows gaps along its path
called the nodes of Ranvier; these represent the spaces between adjacent
Schwann cells along the length of the axon. Interdigitating processes of
Schwann cells partially cover the node. The distance between two nodes is
called an internode and consists of one Schwann cell. The length of the
inter-node varies between 1 and
There are no Schwann cells in the central nervous
system; there, the myelin sheath is formed by the processes of the
oligodendrocytes. Oligodendrocytes differ from Schwann cells in that different
branches of one cell can envelop segments of several axons.
Myelin sheath of the central nervous system. The same
oligodendrocyte forms myelin sheaths for several (3–50) nerve fibers. In the
central nervous system, processes of other cells sometimes cover the nodes of
Ranvier, or there is considerable extracellular space (ES) at that point. The
axolemma shows a thickening where the cell membrane of the oligodendrocyte
comes into contact with it. This limits the diffusion of materials into the
periaxonal space between the axon and the myelin sheath. At upper left is a
surface view of the cell body of an oligodendrocyte. Cyt, cytoplasm of the
oligodendrocyte.
Drawing
of relation of an oligodendrocyte to a neuronal axon in the CNS, as seen in
E.M. An extension of cell cytoplasm wraps around the axon, making a
multi-layered myelin sheath. Ordinarily there is one oligodendrocyte between
two successive nodes of Ranvier. Notice that the cell has other cytoplasmic
extensions up above, which are free to as sociate with other axons. This same
principle of lamellated (layered) myelin sheath formation holds true also for
Schwann cells and peripheral nerves. One difference, however, is that a Schwann
cell is believed to wrap only one axon instead of several. Notice that the
plasma membrane of the axon is bare at the point of the node; this allows for
rapid saltatory conduction as the impulse jumps from node to node to node.
Unmyelinated Fibers
In both the central and peripheral nervous systems,
not all axons are sheathed in myelin. In the
peripheral system, all unmyelinated axons are enveloped within simple clefts
of the Schwann cells. Unlike their association with individual
myelinated axons, each Schwann cell can sheathe many unmyelinated axons.
Unmyelinated nerve fibers do not have nodes of Ranvier, because abutting
Schwann cells are united to form a continuous sheath.
The central nervous system is rich in unmyelinated
axons; unlike those in the peripheral system, these axons are not sheathed. In
the brain and spinal cord, unmyelinated axonal processes run free among the
other neuronal and glial processes.
NERVES
In
the peripheral nervous system, the nerve fibers are grouped in bundles to form
the nerves. Except for a few very
thin nerves made up of unmyelinated fibers, nerves have a whitish, homogeneous,
glistening appearance because of their myelin and collagen content. A nerve is an enclosed, cable-like bundle of axons (the long, slender projection of a
neuron). Neurons are sometimes called nerve cells, though this
term is technically imprecise since many neurons do not form nerves, and nerves
also include the glial cells that ensheath the axons in myelin.
Nerves are part of
the peripheral nervous system. Afferent nerves convey sensory signals to the central nervous system,
for example from skin or organs, while efferent nerves conduct stimulatory signals from the central
nervous system to the musclesand glands. Afferent and efferent nerves are often arranged
together, forming mixed nerves.
Each peripheral nerve is covered externally by a dense
sheath of connective tissue, the epineurium. Underlying this is a layer of flat
cells forming a complete sleeve, the perineurium. Perineurial septa extend into
the nerve and subdivide it into several bundles of fibres. Surrounding each
such fibre is the endoneurial sheath. This is a tube which extends, unbroken,
from the surface of the spinal cord to the level at which the axon synapses
with its muscle fibres or ends in sensory endings. The endoneurial sheath
consists of an inner sleeve of material called the glycocalyx and an outer,
delicate, meshwork of collagen fibres. Peripheral nerves are richly supplied
with blood.
Most nerves connect to the central nervous system through the spinal cord.
The twelve cranial nerves,
however, connect directly to parts of the brain. Spinal nerves are given letter-number combinations according to the
vertebrathrough which they connect to the spinal column.
Cranial nerves are assigned numbers, usually expressed as Roman numerals from I to XII. In addition, most nerves and major
branches of nerves have descriptive names. Inside the central nervous system,
bundles of axons are termed tracts rather
than nerves.
The signals that nerves carry, sometimes called nerve
impulses, are also known as action potentials: rapidly
(up to 120 m/s) traveling electrical waves, which begin typically in the cell
body of a neuron and propagate rapidly down the axon to its tip or
"terminus." The signals cross over from the terminus to the adjacent
neurotransmitter receptor through a gap called the synapse. Motor neuronsinnervate
or activate muscles groups. The nerve system runs through the spinal cord.
Cross
sections of two small nerves with a thin covering layer. Note the Schwann cell
nuclei (arrowheads) and the axons (arrows). PT stain. Medium magnification.
Electron
micrograph of a peripheral nerve containing both myelinated (M) and
unmyelinated (U) nerve fibers. The reticular fibers (RF) seen in cross section
belong to the endoneurium. Near the center of the figure is a Schwann cell
nucleus (S). The perineurial cells (P [over a nucleus], arrows) form a barrier
that controls access of materials to nerve tissue. x30,000. Inset: Part
of an axon, where numerous neurofilaments and microtubules are seen in cross
section. x60,000.
EM of myelinated axons of peripheral nerve. The dark, many-layered
myelin sheaths surround pale axons. At the upper edge of the picture is a
nucleus of a Schwann cell, with its outer rim of cytoplasm continuous with the
outer rim of the myelin sheath of the axon in the left corner. (Remember that
non-myelinated axons are also closely related to Schwann cells, but the Schwann
cells form no layered wrappings around them. Note, too, that one Schwann cell
can be related to several axons when these are non-myelinated.)
Nerves have an external fibrous coat of
dense connective tissue called epineurium, which also fills the space between the bundles of
nerve fibers. Each bundle is surrounded by the perineurium,
a sleeve formed by layers of flattened epithelium-like cells. The cells of
each layer of the perineurial sleeve are joined at their edges by tight
junctions, ah arrangement that makes the perineurium a barrier to the passage
of most macromolecules and has the important function of protecting the nerve
fibers from aggression. Within the perineurial sheath run the Schwann
cell-sheathed axons and their enveloping connective tissue, the endoneurium. The endoneurium consists of a thin layer
of reticular fibers. Endoneurial reticular fibers are produced by Schwann
cells.
The nerves establish communication between brain and
spinal cord centers and the sense organs and effectors (muscles, glands, etc).
They possess afferent and efferent fibers to and from the central nervous
system. Afferent fibers carry the information obtained from the interior of
the body and the environment to the central nervous system. Efferent fibers
carry impulses from the central nervous system to the effector organs commanded
by these centers. Nerves possessing only sensory fibers are called sensory
nerves; those composed only of fibers carrying impulses to the effectors are
called motor nerves. Most nerves have both sensory and motor fibers and are
called mixed nerves; these nerves have both myelinated and unmyelinated axons.
Electron
micrograph of a cross section through a nerve, showing the epineurium, the
perineurium, and the endoneurium. The epineurium is a dense connective tissue
rich in collagen fibers (Col) and fibroblasts (arrow). The perineurium is made
up of several layers of flat cells tightly joined together to form a barrier to
the penetration of the nerve by macromolecules. The endoneurium is composed mainly
of reticular fibers (RF) synthesized by Schwann cells (SC). X1200.
Cross-cuts
of small peripheral nerve bundles as seen in ordinary tissue sections. The
processes have a typically wavy appearance.
Cross section of a
thick nerve stained to show its collagenous components. Picrosirius–polarized
light stain. Medium magnification
A
higher magnification of one bundle of peripheral nerve, showing cross-cuts of
individual processes. The ones in the center are the truest cut; those on
either side are tangentially cut. The best ones show a darker axon in the
center of the fiber, surrounded by a paler myelin sheath. Remember that some of
these fibers are axons of motor neurons, whose cell bodies are in the anterior
horn of the spinal cord, while other fibers are dendrites of the pseudounipolar
sensory cells of the spinal ganglion. This is the one instance where functional
dendrites (i.e., processes coming into the cell body) are structurallv like
axons with myelin sheaths. The dense sheath at the outer edge of the bundle
here is perineurium. The lines of pink surrounding each process represent
endoneurium.
Higher
magnification of longitudinally cut nerve, showing a clear node of Ranvier in
the center of the field. Note that the axon is continuous through the node.
Notice also the "foamy", grainy appearance of the myelin sheaths;
this represents the proteinaceous material of the cell membrane wrappings of
the sheath, often called "neurokeratin" although this is a misnomer.
The lipid portion of the membranes has been dissolved out during tissue
fixation.
Detail
of node of Ranvier, with axon continuing through it. Axons stain deep pink.
Myelin is pale because the lipid material disolves out. The dark strands of
protein neurokeratin give the "foamy" look to the myelin in light
microscopy. Nuclei, seen here near the bottom of the picture, lie between nerve
processes and belong to either Schwann cells or endoneurial connective tissue
cells (such as fibroblasts).
Detail
of a motor nerve ending upon a skeletal muscle cell (voluntary muscle). The
axon terminal is highly branched to form an oval motor end plate. The cell body
which sends out this axon is a multipolar motor neuron, such as those in the
anterior horn of the spinal cord.
Electronmicrograph
of axo-muscular synapse in skeletal muscle. X 33000.
Diagram of motor end plate (myoneural junction) as seen with electron
microscopy. This drawing shows a detail of one knob of an end plate as it rests
in a trough on the surface of a muscle cell. The "subneural clefts"
labelled here are also called "gutters" in the sarcolemmal membrane.
The label "glycoprotein" indicates the position of the basal lamina
of the muscle cell.
EM detail of neuro-neural synapse in the brain or spinal cord. The axon
terminal contains many seed-like synpatic vesicles containing transmitter
substances. The intercellular cleft between the axon and the contacted dendrite
can be seen. Just below the dendritic cell membrane is a dark, filamentous
post-synaptic density. Other profiles in this field, most of them very
irregular in outline, belong to both neuronal processes and glial processes.
There is one large and one small mitochondrion just left of the synaptic
vesicles.
Muscle spindle – a specialized sensory receptor for muscle stretch and
position sense, as related particularly to unconscious maintenance of skeletal
muscle tone and proper balance of postural muscle activity. The spindle is the
encapsulated group of muscle fibers lying in the center of the field of regular
skeletal muscle fibers, all cut in cross-section. The sensory nerve endings
themselves (not visible here) wrap around the muscle fibers within the spindle.
Such endings relay sensory information along dendrites within peripheral
nerves, back to pseudounipolar cell bodies in a spinal ganglion, and thence to
the spinal cord.
Pacinian corpuscle – another specialized sensory ending, this time for
deep pressure. This particular view is from a whole mount of mesentery, so you
are seeing the corpuscle three-dimensionally. They are also found in
subcutaneous tissue, deep to skin. Notice the onion-like layers of specialized
connective tissue surrounding a dark pink dendritic terminal. Again, the cell
body for this dendrite lies in a spinal ganglion, and the axon of that same
cell then proceeds into the spinal cord.
Ruffini’s
body (mechanoreceptor)
Bulb of
Crause (thermoreceptor)
Meissner’s
body (scheme)
GANGLIA
Ganglia are ovoid
structures containing neuronal cell bodies and glial cells supported by
connective tissue. Because they serve as relay stations to transmit nerve
impulses, one nerve enters and another exits from each ganglion. The direction
of the nerve impulse determines whether the ganglion will be a sensory or an
autonomic ganglion.
Sensory Ganglia
Sensory ganglia receive afferent impulses that go to
the central nervous system. Two types of
sensory ganglia exist. Some are associated with cranial nerves (cranial
ganglia); others are associated with the dorsal root of the spinal nerves and
are called spinal ganglia. The latter comprise large neuronal cell
bodies with prominent fine Nissl bodies surrounded by abundant small glial
cells called satellite cells.
This slide is an overview of one half of a transverse section of the
spinal cord, along with its ventral and dorsal roots and a spinal ganglion. At
the extreme left (which is close to the midline of the cord) notice a small
central canal lined by a dark layer of ependymal this contains cerebrospinal
fluid in life. Above the canal lies the narrow slit of the posterior median
sulcus, and below the canal is a wider, bulging separation called the anterior
median fissure. Lateral to all these spaces lies the gray matter of the cord
(quite pink here), where neuronal cell bodies lie. Surrounding the gray matter
is a layer of white matter, consisting of nerve cell processes, all of them
axons, running up or down the length of the cord and therefore cut in
cross-section here. Outside the cord, to the right, lies a mass of nerve cell
bodies, the spinal ganglion, interrupting the course of the dorsal root. Below
the ganglion lies the ventral root. Surrounding the entire complex is a
well-defined, pink band of dura mater which consists of dense collagenous
connective tissue. The wedge of delicate areolar connective tissue at the
bottom of the anterior median fissure is the arachnoid; note the round
cross-cut of a blood vessel lying in it. The pia mater, invisible here, is an
extremely thin connective tissue layer immediately investing the spinal cord.
A connective tissue framework and capsule support the
ganglion cells. The neurons of these ganglia are pseudounipolar and relay
information from the ganglion's nerve endings to the gray matter of the spinal
cord via synapses with local neurons.
Silver-impregnated
sensory ganglion consisting of pseudounipolar neurons.Medium magnification.
Spinal ganglion in Mallory connective tissue stain.
The pseudounipolar cells are in characteristic groups or clumps, separated by
bands of nerve processes. The processes might be either dendrites arriving from
the body periphery or axons proceeding on to the spinal cord. Either way, the
cell bodies or origin for the processes lie within the spinal ganglion and are
sensory neurons. The dark blue sheath outside the ganglion is the dense
collagenous connective tissue dura mater.
Detail of pseudounipolar spinal ganglion each one
encapsulated by a layer of small satellite cells. Bright blue material is the
supportive connective tissue, which is directly continuous with the endoneurium
surrounding the individual nerve processes entering and leaving the ganglion.
Remember that connective tissue is the supportive tissue of the peripheral
nervous system.
Higher power of spinal ganglion stained with H&E.
Satellite cell capsules are clear. The large neuron in the center of the field
has a pale axon hillock where the seemingly single process enters and leaves.
In such a pseudounipolar cell, the incoming dendrite and outgoing axon seem to
be related to the cell body by means of a single "stalk". The
paleness of the hillock is due to the absence of RER (Nissl substance) in this
area.
DEGENERATION & REGENERATION OF NERVE
TISSUE
Although it has
been shown that neurons can divide in the brain of adult birds, mammalian neurons usually do not divide,
and their degeneration represents a permanent loss. Neuronal processes in the
central nervous system arc, within very narrow limits, replaceable by growth
through the synthetic activity of their perikaryons.
Peripheral
nerve fibers can also regenerate if their perikaryons are not destroyed.
Death of a nerve
cell is limited to its perikaryon and processes. The neurons functionally
connected to the dead neuron do not die, except for those with only one link.
In this latter instance, the isolated neuron undergoes transneuronal
degeneration.
In contrast to nerve cells, neuroglia of the central
nervous system – and Schwann cells and ganglionic satellite cells of the
peripheral nervous system – are able to divide by mitosis. Spaces in the
central nervous system left by nerve cells lost by disease or injury are
invaded by neuroglia.
Since nerves are widely distributed throughout the
body, they are often injured. When a nerve axon is transected, degenerative
changes take place, followed by a reparative phase.
In
a wounded nerve fiber, it is important to distinguish the changes occurring in
the proximal segment from those in the distal segment. The proximal segment
maintains its continuity with the trophic center (perikaryon) and frequently
regenerates. The distal segment, separated from the nerve cell body, degenerates.
Axonal injury causes several changes in the
perikaryon: chromatolysis, i.e., dissolution of Nissl substances with a
consequent decrease in cytoplasmic basophilia; an increase in the volume of the
perikaryon; and migration of the nucleus to a peripheral position in the
perikaryon. The proximal segment of the axon degenerates close to the wound for
a short distance, but growth starts as soon as debris is removed by macrophages.
Macrophages produce interleukin-1, which stimulates Schwann cells to secrete
substances that promote nerve growth.
In the nerve stub distal to the injury, both the axon
(now separated from its trophic center) and the myelin sheath degenerate completely,
and their remnants, excluding their connective tissue and perineurial sheaths,
are removed by macrophages. While these regressive changes take place, Schwann
cells proliferate within the remaining connective tissue sleeve, giving rise to
solid cellular columns. These rows of Schwann cells serve as guides to the
sprouting axons formed during the reparative phase.
After the regressive changes, the proximal segment of
the axon grows and branches, forming several filaments that progress in the
direction of the columns of Schwann cells. Only fibers that penetrate these
Schwann cell columns will continue to grow and reach an effector organ.
When there is an extensive gap between the distal and
proximal segments, or when the distal segment disappears altogether (as in the
case of amputation of a limb), the newly grown nerve fibers may form a
swelling, or neuroma, that can be the source of spontaneous pain.
Regeneration is functionally efficient only when the
fibers and the columns of Schwann cells are directed to the correct place. The
possibility is good, however, since each regenerating fiber gives origin to
several processes, and each column of Schwann cells receives processes from
several regenerating fibers. In an injured mixed nerve, however, if regenerating
sensory fibers grow into columns connected to motor end-plates that were
occupied by motor fibers, the function of the muscle will not be reestablished.
Damage to nerves can be caused by physical injury,
swelling (e.g. carpal
tunnel syndrome),
autoimmune diseases (e.g. Guillain-Barré syndrome), infection (neuritis), diabetes, or failure of the blood vessels surrounding the
nerve. Pinched nerves occur when pressure is placed on a nerve, usually from
swelling due to an injury or pregnancy. Nerve damage or pinched nerves are
usually accompanied by pain, numbness, weakness, or paralysis. Patients may feel these symptoms in areas far from
the actual site of damage, a phenomenon called referred pain. Referred
pain occurs because when a nerve is damaged, signaling is defective from all parts
of the area which the nerve receives input, not just the site of the damage.
Neurologists usually diagnose disorders of the nerves by a physical
examination,
including the testing of reflexes, walking and other directed movements, muscle weakness, proprioception, and the sense of touch. This initial exam can be followed with tests such as
nerve
conduction study and electromyography (EMG).
Plasticity of nerve
tissue
Despite its general stability, the nervous system
exhibits some plasticity in adults. Plasticity is very high during embryonic
development, when an excess of nerve cells is formed and the ones that do not
establish correct synapses with other neurons are eliminated. Several studies
made in adult mammals have shown that, after an injury, the neuronal circuits
may be reorganized by the growth of neuronal processes, forming new synapses to
replace the ones lost by injury. Thus, new communications arc established with
some degree of functional recovery. This property of nerve tissue is known as
neuronal plasticity. The regenerative processes in the nervous system are controlled
by several growth factors produced by neurons, glial cells, Schwann cells, and
target cells. These growth factors form a family of molecules called
neurotrophins.
Tumors of the Nervous System
Virtually all cells of the nerve tissue generate
tumors. Glial cells produce gliomas. immature nerve cells produce
medulloblastomas, and Schwann cells produce schwannumas. Because adult neurons
do not divide, they do not produce tumors.
Students’ Practical Activities
Students
must know and illustrate such histologic specimens:
Specimen 1.Spinal cord.
Stained
with silver impregnation.
The structure of
the spinal cord is basically similar over its whole length. In transverse
section, the central mass of grey matter has the shape of a butterfly, the
ventral horns being most prominent and containing the cell bodies of the large
lower motor neurons. The dorsal horns are much less prominent and contain the
cell bodies of small second order sensory neurons, which relay sensory
information to the brain from primary afferent neurons for the modalities of
temperature and pain whose cell bodies lie in the dorsal root ganglia. Small
lateral horns, which contain the cell bodies of preganglionic, sympathetic
efferent neurons, are found in the thoracic and upper lumbar regions
corresponding to the level of the sympathetic outflow from the cord. The volume
of grey matter is much more extensive in the cervical and lumbar regions
corresponding to the great sensory and motor innervation of the limbs and this
is reflected in the much greater diameter of the spinal cord in these areas.
The central containing CSF and lined with ependymal cells lies in the central
commeasure of grey matter.
The white matter of the spinal cord consist of
ascending tracts of sensory fibers and descending motor tracts; passing up the
spinal cord towards the brain, more and more fibers enter and leave the cord so
that the volume of white matter increases progressively from the sacral to
cervical regions.
Externally, the spiral cord has a deep ventral median
fissure but dorsally there is only a shallow dorsal midline sulcus. On each
side, a dorso-lateral sulcus marks the line of entry of the dorsal nerve roots.
Illustrate and indicate: 1.Anterior median sulcus; 2.Posterior median septum;
3.Central canal; 4.Ventral (anterior) horn; 5.Dorsal (posterior) horn;
6.Lateral (intermediate) horn; 7.Ventral white column; 8.Lateral white
column; 9.Dorsal white column; 10.Multipolar neurons.
Specimen 2. Spinal node (dorsalroot ganglion).
Stained
with hematoxylin and eosin.
At
a low magnification of microscope find anterior and posterior spinal cord roots
and on the last one - spinal node, which is cowered by the connective tissue
capsule. Distinct (exact) disposition of pericarions and nerve cells processes
is the characteristic sign of spinal ganglion. Large pseudounipolar neurons
with light vesiclelike nuclei are disposed peripherally right under the
capsule. nerve processes occupy the middle part of the node. At a high
microscope magnification find the sheath of small glial (mantial) cells with
large dark nuclei around the nerve cells. Thin connective tissue layer
surrounds the neurons outside. In fibroblasts and fibrocytes you may observe
elongated nuclei with heterochromatine.
Illustrate and indicate: 1. Capsule 2. Dorsal root 3.Ventral root 4.Ganglion cells 5. Satellite cells 6.Bundle of the nerve fibers 7. Fibroblasts
(nuclei) in the interstitial tissue.
Specimen 3. Nerve (transverse section in human skin).
Stained
with haematoxylin and eosin.
At
a low magnification of the microscope nerve trunk is seen to consist of
separate nerve fibers bandles. Outside it is cowered by the connective tissue
capsule - epineurium. Each bandle of nerve fibers is surrounded by perineurium.
Thin connective tissue layer, which arise from perineurium inside inbetween the
nerve fibers, forms the endoneurium.
Illustrate
and indicate: 1. Nerve (nerve trunk). 2. Fasciculi of mielinated
nerve fibers. 3. Nerve fiber. 4.Endoneurium. 5. Perineurium. 6.
Epineurium.
Cross
section of a thick nerve showing the epineurium, perineurium, and endoneurium.
The myelin sheath that envelops each axon was partially removed by the
histologic technique. PT stain. Medium magnification.
Low
power view of longitudinal section of peripheral nerve, again showing distinct
division into bundles of processes. The "'waviness" of the processes
themselves is often typical of nerve.
Cross-cut
of a peripheral nerve showing characteristically round bundles of nerve
processes surrounded by pale gray-blue connective tissue sheaths. The outer
connective tissue sheath surrounding the entire nerve is the epineurium. The
connective tissue sheath surrounding each round bundle is the perineurium.
Surrounding each individual nerve process within a bundle is the delicate
connective tissue endoneurium (not visible at this magnification).
Reflex arc
In terms of a simple reflex arc (sensory information comes to the cord
and motor information is sent from the cord) picture some basic nerve cell
bodies and processes as follows:
A pseudounipolar, sensory cell body lies within the spinal ganglion. It has
one long dendrite coming in from the extreme right in this picture, from the
body periphery (either from muscle or skin). This dendrite is continuous with
the cell body (no synapses are involved here). The cell's axon leaves along the
same "stalk" with the dendrite and then turns to course through the
dorsal root, into the spinal cord. There its axonal endings synapse upon the
dendrites of a small, intermediate multipolar neuron lying in the dorsal horn
of the gray matter. This intermediary cell
sends its axon to the ventral horn of the, gray matter and synapses upon the
dendrites of a large, multipolar, motor neuron lying there. The axon of the
motor neuron courses out of the cord via the ventral root and proceeds
out of this picture, to the right, until it ends Upon voluntary muscle.
Schematic representation of a nerve and a reflex arc.
In this example, the sensory stimulus starts in the skin and passes to the
spinal cord via the dorsal root ganglion. The sensory stimulus is transmitted
to an interneuron that activates a motor neuron that innervates skeletal
muscle. Examples of the operation of this reflex are withdrawal of the finger
from a hot surface and the knee-jerk reflex.
References:
a)
basic
1.
Practical classes materials.
3.
Stevens A. Human Histology / A.
Stevens, J. Lowe. – [second
edition]. –Mosby, 2000. –
P. 92-98.
4.
Wheter’s Functional Histology : A
Text and Colour Atlas / [Young B.,
Lowe J., Stevens A., Heath J.]. – Elsevier
Limited, 2006. – P. 135-147, 392-399.
5.
Ross M. Histology : A Text and Atlas
/ M. Ross W.Pawlina. – [sixth
edition]. – Lippincott Williams and Wilkins, 2011. – P. 390-393.
b)
additional
6. Eroschenko V.P. Atlas of Histology with functional
correlations / Eroschenko V.P. [tenthedition]. – Lippincott Williams and
Wilkins, 2008. – P. 157-171
7. Charts:
http://217.196.164.19/index.php?dir_name=kafedra&file_name=tl_34.php#n15
8. Volkov K. S. Ultrastructure
of cells and tissues / K. S. Volkov, N. V. Pasechko. – Ternopil
:Ukrmedknyha, 1997. – P. 6-10.
http://en.wikipedia.org/wiki/Histology
http://www.meddean.luc.edu/LUMEN/MedEd/Histo/frames/histo_frames.html
http://www.udel.edu/biology/Wags/histopage/histopage.htm