PHYSIOLOGY IS THE THEORETICAL BASES OF MEDICINE.
BIOELECTRICAL PHENOMENA IN NERVE CELLS.
PHYSIOLOGY OF SPINAL CORD.
Common characteristic of physiology:
a) Defining of “physiology” notion (Physiology is the science about the regularities of organisms‘vital activity in connection with the external environment.)
b) Tasks of physiological subjects (A deep studying of the mechanisms of vital activity of health man with the matter of expose the causes and characters‘breaches of this mechanisms in different diseases.)
c) Connection of physiology with other sciences (Connection of physiology with anatomy course – the names, localization, functions of nerves, muscles, bones, vessels, inner organs, endocrine glands; with histology – the structure of nerves, muscles, bones, vessels, inner organs, endocrine glands; with chemistry – osmotic and oncotic pressure, gradient of concentration, with physics – electric conductivity, with biology – blood groups inheritance. This is the connection of physiology with the subjects which was studied. The physiology is necessary for pathologic physiology, pathologic anatomy, surgery, obstetrics and therapeutics.)
Method of physiology
a) Observation (This is the method in which the scientists don‘t mix in course of vital processes. They only make use of vision and description of all changes. On the base of these changes they make conclusions.)
b) Experiment (There are two kinds of experiments: acute and chronic. Acute experiment was doing with the helps of anesthesia. It may be accompanied by cut off the nerves, introduction the different substances. The chronic experiment was doing in vital animals, for example, after the acute experiment scientists can used the observation.)
c) Examination (This is the method of examine the patient with different diseases, for example, with using the different apparatuses.)
d) Simulation (We can simulation different processes as a laboratory simulation or realistic simulation, for example, apparatus of artificial kidney or apparatus of artificial circulation. It may be the simulation the different processes by means of computers.)
History of physiology development
a) Till 17 century (The first medicine used the knowledge about function of health and ill person and animal, which was based on the observation method. This summarizing of receiving results was doing by Hippocrates, Gallen, Aristotel, Ibn Sina (Avicenna). One of the doctors was Akmean. He come to the conclusion that brain is the organ of consciousness and answer for memory, thought, filling. Empedocl determined that the breathing is doing not only by nose and mouth, but through skin too. Pracsagor distinguished the arterial and venous vessels, but it thought that they have air. Gallen considered that arteries had blood, established the knowledge about the breathing and described the nerves.)
b) In 17-18 centuris (Garvey in 1628 published the work about the small and big circles of blood circulation and about the heart as an engine of blood. Decart was the author of the first text-book “About the Man”, developed the theory of pain, hunger, thirst, digestion, vision, memory. The high of his physiology investigation was the description of the organism‘s reaction on the external irritations. Prokhaska considered that the reflex act may arise in internal and external stimulus. Levenguk and Malpigy described the capillaries. This development widens the vision on blood circulation. Discovering Azelly and Bartolini lymphatic vessels maintain the lymph circulation. Galvani put the bases of electric physiology.)
c) From 19 century to our days (In 19 century physiology separated of anatomy and became the independent science. Majandi studied the physiology of nerves system. Bernar studied the physiology mechanisms of development of digestive juice and their digestion properties, the role of liver in supporting the sugar level in blood, meaning of constant of pupils‘internal surrounding. Yung worked out the three component theory of color perception. Gelmgolths developed this theory and creation the theory of hears perception. Phylomaphytsky worked out the theory of cyclic functioning of nerves system. Phylomaphytsky and Basov worked the operation of suturing the gastric fistula on dogs. Phylomaphytsky and Pirogov worked the method of anesthesia intravenous. Gering worked out the theory of the color vision. Gering and Braier described the reflex of nervus vagus, which control the breathing. Boydich formulated the “all or none” low, which say that cardiac muscle can contract in full or noncontract. In 1846 Ludvig described the theory of uropoiesis. Kennon created the doctrine about homeostasis. G.Selye studied the stress-syndrome.)
d) Standing of physiology in
Cell Membrane
The membrane that surrounds the cell is a remarkable structure. It is not only semipermeable, allowing some substances to pass through it and excluding others, but its permeability can be varied. It is generally referred to as the plasma membrane. The nucleus is also surrounded by a membrane, and the organelles are surrounded by or made up of membrane.
Although the chemical structure of membranes and their properties vary onsiderably from one location to another, they have certain common features. They are generally about 7,5 nm (75 Angstrom units) thick. They are made up primarily of protein and lipids. The major lipids are phospholipids such as phosphatidylcholine and phosphatidylethanolamine. The head end of the molecule contains the phosphate portion, is positively charged, and is quite soluble in water (polar, hydrophilic). The tails are quite insoluble (nonpolar, hydrophobia). In the membrane, the hydrophilic ends are exposed to the aqueous environment that bathes the exterior of the cells and the aqueous cytoplasm; the hydrophobic ends meet in the water-poor interior of the membrane. However, there is a degree of asymmetry in the distribution of lipid in the membrane; in human red cells, for example, there is more phosphatidylethanolamine and phosphatidylserine in the inner lamella and more lecithin and sphingomyelin in me outer lamella. The significance of this asymmetry is unknown. In prokaryotes (cells like bacteria in which there is no nucleus), phospholipids are generally the only membrane lipids, but in eukaryotes (cells containing nuclei), cell membranes also contain cholesterol (in animals) or other steroids (in plants). The cholesterol/phospholipid ratio in the membrane is inversely proportionate to the fluidity of the membrane. Variations in the ratio lead to abnormalities of cell function. In healthy animals, the ratio is maintained at a nearly constant level by a variety of regulatory mechanisms.
There are many different proteins embedded in the membrane. They exist as separate globular units and stud the inside and outside of the membrane in a random array. Some are located in the inner surface of the membrane; some are located on the outer surface; and some extend through the membrane (through and through proteins). In general, the uncharged, hydrophobic portions of the protein molecules are located in the interior of the membrane and the charged, hydrophilic portions are located on the surfaces. Some of the proteins contain lipids (lipoproteins) and some contain arbohydrates (glycoproteins). There are 5 types of proteins in the membrane. In addition to structural proteins, there are proteins that function as pumps, actively transporting ions across the membrane. Other proteins function as passive channels for ions that can be opened or closed by changes in the conformation of the protein. A fourth group of proteins functions as receptors that bind neurotransmitters and hormones, initiating physiologic changes inside the cell. A fifth group functions as enzymes, catalyzing reactions at the surfaces of the membrane.
The protein structure – and particularly the enzyme content – of biologic membranes varies not only from cell to cell but also within the same cell. For example, there are different enzymes embedded in cell membranes than in mitochondrial membranes; in epithelial cells, the enzymes in the cell membrane on the mucosal surface differ from those in the cell membrane on the lateral margins of the cells. The membranes are dynamic structures, and their constituents are being constantly renewed at different rates. In addition, some proteins move laterally in the membrane. For example, receptors move in the membrane and aggregate at sites of endocytosis. There is evidence that the lateral movement of components in the membrane is not random but is controlled by mtracellular mechanisms that probably involve microfilaments and nucrotubules.
Underlying most cells is a thin, fuzzy layer plus some fibrils that collectively make up the basement membrane or, more properly, the basal lamina. The material that makes up the basal lamina has been shown to be made up of a collagen derivative plus 2 glycoproteins.
The Sodium-Potassium Pump
The sodium-potassium pump responsible for the coupled active transport of Na+ out of cells and K+ into cells is a unique protein in the cell membrane Figure 1. This protein is also an adenosine triphosphatase, ie, an enzyme that catalyzes the hydrolysis of ATP to adenosine diphosphate (ADP), and it is activated by Na+ and K+. Consequently, it is known as sodium-potassium-activated adenosine triphosphatase (Na+-K+ ATPase). The ATP provides the energy for transport. The pump extrudes three Na+ from the cell for each two K+ it takes into the cell, ie, it has a coupling ratio of 3/2. Its activity is inhibited by ouabain and related digitalis glycosides used in the treatment of heart failure. It is made up of two a subunits, each with a molecular weight of about 95,000, and two b subunits, each with a molecular weight of about 40,000. Separation of the subunits leads to loss of ATPase activity.
The a subunits contain binding sites for ATP and ouabain, whereas the b subunits are glycoproteins. Application of ATP by micropipette to the inside of the membrane increases transport, whereas application of ATP to the outside of the membrane has no effect. Conversely, ouabain inhibits transport when applied to the outside but not to the inside of the membrane. Consequently, the a subunits must extend through the cell membrane. The protein could exist in 2 conformational states. In one, three Na+ bind to sites accessible only from the inside of the membrane. This triggers hydrolysis of ATP, and the protein changes its conformation so that the three Na+ are extruded into the ECP. In the second conformation, two K+ bind to sites accessible only from the outside of the membrane. This triggers a return to the original conformation while extruding two K+ into the interior of the cell. It appears that Na+ binding is associated with phosphorylation of the protein and K+ binding with dephosphorylation.
Genesis of the Membrane Potential
The distribution of ions across the cell membrane and the nature of this membrane provide the explanation for the membrane potential. K+ diffuses out of the cell along its concentration gradient, while the nondifftisible anion component stays in the cell, creating a potential difference across the membrane. There is thus a slight excess of cations outside the membrane and a slight excess of anions inside it. It should be emphasized, however, that the number of ions responsible for the membrane potential is a minute fraction of the total number present. Na+ influx does not compensate for the K+ efflux because the membrane at rest is much less permeable to Na+ than K+. Cl– diffuses inward down its concentration gradient, but its movement is balanced by the electrical gradient. The sodium-potassium pump contributes to the membrane potential, but its main function in this context is to maintain the concentration gradients on which the existence of the membrane potential depends. If the pump is shut off by the administration of metabolic inhibitors, Na+ enters the cell, K+ leaves it, and the membrane potential declines. The rate of this decline varies with the size of the cell. In large cells, it takes hours, but ierve fibers with diameters of less than 1 mm, complete depolarization can occur in less than 4 minutes.
The magnitude of the membrane potential at any given time depends, of course, upon the distribution of Na+, K+, and Cl– and the permeability of the membrane to each of these ions.
Variations in Membrane Potential
If the resting membrane potential is decreased by the passage of a current through the membrane, the electrical gradient that keeps K+ inside the cell is decreased, and there is an increase in K+ diffusion out of the cell.
This K+ efflux and the simultaneous movement of Cl– into the cell result in a net movement of positive charge out of the cell, with consequent restoration of the resting membrane potential. When the membrane potential increases, these ions move in the opposite direction. These processes occur in all polarized cells and tend to keep the resting membrane potential of the cells constant withiarrow limits. However, ierve and muscle cells, reduction of the membrane potential triggers a voltage-dependent increase in Na+ permeability. This unique feature permits these cells to generate self-propagating impulses that are transmitted along their membranes for great distances.
Resting membrane potential
a) Common characteristic (There is a potential difference across the membranes of most if not all cells, with the inside of the cells negative to the exterior. By convention, this resting membrane potential (steady potential) is written with a minus sign, signifying that the inside is negative relative to the exterior. Its magnitude vanes considerably from tissue to tissue, ranging from -9 to –100 mV. When 2 electrodes are connected through a suitable amplifier to a CRO and placed on the surface of a single axon, no potential difference is observed. However, if one electrode is inserted into the interior of the cell, a constant potential difference is observed, with the inside negative relative to the outside of the cell at rest. This resting membrane potential is found in almost all cells. Ieurons, it is usually about –70 mV Figure 1.
b) Mechanism of development (There are two kind of ion’s transport: active and passive. Active transport is doing due to the energy of ATP. The sodium-potassium pump responsible for the coupled active transport of Na+ out of cells and K+ into cells is a unique protein in the cell membrane. This protein is also an adenosine triphosphatase, ie, an enzyme that catalyzes the hydrolysis of ATP to adenosine diphosphate (ADP), and it is activated by Na+ and K+ Figure 2.
Consequently, it is known as sodium-potassium-activated adenosine triphosphatase (Na+-K+ ATPase). The ATP provides the energy for transport. The pump extrudes three Na+ from the cell for each two K+ it takes into the cell, ie, it has a coupling ratio of 3/2. Its activity is inhibited by ouabain and related digitalis glycosides used in the treatment of heart failure. It is made up of two a subunits, each with a molecular weight of about 95,000, and two b subunits, each with a molecular weight of about 40,000. Separation of the subunits leads to loss of ATPase activity. The a subunits contain binding sites for ATP and ouabain, whereas the b subunits are glycoproteins. Application of ATP by micropipette to the inside of the membrane increases transport, whereas application of ATP to the outside of the membrane has no effect. Conversely, ouabain inhibits transport when applied to the outside but not to the inside of the membrane. Consequently, the a subunits must extend through the cell membrane. The protein could exist in 2 conformational states. In one, three Na+ bind to sites accessible only from the inside of the membrane. This triggers hydrolysis of ATP, and the protein changes its conformation so that the three Na+ are extruded into the ECP. In the second conformation, two K+ bind to sites accessible only from the outside of the membrane. This triggers a return to the original conformation while extruding two K+ into the interior of the cell. It appears that Na+ binding is associated with phosphorylation of the protein and K+ binding with dephosphorylation.
The origin of excitation
a) Characteristic of experimental stimulus (For the force it divided on the under threshold, threshold and upper threshold.)
b) Local answer, critical range of depolarization (Local answer is arised only on under threshold stimulus. Critical range of depolarization is the point from which the action membrane potential can develop.)
c) Genesis of the membrane potential (The stimulus artifact is followed by an isopotential interval (latent period) that ends with the next potential change and corresponds to the time it takes the impulse to travel along the axon from the site of stimulation to the recording electrodes. Its duration is proportionate to the distance between the stimulating and recording electrodes and the speed of conduction of the axon. If the duration of the latent period and the distance between the electrodes are known, the speed of conduction in the axon can be calculated. For example, assume that the distance between the cathode stimulating electrode and the exterior electrode is
The first manifestation of the approaching impulse is a beginning depolarization of the membrane. After an initial 15 mV of depolarization, the rate of depolarization increases. The point at which this change in rate occurs is called the firing level. Thereafter, the tracing on the oscilloscope rapidly reaches and overshoots the isopotential (zero potential) line to approximately +35 mV. It then reverses and falls rapidly toward the resting level. When repolarization is about 70 % completed, the rate of repolarization decreases and the tracing approaches the resting level more slowly. The sharp rise and rapid fall are the spike potential of the axon, and the slower fall at the end of the process is the after-depolarization. After reaching the previous resting level, the tracing overshoots slightly in the hyperpolarizing direction to form the small but prolonged after-hyperpolarization. The after-depolarization is sometimes called the negative after-potential and the after-hyperpolarization the positive after-potential, but the terms are now rarely used. The whole sequence of potential changes is called the action potential. It is a monophasic action potential because it is primarily in one direction. Before electrodes could be inserted in the axons, the response was approximated by recording between an electrode on intact membrane and an electrode on an area of nerve that had been damaged by crushing, destroying the integrity of the membrane. The potential difference between an intact area and such a damaged area is called a demarcation potential.)
d) Changing of excitability in the time of excitation (During the action potential as well as during catelectrotonic and anelectrotonic potentials and the local response, there are changes in the threshold of the neuron to stimulation. Hyperpolarizing anelectrotonic responses elevate the threshold and catelectrotonic potentials lower it as they move the membrane potential closer to the firing level. During the local response the threshold is also lowered, but during the rising and much of the falling phases of the spike potential the neuron is refractory to stimulation. This refractory period is divided into an absolute refractory period, corresponding to the period from the time the firing level is reached until repolarization is about one-third complete; and a relative refractory period, lasting from this point to the start of after-depolarization. During the absolute refractory period no stimulus, no matter how strong, will excite the nerve, but during the relative refractory period stronger thaormal stimuli can cause excitation. During after-depolarization the threshold is again decreased, and during after-hyperpolarization it is increased. These changes in threshold are correlated with the phases of the action potential.)
Receptor potential (Stimulation of dendrites of nerves cells lead to oscillation of resting membrane potential. This change called receptor potential. Duration of receptor potential correspond to duration of stimulation. Receptor potential cause by increasing of nerves permeability of dendrites membrane. Receptor potential lead to axon hillock. Spreading of receptor potential depend on diameter of dendrites, resistance of cytoplasm and resistance of cell membrane.
Changes in Excitability During Electrotonic Potentials & the Action Potential
During the action potential as well as during catelectrotonic and anelectrotonic potentials and the local response, there are changes in the threshold of the neuron to stimulation. Hyperpolarizing anelectrotonic responses elevate the threshold and catelectrotonic potentials lower it as they move the membrane potential closer to the firing level. During the local response the threshold is also lowered, but during the rising and much of the falling phases of the spike potential the neuron is refractory to stimulation. This refractory period is divided into an absolute refractory period, corresponding to the period from the time the firing level is reached until repolarization is about one-third complete; and a relative refractory period, lasting from this point to the start of after-depolarization. During the absolute refractory period no stimulus, no matter how strong, will excite the nerve, but during the relative refractory period stronger thaormal stimuli can cause excitation. During after-depolarization the threshold is again decreased, and during after-hyperpolarization it is increased. These changes in threshold are correlated with the phases of the action potential.
Saltatory Conduction
Conduction in myelinated axons depends upon a similar pattern of circular current flow. However, myelin is an effective insulator, and current flow through it is negligible. Instead, depolarization in myelinated axons jumps from one node of Ranvier to the next, with the current sink at the active node serving to electrotonically depolarize to the firing level the node ahead of the action potential. This jumping of depolarization from node to node is called saltatory conduction. It is a rapid process, and myelinated axons conduct up to 50 times faster than the fastest unmyelinated fibers. Indeed, myelination saves considerable space in the nervous system, because in unmyelinated neurons conduction velocity increases with the square root of the diameter of the axon, whereas in myelinated axons the conduction velocity increases directly with the diameter of the axon. Thus, a nervous system made up of unmyelinated axons would have to be many times larger.
Ionic Basis of Resting Membrane Potential
Ierves, as in other tissues, Na+ is actively transported out of the cell and K+ is actively transported in. K+ diffuses back out of the cell down its concentration gradient, and Na+ diffuses back in, but since the permeability of the membrane to K+ is much greater than it is to Na+ at rest, the passive K+ efflux is much greater than the passive Na+ influx. Since the membrane is impermeable to most of the anions in the cell, the K+ efflux is not accompanied by an equal flux of anions and the membrane is maintained in a polarized state, with the outside positive relative to the inside.
Ionic Fluxes During the Action Potential
Ierve, as in other tissues, a slight decrease in resting membrane potential leads to increased movement of K+ out of and C1– into the cell, restoring the resting membrane potential. Ierve and muscle, however, there is a unique change in the cell membrane when depolarization exceeds 7 mV. This change is a voltage-dependent increase in membrane permeability to Na+, so that the closer the membrane potential is to the firing level the greater the Na+ permeability. The electrical and concentration gradients for Na+ are both directed inward. During the local response, Na+ permeability is slightly increased, but K+ efflux is able to restore the potential to the resting value. When the firing level is reached, permeability is great enough so that Na+ influx further lowers the membrane potential and Na+ permeability is further increased. The consequent Na+ influx swamps the repolarizing processes, and runaway depolarization results, producing the spike potential.
The equilibrium potential for Na+ in mammaliaeurons, calculated by using the Nemst equation, is about +60 mV. With the great increase in Na+ permeability at the start of the action potential, the membrane potential approaches this value. It does not reach it, however, primarily because the change in Na+ permeability is short-lived. Na+ permeability starts to return to the resting value during the rising phase of the spike potential, and Na+ conductance is decreased during repolarization. In addition, the direction of the electrical gradient for Na+ is reversed during the overshoot because the membrane potential is reversed. These factors limit Na+ influx and help bring about repolarization.
Another important factor producing repolarization of the nerve membrane is the increase in K+ permeability that follows the increase in Na+ permeability. The change in K+ permeability starts more slowly and reaches a peak during the falling phase of the action potential. The increase in permeability decreases the barrier to K+ diffusion, and K+ consequently leaves the cell. The resulting net transfer of positive charge out of the cell completes repolarization.
The changes in membrane permeability during the action potential have been documented in a number of ways, perhaps most clearly by the voltage clamp technique. This research technique, the details of which are beyond the scope of this book, has made it possible to measure changes in the conductance of then membrane for various ions. The conductance of an ion is the reciprocal of its electrical resistance in a membrane and is a measure of membrane permeability to that ion. There is no change in C1– conductance. Decreasing the external Na+ concentration decreases the size of the action potential but has little effect on the resting membrane potential. The lack of much effect on the resting membrane potential would be predicted from the Goldman equation, since the permeability of the membrane to Na+ at rest is relatively low. Conversely, increasing the external K+ concentration decreases the resting membrane potential.
The Na+ and K+ channels in the axon are separate, and both are voltage-gated. The Na+ channel is a protein with a molecular weight of about 275,000 and a pore diameter of about 0,5 nm. There are many charged groups on the surface of the channel, and these groups shift when the channel moves from the closed to the open position. Opening of the channel is also known as sodium channel activation. The voltage differences across the membrane that cause the channel to open also drive it more slowly into a special closed state called the inactivated state. The channel remains in the inactivated state for a few milliseconds before returning to the resting state. Na+ channels can be blocked by a poison called tetrodotoxin (TTX) without affecting the K+ channels. Conversely, the K+ channels can be blocked by tetraethylammonium (TEA) without any changes in Na+ conductance.
Although Na+ enters the nerve cell and K+ leaves it during the action potential, the number of ions involved is not large relative to the total numbers present. The fact that the nerve gains Na+ and loses K+ during activity has been demonstrated experimentally, but significant differences in ion concentrations can be measured only after prolonged, repeated stimulation.
As noted above, the after-depolarization and the after-hyperpolarization represent restorative processes in the cell that are separate from those causing the action potential. Relatively little is known about their origin. The after-depolarization is reduced by agents that inhibit metabolism. The after-hyperpolarization is also reduced, and it now seems clear that it is due to the electrogenic action of the sodium pump. The net flux of Na+ to the exterior hyperpolarizes the membrane until equilibrium conditions are restored.
A decrease in extracellular Ca2+ increases the excitability of nerve and muscle cells by decreasing the amount of depolarizatioecessary to initiate the changes in the Na+ and K+ conductance that produce the action potential. Conversely, an increase in extracellular Ca2+ “stabilizes the membrane” by decreasing excitability. The concentration and electrical gradients for Ca2+ are directed inward, and Ca2+ enters neurons during the action potential. The early phase of Ca2+ entry is blocked by TTX, and it appears that even though Na+ permeability is much greater than Ca2+ permeability, Ca2+ is entering via the Na+ channels. An additional late phase of Ca2+ entry is unaffected by TTX and TEA and apparently occurs via a separate voltage-sensitive Ca2+ pathway. Ca2+ entering during the delayed phase plays an important role in the secretion of synaptic transmitters, a Ca2+-dependent process. In addition, Ca2+ entry contributes to depolarization, and in some instances in invertebrates it is primarily responsible for the action potential.
Carrying of excitation by axons
a) Condition of carrying (1. Anatomic integrity of nerve‘s filament. 2. Physiological full value.)
b) Laws of carrying (1. Double-sided conduction. 2. Isolated of conducting. 3. Conducting of excitation without attenuation.)
c) Carrying in myelinated nerves (In myelin filaments conducting of excitation is doing from node of Ranvier to node of Ranvier Figure 3, 4.)
d) Carrying ionmyelinated nerves (Ionmyelin filaments conducting of excitation is doing uninterrupted.)
Figure 5, Common characteristic of synaptic and junction transmission (Impulses are transmitted from one nerve cell to another at synapses. These are the junctions where the axon or some other portion of one cell (the presynaptic cell) terminates on the soma, the dendrites, or some other portion of another neuron (the postsynaptic cell). It is worth noting that dendrites as well as axons can be presynaptic or postsynaptic. Transmission at most of the junctions is chemical; the impulse in the presynaptic axon liberates a chemical mediator. The chemical mediator binds to receptors on the surface of the postsynaptic cell, and this triggers intracellular events that alter the permeability of the membrane of the postsynaptic neuron. At some of the junctions, however, transmission is electrical, and at a few conjoint synapses it is both electrical and chemical. In any case, impulses in the presynaptic fibers usually contribute to the initiation of conducted responses in the postsynaptic cell, but transmission is not a simple jumping of one action potential from the presynaptic to the postsynaptic neuron. It is a complex process that permits the grading and modulation of neural activity necessary for normal function.
In the case of electrical synapses, the membranes of the presynaptic and postsynaptic neurons come close together, forming a gap junction. Like the intercellular junctions in other tissues, these junctions form low-resistance bridges through which ions pass with relative ease. Electrical and conjoint synapses occur in mammals, and there is electrical coupling, for example, between some of the neurons in the lateral vestibular nucleus. However, most synaptic transmission is chemical.
Transmission from nerve to muscle resembles chemical synaptic transmission. The myoneural junction, the specialized area where a motor nerve terminates on a skeletal muscle fiber, is the site of a stereotyped transmission process. The contacts between autonomic neurons and smooth and cardiac muscle are less specialized, and transmission is a more diffuse process.)
a) Classification of synapses (For localization – central (neuro-neuronal) and peripheral (neuro-muscles, neuro-secretion; central synapses may be axo-somatic, axo-dendrites, axo-axonal, dendro-dendrites; for functional meaning – excitatory and inhibitory; for the method of transmission – electrical and chemical.
There is considerable variation in the anatomic structure of synapses in various parts of the mammaliaervous system. The ends of the presynaptic fibers are generally enlarged to form terminal buttons (synaptic knobs). Endings are commonly located on dendritic spines, which are small knobs projecting from dendrites. In some instances, the terminal branches of the axon of the presynaptic neuron form a basket or net around the soma of the postsynaptic cell (“basket cells” of the cerebellum and autonomic ganglia). In other locations, they intertwine with the dendrites of the postsynaptic cell (climbing fibers of the cerebellum) or end on the dendrites directly (apical dendrites of cortical pyramids) or on the axons (axo-axonal endings). In the spinal cord, the presynaptic endings are closely applied to the soma and the proximal portions of the dendrites of the postsynaptic neuron. The number of synaptic knobs varies from one per postsynaptic cell (in the midbrain) to a very large number. The number of synaptic knobs applied to a single spinal motor neuron has been calculated to be about 10,000, with 2000 on the cell body and 8000 on the dendrites. Indeed, there are so many knobs that the neuron appears to be encrusted with them. The portion of the soma membrane covered by any single synaptic knob is small, but the synaptic knobs are so numerous that, in aggregate, the area covered by them all is often 40 % of the total membrane area. In the cerebral cortex, it has been calculated that 98 % of the synapses are on dendrites and only 2 % are on cell bodies.)
b) Figure 6, Structure of synapses (All synapses consist of: presynaptic membrane, postsynaptic membrane, synaptic cleft, subsynaptic membrane.)
c) Excitatory postsynaptic porential (Single stimuli applied to the sensory nerves in the experimental situation described above characteristically do not lead to the formation of a propagated action potential in the postsynaptic neuron. Instead, the stimulation produces either a transient, partial depolarization or a transient hyperpolarization.
The depolarizing response produced by a single stimulus to the proper input begins about 0,5 ms after the afferent impulse enters the spinal cord. It reaches its peak 1-1,5 ms later and then declines exponentially, with a time constant (time required for the response to decay to 1/e, or 1/2,718 of its maximum) of about 4 ms. During this potential, the excitability of the neuron to other stimuli is increased, and consequently the potential is called an excitatory postsynaptic potential (EPSP).
The EPSP is due to depolarization of the postsynaptic cell membrane immediately under the active synaptic knob. The area of inward current flow thus created is so small that it will not drain off enough positive charges to depolarize the whole membrane. Instead, an EPSP is inscribed. The EPSP due to activity in one synaptic knob is small, but the depolarizations produced by each of the active knobs summate.
Summation may be spatial or temporal. When activity is present in more than one synaptic knob at the same time, spatial summation occurs and activity in one synaptic knob is said to facilitate activity in another to approach the firing level. Temporal summation occurs if repeated afferent stimuli cause new EPSPs before previous EPSPs have decayed. The EPSP is therefore not an all or none response but is proportionate in size to the strength of the afferent stimulus. If the EPSP is large enough to reach the firing level of the cell, a full-fledged action potential is produced.)
d) Synaptic delay (When an impulse reaches the presynaptic terminals, there is an interval of at least 0,5 ms, the synaptic delay, before a response is obtained in the postsynaptic neuron. The delay following maximal stimulation of the presynaptic neuron corresponds to the latency of the EPSP and is due to the time it takes for the synaptic mediator to be released and to act on the membrane of the postsynaptic cell. Because of it, conduction along a chain of neurons is slower if there are many synapses in the chain than if there are only a few. This fact is important in comparing, for example, transmission in the lemniscal sensory pathways to the cerebral cortex and transmission in the reticular activating system. Since the minimum time for transmission across one synapse is 0,5 ms, it is also possible to determine whether a given reflex pathway is monosynaptic or polysynaptic (contains more than one synapse) by measuring the delay in transmission from the dorsal to the ventral root across the spinal cord.)
A synapse, or synaptic cleft, is the gap that separates adjacent neurons or a neuron and a muscle. Transmission of an impulse across a synapse, from presynaptic cell to postsynaptic cell, may be electrical or chemical. In electrical synapses, the action potential travels along the membranes of gap junctions, small tubes of cytoplasm along the membranes of gap junctions, small tubes of cytoplasm that allow the transfer of ions between adjacent cells. In chemical synapses, action potentials are transferred across the synapse by the diffusion of chemicals, as follows:
· Calcium (Ca2+) gates open. When an action potential reaches the end of an axon, the depolarization of the membrane causes gated channels to open that allow Ca2+ to enter.
· Synaptic vesicles release neurotransmitter. The influx of Ca2+ into the terminal end of the axon causes synaptic vesicles to merge with the presynaptic membrane, releasing a neurotransmitter into the synaptic cleft.
· Neurotransmitter binds with postsynaptic receptors. The neurotransmitter diffuses across the synaptic cleft and binds with specialized protein receptors on the postsynaptic membrane. Different proteins are receptors for different neurotransmitters.
· The postsynaptic membrane is excited or inhibited. Depending upon the kind of neurotransmitter and the kind of membrane receptor, there are two possible outcomes for the postsynaptic membrane, both of which are graded potentials.
o If positive ion gates open (which allow more Na+ and Ca2+ to enter than K+ to exit), the membrane becomes depolarized, which results in an excitatory postsynaptic potential (EPSP). If the threshold potential is exceeded, an action potential is generated.
o If K+ or chlorine ion (Cl−) gates open (allowing K+ to exit or Cl− to enter), the membrane becomes more polarized (hyperpolarized), which results in an inhibitory postsynaptic potential (IPSP). As a result, it becomes more difficult to generate an action potential on this membrane.
· The neurotransmitter is degraded and recycled. After the neurotransmitter binds to the postsynaptic membrane receptors, it is either transported back to and reabsorbed by the secreting neuron, or it is broken down by enzymes in the synaptic cleft. For example, the commoeurotransmitter acetylcholine is broken down by cholinesterase. Reabsorbed and degraded neurotransmitters are recycled by the presynaptic cell.
Common characteristic of electrical synapses (Electrical synapses is the junctions in which the transmission of information do through the direct passage of bioelectrical signal from cell to cell. This synapses has small synaptic split (to 5 nm), low specific resistance between the presynaptic and postsynaptic membranes. There are the transverse canals in both membranes with the diameter of 1 nm..)
a) Excitatory transmitter (Excitatory impulses go to the synapse and increase permeability of postsynaptic cell membrane to Na+.)
b) Inhibitory transmitter (Inhibitory impulses go to the synapse and increase permeability of postsynaptic cell membrane to Cl–, not to Na+.)
Common characteristic of chemical synapses (Chemical synapses is the junctions in which the transmission of information do through the direct passage with chemical substances from cell to cell. These substances named mediators.)
a) Classification of chemical synapses (These synapses named for the type of mediator – cholinergic (mediator – acetylcholine), adrenergic (mediator – epinephrine, norepinephrine), serotonin (mediator – serotonin), dopaminenergic (mediator – dopamin), GABA-ergic (mediator – gamma-aminobutyric acid).
b) Chemical transmission of synaptic activity (Active membrane potential go along the nerve to presynaptic end – presynaptic membrane have depolarilazed – the Ca2+-cannals activated – Ca2+-go to the presynaptic end – Ca2+-activated transport of vesiccles with the mediator along the neurofilaments to presynaptic membrane – the mediator pick out from presynaptic ends to the synaptic split – molecules of mediator diffuse through the synaptic split to postsynaptic membrane – molecules of mediator interact with the receptors on the postsynaptic membrane – this interaction lead to the conformation of receptors and activation of corresponding substances.)
Connective Tissue
A summary of the various kinds of connective tissues is given in Figure 1 and Table 1 . Some general characteristics of connective tissues follow.
|
|
|||
|
|||
|
|
|||
|
|||
|
|
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Functionally-structures peculiarities of nervous system Figure 7.
a) Neuron as a structurally-functional unit (The bases of nervous system are neurons. They have body or soma and axons and dendrites. Physiological role of axons is transmition of neurons impulses from soma to other neurons or organs, the physiological role of dendrites is supplying of information to the neurons body. The axon hillock has a maximal excitability ieurons. Neuron may be in the rest condition (absent change of rest membrane potential) and active condition (may generate active potential), and inhibition condition (stop the impulses activity of the neuron).
Classification of nervous cells according to functioning meaning: 1) Sensory or afferent (to perceive irritation and transmit excitement in central nervous system); 2) Interneurons (to help spreading of excitement ieurons nets or act inhibition); 3) moving (motor or efferent) – to transmit the excitement to working organs.
Mediators help to connect the neurons. There are excitable (glutamine acid, acetylcholine) and inhibitory (gamma-ammino-butiric acid – GABA, glycin) mediators.
b) Axon transport (There are 2 kinds of axon transport: fast and slow. Fast transport provides transmition of mitochondria, vesicles with mediators. It speed is 250-
c) Role of glia cells (The neurons of the central nervous system are supported by several varieties of non-excitable cells that are called the neuroglia. There are four types of neuroglial cells: astroglia, oligodendroglia, microglia, epindima. They have insulating function, produce myelin, have secretor role, absorbed from intercells fluid K+.)
Nervous tissue is composed of two main cell types: neurons and glial cells. Neurons transmit nerve messages. Glial cells are in direct contact with neurons and often surround them.
Nerve Cells and Astrocyte (SEM x2,250). This image is copyright Dennis Kunkel at, used with permission.
The neuron is the functional unit of the nervous system. Humans have about 100 billion neurons in their brain alone! While variable in size and shape, all neurons have three parts. Dendrites receive information from another cell and transmit the message to the cell body. The cell body contains the nucleus, mitochondria and other organelles typical of eukaryotic cells. The axon conducts messages away from the cell body.
Structure of a typical neuron.
Three types of neurons occur. Sensory neurons typically have a long dendrite and short axon, and carry messages from sensory receptors to the central nervous system. Motor neurons have a long axon and short dendrites and transmit messages from the central nervous system to the muscles (or to glands). Interneurons are found only in the central nervous system where they connect neuron to neuron.
Structure of a neuron and the direction of nerve message transmission.
Some axons are wrapped in a myelin sheath formed from the plasma membranes of specialized glial cells known as Schwann cells. Schwann cells serve as supportive, nutritive, and service facilities for neurons. The gap between Schwann cells is known as the node of Ranvier, and serves as points along the neuron for generating a signal. Signals jumping from node to node travel hundreds of times faster than signals traveling along the surface of the axon. This allows your brain to communicate with your toes in a few thousandths of a second.
Cross section of myelin sheaths that surround axons
Structure of a nerve bundle.
The Nerve Message
The plasma membrane of neurons, like all other cells, has an unequal distribution of ions and electrical charges between the two sides of the membrane. The outside of the membrane has a positive charge, inside has a negative charge. This charge difference is a resting potential and is measured in millivolts. Passage of ions across the cell membrane passes the electrical charge along the cell. The voltage potential is -65mV (millivolts) of a cell at rest (resting potential). Resting potential results from differences between sodium and potassium positively charged ions and negatively charged ions in the cytoplasm. Sodium ions are more concentrated outside the membrane, while potassium ions are more concentrated inside the membrane. This imbalance is maintained by the active transport of ions to reset the membrane known as the sodium potassium pump. The sodium-potassium pump maintains this unequal concentration by actively transporting ions against their concentration gradients.
Transmission of an action potential.
Changed polarity of the membrane, the action potential, results in propagation of the nerve impulse along the membrane. An action potential is a temporary reversal of the electrical potential along the membrane for a few milliseconds. Sodium gates and potassium gates open in the membrane to allow their respective ions to cross. Sodium and potassium ions reverse positions by passing through membrane protein channel gates that can be opened or closed to control ion passage. Sodium crosses first. At the height of the membrane potential reversal, potassium channels open to allow potassium ions to pass to the outside of the membrane. Potassium crosses second, resulting in changed ionic distributions, which must be reset by the continuously running sodium-potassium pump. Eventually enough potassium ions pass to the outside to restore the membrane charges to those of the original resting potential.The cell begins then to pump the ions back to their original sides of the membrane.
The action potential begins at one spot on the membrane, but spreads to adjacent areas of the membrane, propagating the message along the length of the cell membrane. After passage of the action potential, there is a brief period, the refractory period, during which the membrane cannot be stimulated. This prevents the message from being transmitted backward along the membrane.
Steps in an Action Potential
1. At rest the outside of the membrane is more positive than the inside.
2. Sodium moves inside the cell causing an action potential, the influx of positive sodium ions makes the inside of the membrane more positive than the outside.
3. Potassium ions flow out of the cell, restoring the resting potential net charges.
4. Sodium ions are pumped out of the cell and potassium ions are pumped into the cell, restoring the original distribution of ions.
Synapses
The junction between a nerve cell and another cell is called a synapse. Messages travel within the neuron as an electrical action potential. The space between two cells is known as the synaptic cleft. To cross the synaptic cleft requires the actions of neurotransmitters. Neurotransmitters are stored in small synaptic vessicles clustered at the tip of the axon.
A synapse.
Excitatory Synapse from the Central Nervous System.
Arrival of the action potential causes some of the vesicles to move to the end of the axon and discharge their contents into the synaptic cleft. Released neurotransmitters diffuse across the cleft, and bind to receptors on the other cell’s membrane, causing ion channels on that cell to open. Some neurotransmitters cause an action potential, others are inhibitory.
Neurotransmitters tend to be small molecules, some are even hormones. The time for neurotransmitter action is between 0,5 and 1 millisecond. Neurotransmitters are either destroyed by specific enzymes in the synaptic cleft, diffuse out of the cleft, or are reabsorbed by the cell. More than 30 organic molecules are thought to act as neurotransmitters. The neurotransmitters cross the cleft, binding to receptor molecules on the next cell, prompting transmission of the message along that cell’s membrane. Acetylcholine is an example of a neurotransmitter, as is norepinephrine, although each acts in different responses. Once in the cleft, neurotransmitters are active for only a short time. Enzymes in the cleft inactivate the neurotransmitters. Inactivated neurotransmitters are taken back into the axon and recycled.
Diseases that affect the function of signal transmission can have serious consequences. Parkinson’s disease has a deficiency of the neurotransmitter dopamine. Progressive death of brain cells increases this deficit, causing tremors, rigidity and unstable posture. L-dopa is a chemical related to dopamine that eases some of the symptoms (by acting as a substitute neurotransmitter) but cannot reverse the progression of the disease.
The bacterium Clostridium tetani produces a toxin that prevents the release of GABA. GABA is important in control of skeletal muscles. Without this control chemical, regulation of muscle contraction is lost; it can be fatal when it effects the muscles used in breathing.
Clostridium botulinum produces a toxin found in improperly canned foods. This toxin causes the progressive relaxation of muscles, and can be fatal. A wide range of drugs also operate in the synapses: cocaine, LSD, caffeine, and insecticides.
Nervous Systems
Multicellular animals must monitor and maintain a constant internal environment as well as monitor and respond to an external environment. In many animals, these two functions are coordinated by two integrated and coordinated organ systems: the nervous system and the endocrine system. Click here for a diagram of the Nervous System.
Three basic functions are prformed by nervous systems:
1. Receive sensory input from internal and external environments
2. Integrate the input
3. Respond to stimuli
Sensory Input
Receptors are parts of the nervous system that sense changes in the internal or external environments. Sensory input can be in many forms, including pressure, taste, sound, light, blood pH, or hormone levels, that are converted to a signal and sent to the brain or spinal cord.
Integration and Output
In the sensory centers of the brain or in the spinal cord, the barrage of input is integrated and a response is generated. The response, a motor output, is a signal transmitted to organs than can convert the signal into some form of action, such as movement, changes in heart rate, release of hormones, etc.
Endocrine Systems
Some animals have a second control system, the endocrine system. The nervous system coordinates rapid responses to external stimuli. The endocrine system controls slower, longer lasting responses to internal stimuli. Activity of both systems is integrated.
Divisions of the Nervous System
The nervous system monitors and controls almost every organ system through a series of positive and negative feedback loops.The Central Nervous System (CNS) includes the brain and spinal cord. The Peripheral Nervous System (PNS) connects the CNS to other parts of the body, and is composed of nerves (bundles of neurons).
Not all animals have highly specialized nervous systems. Those with simple systems tend to be either small and very mobile or large and immobile. Large, mobile animals have highly developed nervous systems: the evolution of nervous systems must have been an important adaptation in the evolution of body size and mobility.
Coelenterates, cnidarians, and echinoderms have their neurons organized into a nerve net. These creatures have radial symmetry and lack a head. Although lacking a brain or either nervous system (CNS or PNS) nerve nets are capable of some complex behavior.
Nervous systems in radially symmetrical animals.
Bilaterally symmetrical animals have a body plan that includes a defined head and a tail region. Development of bilateral symmetry is associated with cephalization, the development of a head with the accumulation of sensory organs at the front end of the organism. Flatworms have neurons associated into clusters known as ganglia, which in turn form a small brain. Vertebrates have a spinal cord in addition to a more developed brain.
Some nervous systems in bilaterally symmetrical animals.
Chordates have a dorsal rather than ventral nervous system. Several evolutionary trends occur in chordates: spinal cord, continuation of cephalization in the form of larger and more complex brains, and development of a more elaborate nervous system. The vertebrate nervous system is divided into a number of parts. The central nervous system includes the brain and spinal cord. The peripheral nervous system consists of all body nerves. Motor neuron pathways are of two types: somatic (skeletal) and autonomic (smooth muscle, cardiac muscle, and glands). The autonomic system is subdivided into the sympathetic and parasympathetic systems.
Definition of speed conduction of excitation by moving nerve:
To establish stimulating electrodes over the nervus ulnaris more medial to processus ulnaris. The leading electrodes place over the abducens muscle of fifth finger. Inflict the upper threshold stimulus.
On the electromyograph‘s screen to definite the time from moment of infliction of stimulus to moment of origin the active potential (latent period) – t1. To carry stimulating electrodes in distant place and definite this latent period – t2. To measure the distance between the places of stimulating electrodes – S.
The speed‘s conduction of excitation which are moving by nerve determine according to the formula: V=S: (t1-t2) (m/s).
Evaluation of neuro-muscular transmission:
To establish stimulating electrodes on the forearm (antebrachium) over the nervus ulnaris. The leading electrodes place over the muscle flexion wrist ulnar. Inflict the upper threshold stimulus with the gradual increasing of frequency to 70 impulses per second. On the electromyograph‘s screen look after the changing of amplitude of active membrane potential.
Space summation of under threshold stimulation:
To establish stimulating electrodes on the project of m. flexor carpi radialis in the middle third of antebrachii with the common square 1,2 sm2. To establish level of threshold stimulus, which elicit prolonged contraction of muscle.
After that on this muscle put the electrodes with common square 4,0 sm2. Inflict the under threshold stimulus that was for electrodes with the common square 1,2 sm2.
Peculiarities of excitement transmition ieural chain.
a) Divergence and irradiation (Neurons may to connected by synapses with different nervous cells. This property has the notion divergence. If it will be an active transmition of excitement it is called irradiation.)
b) Convergence (On the each neuron of central nervous system may come together different afferent impulses. That is why in one neuron coming at the same time different excitement. Later it analyzing and formed in one axon excitement that goes to other chain of nervous net.)
c) Reverberation (In central nervous system are present the nervous chain of own excitement. They arise at the answer of stimulus and excitements in these chains are circulated to the time, when other external stimulus inhibits it or it tired. Reverberation is a base of short-time memory.)
d) Time summation (It is a rise of excitement in the case of action under threshold stimulus. If the frequency of stimulus sufficiently big, excitive postsynaptic potential amount to firing level of depolarization and active potential arise.)
e) Space summation (It is a rise of excitement because of simultaneous action of several under-threshold stimuli. In these cases excitive postsynaptic potential amount to the firing level of depolarization or may be more and active potential arise.)
f) Occlusion (On account of divergence one neuron may pass excitive signals on the other neurons. Another neuron may excite several neurons. But if from both neurons which is divergented excitement will be simultaneously the total quantity of excited neurons will be decrease.)
Inhibition in the nervous system (Inhibition is an active reaction which add to the oppression or prevention of excitement.)
a) Postsynaptic inhibition (Excitement, which are coming to the inhibitory neuron – Renshow cell of spinal cord, Purkinje cell of cerebellum, astrocites of cortex of big hemisphere – help to produce an inhibitory mediator of this cell (GABA, glycin). According to this act, increase activity of K+ channels of postsynaptic cells, which lead to the hyperpolarization. As result – decrease of activity of Na+ channels and possibility of development of depolarization in the excitive cell.)
b) Presynaptic inhibition (It may be between axons of excitive and inhibitory neurons. Inhibitory mediator cause hyperpolarization of axon of excitable neuron, prevent arriving of active potential to presynaptic end and as result decrease production of mediator for the development of excitement in postsynaptic cell.)
c) Opposite inhibition (Collaterals of axons of excitive nervous cells are form synaptic connection with inhibitory neurons. These inhibitory neurons have synaptic connection with these excitive neurons. In the case of excitement of excitive neuron activated inhibitory neuron, which produce GABA or glycin in synaptic cleft. As a result, occur hyperpolarization of membrane of excitive neuron and it activity inhibited. Opposite inhibition may be presynaptic or postsynaptic.)
d) Lateral inhibition (If in a neurons’ chain, which secure opposite inhibition collaterals of axons of inhibitioeurons form synaptic connection with neighboring excitive cells in these cells develop lateral inhibition.)
Common characteristic of reflexes (Reflex is a change of functional activity of tissues, organs or whole organism as an answer on stimulus by help of central neural system.)
a) Structure of reflector arc (Reflector arc is a structure base of reflex. It consists of 1. Receptors, which are perceive different influences which are act on organism; 2. Afferent neurons, which connect receptors with central nervous system; 3. Central part of central nervous system, which realize analyses and synthesis of afferent information; 4. Efferent chain secure going out of excitement from central nervous system; 5. Effector is executive organ; 6. Opposite connection.)
b) Classification of reflexes (1. According to the biological meaning: food, defensive, oriental, homeostatic, sex. 2. According to the position of receptors: exteroreceptors (skin, vision, hearing, smell), interoreceptors (visceroreceptors –from inner organs) and proprioreceptors (from muscular, tendons, joints). 3. According to the level of close of reflector arc: spinal, bulbar, mesencephalon, diencephalon, cortex. 4. According to the character of answer: moving, secretory, vessel-moving. 5. According to the duration of answer: fase, tonic. 6. According to the quantity of synapses in central chain: monosynaptic, polisynaptic. 7. According to the kind of efferent part of reflectory arc: somatic, autonomic. 8. According to the position of effector: moving, vessels, heart, secretory, etc. 9. According to the adaptative meaning: physiological, pathological.)
The spinal cord is the most important structure between the body and the brain. The spinal cord extends from the foramen magnum where it is continuous with the medulla to the level of the first or second lumbar vertebrae. It is a vital link between the brain and the body, and from the body to the brain. The spinal cord is 40 to 50 cm long and 1 cm to 1.5 cm in diameter. Two consecutive rows of nerve roots emerge on each of its sides. These nerve roots join distally to form 31 pairs of spinal nerves. The spinal cord is a cylindrical structure of nervous tissue composed of white and gray matter, is uniformly organized and is divided into four regions: cervical (C), thoracic (T), lumbar (L) and sacral (S), each of which is comprised of several segments. The spinal nerve contains motor and sensory nerve fibers to and from all parts of the body. Each spinal cord segment innervates a dermatome.
General Features
1. Similar cross-sectional structures at all spinal cord levels.
2. It carries sensory information (sensations) from the body and some from the head to the central nervous system (CNS) via afferent fibers, and it performs the initial processing of this information.
3. Motor neurons in the ventral horn project their axons into the periphery to innervate skeletal and smooth muscles that mediate voluntary and involuntary reflexes.
4. It contains neurons whose descending axons mediate autonomic control for most of the visceral functions.
5. It is of great clinical importance because it is a major site of traumatic injury and the locus for many disease processes.
Although the spinal cord constitutes only about 2% of the central nervous system (CNS), its functions are vital. Knowledge of spinal cord functional anatomy makes it possible to diagnose the nature and location of cord damage and many cord diseases.
The spinal cord is divided into four different regions: the cervical, thoracic, lumbar and sacral regions. The different cord regions can be visually distinguished from one another. Two enlargements of the spinal cord can be visualized: The cervical enlargement, which extends between C3 to T1; and the lumbar enlargements which extends between L1 to S2.
The cord is segmentally organized. There are 31 segments, defined by 31 pairs of nerves exiting the cord. These nerves are divided into 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerve (Figure 3.2). Dorsal and ventral roots enter and leave the vertebral column respectively through intervertebral foramen at the vertebral segments corresponding to the spinal segment.
The cord is sheathed in the same three meninges as is the brain: the pia, arachnoid and dura. The dura is the tough outer sheath, the arachnoid lies beneath it, and the pia closely adheres to the surface of the cord (Figure 3.3). The spinal cord is attached to the dura by a series of lateral denticulate ligaments emanating from the pial folds.
During the initial third month of embryonic development, the spinal cord extends the entire length of the vertebral canal and both grow at about the same rate. As development continues, the body and the vertebral column continue to grow at a much greater rate than the spinal cord proper. This results in displacement of the lower parts of the spinal cord with relation to the vertebrae column. The outcome of this uneven growth is that the adult spinal cord extends to the level of the first or second lumbar vertebrae, and the nerves grow to exit through the same intervertebral foramina as they did during embryonic development. This growth of the nerve roots occurring within the vertebral canal, results in the lumbar, sacral, and coccygeal roots extending to their appropriate vertebral levels.
All spinal nerves, except the first, exit below their corresponding vertebrae. In the cervical segments, there are 7 cervical vertebrae and 8 cervical nerves (Figure 3.2). C1-C7 nerves exit above their vertebrae whereas the C8 nerve exits below the C7 vertebra. It leaves between the C7 vertebra and the first thoracic vertebra. Therefore, each subsequent nerve leaves the cord below the corresponding vertebra. In the thoracic and upper lumbar regions, the difference between the vertebrae and cord level is three segments. Therefore, the root filaments of spinal cord segments have to travel longer distances to reach the corresponding intervertebral foramen from which the spinal nerves emerge. The lumbosacral roots are known as the cauda equina.
Each spinal nerve is composed of nerve fibers that are related to the region of the muscles and skin that develops from one body somite (segment). A spinal segment is defined by dorsal roots entering and ventral roots exiting the cord, (i.e., a spinal cord section that gives rise to one spinal nerve is considered as a segment.)
A dermatome is an area of skin supplied by peripheral nerve fibers originating from a single dorsal root ganglion. If a nerve is cut, one loses sensation from that dermatome. Because each segment of the cord innervates a different region of the body, dermatomes can be precisely mapped on the body surface, and loss of sensation in a dermatome can indicate the exact level of spinal cord damage in clinical assessment of injury. It is important to consider that there is some overlap betweeeighboring dermatomes. Because sensory information from the body is relayed to the CNS through the dorsal roots, the axons originating from dorsal root ganglion cells are classified as primary sensory afferents, and the dorsal root’s neurons are the first order (1°) sensory neuron. Most axons in the ventral roots arise from motor neurons in the ventral horn of the spinal cord and innervate skeletal muscle. Others arise from the lateral horn and synapse on autonomic ganglia that innervate visceral organs. The ventral root axons join with the peripheral processes of the dorsal root ganglion cells to form mixed afferent and efferent spinal nerves, which merge to form peripheral nerves. Knowledge of the segmental innervation of the cutaneous area and the muscles is essential to diagnose the site of an injury.
A transverse section of the adult spinal cord shows white matter in the periphery, gray matter inside, and a tiny central canal filled with CSF at its center. Surrounding the canal is a single layer of cells, the ependymal layer. Surrounding the ependymal layer is the gray matter – a region containing cell bodies – shaped like the letter “H” or a “butterfly”. The two “wings” of the butterfly are connected across the midline by the dorsal gray commissure and below the white commissure (Figure 3.6). The shape and size of the gray matter varies according to spinal cord level. At the lower levels, the ratio between gray matter and white matter is greater than in higher levels, mainly because lower levels contain less ascending and descending nerve fibers.
The gray matter mainly contains the cell bodies of neurons and glia and is divided into four main columns: dorsal horn, intermediate column, lateral horn and ventral horn column.
The dorsal horn is found at all spinal cord levels and is comprised of sensory nuclei that receive and process incoming somatosensory information. From there, ascending projections emerge to transmit the sensory information to the midbrain and diencephalon. The intermediate column and the lateral horn comprise autonomic neurons innervating visceral and pelvic organs. The ventral horn comprises motor neurons that innervate skeletal muscle.
At all the levels of the spinal cord, nerve cells in the gray substance are multipolar, varying much in their morphology. Many of them are Golgi type I and Golgi type II nerve cells. The axons of Golgi type I are long and pass out of the gray matter into the ventral spinal roots or the fiber tracts of the white matter. The axons and dendrites of the Golgi type II cells are largely confined to the neighboring neurons in the gray matter.
A more recent classification of neurons within the gray matter is based on function. These cells are located at all levels of the spinal cord and are grouped into three main categories: root cells, column or tract cells and propriospinal cells.
The root cells are situated in the ventral and lateral gray horns and vary greatly in size. The most prominent features of the root cells are large multipolar elements exceeding 25 µm of their somata. The root cells contribute their axons to the ventral roots of the spinal nerves and are grouped into two major divisions: 1) somatic efferent root neurons, which innervate the skeletal musculature; and 2) the visceral efferent root neurons, also called preganglionic autonomic axons, which send their axons to various autonomic ganglia.
The column or tract cells and their processes are located mainly in the dorsal gray horn and are confined entirely within the CNS. The axons of the column cells form longitudinal ascending tracts that ascend in the white columns and terminate upoeurons located rostrally in the brain stem, cerebellum or diencephalon. Some column cells send their axons up and down the cord to terminate in gray matter close to their origin and are known as intersegmental association column cells. Other column cell axons terminate within the segment in which they originate and are called intrasegmental association column cells. Still other column cells send their axons across the midline to terminate in gray matter close to their origin and are called commissure association column cells.
The propriospinal cells are spinal interneurons whose axons do not leave the spinal cord proper. Propriospinal cells account for about 90% of spinal neurons. Some of these fibers also are found around the margin of the gray matter of the cord and are collectively called the fasciculus proprius or the propriospinal or the archispinothalamic tract.
Marginal zone nucleus or posterior marginalis, is found at all spinal cord levels as a thin layer of column/tract cells (column cells) that caps the tip of the dorsal horn. The axons of its neurons contribute to the lateral spinothalamic tract which relays pain and temperature information to the diencephalon.
Substantia gelatinosa is found at all levels of the spinal cord. Located in the dorsal cap-like portion of the head of the dorsal horn, it relays pain, temperature and mechanical (light touch) information and consists mainly of column cells (intersegmental column cells). These column cells synapse in cell at Rexed layers IV to VII, whose axons contribute to the ventral (anterior) and lateral spinal thalamic tracts. The homologous substantia gelatinosa in the medulla is the spinal trigeminal nucleus.
Nucleus proprius is located below the substantia gelatinosa in the head and neck of the dorsal horn. This cell group, sometimes called the chief sensory nucleus, is associated with mechanical and temperature sensations. It is a poorly defined cell column which extends through all segments of the spinal cord and its neurons contribute to ventral and lateral spinal thalamic tracts, as well as to spinal cerebellar tracts. The axons originating iucleus proprius project to the thalamus via the spinothalamic tract and to the cerebellum via the ventral spinocerebellar tract (VSCT).
Dorsal nucleus of Clarke is a cell column located in the mid-portion of the base form of the dorsal horn. The axons from these cells pass uncrossed to the lateral funiculus and form the dorsal (posterior) spinocerebellar tract (DSCT), which subserve unconscious proprioception from muscle spindles and Golgi tendon organs to the cerebellum, and some of them innervate spinal interneurons. The dorsal nucleus of Clarke is found only in segments C8 to L3 of the spinal cord and is most prominent in lower thoracic and upper lumbar segments. The homologous dorsal nucleus of Clarke in the medulla is the accessory cuneate nucleus, which is the origin of the cuneocerebellar tract (CCT).
Intermediolateral nucleus is located in the intermediate zone between the dorsal and the ventral horns in the spinal cord levels. Extending from C8 to L3, it receives viscerosensory information and contains preganglionic sympathetic neurons, which form the lateral horn. A large proportion of its cells are root cells which send axons into the ventral spinal roots via the white rami to reach the sympathetic tract as preganglionic fibers. Similarly, cell columns in the intermediolateral nucleus located at the S2 to S4 levels contains preganglionic parasympathetic neurons.
Lower motor neurouclei are located in the ventral horn of the spinal cord. They contain predominantly motor nuclei consisting of α, β and γ motor neurons and are found at all levels of the spinal cord–they are root cells. The a motor neurons are the final common pathway of the motor system, and they innervate the visceral and skeletal muscles.
3.7 Rexed Laminae
The distribution of cells and fibers within the gray matter of the spinal cord exhibits a pattern of lamination. The cellular pattern of each lamina is composed of various sizes or shapes of neurons (cytoarchitecture) which led Rexed to propose a new classification based on 10 layers (laminae). This classification is useful since it is related more accurately to function than the previous classification scheme which was based on major nuclear groups.
Laminae I to IV, in general, are concerned with exteroceptive sensation and comprise the dorsal horn, whereas laminae V and VI are concerned primarily with proprioceptive sensations. Lamina VII is equivalent to the intermediate zone and acts as a relay between muscle spindle to midbrain and cerebellum, and laminae VIII-IX comprise the ventral horn and contain mainly motor neurons. The axons of these neurons innervate mainly skeletal muscle. Lamina X surrounds the central canal and contains neuroglia.
Rexed lamina I – Consists of a thin layer of cells that cap the tip of the dorsal horn with small dendrites and a complex array of nonmyelinated axons. Cells in lamina I respond mainly to noxious and thermal stimuli. Lamina I cell axons join the contralateral spinothalamic tract; this layer corresponds to nucleus posteromarginalis.
Rexed lamina II – Composed of tightly packed interneurons. This layer corresponds to the substantia gelatinosa and responds to noxious stimuli while others respond to non-noxious stimuli. The majority of neurons in Rexed lamina II axons receive information from sensory dorsal root ganglion cells as well as descending dorsolateral fasciculus (DLF) fibers. They send axons to Rexed laminae III and IV (fasciculus proprius). High concentrations of substance P and opiate receptors have been identified in Rexed lamina II. The lamina is believed to be important for the modulation of sensory input, with the effect of determining which pattern of incoming information will produce sensations that will be interpreted by the brain as being painful.
Rexed lamina III – Composed of variable cell size, axons of these neurons bifurcate several times and form a dense plexus. Cells in this layer receive axodendritic synapses from Aβ fibers entering dorsal root fibers. It contains dendrites of cells from laminae IV, V and VI. Most of the neurons in lamina III function as propriospinal/interneuron cells.
Rexed lamina IV – The thickest of the first four laminae. Cells in this layer receive Aß axons which carry predominantly non-noxious information. In addition, dendrites of neurons in lamina IV radiate to lamina II, and respond to stimuli such as light touch. The ill-defined nucleus proprius is located in the head of this layer. Some of the cells project to the thalamus via the contralateral and ipsilateral spinothalamic tract.
Rexed lamina V – Composed neurons with their dendrites in lamina II. The neurons in this lamina receive monosynaptic information from Aß, Ad and C axons which also carry nociceptive information from visceral organs. This lamina covers a broad zone extending across the neck of the dorsal horn and is divided into medial and lateral parts. Many of the Rexed lamina V cells project to the brain stem and the thalamus via the contralateral and ipsilateral spinothalamic tract. Moreover, descending corticospinal and rubrospinal fibers synapse upon its cells.
Rexed lamina VI – Is a broad layer which is best developed in the cervical and lumbar enlargements. Lamina VI divides also into medial and lateral parts. Group Ia afferent axons from muscle spindles terminate in the medial part at the C8 to L3 segmental levels and are the source of the ipsilateral spinocerebellar pathways. Many of the small neurons are interneurons participating in spinal reflexes, while descending brainstem pathways project to the lateral zone of Rexed layer VI.
Rexed lamina VII – This lamina occupies a large heterogeneous region. This region is also known as the zona intermedia (or intermediolateral nucleus). Its shape and boundaries vary along the length of the cord. Lamina VII neurons receive information from Rexed lamina II to VI as well as visceral afferent fibers, and they serve as an intermediary relay in transmission of visceral motor neurons impulses. The dorsal nucleus of Clarke forms a prominent round oval cell column from C8 to L3. The large cells give rise to uncrossed nerve fibers of the dorsal spinocerebellar tract (DSCT). Cells in laminae V to VII, which do not form a discrete nucleus, give rise to uncrossed fibers that form the ventral spinocerebellar tract (VSCT). Cells in the lateral horn of the cord in segments T1 and L3 give rise to preganglionic sympathetic fibers to innervate postganglionic cells located in the sympathetic ganglia outside the cord. Lateral horeurons at segments S2 to S4 give rise to preganglionic neurons of the sacral parasympathetic fibers to innervate postganglionic cells located in peripheral ganglia.
Rexed lamina VIII – Includes an area at the base of the ventral horn, but its shape differs at various cord levels. In the cord enlargements, the lamina occupies only the medial part of the ventral horn, where descending vestibulospinal and reticulospinal fibers terminate. The neurons of lamina VIII modulate motor activity, most probably via g motor neurons which innervate the intrafusal muscle fibers.
Rexed lamina IX – Composed of several distinct groups of large a motor neurons and small γ and β motor neurons embedded within this layer. Its size and shape differ at various cord levels. In the cord enlargements the number of α motor neurons increase and they form numerous groups. The α motor neurons are large and multipolar cells and give rise to ventral root fibers to supply extrafusal skeletal muscle fibers, while the small γ motor neurons give rise to the intrafusal muscle fibers. The α motor neurons are somatotopically organized.
Rexed lamina X – Neurons in Rexed lamina X surround the central canal and occupy the commissural lateral area of the gray commissure, which also contains decussating axons.
In summary, laminae I-IV are concerned with exteroceptive sensations, whereas laminae V and VI are concerned primarily with proprioceptive sensation and act as a relay between the periphery to the midbrain and the cerebellum. Laminae VIII and IX form the final motor pathway to initiate and modulate motor activity via α, β and γ motor neurons, which innervate striated muscle. All visceral motor neurons are located in lamina VII and innervate neurons in autonomic ganglia.
Surrounding the gray matter is white matter containing myelinated and unmyelinated nerve fibers. These fibers conduct information up (ascending) or down (descending) the cord. The white matter is divided into the dorsal (or posterior) column (or funiculus), lateral column and ventral (or anterior) column (Figure 3.8). The anterior white commissure resides in the center of the spinal cord, and it contains crossing nerve fibers that belong to the spinothalamic tracts, spinocerebellar tracts, and anterior corticospinal tracts. Three general nerve fiber types can be distinguished in the spinal cord white matter: 1) long ascending nerve fibers originally from the column cells, which make synaptic connections to neurons in various brainstem nuclei, cerebellum and dorsal thalamus, 2) long descending nerve fibers originating from the cerebral cortex and various brainstem nuclei to synapse within the different Rexed layers in the spinal cord gray matter, and 3) shorter nerve fibers interconnecting various spinal cord levels such as the fibers responsible for the coordination of flexor reflexes. Ascending tracts are found in all columns whereas descending tracts are found only in the lateral and the anterior columns.
Four different terms are often used to describe bundles of axons such as those found in the white matter: funiculus, fasciculus, tract, and pathway. Funiculus is a morphological term to describe a large group of nerve fibers which are located in a given area (e.g., posterior funiculus). Within a funiculus, groups of fibers from diverse origins, which share common features, are sometimes arranged in smaller bundles of axons called fasciculus, (e.g., fasciculus proprius [Figure 3.8]). Fasciculus is primarily a morphological term whereas tracts and pathways are also terms applied to nerve fiber bundles which have a functional connotation. A tract is a group of nerve fibers which usually has the same origin, destination, and course and also has similar functions. The tract name is derived from their origin and their termination (i.e., corticospinal tract – a tract that originates in the cortex and terminates in the spinal cord; lateral spinothalamic tract – a tract originated in the lateral spinal cord and ends in the thalamus). A pathway usually refers to the entire neuronal circuit responsible for a specific function, and it includes all the nuclei and tracts which are associated with that function. For example, the spinothalamic pathway includes the cell bodies of origin (in the dorsal root ganglia), their axons as they project through the dorsal roots, synapses in the spinal cord, and projections of second and third order neurons across the white commissure, which ascend to the thalamus in the spinothalamic tracts.
Spinal Cord Tracts
The spinal cord white matter contains ascending and descending tracts.
Ascending tracts. The nerve fibers comprise the ascending tract emerge from the first order (1°) neuron located in the dorsal root ganglion (DRG). The ascending tracts transmit sensory information from the sensory receptors to higher levels of the CNS. The ascending gracile and cuneate fasciculi occupying the dorsal column, and sometimes are named the dorsal funiculus. These fibers carry information related to tactile, two point discrimination of simultaneously applied pressure, vibration, position, and movement sense and conscious proprioception. In the lateral column (funiculus), the neospinothalamic tract (or lateral spinothalamic tract) is located more anteriorly and laterally, and carries pain, temperature and crude touch information from somatic and visceral structures. Nearby laterally, the dorsal and ventral spinocerebellar tracts carry unconscious proprioception information from muscles and joints of the lower extremity to the cerebellum. In the ventral column (funiculus) there are four prominent tracts: 1) the paleospinothalamic tract (or anterior spinothalamic tract) is located which carry pain, temperature, and information associated with touch to the brain stem nuclei and to the diencephalon, 2) the spinoolivary tract carries information from Golgi tendon organs to the cerebellum, 3) the spinoreticular tract, and 4) the spino-tectal tract. Intersegmental nerve fibers traveling for several segments (2 to 4) and are located as a thin layer around the gray matter is known as fasciculus proprius, spinospinal or archispinothalamic tract. It carries pain information to the brain stem and diencephalon.
Descending tracts. The descending tracts originate from different cortical areas and from brain stem nuclei. The descending pathway carry information associated with maintenance of motor activities such as posture, balance, muscle tone, and visceral and somatic reflex activity. These include the lateral corticospinal tract and the rubrospinal tracts located in the lateral column (funiculus). These tracts carry information associated with voluntary movement. Other tracts such as the reticulospinal vestibulospinal and the anterior corticospinal tract mediate balance and postural movements. Lissauer’s tract, which is wedged between the dorsal horn and the surface of the spinal cord carry the descending fibers of the dorsolateral funiculus (DFL), which regulate incoming pain sensation at the spinal level, and intersegmental fibers. Additional details about ascending and descending tracts are described in the next few chapters.
Information from the skin, skeletal muscle and joints is relayed to the spinal cord by sensory cells located in the dorsal root ganglia. The dorsal root fibers are the axons originated from the primary sensory dorsal root ganglion cells. Each ascending dorsal root axon, before reaching the spinal cord, bifurcates into ascending and descending branches entering several segments below and above their own segment. The ascending dorsal root fibers and the descending ventral root fibers from and to discrete body areas form a spinal nerve. There are 31 paired spinal nerves. The dorsal root fibers segregate into lateral and medial divisions. The lateral division contains most of the unmyelinated and small myelinated axons carrying pain and temperature information to be terminated in the Rexed laminae I, II, and IV of the gray matter. The medial division of dorsal root fibers consists mainly of myelinated axons conducting sensory fibers from skin, muscles and joints; it enters the dorsal/posterior column/funiculus and ascend in the dorsal column to be terminated in the ipsilateral nucleus gracilis or nucleus cuneatus at the medulla oblongata region, i.e., the axons of the first-order (1°) sensory neurons synapse in the medulla oblongata on the second order (2°) neurons (iucleus gracilis or nucleus cuneatus). In entering the spinal cord, all fibers send collaterals to different Rexed lamina.
Axons entering the cord in the sacral region are found in the dorsal columear the midline and comprise the fasciculus gracilis, whereas axons that enter at higher levels are added in lateral positions and comprise the fasciculus cuneatus. This orderly representation is termed “somatotopic representation”.
Functionally-structural characteristic of spinal cord.
a) Functions, macroscopic structure (The functions of spinal cord are transmitting the impulses and reflectory function. Spinal cord has segmental structure. It consists of 31-33 segments: 8 cervical, 12 thoracic, 5 lumbal, 5 sacral and from 1 to 3 coccigea. Every segment has two pairs of ventral and dorsal roots: right and left. In outer layer present the white substance, where pass conduction tracts, and in inner layer present grey matter, where present nucleus. Segmental principle of work connects with segmental structure of spinal cord. Spinal reflexes are reflexes, which reflex arcs are locked on level of segment of spinal cord. They are tendinous and dermal reflexes. For example, knee reflex is locked on LIII-LIV level. Reflexes between segments whose reflex arcs are locked on many segments. For example, vessels` reflex – on ThI– LII level.)
b) Property of neurons elements (Body of sensory cell are present outside the spinal cord. Some of them are present in spinal ganglion (they innervate the sceletal muscles). Other is present in extra- and intramural ganglions of autonomic nervous system and provide sensitivity of inner organs. The nervous fibers of sensory cells may be myelinated (v=12-120 m/s) and nonmyelinated (v=2 m/s). They go to spinal cord from pain, chemo- and some mechanoreceptors. 3 % of all neurons are moving, 97 % are interneurons. There are α- and γ-motoneurons. α-motoneurons transmit signals from spinal cord to celetal muscles. γ-motoneurons (30 %) innervate intrausal muscles fibers. Excitement of the fibers lead to the contraction or relaxation of extrafusal muscles fibers.)
c) Interposed of afferent and efferent fibers on peripheral part (Bell-Magandy`s law: in the spinal cord the dorsal are sensory, the ventral roots are motors. Quantity of sensory fibers in posterior roots in 20 times more than moving in arterior.)
Reflectory activity of spinal cord.
a) Elicity and reflectory arc of miotatic reflexes: elbow, knee, achill (Spinal somatic reflexes are totally of simple pose and motor acts, which can be realized without participation of higher parts of central nervous system. Stretch reflexes are monosynaptic. Extention, flexor reflexes – polysynaptic, spinal locomotor reflexes are transference. They provide by the help of coordinated movements of limbs. Programated on spinal level it is autonomic movement. Characteristics of elbow, knee, Achilles, plantar and abdominal reflexes. Elbow, knee, Achilles reflexes are monosynaptic, myotatic reflexes. They have segmental character. They are locked on level: elbow – CV– CVI , knee – LII– LIV, Achilles SI– SII.
Plantar and abdominal are dermal monosynaptic reflexe. Are locked on level: plantar – Th12. Superior abdominal Th8 – Th9, medius abdominal Th9–Th10, inferior abdominal – Th11–Th12.
Reflex arcs of tendinous relflex are: knee reflex – intrafusal fibers of m. quadriceps femoris – n. femoralis – LII – LIV – n. femoralis – extrafusal fibers of m. quadriceps femoris.
Achilles reflex – intrafusal fibers of m. gastrocnemius – n. ischiadicus – SI-SII – n. tibialis – extrafusal fibers of m. gastrocnemius.
Elbow flexor reflex – intrafusal fibers of m. biceps brachii – n. musculocutaneus – CV-CVI – n. musculocutaneus – extrafusal fibers of m. biceps brachii.
Elbow extension reflex – intrafusal fibers of m. triceps brachii – n. radialis – CVII-CVIII – n. radialis – extrafusal fibers of m. triceps brachii.
b) Mechanism of development of miotatic reflexes (Muscle spindle consist of nucleus bag (central part) and intrafusal muscles fibers. Spindles connect to the exstrafusal fibers. The quantity of spindles increase in the case of direct muscles’ moving. In the nucleus bag present nervous ending (like-spirale), this has receptor function. From begining of afferent fiber, which transmit excitement fast. Nervous ending may excited in the case of cotraction of muscles fibers. γ-motoneurons have the influence on contraction of it too. γ-motoneurons help to contract the intrafusal fibers. This is a course of stretch of nervous ending iucleus bag. The quantity of impulses to spinal cord is increase. Sensor neurons end near α-motoneurons, excite them and as result extrafusal fibers are contract. This is a base of myotatic reflexes.)
c) Meaning of invistigation of spinal reflexes (It very important for neurologic department to determing the place of destruction of spinal cord.)
d) Bent and cross-unbend reflexes (Cross-unbend reflexes are in the base of locomotor acts and characterised by inhibition of motoneurons of extensor muscles in the same time of excitement of motoneurons of flexor muscles. At this time on the leg and arm of opposite side present opposite reaction. In the stretch reflexes more high tone are in muscles extensor. They help support the static and pose of body.)
Functional meaning of spinal cord’ tracts (There are 2 kinds of tracts: ascendens and descendens. Ascendens tract are sensory, descendens are motor.)
a) Ascendens (Ascending tracts are sensory. They conduct information from external environment to the higher situated centers of encephalon. They are conductors of information from enternal surroundings to the higher part of central nervous system. Goll`s tract (fasciculus gracilis), Burdach`s tract (fasciculus cuneatus) are situated in postirior columnus. They are conducters of tactive and proprioceptive (for example, muscles-elbow) sensorities from down and upper part of the body. Tractus spinothalamicus is situated in lateral columnus. They carry pain, temperature – tractus spinothalamicus dorsalis and spinotectalis – and tactil – tractus spinothalamicus ventralis – sensetivities from body to thalamus.)
b) Descendens (Descending tracts are motors. Corticospinal tract (tractus corticospinalis lateralis) is basic motor tract. It is passing in side columns. It is a conductor of impulses to the skeletal muscles, is regulating free movements.
Monacow`s tract (rubrospinalis) – in side columns, regulate tone of skeletal muscles.
Tractus vestibulospinal dorsalis is present in side columns regilate equilibrium and supporting of pose. Olivospinal tract – in side columns – may be takes part in thalamospinal reflexes. Reticulospinal tract – in front columns – is regulating the tone of skeletal muscles, vegetative spinal centers. Vestibulospinal tract – in front columns – regulates equilibrium and supporting of pose.)
References:
1. Review of Medical Physiology // W.F. Ganong. – Twentieth edition, 2001. – P. 49-58, 62-84, 90-91, 95-96.
2. Textbook of Medical Physiology // A.C. Guyton, J.E. Hall. – Tenth edition, 2002. – P. 52-65, 67-78, 80-94, 96-99.
