The spinal cord is an organ of the central nervous system of vertebrates located in the spinal canal. It is generally accepted that the border between the spinal cord and the brain passes at the level of the intersection of the pyramidal fibers (although this border is very arbitrary). Inside the spinal cord there is a cavity called the central canal. The spinal cord is protected by the pia, arachnoid and dura mater. The spaces between the membranes and the spinal canal are filled with cerebrospinal fluid. The space between the outer hard shell and the bone of the vertebrae is called the epidural and is filled with fat and venous network.

Histology of the spinal cord

The spinal cord consists of two symmetrical halves, separated from each other in front by a deep median fissure, and behind by a connective tissue septum. On fresh preparations of the spinal cord, it can be seen with the naked eye that its substance is inhomogeneous. The inner part of the organ is darker - this is its gray matter. On the periphery of the spinal cord is a lighter white matter. The protrusions of the gray matter are called horns. There are anterior (ventral), posterior (dorsal) and lateral (lateral) horns. Throughout the spinal cord, the ratio of gray and white matter changes. The gray matter is represented by the smallest number of cells in the thoracic region, the largest - in the lumbar.

The gray matter of the spinal cord consists of the bodies of neurons, unmyelinated and thin myelinated fibers and neuroglia. The main component of gray matter, which distinguishes it from white, are multipolar neurons. Cells similar in size, fine structure and functional significance lie in gray matter in groups called nuclei. Separate areas of the gray matter of the spinal cord differ significantly from each other in the composition of neurons, nerve fibers and neuroglia.

Among the neurons of the spinal cord, the following types of cells can be distinguished:

radicular cells whose axons leave the spinal cord as part of its anterior roots

internal cells whose processes terminate in synapses within the gray matter of the spinal cord

beam cells, the axons of which pass through the white matter in separate bundles of fibers that carry nerve impulses from certain nuclei of the spinal cord to its other segments or to the corresponding parts of the brain, forming pathways.

In the posterior horns, a spongy layer, a gelatinous substance, a proper nucleus of the posterior horn and a thoracic nucleus are distinguished. Between the posterior and lateral horns, the gray matter protrudes into the white in strands, as a result of which a network-like loosening is formed, called the mesh formation. The spongy layer of the posterior horns is characterized by a wide-loop glial scaffold, which contains a large number of small intercalary neurons. Glial elements predominate in the gelatinous substance. Nerve cells here are small and their number is negligible. The posterior horns are rich in diffusely located intercalary cells. These are small multipolar associative and commissural cells, the axons of which terminate within the gray matter of the spinal cord of the same side (associative cells) or the opposite side (commissural cells). The neurons of the spongy zone, the gelatinous substance and the intercalary cells communicate between the sensory cells of the spinal ganglia and the motor cells of the anterior horns, closing the local reflex arcs. In the middle of the posterior horn is its own nucleus of the posterior horn. It consists of intercalary neurons, the axons of which pass through the anterior white commissure to the opposite side of the spinal cord into the lateral funiculus of the white matter, where they are part of the ventral spinal-cerebellar and spinal-thalamic pathways and go to the cerebellum and thalamus. The thoracic nucleus (Clark's nucleus) consists of large intercalary neurons with highly branched dendrites. Their axons exit into the lateral funiculus of the white matter of the same side and, as part of the posterior spinal-cerebellar tract (Flexig's path), rise to the cerebellum. In the intermediate zone, a medial intermediate nucleus is distinguished, the axons of the cells of which join the anterior spinal cerebellar path (Govers path) of the same side, and the lateral intermediate nucleus, located in the lateral horns and representing a group of associative cells of the sympathetic reflex arc. The axons of these cells leave the brain together with the somatic motor fibers as part of the anterior roots and separate from them in the form of white connecting branches of the sympathetic trunk. The largest neurons of the spinal cord are located in the anterior horns, which have a body diameter of 100–150 microns and form nuclei of considerable volume. This is the same as the neurons of the nuclei of the lateral horns, radicular cells, since their axons make up the bulk of the fibers of the anterior roots. As part of the mixed spinal nerves, they enter the periphery and form motor endings in the skeletal muscles. Thus, these nuclei are motor somatic centers. In the anterior horns, the medial and lateral groups of motor cells are most pronounced.

The first innervates the muscles of the trunk and is well developed throughout the spinal cord. The second is located in the region of the cervical and lumbar thickenings and innervates the muscles of the limbs. Motoneurons provide efferent information to skeletal striated muscles, they are large cells (diameter - 100-150 microns). There are many scattered bundle neurons in the gray matter of the spinal cord. The axons of these cells exit into the white matter and immediately divide into longer ascending and shorter descending branches. Together, these fibers form their own, or main, bundles of white matter, directly adjacent to the gray matter.

White matter surrounds gray matter. The grooves of the spinal cord divide it into cords: anterior, lateral and posterior. The cords are nerve tracts that connect the spinal cord with the brain.

The widest and deepest sulcus is the anterior median fissure, which separates the white matter between the anterior horns of the gray matter. Opposite it is the posterior median sulcus.

A pair of lateral grooves go, respectively, to the posterior and anterior horns of the gray matter.

The posterior funiculus is divided, forming two ascending tracts: the closest to the posterior median sulcus (gentle, or thin bundle) and more lateral (wedge-shaped bundle). The inner bundle, thin, rises from the lowest parts of the spinal cord, while the wedge-shaped one is formed only at the level of the thoracic region.

Histological structure of the spinal cord.

The spinal cord (SM) consists of 2 symmetrical halves, separated in front by a deep fissure, and behind by a commissure. The transverse section clearly shows the gray and white matter. The gray matter of the SM on the cut has the shape of a butterfly or the letter "H" and has horns - anterior, posterior and lateral horns. The gray matter of the SM consists of bodies of neurocytes, nerve fibers and neuroglia.

The abundance of neurocytes determines the gray color of the gray matter of the SM. Morphologically, SM neurocytes are predominantly multipolar. Neurocytes in the gray matter are surrounded by nerve fibers tangled like felt - neuropil. The axons in the neuropil are weakly myelinated, while the dendrites are not myelinated at all. Similar in size, fine structure, and functions, SC neurocytes are arranged in groups and form nuclei.

Among SM neurocytes, the following types are distinguished:

1. Radicular neurocytes - located in the nuclei of the anterior horns, they are motor in function; axons of radicular neurocytes as part of the anterior roots leave the spinal cord, conduct motor impulses to the skeletal muscles.

2. Internal cells - the processes of these cells do not leave the limits of the gray matter of the SM, end within the given segment or the adjacent segment͵ ᴛ.ᴇ. are associative in function.

3. Beam cells - the processes of these cells form nerve bundles of white matter and are sent to neighboring segments or outlying sections of the NS, ᴛ.ᴇ. are also associative in function.

The posterior horns of the SM are shorter, narrower and contain the following types of neurocytes:

a) beam neurocytes - are located diffusely, receive sensitive impulses from the neurocytes of the spinal ganglia and transmit along the ascending pathways of the white matter to the outlying sections of the NS (to the cerebellum, to the cerebral cortex);

b) internal neurocytes - transmit sensitive impulses from the spinal ganglia to the motor neurocytes of the anterior horns and to neighboring segments.

There are 3 zones in the posterior horns of the SM:

1. Spongy substance - consists of small bundled neurocytes and gliocytes.

2. Gelatinous substance - contains a large number of gliocytes, has practically no neurocytes.

3. Proprietary SM nucleus - consists of bundled neurocytes that transmit impulses to the cerebellum and thalamus.

4. Clark's nucleus (Thoracic nucleus) - consists of bundled neurocytes, the axons of which, as part of the lateral cords, are sent to the cerebellum.

In the lateral horns (intermediate zone) there are 2 medial intermediate nuclei and a lateral nucleus. The axons of the bundle associative neurocytes of the medial intermediate nuclei transmit impulses to the cerebellum. The lateral nucleus of the lateral horns in the thoracic and lumbar SM is the central nucleus of the sympathetic division of the autonomic NS. The axons of the neurocytes of these nuclei go as part of the anterior roots of the spinal cord as preganglionic fibers and terminate on the neurocytes of the sympathetic trunk (prevertebral and paravertebral sympathetic ganglia). The lateral nucleus in the sacral part of the SM is the central nucleus of the parasympathetic part of the autonomic NS.

The anterior horns of the SM contain a large number of motor neurons (motor neurons) that form 2 groups of nuclei:

1. Medial group of nuclei - innervates the muscles of the body.

2. The lateral group of nuclei is well expressed in the region of the cervical and lumbar thickening - it innervates the muscles of the extremities.

According to their function, among the motoneurons of the anterior horns of the SC are distinguished:

1. - motor neurons are large - have a diameter of up to 140 microns, transmit impulses to extrafusal muscle fibers and provide rapid muscle contraction.

2. small motor neurons - maintain the tone of the skeletal muscles.

3. -motoneurons - transmit impulses to intrafusal muscle fibers (as part of the neuromuscular spindle).

Motoneurons are an integrative unit of the SM; they are influenced by both excitatory and inhibitory impulses. Up to 50% of the body surface and motor neuron dendrites are covered with synapses. The average number of synapses per 1 human SC motor neuron is 25-35 thousand. At the same time, 1 motor neuron can transmit impulses from thousands of synapses coming from neurons of the spinal and supraspinal levels.

Reverse inhibition of motor neurons is also possible due to the fact that the axon branch of the motor neuron transmits an impulse to inhibitory Renshaw cells, and the axons of Renshaw cells end on the body of the motor neuron with inhibitory synapses.

Axons of motor neurons leave the SM as part of the anterior roots, reach the skeletal muscles, and end on each muscle fiber with a motor plaque.

The white matter of the spinal cord consists of longitudinally oriented predominantly myelinated nerve fibers that form the posterior (ascending), anterior (descending), and lateral (both ascending and descending) cords, as well as glial elements.

Spinal cord

The spinal cord consists of two symmetrical halves, separated from each other in front by a deep median fissure, and behind by a median sulcus. The spinal cord is characterized by a segmental structure; each segment is associated with a pair of anterior (ventral) and a pair of posterior (dorsal) roots.

In the spinal cord there are Gray matter located in the central part, and white matter lying on the periphery.

The white matter of the spinal cord is a collection of longitudinally oriented predominantly myelinated nerve fibers. Bundles of nerve fibers that communicate between different parts of the nervous system are called tracts, or pathways, of the spinal cord.

The outer border of the white matter of the spinal cord forms glial border membrane, consisting of fused flattened processes of astrocytes. This membrane is permeated by nerve fibers that make up the anterior and posterior roots.

Throughout the entire spinal cord in the center of the gray matter runs the central canal of the spinal cord, which communicates with the ventricles of the brain.

The gray matter on the transverse section has the appearance of a butterfly and includes front, or ventral, rear, or dorsal, and lateral, or lateral, horns. The gray matter contains the bodies, dendrites and (partly) axons of neurons, as well as glial cells. The main component of gray matter, which distinguishes it from white, are multipolar neurons. Between the bodies of neurons there is a neuropil - a network formed by nerve fibers and processes of glial cells.

As the spinal cord develops from the neural tube, neurons cluster into 10 layers, or Rexed's plates. At the same time, plates I-V correspond to the posterior horns, plates VI-VII correspond to the intermediate zone, plates VIII-IX correspond to the anterior horns, plate X corresponds to the zone near the central canal. This division into plates complements the organization of the structure of the gray matter of the spinal cord, based on the localization of the nuclei. On transverse sections, nuclear groups of neurons are more clearly visible, and on sagittal sections, the lamellar structure is better seen, where neurons are grouped into Rexed columns. Each column of neurons corresponds to a specific area on the periphery of the body.

Cells similar in size, fine structure and functional significance lie in gray matter in groups called nuclei.

Among the neurons of the spinal cord, three types of cells can be distinguished:

• radicular,
• internal,
• beam.

Axons of radicular cells leave the spinal cord as part of its anterior roots. The processes of internal cells end in synapses within the gray matter of the spinal cord. The axons of the beam cells pass through the white matter as separate bundles of fibers that carry nerve impulses from certain nuclei of the spinal cord to its other segments or to the corresponding parts of the brain, forming pathways. Separate areas of the gray matter of the spinal cord differ significantly from each other in the composition of neurons, nerve fibers and neuroglia.

AT posterior horns Distinguish between the spongy layer, the gelatinous substance, the nucleus proper of the posterior horn and the thoracic nucleus of Clark. Between the posterior and lateral horns, the gray matter juts into the white as strands, as a result of which its mesh-like loosening is formed, which is called the mesh formation, or reticular formation, of the spinal cord.

The posterior horns are rich in diffusely located intercalary cells. These are small multipolar associative and commissural cells, the axons of which terminate within the gray matter of the spinal cord of the same side (associative cells) or the opposite side (commissural cells).

The neurons of the spongy zone and the gelatinous substance communicate between the sensitive cells of the spinal ganglia and the motor cells of the anterior horns, closing the local reflex arcs.

Clark's nucleus neurons receive information from muscle, tendon, and joint receptors (proprioceptive sensitivity) along the thickest radicular fibers and transmit it to the cerebellum.

In the intermediate zone, there are centers of the autonomic (autonomous) nervous system - preganglionic cholinergic neurons of its sympathetic and parasympathetic divisions.

AT anterior horns the largest neurons of the spinal cord are located, which form nuclei of considerable volume. This is the same as the neurons of the nuclei of the lateral horns, radicular cells, since their neurites make up the bulk of the fibers of the anterior roots. As part of the mixed spinal nerves, they enter the periphery and form motor endings in the skeletal muscles. Thus, the nuclei of the anterior horns are motor somatic centers.

Glia of the spinal cord

The main part of the glial backbone of the gray matter is protoplasmic and fibrous astrocytes. The processes of fibrous astrocytes extend beyond the gray matter and, together with elements of connective tissue, take part in the formation of partitions in the white matter and glial membranes around blood vessels and on the surface of the spinal cord.

Oligodendrogliocytes are part of the sheaths of nerve fibers, predominate in the white matter.

The ependymal glia lines the central canal of the spinal cord. Ependymocytes participate in the production of cerebrospinal fluid (CSF). A long process departs from the peripheral end of the ependymocyte, which is part of the outer boundary membrane of the spinal cord.

Directly under the ependymal layer is a subependymal (periventricular) boundary glial membrane formed by processes of astrocytes. This membrane is part of the so-called. hemato-liquor barrier.

Microglia enter the spinal cord as blood vessels grow into it and are distributed in the gray and white matter.

The connective tissue membranes of the spinal cord correspond to the membranes of the brain.

Brain

In the brain, gray and white matter are distinguished, but their distribution here is much more complicated than in the spinal cord. Most of the gray matter of the brain is located on the surface of the cerebrum and cerebellum, forming them bark. A smaller part forms numerous subcortical nuclei surrounded by white matter. All gray matter nuclei are composed of multipolar neurons.

Cerebellum

The cerebellum is the central organ balance and coordination of movements. It is formed by two hemispheres with a large number of grooves and convolutions, and a narrow middle part - a worm.

The bulk of the gray matter in the cerebellum is located on the surface and forms its cortex. A smaller part of the gray matter lies deep in the white matter in the form of the central nuclei of the cerebellum.

The cerebellar cortex is a nerve center of the screen type and is characterized by a highly ordered arrangement of neurons, nerve fibers and glial cells. There are three layers in the cerebellar cortex: molecular, ganglionic and granular.

Outer molecular layer contains relatively few cells. It distinguishes between basket and stellate neurons.

Average ganglion layer formed by one row of large pear-shaped cells, first described by the Czech scientist Jan Purkinje.

Interior granular layer characterized by a large number of tightly lying cells, as well as the presence of the so-called. glomeruli of the cerebellum. Among neurons, granule cells, Golgi cells, and fusiform horizontal neurons are distinguished here.

More detailed structure of the cerebellar cortex

molecular layer contains two main types of neurons: basket and stellate. Basket neurons are located in the lower third of the molecular layer. Their dendrites form bonds with parallel fibers in the outer part of the molecular layer. Long axons of basket cells run across the gyrus and give off collaterals to the bodies of pear-shaped neurons, densely braiding them like a basket. The activity of the basket neurons causes inhibition of the piriform neurons of Purkinje.

The stellate neurons lie above the basket cells and are of two types. Small stellate neurons are equipped with short dendrites and weakly branched axons that form synapses on the dendrites of pear-shaped cells. Large stellate neurons, unlike small ones, have long and highly branched dendrites and axons. The branches of their axons are part of the so-called baskets. Basket and stellate neurons of the molecular layer are a single system of intercalary neurons that transmit inhibitory nerve impulses to the dendrites and bodies of pear-shaped cells in a plane transverse to the convolutions.

Ganglion layer contains bodies of Purkinje cells lying in one row, braided with collaterals of axons of basket cells. From the large pear-shaped body of these neurons, 2-3 dendrites extend into the molecular layer, which branch abundantly and penetrate the entire thickness of the molecular layer. All branches of the dendrites are located only in one plane, perpendicular to the direction of the convolutions. On the dendrites there are spines - the contact zones of excitatory synapses formed by parallel fibers and inhibitory synapses formed by climbing fibers.

From the base of the bodies of Purkinje cells, axons depart, passing through the granular layer of the cerebellar cortex into the white matter and ending on the cells of the cerebellar nuclei. This is the initial link of the efferent inhibitory pathways of the cerebellum. Within the granular layer, collaterals depart from these axons, which return to the ganglionic layer and enter into a synaptic connection with neighboring pear-shaped neurons.

Granular layer The cerebellar cortex contains closely spaced bodies of granular neurons, or granule cells. The cell has 3-4 short dendrites, ending in the same layer with terminal branches in the form of a "bird's foot". Entering into a synaptic connection with the endings of the excitatory mossy fibers that enter the cerebellum, the dendrites of the granule cells form characteristic structures called cerebellar glomeruli.

The axons of the granule cells rise into the molecular layer and in it divide in a T-shape into two branches oriented parallel to the surface of the cortex along the gyri of the cerebellum. Overcoming long distances, these parallel fibers cross the branching of the dendrites of many pear-shaped cells and form synapses with them and the dendrites of basket and stellate neurons. Thus, the axons of the granule cells transmit the excitation they receive from mossy fibers over a considerable distance to many pear-shaped cells.

The second type of cells in the granular layer of the cerebellum are inhibitory stellate neurons, they are also large granule cells, they are also Golgi cells. There are two types of such cells: with short and long axons. Neurons with short axons lie near the ganglion layer. Their branched dendrites spread in the molecular layer and form synapses with parallel fibers - axons of granule cells. Short axons go to the glomeruli of the cerebellum and end in synapses at the terminal branches of the dendrites of granule cells proximal to the synapses of mossy fibers. Excitation of stellate neurons can block impulses coming through mossy fibers.

A few stellate neurons with long axons have dendrites and axons abundantly branching in the granular layer, extending into the white matter. These cells are thought to provide communication between different areas of the cerebellar cortex.

The third type of cells in the granular layer are spindle-shaped horizontal cells. They have a small elongated body, from which long horizontal dendrites extend in both directions, ending in the ganglionic and granular layers. The axons of these cells give collaterals to the granular layer and go to the white matter.

Afferent fibers, entering the cerebellar cortex, are represented by two types - mossy and climbing fibers. Mossy fibers through granule cells have an exciting effect on pear-shaped cells. They end in the glomeruli of the granular layer of the cerebellum in the form of extensions-rosettes, where they come into contact with the dendrites of the granule cells. Each bryophyte gives branches to many glomeruli of the cerebellum, and each glomerulus receives branches from many bryophytes. Axons of granule cells along parallel fibers of the molecular layer transmit impulses to the dendrites of pear-shaped, basket-shaped, stellate neurons of the granular layer.

Climbing, or liana-like, fibers cross the granular layer, adjoin pear-shaped neurons and spread along their dendrites, ending on their surface with excitatory synapses. Climbing fibers transmit excitation directly to piriform neurons. Each Purkinje cell usually has one such fiber in contact.

Thus, excitatory impulses entering the cerebellar cortex reach pear-shaped neurons either directly along climbing fibers or along parallel fibers of granule cells.

Inhibition in the cerebellum is a function of stellate neurons of the molecular layer, basket neurons, and Golgi cells of the granular layer. The axons of the first two, following across the convolutions and inhibiting the activity of pear-shaped cells, limit their excitation to narrow discrete zones of the cortex. The entry of excitatory signals into the cerebellar cortex through mossy fibers, through granule cells and parallel fibers can be interrupted by inhibitory synapses of large stellate neurons localized on the terminal branches of granule cell dendrites proximal to excitatory synapses.

Efferent fibers the cerebellar cortex is represented by axons of Purkinje cells, which in the form of myelin fibers are sent to the white matter and reach the deep nuclei of the cerebellum and the vestibular nucleus, on the neurons of which they form inhibitory synapses.

The cerebellar cortex contains various glial elements. The granular layer contains fibrous and protoplasmic astrocytes. The legs of fibrous astrocyte processes form perivascular membranes, which are a component of the blood-brain barrier, as well as membranes around the glomeruli of the cerebellum. All layers in the cerebellum contain oligodendrocytes. The granular layer and white matter of the cerebellum are especially rich in these cells. In the ganglion layer between pear-shaped neurons lie special astrocytes with dark nuclei - Bergman cells. The processes of these cells are sent to the surface of the cortex and form glial fibers of the molecular layer of the cerebellum (Bergman fibers), which support the branching of the dendrites of pear-shaped cells. Microglia are found in large quantities in the molecular and ganglionic layers.

(see also from general histology)

Some terms from practical medicine:

• ataxia- violation of movements, manifested by a disorder in their coordination;
• alcoholic ataxia- taxia in alcohol intoxication, due to a functional disorder of the vestibulocerebellar system;
• myelodysplasia- the general name of anomalies in the development of the spinal cord;
• myelomeningocele- spinal hernia, the hernial sac of which contains cerebrospinal fluid and a section of the spinal cord along with its membranes and roots of the spinal nerves;

In the spinal cord distinguish between gray and white matter. On a transverse section of the spinal cord, the gray matter looks like the letter H. There are anterior (ventral), lateral, or lateral (lower cervical, thoracic, two lumbar), and posterior (dorsal) horns of the gray matter of the spinal cord.

Gray matter represented by the bodies of neurons and their processes, nerve endings with a synaptic apparatus, macro- and microglia and blood vessels.

white matter surrounds the gray matter on the outside and is formed by bundles of pulpy nerve fibers that form pathways throughout the entire spinal cord. These paths are directed towards the brain or descend from it. This also includes fibers that go to the higher or lower segments of the spinal cord. In addition, white matter contains astrocytes, individual neurons, and hemocapillaries.

in white matter each half of the spinal cord (on a transverse section) there are three pairs of columns (cords): posterior (between the posterior median septum and the medial surface of the posterior horn), lateral (between the anterior and posterior horns) and anterior (between the medial surface of the anterior horn and the anterior median fissure ).

In the center of the spinal cord passes through a channel lined with ependymocytes, among which there are poorly differentiated forms capable, according to some authors, of migration and differentiation into neurons. In the lower segments of the spinal cord (lumbar and sacral), after puberty, proliferation of gliocytes and overgrowth of the canal, the formation of an intraspinal organ occurs. The latter contains gliocytes and secretory cells that produce a vasoactive neuropeptide. The organ undergoes involution after 36 years.

gray matter neurons spinal cord are multipolar. Among them, neurons with a few weakly branching dendrites, neurons with branching dendrites, as well as transitional forms are distinguished.

Depending on where the shoots go neurons, emit: internal neurons, the processes of which end in synapses within the spinal cord; bundle neurons, the neurite of which goes as part of bundles (conducting pathways) to other parts of the spinal cord or to the brain; radicular neurons, the axons of which leave the spinal cord as part of the anterior roots.

In cross section, neurons are grouped into nuclei, which contain neurons similar in structure and function. On a longitudinal section, these neurons are arranged in layers in the form of a column, which is clearly visible in the region of the posterior horn. The neurons of each column innervate strictly defined areas of the body. The regularities of the grouping of neurons and their functions can be judged by the Rexed plates (1-X). In the center of the posterior horn is its own nucleus of the posterior horn, at the base of the posterior horn is the thoracic nucleus (Clark), lateral and somewhat deeper are the basilar nuclei, in the intermediate zone is the medial intermediate nucleus. In the dorsal part of the posterior horn, small neurons of the gelatinous substance (Roland's) are successively located from the depth to the outside, then small neurons of the spongy zone and, finally, the border zone containing small neurons.

Axons of sensory neurons from the spinal ganglia enter the spinal cord through the posterior roots and further into the marginal zone, where they are divided into two branches: a short descending and a long ascending. Along the lateral branches from these branches of the axon, impulses are transmitted to the associative neurons of the gray matter. Pain, temperature and tactile sensitivity is projected onto the neurons of the gelatinous substance and the own nucleus of the posterior horn. The gelatinous substance contains interneurons that produce opioid peptides that affect pain sensations (the so-called "pain gates"). Impulses from the internal organs are transmitted to the neurons of the nuclei of the intermediate zone. Signals from muscles, tendons, joint capsules, etc. (proprioception) are directed to Clark's nucleus and other nuclei. The axons of the neurons of these nuclei form ascending pathways.

In the posterior horns of the spinal cord many diffusely located neurons whose axons terminate within the spinal cord on the same or opposite side of the gray matter. The axons of these neurons enter the white matter and immediately divide into descending and ascending branches. Spreading at the level of 4-5 spinal segments, these branches together form their own bundles of white matter, directly adjacent to the gray matter. At the same time, the posterior, lateral and anterior proper bundles are distinguished. All these bundles of white matter belong to the own apparatus of the spinal cord. From the axons that are part of their own bundles, collaterals depart, ending in synapses on motor neurons. Due to this, conditions are created for an avalanche-like increase in the number of neurons that transmit impulses along the reflex arcs of the spinal cord's own apparatus.

Spinal cord- medulla spinalis - lies in the spinal canal, occupying approximately 2/3 of its volume. In cattle and horses, its length is 1.8-2.3 m, weight 250-300 g, in pigs - 45-70 g. It looks like a cylindrical cord, somewhat flattened dorsoventrally. There is no clear boundary between the brain and spinal cord. It is believed that it runs along the anterior margin of the atlas. In the spinal cord, cervical, thoracic, lumbar, sacral and caudal parts are distinguished according to their location. In the embryonic period of development, the spinal cord fills the entire spinal canal, but due to the high growth rate of the skeleton, the difference in their length becomes larger. As a result, the brain in cattle ends at the level of the 4th, in the pig - in the region of the 6th lumbar vertebra, and in the horse - in the region of the 1st segment of the sacral bone. Along the entire spinal cord along its dorsal surface passes median dorsal groove. Connective tissue departs from it deep into dorsal septum. On the sides of the median sulcus are smaller dorsal lateral grooves. On the ventral surface there is a deep median ventral fissure, and on the sides of it - ventral lateral grooves. At the end, the spinal cord sharply narrows, forming cerebral cone, which goes into terminal thread. It is formed by connective tissue and ends at the level of the first tail vertebrae.

There are thickenings in the cervical and lumbar parts of the spinal cord. In connection with the development of the limbs, the number of neurons and nerve fibers in these areas increases. At the pig cervical thickening formed by 5–8 neurosegments. Its maximum width at the level of the 6th cervical vertebra is 10 mm. Lumbar thickening falls on the 5th-7th lumbar neurosegments. In each segment, a pair of spinal nerves departs from the spinal cord with two roots - on the right and on the left. The dorsal root arises from the dorsal lateral groove, the ventral root from the ventral lateral groove. The spinal nerves leave the spinal canal through the intervertebral foramen. The area of ​​the spinal cord between two adjacent spinal nerves is called neurosegment.

Neurosegments are of different lengths and often do not correspond in size to the length of the bone segment. As a result, the spinal nerves depart at different angles. Many of them travel some distance inside the spinal canal before leaving the intervertebral foramen of their segment. In the caudal direction, this distance increases and from the nerves running inside the spinal canal, behind the cerebral cone, a kind of brush is formed, which is called the "ponytail".

Histological structure. On a transverse section of the spinal cord with the naked eye, its division into white and gray matter is visible.

Gray matter is in the middle and looks like the letter H or a flying butterfly. A small hole is visible in its center - a cross section central spinal canal. The area of ​​gray matter around the central canal is called gray commissure. Directed upwards from her dorsal pillars(on a cross section - horns), way down - ventral columns (horns) gray matter. In the thoracic and lumbar parts of the spinal cord, there are thickenings on the sides of the ventral columns - lateral pillars, or horns gray matter. The composition of the gray matter includes multipolar neurons and their processes that are not covered with a myelin sheath, as well as neuroglia.

Fig.142. Spinal cord (according to I.V. Almazov, L.S. Sutulov, 1978)

1 - dorsal median septum; 2 - ventral median fissure; 3 - ventral root; 4 - ventral gray commissure; 5 - dorsal gray commissure; 6 - spongy layer; 7 - gelatinous substance; 8 - dorsal horn; 9 - mesh reticular formation; 10 - lateral horn; 11 - ventral horn; 12 - own nucleus of the posterior horn; 13 - dorsal nucleus; 14 - cores of the intermediate zone; 15 - lateral core; 16 - nuclei of the ventral horn; 17 - shell of the brain.

Neurons in different parts of the brain differ in structure and function. In this regard, various zones, layers and cores are distinguished in it. The bulk of the neurons of the dorsal horns are associative, intercalary neurons that transmit the nerve impulses that come to them either to motor neurons, or to the lower and upper parts of the spinal cord, and then to the brain. The axons of sensory neurons of the spinal ganglia approach the dorsal columns. The latter enter the spinal cord in the region of the dorsal lateral grooves in the form of dorsal roots. The degree of development of the dorsal lateral columns (horns) is directly dependent on the degree of sensitivity.

The ventral horns contain motor neurons. These are the largest multipolar nerve cells in the spinal cord. Their axons form the ventral roots of the spinal nerves, extending from the spinal cord in the region of the ventral lateral sulcus. The development of the ventral horns depends on the development of the locomotor apparatus. The lateral horns contain neurons belonging to the sympathetic nervous system. Their axons leave the spinal cord as part of the ventral roots and form the white connecting branches of the borderline sympathetic trunk.

white matter forms the periphery of the spinal cord. In the area of ​​thickening of the brain, it prevails over the gray matter. Consists of myelinated nerve fibers and neuroglia. The myelin sheath of the fibers gives them a whitish-yellowish color. The dorsal septum, ventral fissure and pillars (horns) of the gray matter divide the white matter into cords: dorsal, ventral and lateral. Dorsal cords do not connect with each other, since the dorsal septum reaches the gray commissure. Lateral cords separated by a mass of gray matter. Ventral cords communicate with each other in the area white spike- an area of ​​white matter lying between the ventral fissure and the gray commissure.

Complexes of nerve fibers passing in the cords form pathways. More deeply lying complexes of fibers form conducting paths connecting different segments of the spinal cord. Together they amount to own apparatus spinal cord. More superficially located complexes of nerve fibers form afferent (sensory, or ascending) and efferent (motor, or descending) projection pathways connecting the spinal cord to the brain. Sensory pathways from the spinal cord to the brain run in the dorsal cords and in the superficial layers of the lateral cords. The motor pathways from the brain to the spinal cord run in the ventral cords and in the middle sections of the lateral cords.