Localization of the motor nuclei of the spinal cord physiology. Anatomical and physiological features of the spinal cord. Structural formations of the hindbrain

The spinal cord performs conduction and reflex functions.

Conductor function carried out by ascending and descending pathways passing through the white matter of the spinal cord. They connect individual segments of the spinal cord with each other, as well as with the brain.

reflex function It is carried out by means of unconditioned reflexes, which close at the level of certain segments of the spinal cord and are responsible for the simplest adaptive reactions. The cervical segments of the spinal cord (C3 - C5) innervate the movements of the diaphragm, the thoracic (T1 - T12) - the external and internal intercostal muscles; cervical (C5 - C8) and thoracic (T1 - T2) are the centers of movement of the upper limbs, lumbar (L2 - L4) and sacral (S1 - S2) are the centers of movement of the lower extremities.

In addition, the spinal cord is involved in implementation of autonomic reflexes - response of internal organs to irritation of visceral and somatic receptors. The vegetative centers of the spinal cord, located in the lateral horns, are involved in the regulation of blood pressure, cardiac activity, secretion and motility of the digestive tract, and the function of the genitourinary system.

In the lumbosacral region of the spinal cord there is a defecation center, from which impulses arrive through the parasympathetic fibers in the composition of the pelvic nerve, which increase the motility of the rectum and provide a controlled act of defecation. An arbitrary act of defecation is performed due to the descending influences of the brain on the spinal center. In the II-IV sacral segments of the spinal cord there is a reflex center of urination, which provides a controlled separation of urine. The brain controls urination and provides one hundred arbitrariness. In a newborn child, urination and defecation are involuntary acts, and only as the regulatory function of the cerebral cortex matures do they become voluntarily controlled (usually this occurs in the first 2-3 years of a child's life).

Brain- the most important department of the central nervous system - surrounded by the meninges and located in the cranial cavity. It consists of brain stem : medulla oblongata, pons, cerebellum, midbrain, diencephalon, and the so-called telencephalon, consisting of subcortical, or basal, ganglia and cerebral hemispheres (Fig. 11.4). The upper surface of the brain in shape corresponds to the inner concave surface of the cranial vault, the lower surface (the base of the brain) has a complex relief corresponding to the cranial fossae of the inner base of the skull.

Rice. 11.4.

The brain is intensively formed during embryogenesis, its main parts are allocated already by the 3rd month of intrauterine development, and by the 5th month the main furrows of the cerebral hemispheres are clearly visible. In a newborn, the mass of the brain is about 400 g, its ratio with body weight is significantly different from that of an adult - it is 1/8 of the body weight, while in an adult it is 1/40. The most intensive period of growth and development of the human brain falls on the period of early childhood, then its growth rates decrease somewhat, but remain high until the age of 6-7, by which time the brain mass already reaches 4/5 of the adult brain mass. The final maturation of the brain ends only by the age of 17–20, its mass increases by 4–5 times compared to newborns and averages 1400 g for men and 1260 g for women (the mass of an adult brain ranges from 1100 to 2000 g). ). The length of the brain in an adult is 160–180 mm, and the diameter is up to 140 mm. In the future, the mass and volume of the brain remain maximum and constant for each person. It is interesting that the mass of the brain does not directly correlate with the mental abilities of a person, however, with a decrease in brain mass below 1000 g, a decrease in intelligence is natural.

Changes in the size, shape, and mass of the brain during development are accompanied by changes in its internal structure. The structure of neurons, the form of interneuronal connections become more complicated, white and gray matter become clearly demarcated, various pathways of the brain are formed.

The development of the brain, like other systems, is heterochronous (uneven). Before others, those structures on which the normal vital activity of the organism depends at this age stage mature. Functional usefulness is first achieved by stem, subcortical and cortical structures that regulate the vegetative functions of the body. These departments in their development approach the brain of an adult by the age of 2-4 years.

The spinal cord is the most ancient formation of the CNS. A characteristic feature of the structure is segmentation.

The neurons of the spinal cord form it Gray matter in the form of anterior and posterior horns. They perform a reflex function of the spinal cord.

The posterior horns contain neurons (interneurons) that transmit impulses to the overlying centers, to the symmetrical structures of the opposite side, to the anterior horns of the spinal cord. The posterior horns contain afferent neurons that respond to pain, temperature, tactile, vibration, and proprioceptive stimuli.

The anterior horns contain neurons (motoneurons) that give axons to the muscles, they are efferent. All descending pathways of the CNS for motor reactions terminate in the anterior horns.

In the lateral horns of the cervical and two lumbar segments there are neurons of the sympathetic division of the autonomic nervous system, in the second-fourth segments - of the parasympathetic.

The spinal cord contains many intercalary neurons that provide communication with the segments and with the overlying parts of the CNS; they account for 97% of the total number of spinal cord neurons. They include associative neurons - neurons of the spinal cord's own apparatus, they establish connections within and between segments.

white matter the spinal cord is formed by myelin fibers (short and long) and performs a conductive role.

Short fibers connect neurons of one or different segments of the spinal cord.

Long fibers (projection) form the pathways of the spinal cord. They form ascending pathways to the brain and descending pathways from the brain.

The spinal cord performs reflex and conduction functions.

The reflex function allows you to realize all the motor reflexes of the body, reflexes of internal organs, thermoregulation, etc. Reflex reactions depend on the location, strength of the stimulus, the area of ​​​​the reflexogenic zone, the speed of the impulse through the fibers, and the influence of the brain.

Reflexes are divided into:

1) exteroceptive (occur when irritated by environmental agents of sensory stimuli);

2) interoceptive (occur when irritated by presso-, mechano-, chemo-, thermoreceptors): viscero-visceral - reflexes from one internal organ to another, viscero-muscular - reflexes from internal organs to skeletal muscles;

3) proprioceptive (own) reflexes from the muscle itself and its associated formations. They have a monosynaptic reflex arc. Proprioceptive reflexes regulate motor activity due to tendon and postural reflexes. Tendon reflexes (knee, Achilles, with the triceps of the shoulder, etc.) occur when the muscles are stretched and cause relaxation or contraction of the muscle, occur with every muscle movement;

4) postural reflexes (occur when the vestibular receptors are excited when the speed of movement and the position of the head relative to the body change, which leads to a redistribution of muscle tone (increase in extensor tone and decrease in flexors) and ensures body balance).

The study of proprioceptive reflexes is performed to determine the excitability and degree of damage to the central nervous system.

The conduction function ensures the connection of the neurons of the spinal cord with each other or with the overlying sections of the central nervous system.

Structural formations of the hindbrain.

1. V-XII pair of cranial nerves.

2. Vestibular nuclei.

3. Kernels of the reticular formation.

The main functions of the hindbrain are conductive and reflex.

Descending paths pass through the hindbrain (corticospinal and extrapyramidal), ascending - reticulo- and vestibulospinal, responsible for the redistribution of muscle tone and maintaining body posture.

The reflex function provides:

1) protective reflexes (lacrimation, blinking, coughing, vomiting, sneezing);

3) posture maintenance reflexes (labyrinth reflexes). Static reflexes maintain muscle tone to maintain body posture, statokinetic ones redistribute muscle tone to take a pose corresponding to the moment of rectilinear or rotational movement;

4) centers located in the hindbrain regulate the activity of many systems.

The vascular center regulates vascular tone, the respiratory center regulates inhalation and exhalation, the complex food center regulates the secretion of the gastric, intestinal glands, pancreas, liver secretory cells, salivary glands, provides reflexes of sucking, chewing, swallowing.

Damage to the hindbrain leads to a loss of sensitivity, volitional motility, and thermoregulation, but breathing, blood pressure, and reflex activity are preserved.

Structural units of the midbrain:

1) tubercles of the quadrigemina;

2) red core;

3) black core;

4) nuclei of the III-IV pair of cranial nerves.

The tubercles of the quadrigemina perform an afferent function, the rest of the formations perform an efferent function.

The tubercles of the quadrigemina closely interact with the nuclei of III-IV pairs of cranial nerves, the red nucleus, with the optic tract. Due to this interaction, the anterior tubercles provide an orienting reflex reaction to light, and the posterior tubercles to sound. They provide vital reflexes: a start reflex is a motor reaction to a sharp unusual stimulus (increased flexor tone), a landmark reflex is a motor reaction to a new stimulus (turning the body, head).

The anterior tubercles with the nuclei of the III-IV cranial nerves provide a convergence reaction (convergence of the eyeballs to the midline), the movement of the eyeballs.

The red nucleus takes part in the regulation of the redistribution of muscle tone, in restoring the body posture (increases the tone of the flexors, lowers the tone of the extensors), maintains balance, and prepares the skeletal muscles for voluntary and involuntary movements.

The substantia nigra of the brain coordinates the act of swallowing and chewing, breathing, blood pressure (the pathology of the substantia nigra of the brain leads to an increase in blood pressure).

The diencephalon consists of the thalamus and hypothalamus, they connect the brain stem with the cerebral cortex.

thalamus- a paired formation, the largest accumulation of gray matter in the diencephalon.

Topographically, the anterior, middle, posterior, medial and lateral groups of nuclei are distinguished.

By function, they are distinguished:

1) specific:

a) switching, relay. They receive primary information from various receptors. The nerve impulse along the thalamocortical tract goes to a strictly limited area of ​​the cerebral cortex (primary projection zones), due to this, specific sensations arise. The nuclei of the ventrabasal complex receive an impulse from skin receptors, tendon proprioceptors, and ligaments. The impulse is sent to the sensorimotor zone, the body orientation in space is regulated. The lateral nuclei switch the impulse from the visual receptors to the occipital visual zone. The medial nuclei respond to a strictly defined sound wave length and conduct an impulse to the temporal zone;

b) associative (internal) nuclei. The primary impulse comes from the relay nuclei, is processed (an integrative function is carried out), transmitted to the associative zones of the cerebral cortex, the activity of the associative nuclei increases under the action of a painful stimulus;

2) non-specific nuclei. This is a non-specific way of transmitting impulses to the cerebral cortex, the frequency of the biopotential changes (modeling function);

3) motor nuclei involved in the regulation of motor activity. Impulses from the cerebellum, basal ganglia go to the motor zone, carry out the relationship, consistency, sequence of movements, spatial orientation of the body.

The thalamus is a collector of all afferent information, except for olfactory receptors, the most important integrative center.

Hypothalamus located on the bottom and sides of the third ventricle of the brain. Structures: gray tubercle, funnel, mastoid bodies. Zones: hypophysiotropic (preoptic and anterior nuclei), medial (middle nuclei), lateral (outer, posterior nuclei).

Physiological role - the highest subcortical integrative center of the autonomic nervous system, which has an effect on:

1) thermoregulation. The anterior nuclei are the center of heat transfer, where the process of sweating, respiratory rate and vascular tone are regulated in response to an increase in ambient temperature. The posterior nuclei are the center of heat production and the preservation of heat when the temperature drops;

2) pituitary. Liberins promote the secretion of hormones of the anterior pituitary gland, statins inhibit it;

3) fat metabolism. Irritation of the lateral (nutrition center) nuclei and ventromedial (satiation center) nuclei leads to obesity, inhibition leads to cachexia;

4) carbohydrate metabolism. Irritation of the anterior nuclei leads to hypoglycemia, the posterior nuclei to hyperglycemia;

5) the cardiovascular system. Irritation of the anterior nuclei has an inhibitory effect, the posterior nuclei - an activating one;

6) motor and secretory functions of the gastrointestinal tract. Irritation of the anterior nuclei increases motility and secretory function of the gastrointestinal tract, while the posterior nuclei inhibit sexual function. The destruction of the nuclei leads to a violation of ovulation, spermatogenesis, a decrease in sexual function;

7) behavioral responses. Irritation of the starting emotional zone (front nuclei) causes a feeling of joy, satisfaction, erotic feelings, the stop zone (rear nuclei) causes fear, a feeling of anger, rage.

Reticular formation of the brain stem- accumulation of polymorphic neurons along the brain stem.

Physiological feature of neurons of the reticular formation:

1) spontaneous bioelectrical activity. Its causes are humoral irritation (increase in the level of carbon dioxide, biologically active substances);

2) sufficiently high excitability of neurons;

3) high sensitivity to biologically active substances.

The reticular formation has wide bilateral connections with all parts of the nervous system, according to its functional significance and morphology it is divided into two parts:

1) rastral (ascending) department - reticular formation of the diencephalon;

2) caudal (descending) - the reticular formation of the posterior, midbrain, bridge.

The physiological role of the reticular formation is the activation and inhibition of brain structures.

limbic system- a collection of nuclei and nerve tracts.

Structural units of the limbic system:

1) olfactory bulb;

2) olfactory tubercle;

3) transparent partition;

4) hippocampus;

5) parahippocampal gyrus;

6) almond-shaped nuclei;

7) piriform gyrus;

8) dentate fascia;

9) cingulate gyrus.

The main functions of the limbic system:

1) participation in the formation of food, sexual, defensive instincts;

2) regulation of vegetative-visceral functions;

3) the formation of social behavior;

4) participation in the formation of the mechanisms of long-term and short-term memory;

5) performance of the olfactory function;

6) inhibition of conditioned reflexes, strengthening of unconditioned ones;

7) participation in the formation of the wake-sleep cycle.

Significant formations of the limbic system are:

1) hippocampus. Its damage leads to a disruption in the process of memorization, information processing, a decrease in emotional activity, initiative, a slowdown in the speed of nervous processes, irritation - to an increase in aggression, defensive reactions, and motor function. Hippocampal neurons are characterized by high background activity. In response to sensory stimulation, up to 60% of neurons react, the generation of excitation is expressed in a long-term reaction to a single short impulse;

2) almond-shaped nuclei. Their damage leads to the disappearance of fear, inability to aggression, hypersexuality, reactions of caring for offspring, irritation - to a parasympathetic effect on the respiratory and cardiovascular, digestive systems. The neurons of the amygdala nuclei have a pronounced spontaneous activity, which is inhibited or enhanced by sensory stimuli;

3) olfactory bulb, olfactory tubercle.

The limbic system has a regulatory effect on the cerebral cortex.

The highest department of the CNS is the cerebral cortex, its area is 2200 cm 2.

The cerebral cortex has a five-, six-layer structure. Neurons are represented by sensory, motor (Betz cells), interneurons (inhibitory and excitatory neurons).

The cerebral cortex is built according to the columnar principle. Columns are functional units of the cortex, divided into micromodules that have homogeneous neurons.

According to IP Pavlov's definition, the cerebral cortex is the main manager and distributor of body functions.

The main functions of the cerebral cortex:

1) integration (thinking, consciousness, speech);

2) ensuring the connection of the organism with the external environment, its adaptation to its changes;

3) clarification of the interaction between the body and systems within the body;

4) coordination of movements (the ability to carry out voluntary movements, to make involuntary movements more accurate, to carry out motor tasks).

These functions are provided by corrective, triggering, integrative mechanisms.

I. P. Pavlov, creating the doctrine of analyzers, distinguished three sections: peripheral (receptor), conductive (three-neural pathway for transmitting impulses from receptors), brain (certain areas of the cerebral cortex, where the processing of a nerve impulse takes place, which acquires a new quality ). The brain section consists of the analyzer nuclei and scattered elements.

According to modern ideas about the localization of functions, three types of fields arise during the passage of an impulse in the cerebral cortex.

1. The primary projection zone lies in the region of the central section of the analyzer nuclei, where the electrical response (evoked potential) first appeared, disturbances in the region of the central nuclei lead to a violation of sensations.

2. The secondary zone lies in the environment of the nucleus, is not associated with receptors, the impulse comes through the intercalary neurons from the primary projection zone. Here, a relationship is established between phenomena and their qualities, violations lead to a violation of perceptions (generalized reflections).

3. The tertiary (associative) zone has multisensory neurons. The information has been revised to meaningful. The system is capable of plastic restructuring, long-term storage of traces of sensory action. In case of violation, the form of abstract reflection of reality, speech, purposeful behavior suffer.

Collaboration of the cerebral hemispheres and their asymmetry.

There are morphological prerequisites for the joint work of the hemispheres. The corpus callosum provides a horizontal connection with the subcortical formations and the reticular formation of the brain stem. Thus, the friendly work of the hemispheres and reciprocal innervation are carried out during joint work.

functional asymmetry. Speech, motor, visual and auditory functions dominate in the left hemisphere. The thinking type of the nervous system is left hemisphere, and the artistic type is right hemisphere.

Spinal cord It is a cylindrical elongated cord, somewhat flattened from front to back, located in the spinal canal. The length of the spinal cord in men is about 45 cm, in women - 41-42 cm. The mass of the spinal cord is about 30 g, which is 2.3% of the mass of the brain. The spinal cord is surrounded by three membranes (dura, arachnoid and soft). The spinal cord begins at the level of the lower edge of the foramen magnum, where it passes into the brain. The lower bound of the tapering in the form cones of the spinal cord corresponds to the level of the upper edge of the second lumbar vertebra. Below this level is terminal thread, surrounded by the roots of the spinal nerves and the membranes of the spinal cord, forming a closed sac in the lower part of the spinal canal. As part of the terminal thread, the inner and outer parts are distinguished. The inner part goes from the level of the second lumbar vertebra to the level of the second sacral vertebra, it has a length of about 15 cm. The outer part of the terminal thread does not contain nervous tissue, it is a continuation of the meninges. It is about 8 cm long, fuses with the periosteum of the spinal canal at the level of the second coccygeal vertebra (on the structure of the spine, see the article Structure and Functions of the Spine).
The average diameter of the spinal cord is 1 cm. The spinal cord has two thickenings: cervical and lumbosacral, in the thickness of which nerve cells are located (for the structure of the nervous tissue, see the article General idea of ​​the structure and functions of the nervous system), whose processes go, respectively, to the upper and lower limbs. The anterior median fissure runs along the midline on the anterior surface of the spinal cord from top to bottom. On the posterior surface, it corresponds to a less deep posterior median sulcus. From the bottom of the posterior median sulcus to the posterior surface of the gray matter, the posterior median septum passes through the entire thickness of the white matter of the spinal cord. On the anterior-lateral surface of the spinal cord, on the side of the anterior median fissure, on each side there is an anterior-lateral groove. Through the anterior-lateral groove, the anterior (motor) roots of the spinal nerves exit the spinal cord. On the posterior-lateral surface of the spinal cord, on each side, there is a posterior-lateral groove through which the nerve fibers (sensory) of the posterior roots of the spinal nerves enter the thickness of the spinal cord. These grooves divide the white matter of each half of the spinal cord into three longitudinal strands - the funiculus: anterior, lateral and posterior. Between the anterior median fissure and the anterior-lateral groove on each side is anterior cord spinal cord. Between the anterior-lateral and postero-lateral grooves on the surface of the right and left sides of the spinal cord is visible lateral cord. Behind the posterolateral sulcus, on the sides of the posterior median sulcus, is a paired posterior funiculus spinal cord.

Exiting through the anterior-lateral groove front spine formed by the axons of motor (motor) neurons located in the anterior horn (column) of the gray matter of the spinal cord. back spine, sensitive, is formed by a collection of axons of pseudo-unipolar neurons. The bodies of these neurons form spinal ganglion located in the spinal canal near the corresponding intervertebral foramen. Further, in the intervertebral foramen, both roots are connected to each other, forming a mixed (containing sensory, motor and autonomic nerve fibers) spinal nerve, which then divides into anterior and posterior branches. Throughout the spinal cord on each side there are 31 pairs of roots, forming 31 pairs of spinal nerves.
The part of the spinal cord corresponding to two pairs of spinal nerve roots (two anterior and two posterior) is called segment of the spinal cord. There are 8 cervical (C1-C8), 12 thoracic (Th1-Th12), 5 lumbar (L1-L5), 5 sacral (S1-S5) and 1-3 coccygeal (Co1-Co3) segments (31 segments in total). The upper segments are located at the level of the bodies of the cervical vertebrae corresponding to their serial number ( rice. 2). The lower cervical and upper thoracic segments are one vertebra higher than the corresponding vertebral bodies. In the middle thoracic region, this difference is equal to two vertebrae, in the lower thoracic region, to three vertebrae. The lumbar segments are located at the level of the bodies of the tenth and eleventh thoracic vertebrae, the sacral and coccygeal segments correspond to the levels of the twelfth thoracic and first lumbar vertebrae. This discrepancy between the segments of the spinal cord and the vertebrae is due to the different growth rates of the spine and spinal cord. Initially, in the second month of intrauterine life, the spinal cord occupies the entire spinal canal, and then, due to the faster growth of the spine, lags behind in growth and shifts upward relative to it. So the roots of the spinal nerves are directed not only to the sides, but also down, and the more down, the closer to the tail end of the spinal cord. The direction of the roots in the lumbar part of the spinal cord within the spinal canal becomes almost parallel to the longitudinal axis of the spinal cord, so that the cone of the brain and the terminal filament lie among a dense bundle of nerve roots, which is called ponytail.

In experiments with transection of individual roots in animals, it was found that each segment of the spinal cord innervates three transverse segments, or metameres, of the body: its own, one above and one below. Consequently, each metamere of the body receives sensory fibers from three roots, and in order to desensitize a part of the body, it is necessary to cut three roots (reliability factor). Skeletal muscles (trunk and limbs) also receive motor innervation from three adjacent segments of the spinal cord. (For more information about the segmental division of the spinal cord and areas of sensory and motor innervation, see the American Spinal Injury Association Classification of the Level and Severity of Spinal Cord Injury.)

Internal structure of the spinal cord

The spinal cord consists of gray and white matter. Gray matter is located in the central parts of the spinal cord, white - on its periphery ( fig.1).

Gray matter of the spinal cord

AT gray matter a narrow central channel runs from top to bottom. At the top, the canal communicates with the fourth ventricle of the brain. The lower end of the canal expands and ends blindly in the terminal ventricle (Krause's ventricle). In an adult, the central canal overgrows in places, its uncovered areas contain cerebrospinal fluid. The canal walls are lined with ependymocytes.

The gray matter along the spinal cord on both sides of the central canal forms two irregularly shaped vertical strands - the right and left gray columns. A thin plate of gray matter connecting both gray columns in front of the central canal is called the anterior gray commissure. Behind the central canal, the right and left columns of gray matter are connected by a posterior gray commissure. Each column of gray matter has an anterior part (anterior column) and a posterior part (posterior column). At the level between the eighth cervical segment and the second lumbar segment, inclusive on each side, the gray matter also forms a lateral (lateral) protrusion - a lateral column. Above and below this level there are no side pillars. On a transverse section of the spinal cord, the gray matter looks like a butterfly or the letter H, and three pairs of columns form the anterior, posterior, and lateral horns of the gray matter. The anterior horn is wider, the posterior horn is narrower. The lateral horn topographically corresponds to the lateral column of gray matter.
The gray matter of the spinal cord is formed by the bodies of neurons, unmyelinated and thin myelinated fibers and neuroglia.
AT front horns (pillars) the bodies of the largest neurons of the spinal cord (diameter 100-140 microns) are located. They form five nuclei(clusters). These nuclei are the motor (motor) centers of the spinal cord. The axons of these cells make up the bulk of the fibers of the anterior roots of the spinal nerves. As part of the spinal nerves, they go to the periphery and form motor (motor) endings in the muscles of the trunk, limbs and in the diaphragm (the muscle plate that separates the chest and abdominal cavities and plays a major role during inspiration).
Gray matter back horns (pillars) heterogeneous. In addition to neuroglia, the posterior horns contain a large number of intercalary neurons, with which some of the axons coming from sensory neurons in the posterior roots are in contact. They are small multipolar, so-called associative and commissural cells. Associative neurons have axons that terminate at different levels within the gray matter of their half of the spinal cord. Axons of commissural neurons terminate on the opposite side of the spinal cord. The processes of the nerve cells of the posterior horn communicate with the neurons of the higher and lower adjacent segments of the spinal cord. The processes of these neurons also terminate on neurons located in the anterior horns of their segment.
In the middle of the posterior horn there is a so-called proper nucleus. It is formed by the bodies of intercalary neurons. The axons of these nerve cells pass into the lateral funiculus of the white matter (see below) of their own and opposite half of the spinal cord and participate in the formation of the pathways of the spinal cord (anterior spinal cerebellar and spinal thalamic tracts).
At the base of the posterior horn of the spinal cord is the thoracic nucleus (Clark's column). It consists of large intercalary neurons (Stilling cells) with well-developed, highly branched dendrites. The axons of the cells of this nucleus enter the lateral funiculus of the white matter of their side of the spinal cord and also form pathways (the posterior spinal cerebellar tract).
AT lateral horns spinal cord are the centers of the autonomic nervous system. At the level of C8-Th1, there is a sympathetic center for pupil dilation. In the lateral horns of the thoracic and upper segments of the lumbar spinal cord, there are spinal centers of the sympathetic nervous system that innervate the heart, blood vessels, sweat glands, and the digestive tract. It is here that neurons lie that are directly connected with the peripheral sympathetic ganglia. The axons of these neurons, which form the autonomic nucleus in the segments of the spinal cord from the eighth cervical to the second lumbar, pass through the anterior horn, exit the spinal cord as part of the anterior roots of the spinal nerves. In the sacral spinal cord, there are parasympathetic centers innervating the pelvic organs (reflex centers for urination, defecation, erection, ejaculation).
The nerve centers of the spinal cord are segmental or working centers. Their neurons are directly connected with receptors and working organs. In addition to the spinal cord, such centers are found in the medulla oblongata and midbrain. The suprasegmental centers, for example, the diencephalon, the cerebral cortex, do not have a direct connection with the periphery. They govern it through segmental centers.

Reflex function of the spinal cord

The gray matter of the spinal cord, the posterior and anterior roots of the spinal nerves, forms its own white matter bundles segmental apparatus of the spinal cord. It provides reflex (segmental) spinal cord function.
The nervous system functions according to reflex principles. Reflex represents the body's response to external or internal influences and spreads along the reflex arc. reflex arcs are circuits made up of nerve cells.

Rice. 3.
1 - sensory neuron, 2 - spinal ganglion, 3 - myelinated nerve fiber, 4 - sensory nerve ending, 5 - nerve ending (plaque) on the muscle fiber, 6 - spinal nerve, 7 - spinal nerve roots, 8 - efferent (motor) neuron in the anterior horn of the spinal cord.

The simplest reflex arc includes sensory and effector neurons, along which the nerve impulse moves from the place of origin (from the receptor) to the working organ (effector) ( fig.3). The body of the first sensitive (pseudo-unipolar) neuron is located in the spinal ganglion. The dendrite begins with a receptor that perceives external or internal irritation (mechanical, chemical, etc.) and converts it into a nerve impulse that reaches the body of the nerve cell. From the body of the neuron along the axon, the nerve impulse through the sensory roots of the spinal nerves is sent to the spinal cord, where it forms synapses with the bodies of effector neurons. In each interneuronal synapse, with the help of biologically active substances (mediators), an impulse is transmitted. The axon of the effector neuron exits the spinal cord as part of the anterior roots of the spinal nerves (motor or secretory nerve fibers) and goes to the working organ, causing muscle contraction, increased (inhibition) of gland secretion.
More complex reflex arcs have one or more intercalary neurons. The body of the intercalary neuron in the three-neuron reflex arcs is located in the gray matter of the posterior columns (horns) of the spinal cord and contacts the axon of the sensitive neuron that comes as part of the posterior (sensitive) roots of the spinal nerves. Axons of intercalary neurons are sent to the anterior columns (horns), where the bodies of effector cells are located. The axons of effector cells are sent to the muscles, glands, affecting their function. In the nervous system, there are many complex multi-neuron reflex arcs, which have several intercalary neurons located in the gray matter of the spinal cord and brain.
An example of the simplest reflex is the knee reflex, which occurs in response to a short-term stretching of the quadriceps femoris muscle with a light blow to its tendon below the patella. After a short latent (hidden) period, the quadriceps contraction occurs, as a result of which the freely hanging lower leg rises. The knee jerk is one of the so-called muscle stretch reflexes, the physiological significance of which is to regulate the length of the muscle, which is especially important for maintaining posture. For example, when a person is standing, each flexion in the knee joint, even so weak that it can neither be seen nor felt, is accompanied by stretching of the quadriceps muscle and a corresponding increase in the activity of the sensory endings (muscle spindles) located in it. As a result, there is an additional activation of the motor neurons of the quadriceps muscle (patellar reflex), and an increase in its tone, which counteracts flexion. Conversely, too much contraction of a muscle weakens the stimulation of its stretch receptors. The frequency of their impulses, which excites motor neurons, decreases, and muscle tone weakens.
As a rule, several muscles are involved in the movement, which in relation to each other can act as agonists (acting in one direction) or antagonists (acting in different directions). A reflex act is possible only with a conjugated, so-called reciprocal inhibition motor centers of antagonist muscles. When walking, flexion of the leg is accompanied by relaxation of the extensor muscles and, conversely, during extension, the flexor muscles are inhibited. If this did not happen, then there would be a mechanical struggle of the muscles, convulsions, and not adaptive motor acts. When a sensory nerve is stimulated, causing a flexion reflex, impulses are sent to the centers of the flexor muscles and through special intercalary neurons (Renshaw inhibitory cells) to the centers of the extensor muscles. In the first, they cause the process of excitation, and in the second - inhibition. In response, a coordinated, coordinated reflex act occurs - the flexion reflex.
The interaction of the processes of excitation and inhibition is a universal principle underlying the activity of the nervous system. Of course, it is realized not only at the level of segments of the spinal cord. The higher divisions of the nervous system exercise their regulatory influence, causing the processes of excitation and inhibition of neurons of the lower divisions. It is important to note that the higher the level of the animal, the stronger the power of the highest sections of the central nervous system, the more the higher section is the manager and distributor of the body's activity (IP Pavlov). In humans, such a manager and distributor is the cerebral cortex.
Each spinal reflex has its own receptive field and its own localization (location), its own level. So, for example, the center of the knee jerk is in the II - IV lumbar segment; Achilles - in the V lumbar and I - II sacral segments; plantar - in I - II sacral, the center of the abdominal muscles - in VIII - XII thoracic segments. The most important vital center of the spinal cord is the motor center of the diaphragm, located in the III-IV cervical segments. Damage to it leads to death due to respiratory arrest.
In addition to motor reflex arcs, vegetative reflex arcs are closed at the level of the spinal cord, which control the activity of internal organs.
Intersegmental reflex connections. In the spinal cord, in addition to the reflex arcs described above, limited by the limits of one or more segments, there are ascending and descending intersegmental reflex pathways. The intercalary neurons in them are the so-called propriospinal neurons, whose bodies are located in the gray matter of the spinal cord, and whose axons ascend or descend at various distances in the composition propriospinal tracts white matter, never leaving the spinal cord. Experiments with the degeneration of nervous structures (in which individual parts of the spinal cord are completely isolated) have shown that most of its nerve cells belong to propriospinal neurons. Some of them form independent functional groups responsible for performing automatic movements ( spinal cord automatic programs). Intersegmental reflexes and these programs contribute to the coordination of movements triggered at different levels of the spinal cord, in particular the fore and hind limbs, limbs and neck.
Thanks to these reflexes and automatic programs, the spinal cord is able to provide complex coordinated movements in response to an appropriate signal from the periphery or from the overlying sections of the central nervous system. Here you can talk about integrative (unifying) function of the spinal cord, although it should be borne in mind that in higher vertebrates (in particular, in mammals), the regulation of spinal functions by the higher parts of the central nervous system increases (the process encephalization).
spinal locomotion. It was found that the main characteristics of locomotion, i.e., the movement of a person or animal in the environment with the help of coordinated movements of the limbs, programmed at the level of the spinal cord. Painful irritation of any limb of the spinal animal causes reflex movements of all four; if such stimulation continues long enough, rhythmic flexion and extension movements of the unstimulated limbs may occur. If such an animal is placed on a treadmill (treadmill), then under certain conditions it will perform coordinated walking movements that are very similar to natural ones.
In a spinal animal anesthetized and paralyzed with curare, under certain conditions it is possible to register rhythmically alternating volleys of impulses from extensor and flexor motoneurons, approximately corresponding to those observed during natural walking. Since such impulsation is not accompanied by movements, it is called false locomotion. It is provided by yet unidentified locomotor centers of the spinal cord. Apparently, there is one such center for each limb. The activity of the centers is coordinated by the propriospinal systems and tracts that cross the spinal cord within individual strands.
It is assumed that humans also have spinal locomotor centers. Apparently, their activation upon skin irritation manifests itself in the form neonatal stepping reflex. However, as the central nervous system matures, the higher departments obviously subjugate such centers to themselves to such an extent. that in an adult they lose the ability for independent activity. Nevertheless, the activation of locomotor centers through intensive training underlies various methods for restoring walking in patients with spinal cord injury (see the article The effectiveness of intensive training in restoring motor function).
Thus, programmed (automatic) motor acts are provided even at the level of the spinal cord. Such motor programs independent of external stimulation are more widely represented in higher motor centers. Some of them (for example, breathing) are innate, while others (for example, cycling) are acquired through learning.

White matter of the spinal cord. The conduction function of the spinal cord

The white matter of the spinal cord is formed by a set of longitudinally oriented nerve fibers running in an ascending or descending direction. White matter surrounds gray matter on all sides and is divided, as already mentioned above, into three cords: front, rear, side. In addition, it distinguishes anterior white commissure. It is located posterior to the anterior median fissure and connects the anterior cords of the right and left sides.
Bundles of nerve fibers (a set of processes) in the cords of the spinal cord make up pathways of the spinal cord. There are three beam systems:

1. Short bundles of association fibers connect segments of the spinal cord located at different levels.
2. Ascending (afferent, sensory) pathways are sent to the centers of the brain.
3. Descending (efferent, motor) pathways go from the brain to the cells of the anterior horns of the spinal cord.

In the white matter of the anterior cords, there are mainly descending pathways, in the lateral cords - ascending and descending, in the posterior cords - ascending pathways.
Sensitive (ascending) paths. The spinal cord conducts four types of sensitivity: tactile (a sense of touch and pressure), temperature, pain and proprioception (from muscle and tendon receptors, the so-called joint-muscular feeling, a sense of position and movement of the body and limbs).
The bulk of the ascending pathways proprioceptive sensitivity. This indicates the importance of movement control, the so-called feedback, for the motor function of the body. The pathways of proprioceptive sensitivity are directed to the cerebral cortex and to the cerebellum, which is involved in the coordination of movements. The proprioceptive pathway to the cerebral cortex is represented by two bundles: thin and wedge-shaped. Thin beam (Gaulle beam) conducts impulses from the proprioceptors of the lower extremities and the lower half of the body and is adjacent to the posterior median sulcus in the posterior cord. Wedge-shaped bundle (Burdach's bundle) adjoins it from the outside and carries impulses from the upper half of the body and from the upper limbs. Two go to the cerebellum dorsal tract- front (Flexiga) and rear (Goversa). They are located in the lateral funiculi. The anterior spinal cerebellar tract serves to control the position of the limbs and the balance of the whole body during movement and posture. The posterior spinal cerebellar pathway is specialized for the rapid regulation of fine movements of the upper and lower extremities. Due to the receipt of impulses from proprioceptors, the cerebellum is involved in automatic reflex coordination of movements. This is especially clearly manifested in sudden imbalances during walking, when, in response to a change in body position, a whole complex of involuntary movements arises, aimed at maintaining balance.
impulses painful and temperature sensitivity holds lateral (lateral) dorsal-thalamic pathway. The first neuron of this pathway are the sensory cells of the spinal nodes. Their peripheral processes (dendrites) come as part of the spinal nerves. The central processes form the posterior roots and go to the spinal cord, ending on the intercalary neurons of the posterior horns (2nd neuron). The processes of the second neurons pass through the anterior white commissure to the opposite side (form a decussation) and rise as part of the lateral funiculus of the spinal cord to the brain. As a result of the fact that the fibers cross along the way, impulses from the left half of the trunk and limbs are transmitted to the right hemisphere, and from the right half to the left.
Tactile sensitivity (sense of touch, touch, pressure) holds anterior dorsal thalamic pathway which is part of the anterior funiculus of the spinal cord.
motor pathways represented by two groups:
1. Anterior and lateral (lateral) pyramidal (corticospinal) tracts, conducting impulses from the cortex to the motor cells of the spinal cord, which are the paths of arbitrary (conscious) movements. They are represented by axons of giant pyramidal cells (Betz cells) located in the cortex of the precentral gyrus of the cerebral hemispheres. At the border with the spinal cord, most of the fibers of the common pyramidal pathway passes to the opposite side (forms a decussation) and forms a lateral pyramidal pathway that descends in the lateral funiculus of the spinal cord, ending on the motor neurons of the anterior horn. A smaller part of the fibers does not cross and goes in the anterior funiculus, forming the anterior pyramidal tract. However, these fibers also gradually pass through the anterior white commissure to the opposite side (form a segmental decussation) and end on the motor cells of the anterior horn. The processes of the cells of the anterior horn form the anterior (motor) root and end in the muscle with a motor ending. Thus, both pyramidal paths are crossed. Therefore, with unilateral damage to the brain or spinal cord, movement disorders occur below the injury site on the opposite side of the body. The pyramidal pathways are two-neuronal (the central neuron is the pyramidal cell of the cortex, the peripheral neuron is the motoneuron of the anterior horn of the spinal cord). When the body or axon of the central neuron is damaged, central (spastic) paralysis, and in case of damage to the body or axon of a peripheral neuron - peripheral (flaccid) paralysis.

Extrapyramidal, reflex motor pathways

These include:
- red nuclear-spinal (rubrospinal) path - goes as part of the lateral cords from the cells of the red nucleus of the midbrain to the anterior horns of the spinal cord, carries impulses of subconscious control of movements and tone of skeletal muscles;
- tecto-spinal (cover-spinal) path - goes in the anterior cord, connects the upper hillocks of the midbrain tegmentum (subcortical centers of vision) and the lower hillocks (hearing centers) with the motor nuclei of the anterior horns of the spinal cord, its function is to ensure coordinated eye movements , head and upper limbs to unexpected light and sound effects;
- vestibulo-spinal (vestibulo-spinal) path - goes from the vestibular (vestibular) nuclei (8th pair of cranial nerves) to the motor cells of the anterior horns of the spinal cord, has an exciting effect on the motor nuclei of the extensor muscles (anti-gravity muscles), and mainly on the axial muscles (muscles of the spinal column) and on the muscles of the belts of the upper and lower extremities. The vestibulo-spinal tract has an inhibitory effect on the flexor muscles.

Blood supply to the spinal cord

The spinal cord is supplied with blood by the longitudinally running anterior and two posterior spinal arteries. The anterior spinal artery is formed by the connection of the spinal branches of the right and left vertebral arteries, and runs along the anterior longitudinal fissure of the spinal cord. The posterior spinal artery, steam room, is adjacent to the posterior surface of the spinal cord near the entry into it of the posterior root of the spinal nerve. These arteries continue throughout the spinal cord. They connect with the spinal branches of the deep cervical artery, posterior intercostal, lumbar and lateral sacral arteries, which enter the spinal canal through the intervertebral foramina.
The veins of the spinal cord empty into the internal vertebral venous plexus.

Meninges of the spinal cord

Rice. four. The spinal cord and its membranes in the spinal canal. 1 - hard shell of the spinal cord, 2 - epidural space, 3 - arachnoid, 4 - posterior root of the spinal nerve, 5 - anterior root, 6 - spinal ganglion, 7 - spinal nerve, 8 - subarachnoid (subarachnoid) space, 9 - dentate bundle.

The spinal cord is surrounded by three membranes ( rice. four).
Outside is located dura mater. Between this membrane and the periosteum of the spinal canal is the epidural space. Inside from the dura mater there is arachnoid separated from the dura mater by the subdural space. Directly adjacent to the spinal cord is the inner pia mater. Between the arachnoid and the inner meninges is the subarachnoid (subarachnoid) space filled with cerebrospinal fluid.
Dura mater of the spinal cord It is a blind sac that contains the spinal cord, anterior and posterior roots of the spinal nerves, and the rest of the meninges. The dura mater is dense, formed by fibrous connective tissue, contains a significant amount of elastic fibers. At the top, the dura mater of the spinal cord is firmly fused with the edges of the foramen magnum and passes into the dura mater of the brain. In the spinal canal, the dura mater is strengthened by its processes, which continue into the sheaths of the spinal nerves. These processes fuse with the periosteum in the region of the intervertebral foramina. The dura mater is also strengthened by numerous fibrous bundles leading to the posterior longitudinal ligament of the spine. These bundles are better expressed in the cervical, lumbar and sacral regions and worse in the chest region. In the upper cervical region, the dura covers the right and left vertebral arteries.
The outer surface of the dura is separated from the periosteum epidural space. It is filled with fatty tissue and contains the internal vertebral venous plexus. The inner surface of the dura mater of the spinal cord is separated from the arachnoid by a slit-like subdural space. It is filled with a large number of thin connective tissue bundles. The subdural space of the spinal cord at the top communicates with the space of the same name of the brain, at the bottom it ends blindly at the level of the second sacral vertebra. Below this level, the bundles of fibrous fibers of the dura mater continue into the terminal thread.
arachnoid mater of the spinal cord It is represented by a thin translucent connective tissue plate located medially from the hard shell. The hard and arachnoid membranes grow together only near the intervertebral foramina. Between the arachnoid and soft membranes (in the subarachnoid space) there is a network of crossbeams, consisting of thin bundles of collagen and elastic fibers. These connective tissue bundles connect the arachnoid mater to the pia mater and to the spinal cord.
The soft (vascular) membrane of the spinal cord tightly attached to the surface of the spinal cord. Connective tissue fibers extending from the soft shell accompany the blood vessels, go with them into the tissue of the spinal cord. Between the arachnoid and pia mater is subarachnoid, or subarachnoid space. It contains 120-140 ml of cerebrospinal fluid. In the upper sections, this space continues into the subarachnoid space of the brain. In the lower sections, the subarachnoid space of the spinal cord contains only the roots of the spinal nerves. Below the level of the second lumbar vertebra, it is possible to obtain cerebrospinal fluid for examination by puncture without risking damage to the spinal cord.
From the lateral sides of the pia mater of the spinal cord, between the anterior and posterior roots of the spinal nerves, goes frontally to the right and left dentate ligament. The dentate ligament also grows together with the arachnoid and with the inner surface of the hard shell of the spinal cord; the ligament, as it were, hangs the spinal cord in the subarachnoid space. Having a continuous origin on the lateral surfaces of the spinal cord, the ligament is divided into 20-30 teeth in the lateral direction. The upper tooth corresponds to the level of the large occipital foramen, the lower one is located between the roots of the twelfth thoracic and first lumbar vertebrae. In addition to the dentate ligaments, the spinal cord is fixed in the spinal canal using the posterior subarachnoid septum. This septum starts from the hard, arachnoid and soft membranes and connects to the posterior median septum, which is present between the posterior cords of the white matter of the spinal cord. In the lower lumbar and sacral regions of the spinal cord, the posterior septum of the subarachnoid space, as well as the dentate ligaments, is absent. Adipose tissue and venous plexuses of the epidural space, spinal cord membranes, cerebrospinal fluid and ligamentous apparatus protect the spinal cord from concussions during body movements.

Literature

1. Antonen E.G. Spinal cord (anatomical, physiological and neurological aspects).
2. Sapin M.R., Nikityuk D.B. Human anatomy. - In 3 volumes. - M. - 1998. - V.3.
3. Materials of the site medicinform.net.

The spinal cord is the oldest part of the CNS. It is located in the spinal canal and has a segmental structure. The spinal cord is divided into cervical, thoracic, lumbar and sacral sections, each of which includes a different number of segments. Two pairs of roots depart from the segment - posterior and anterior (Fig. 3.11).

The posterior roots are formed by axons of primary afferent neurons, the bodies of which lie in the spinal sensory ganglia; the anterior roots consist of processes of motor neurons, they are directed to the corresponding effectors (Bell-Magendie law). Each root is a set of nerve fibers.

Rice. 3.11.

On the cross section of the spinal cord (Fig. 3.12), it can be seen that in the center there is gray matter, consisting of the bodies of neurons and resembling the shape of a butterfly, and along the periphery lies white matter, which is a system of neuronal processes: ascending (nerve fibers are sent to different parts of the brain brain) and descending (nerve fibers are sent to certain parts of the spinal cord).

Rice. 3.12.

• 1 - anterior horn of gray matter; 2 - posterior horn of gray matter;
• 3 - lateral horn of gray matter; 4 - anterior root of the spinal cord; 5 - posterior root of the spinal cord.

The appearance and complication of the spinal cord is associated with the development of locomotion (movement). Locomotion, providing the movement of a person or animal in the environment, creates the possibility of their existence.

The spinal cord is the center of many reflexes. They can be divided into 3 groups: protective, vegetative and tonic.

• 1. Protective-pain reflexes are characterized by the fact that the action of stimuli, as a rule, on the skin surface, causes a protective reaction, which leads to the removal of the stimulus from the surface of the body or the removal of the body or its parts from the stimulus. Protective reactions are expressed in the withdrawal of a limb or running away from a stimulus (flexion and extension reflexes). These reflexes are carried out segment by segment, but with more complex reflexes, such as scratching in hard-to-reach places, complex multi-segment reflexes arise.
• 2. Vegetative reflexes are provided by nerve cells located in the lateral horns of the spinal cord, which are the centers of the sympathetic nervous system. Here, vasomotor, urethral reflexes, defecation reflexes, sweating, etc.
• 3. Tonic reflexes are very important. They provide the formation and maintenance of skeletal muscle tone. Tone is a constant, invisible contraction (tension) of the muscles without fatigue. The tone provides the posture and position of the body in space. A posture is a fixed position of the body (head and other parts of the body) of a person or animals in space under the conditions of gravity.

In addition, the spinal cord performs a conductive function, which is carried out by ascending and descending fibers of the white matter of the spinal cord (Table 3.1). As part of the conducting paths, both afferent and efferent fibers pass. Since some of these fibers conduct interoceptive impulses from the internal organs, this allows them to be used for pain relief during intracavitary operations by introducing an anesthetic into the spinal canal (spinal anesthesia).

Table 3.1

The conduction pathways of the spinal cord and their physiological significance

Age features of the spinal cord

The spinal cord develops earlier than other parts of the CNS. During fetal development and in the newborn, it fills the entire cavity of the spinal canal. The length of the spinal cord in a newborn is 14-16 cm. The growth in length of the axial cylinder and the myelin sheath continues up to 20 years. It grows most intensively in the first year of life. However, the rate of its growth lags behind the growth of the spine. Therefore, by the end of the 1st year of life, the spinal cord is located at the level of the upper lumbar vertebrae, just as in an adult.

The growth of individual segments is uneven. The thoracic segments grow most intensively, the lumbar and sacral segments grow weaker. Cervical and lumbar thickenings appear already in the embryonic period. By the end of the 1st year of life and after 2 years, these thickenings reach their maximum development, which is associated with the development of the limbs and their motor activity.

Spinal cord cells begin to develop in utero, but development does not end after birth. In a newborn, the neurons that form the nuclei of the spinal cord are morphologically mature, but differ from an adult in their smaller size and lack of pigment. In a newborn child, in the transverse section of the segments, the posterior horns predominate over the anterior horns. This indicates more developed sensory functions compared to motor ones. The ratio of these parts reaches the level of adults by the age of 7, however, functionally motor and sensory neurons continue to develop.

The diameter of the spinal cord is associated with the development of sensitivity, motor activity and pathways. After 12 years, the diameter of the spinal cord reaches the adult level.

The amount of cerebrospinal fluid in newborns is less than in adults (40-60 g), and the protein content is higher. In the future, from 8-10 years old, the amount of cerebrospinal fluid in children is almost the same as in adults, and the amount of proteins already from 6-12 months corresponds to the level of adults.

The reflex function of the spinal cord is formed already in the embryonic period, and its formation is stimulated by the movements of the child. From the 9th week, the fetus has generalized movements of the arms and legs (simultaneous contraction of the flexors and extensors) with skin irritation. The tonic contraction of the flexor muscles predominates and forms the posture of the fetus, providing its minimum volume in the uterus, periodic generalized contractions of the extensor muscles, starting from the 4-5th month of intrauterine life, are felt by the mother as fetal movement. After birth, reflexes appear, which gradually disappear in ontogenesis:

• stepping reflex (movement of the legs when taking the child under the armpits);
• Babinsky's reflex (abduction of the big toe when the foot is irritated, disappears at the beginning of the 2nd year of life);
• knee reflex (flexion of the knee joint due to the predominance of flexor tone; it transforms into an extensor reflex at the 2nd month);
• grasping reflex (grasping and holding an object when touching the palm, disappears on the 3-4th month);
• the grasping reflex (bringing the arms to the sides, then bringing them together with the rapid lifting and lowering of the child, disappears after the 4th month);
• crawling reflex (in the position lying on the stomach, the child raises his head and makes crawling movements; if you put your palm on the soles, the child will begin to actively push off the obstacle with his feet, disappears by the 4th month);
• labyrinth reflex (in the position of the child on the back, when the position of the head in space changes, the tone of the muscles of the extensor muscles of the neck, back, legs increases; when turning over on the stomach, the tone of the flexors of the neck, back, arms and legs increases);
• torso-rectifying (when the child's feet come into contact with the support, the head is straightened, it is formed by the 1st month);
• Landau reflex (upper - a child in a position on his stomach raises his head and upper body, leaning on a plane with his hands; lower - in a position on his stomach, the child unbends and raises his legs; these reflexes are formed by the 5-6th month), etc.

At first, the reflexes of the spinal cord are very imperfect, uncoordinated, generalized, the tone of the flexor muscles prevails over the tone of the extensor muscles. Periods of motor activity prevail over periods of rest. Reflexogenic zones narrow by the end of the 1st year of life and become more specialized.

With the aging of the body, there is a decrease in the strength and an increase in the latent period of reflex reactions, the cortical control of spinal reflexes decreases (the Babinski reflex appears again, the proboscis labial reflex), coordination of movements deteriorates due to a decrease in the strength and mobility of the main nervous processes.

Topic 4. physiology of the spinal cord.

Purpose and objectives of the study.

The study of the material of this lecture aims to acquaint students with the physiological processes occurring at the level of the spinal cord.

W tasks studies are:

Acquaintance with the morphological and functional features of the organization of the spinal cord;

Study of the reflex functions of the spinal cord;

Familiarize yourself with the consequences of spinal cord injury.

Lecture notes 4. Physiology of the spinal cord.

Morphofunctional organization of the spinal cord.

Functions of the spinal cord.

limb reflexes.

posture reflexes.

Abdominal reflexes

Spinal cord disorders.

Morphofunctional organization of the spinal cord. The spinal cord is the most ancient formation of the central nervous system. A characteristic feature of its organization is the presence of segments that have inputs in the form of posterior roots, a cell mass of neurons (gray matter) and outputs in the form of anterior roots. The human spinal cord has 31 segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal. There are no morphological boundaries between segments of the spinal cord; therefore, the division into segments is functional and is determined by the zone of distribution of the fibers of the posterior root in it and the zone of cells that form the exit of the anterior roots. Each segment innervates three metameres (31) of the body through its roots and receives information also from three metameres of the body. As a result of overlap, each metamere of the body is innervated by three segments and transmits signals to three segments of the spinal cord.

The human spinal cord has two thickenings: cervical and lumbar - they contain a greater number of neurons than in the rest of its parts, which is due to the development of the upper and lower extremities.

The fibers entering the posterior roots of the spinal cord perform functions that are determined by where and on which neurons these fibers end. In experiments with transection and irritation of the roots of the spinal cord, it was shown that the posterior roots are afferent, sensitive, and the anterior roots are efferent, motor.

Afferent inputs to the spinal cord are organized by the axons of the spinal ganglia, which lie outside the spinal cord, and the axons of the ganglia of the sympathetic and parasympathetic divisions of the autonomic nervous system.

The first group (I) of afferent inputs The spinal cord is formed by sensory fibers coming from muscle receptors, tendon receptors, periosteum, and joint membranes. This group of receptors forms the beginning of the so-called proprioceptive sensitivity. Proprioceptive fibers are divided into 3 groups according to the thickness and speed of excitation (Ia, Ib, Ic). The fibers of each group have their own thresholds for the occurrence of excitation. The second group (II) afferent inputs of the spinal cord starts from skin receptors: pain, temperature, tactile, pressure - and is skin receptor system. Third group (III) afferent inputs the spinal cord is represented by inputs from internal organs; this is viscero-receptive system.

The neurons of the spinal cord form it Gray matter in the form of symmetrically located two front and two rear. The gray matter is distributed into nuclei, elongated along the length of the spinal cord, and is located in the cross section in the shape of a butterfly.

The posterior horns perform mainly sensory functions and contain neurons that transmit signals to the overlying centers, to the symmetrical structures of the opposite side, or to the anterior horns of the spinal cord.

In the anterior horns are neurons that give their axons to the muscles (motoneurons).

The spinal cord has, in addition to those named, also lateral horns. Starting from the I thoracic segment of the spinal cord and up to the first lumbar segments, neurons of the sympathetic are located in the lateral horns of the gray matter, and neurons of the parasympathetic division of the autonomic (vegetative) nervous system are located in the sacral.

The human spinal cord contains about 13 million neurons, of which only 3% are motor neurons, and 97% are intercalary.

Functionally, spinal cord neurons can be divided into 4 main groups:

1) motoneurons, or motor, - cells of the anterior horns, the axons of which form the anterior roots;

2) interneurons- neurons that receive information from the spinal ganglia and are located in the posterior horns. These afferent neurons respond to pain, temperature, tactile, vibration, proprioceptive stimuli and transmit impulses to the overlying centers, to the symmetrical structures of the opposite side, to the anterior horns of the spinal cord;

3) sympathetic, parasympathetic neurons are located in the lateral horns. In the lateral horns of the cervical and two lumbar segments, neurons of the sympathetic division of the autonomic nervous system are located, in segments II-IV of the sacral - of the parasympathetic. The axons of these neurons leave the spinal cord as part of the anterior roots and go to the ganglion cells of the sympathetic chain and to the ganglia of the internal organs;

4) association cells- neurons of the spinal cord's own apparatus, establishing connections within and between segments. So, at the base of the posterior horn there is a large accumulation of nerve cells that form intermediate nucleus spinal cord. Its neurons have short axons, which mainly go to the anterior horn and form synaptic contacts with motor neurons there. The axons of some of these neurons extend over 2-3 segments, but never extend beyond the spinal cord.

Nerve cells of different types, diffusely scattered or collected in the form of nuclei. Most of the nuclei in the spinal cord occupy several segments, so the afferent and efferent fibers associated with them enter and leave the spinal cord through several roots. The most significant spinal nuclei are the nuclei of the anterior horns, formed by motor neurons.

All descending pathways of the central nervous system that cause motor reactions terminate on the motor neurons of the anterior horns. In this regard, Sherrington called them "common final path".

There are three types of motor neurons: alpha, beta and gamma.. Alpha motor neurons represented by large multipolar cells with a body diameter of 25-75 microns; their axons innervate motor muscles, which are capable of developing considerable strength. Beta motor neurons are small neurons that innervate the tonic muscles. Gamma motor neurons(9) even smaller - their body diameter is 15-25 microns. They are localized in the motor nuclei of the ventral horns among alpha and beta motor neurons. Gamma motor neurons carry out motor innervation of muscle receptors (muscle spindles (32)). Axons of motor neurons make up the bulk of the anterior roots of the spinal cord (motor nuclei).

Functions of the spinal cord. There are two main functions of the spinal cord: conduction and reflex. Conductor function provides communication of neurons of the spinal cord with each other or with the overlying parts of the central nervous system. reflex function allows you to realize all the motor reflexes of the body, reflexes of internal organs, the genitourinary system, thermoregulation, etc. Own reflex activity of the spinal cord is carried out by segmental reflex arcs.

Let us introduce some important definitions. The minimum stimulus that elicits a reflex is called threshold(43) (or threshold stimulus) of this reflex. Every reflex has receptive field(52), i.e., a set of receptors, the irritation of which causes a reflex with the lowest threshold.

When studying movements, one has to break down a complex reflex act into separate, relatively simple reflexes. At the same time, it should be remembered that under natural conditions an individual reflex appears only as an element of a complex activity.

Spinal reflexes are divided into:

Firstly, receptors, stimulation of which causes a reflex:

a) proprioceptive (own) reflexes from the muscle itself and its associated formations. They have the simplest reflex arc. Reflexes arising from proprioceptors are involved in the formation of the act of walking and the regulation of muscle tone.

b) visceroceptive reflexes arise from the receptors of the internal organs and are manifested in the contraction of the muscles of the abdominal wall, chest and back extensors. The emergence of visceromotor reflexes is associated with the convergence (25) of visceral and somatic nerve fibers to the same interneurons of the spinal cord,

in) skin reflexes occur when skin receptors are irritated by signals from the external environment.

Secondly, by organs:

a) limb reflexes;

b) abdominal reflexes;

c) testicular reflex;

d) anal reflex.

The simplest spinal reflexes that can be easily observed are flexion and extensor. Flexion (55) should be understood as a decrease in the angle of a given joint, and extension as its increase. Flexion reflexes are widely represented in human movements. Characteristic of these reflexes is the great strength they can develop. However, they get tired quickly. The extensor reflexes are also widely represented in human movements. For example, these include the reflexes of maintaining an upright posture. These reflexes, unlike flexion reflexes, are much more resistant to fatigue. Indeed, we can walk and stand for a long time, but for long-term work, such as lifting weights with our hands, our physical capabilities are much more limited.

The universal principle of the reflex activity of the spinal cord is called common end path. The fact is that the ratio of the number of fibers in the afferent (posterior roots) and efferent (anterior roots) pathways of the spinal cord is approximately 5:1. C. Sherrington figuratively compared this principle with a funnel, the wide part of which is the afferent pathways of the posterior roots, and the narrow efferent pathways of the anterior roots of the spinal cord. Often the territory of the final path of one reflex overlaps with the territory of the final path of another reflex. In other words, different reflexes can compete to occupy the final path. This can be illustrated with an example. Imagine that a dog is running away from danger and is being bitten by a flea. In this example, two reflexes compete for a common final path - the muscles of the hind leg: one is the scratching reflex, and the other is the walking-running reflex. At some moments, the scratching reflex can overpower, and the dog stops and starts to itch, but then the walking-running reflex can take over again, and the dog will resume running.

As already mentioned, during the implementation of reflex activity, individual reflexes interact with each other, forming functional systems. One of the most important elements of a functional system - reverse afferentation, thanks to which the nerve centers, as it were, evaluate how the reaction is carried out, and can make the necessary adjustments to it.

Limb reflexes .

Muscle stretch reflexes. There are two types of stretch reflex: phasic (fast) and tonic (slow). An example of a phase reflex is knee jerk, which occurs with a light blow to the tendon of the muscle in the popliteal cup. The stretch reflex prevents overstretching of the muscle, which seems to be resisting stretching. This reflex occurs as a response of a muscle to stimulation of its receptors, therefore it is often referred to as own muscle reflex. Rapid stretching of the muscle, just a few millimeters by a mechanical impact on its tendon, leads to contraction of the entire muscle and extension of the lower leg.

The path of this reflex is as follows:

Muscle receptors of the quadriceps femoris;

spinal ganglion;

back roots;

Posterior horns of III lumbar segment;

Motoneurons of the anterior horns of the same segment;

Fibers of the quadriceps femoris muscle.

The realization of this reflex would be impossible if, simultaneously with the contraction of the extensor muscles, the flexor muscles did not relax. Therefore, during the extensor reflex, the motor neurons of the flexor muscles are inhibited by the intercalary inhibitory Renshaw cells (24) (reciprocal inhibition). Phase reflexes are involved in the formation of walking. The stretch reflex is characteristic of all muscles, but in the extensor muscles, they are well pronounced and easily evoked.

The phasic stretch reflexes also include the Achilles reflex, caused by a light blow to the Achilles tendon, and the elbow reflex, caused by a hammer blow to the quadriceps tendon.

Tonic reflexes arise with prolonged stretching of the muscles, their main purpose is to maintain the posture. In the standing position, tonic contraction of the extensor muscles prevents flexion of the lower extremities under the influence of gravity forces and ensures the maintenance of an upright position. The tonic contraction of the back muscles provides a person's posture. Tonic contraction of skeletal muscles is the background for the implementation of all motor acts carried out with the help of phase muscle contractions. An example of a tonic stretch reflex is the calf muscle's own reflex. This is one of the main muscles, thanks to which the vertical posture of a person is maintained.

The reflex responses are more complex and are expressed in coordinated flexion and extension of the muscles of the extremities. An example is flexion reflexes aimed at avoiding various damaging effects(Fig.4.1.) . The receptive field of the flexion reflex is quite complex and includes various receptor formations and afferent pathways of various speeds. The flexion reflex occurs when the pain receptors of the skin, muscles and internal organs are irritated. The afferent fibers involved in these stimulations have a wide range of conduction velocities - from group A myelinated fibers to group C unmyelinated fibers. flexion reflex afferents.

Flexion reflexes differ from intrinsic muscle reflexes not only by a large number of synaptic switches on the way to motor neurons, but also by the involvement of a number of muscles, the coordinated contraction of which determines the movement of the entire limb. Simultaneously with the excitation of the motor neurons innervating the flexor muscles, reciprocal inhibition of the motor neurons of the extensor muscles occurs.

With sufficiently intense stimulation of the receptors of the lower limb, irradiation of excitation occurs and the muscles of the upper limb and trunk are involved in the reaction. When motor neurons of the opposite side of the body are activated, not flexion, but extension of the muscles of the opposite limb is observed - a cross-extension reflex.

posture reflexes. Even more complex are posture reflexes- redistribution of muscle tone, which occurs when the position of the body or its individual parts changes. They represent a large group of reflexes. Flexion tonic posture reflex can be observed in a frog and in mammals, which are characterized by a bent position of the limbs (rabbit).

For most mammals and humans, the main importance for maintaining body position is not flexion, but extensor reflex tone. At the level of the spinal cord, a particularly important role in the reflex regulation of extensor tone is played by cervical postural reflexes. Their receptors are found in the muscles of the neck. The reflex arc is polysynaptic, closes at the level of I-III cervical segments. Impulses from these segments are transmitted to the muscles of the trunk and limbs, causing a redistribution of their tone. There are two groups of these reflexes - arising when tilting and when turning the head.

The first group of cervical postural reflexes exists only in animals and occurs when the head is tilted down (Fig. 4.2.). At the same time, the tone of the flexor muscles of the forelimbs and the tone of the extensor muscles of the hind limbs increase, as a result of which the forelimbs bend and the hind limbs unbend. When the head is tilted up (posteriorly), opposite reactions occur - the forelimbs unbend due to an increase in the tone of their extensor muscles, and the hind limbs bend due to an increase in the tone of their flexor muscles. These reflexes arise from the proprioceptors of the muscles of the neck and fascia covering the cervical spine. Under conditions of natural behavior, they increase the animal's chance to get food that is above or below head level.

Reflexes of the posture of the upper limbs in humans are lost. Reflexes of the lower extremities are expressed not in flexion or extension, but in the redistribution of muscle tone, which ensures the preservation of a natural posture.

The second group of cervical postural reflexes arises from the same receptors, but only when the head is turned to the right or left (Fig. 4.3). At the same time, the tone of the extensor muscles of both limbs on the side where the head is turned increases, and the tone of the flexor muscles on the opposite side increases. The reflex is aimed at maintaining a posture that can be disturbed due to a change in the position of the center of gravity after turning the head. The center of gravity shifts in the direction of head rotation - it is on this side that the tone of the extensor muscles of both limbs increases. Similar reflexes are observed in humans.

At the level of the spinal cord, they also close rhythmic reflexes- repeated flexion and extension of the limbs. Examples are the scratching and walking reflexes. Rhythmic reflexes are characterized by the coordinated work of the muscles of the limbs and the trunk, the correct alternation of flexion and extension of the limbs, along with the tonic contraction of the adductor muscles, which set the limb in a certain position to the skin surface.

Abdominal reflexes (upper, middle and lower) appear with dashed irritation of the skin of the abdomen. They are expressed in the reduction of the corresponding sections of the muscles of the abdominal wall. These are protective reflexes. To call the upper abdominal reflex, irritation is applied parallel to the lower ribs directly below them, the arc of the reflex closes at the level of the VIII-IX thoracic segment of the spinal cord. The middle abdominal reflex is caused by irritation at the level of the navel (horizontally), the arc of the reflex closes at the level of the IX-X thoracic segment. To obtain a lower abdominal reflex, irritation is applied parallel to the inguinal fold (next to it), the arc of the reflex closes at the level of the XI-XII thoracic segment.

Cremasteric (testicular) reflex is to reduce m. sremaster and lifting the scrotum in response to a dashed irritation of the upper inner surface of the skin of the thigh (skin reflex), this is also a protective reflex. Its arc closes at the level of the I-II lumbar segment.

anal reflex expressed in the reduction of the external sphincter of the rectum in response to dashed irritation or skin prick near the anus, the arc of the reflex closes at the level of the IV-V sacral segment.

Vegetative reflexes. In addition to the reflexes discussed above, which belong to the somatic category, since they are expressed in the activation of skeletal muscles, the spinal cord plays an important role in the reflex regulation of internal organs, being the center of many visceral reflexes. These reflexes are carried out with the participation of neurons of the autonomic nervous system located in the lateral horns of the gray matter. The axons of these nerve cells leave the spinal cord through the anterior roots and end on the cells of the sympathetic or parasympathetic autonomic ganglia. Ganglion neurons, in turn, send axons to the cells of various internal organs, including smooth muscles of the intestine, blood vessels, bladder, glandular cells, and cardiac muscle. Vegetative reflexes of the spinal cord are carried out in response to irritation of the internal organs and end with a contraction of the smooth muscles of these organs.