Functions of the spinal cord physiology briefly. Functions of the spinal cord
Topic 4. physiology of the spinal cord.
Purpose and objectives of the study.
Studying the material in this lecture aims to familiarize students with the physiological processes occurring at the level of the spinal cord.
Z adachami studies are:
Familiarization with the morphofunctional features of the organization of the spinal cord;
Study of reflex functions of the spinal cord;
Familiarization 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.
Reflexes of the limbs.
Posture reflexes.
Abdominal reflexes
Spinal cord dysfunctions.
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 with inputs in the form of dorsal 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 the segments of the spinal cord, therefore the division into segments is functional and is determined by the zone of distribution of dorsal root fibers in it and the zone of cells that form the exit of the anterior roots. Each segment, through its roots, innervates three metameres (31) of the body and also receives information from three metameres of the body. As a result of the overlap, each metamer 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 larger number of neurons than in other parts of it, which is due to the development of the upper and lower extremities.
Fibers traveling along the dorsal 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 spinal cord roots, it was shown that the dorsal roots are afferent, sensitive, and the anterior ones are efferent, motor.
Afferent inputs to the spinal cord are organized by the axons of the spinal ganglia lying outside the spinal cord, and by the axons of the ganglia of the sympathetic and parasympathetic divisions of the autonomic nervous system.
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 thickness and speed of excitation (Ia, Ib, Ic). The fibers of each group have their own thresholds for the occurrence of excitation. Second group (II) of afferent inputs of the spinal cord starts from skin receptors: pain, temperature, tactile, pressure - and represents cutaneous receptive system. The third group (III) of afferent inputs the spinal cord is represented by inputs from internal organs; This viscero-receptive system.
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 that extend along the length of the spinal cord and is butterfly shaped in cross section.
The posterior horns perform mainly sensory functions and contain neurons that transmit signals to overlying centers, to symmetrical structures on the opposite side, or to the anterior horns of the spinal cord.
The anterior horns contain neurons that send their axons to the muscles (motoneurons).
The spinal cord has, in addition to those mentioned, also lateral horns. Starting from the first thoracic segment of the spinal cord and up to the first lumbar segments, neurons of the sympathetic, and in the sacral - of the parasympathetic division of the autonomic (autonomic) nervous system are located in the lateral horns of the gray matter.
The human spinal cord contains about 13 million neurons, of which only 3% are motor neurons, and 97% are intercalary neurons.
Functionally, spinal cord neurons can be divided into 4 main groups:
1) motor neurons, or motor neurons, - 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 dorsal horns. These afferent neurons respond to pain, temperature, tactile, vibration, proprioceptive stimulation 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. The neurons of the sympathetic division of the autonomic nervous system are located in the lateral horns of the cervical and two lumbar segments, and the parasympathetic ones are located in the II-IV segments of the sacral segments. The axons of these neurons leave the spinal cord as part of the anterior roots and are directed to the cells of the ganglia 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. Thus, 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 are scattered diffusely or collected in the form of nuclei. Most nuclei in the spinal cord occupy several segments, so the afferent and efferent fibers associated with them enter and exit the spinal cord along 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 responses end 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 µm; their axons innervate motor muscles, which are capable of developing significant force. Beta motor neurons- these are small neurons that innervate the tonic muscles. Gamma motor neurons(9) even smaller - the diameter of their body is 15-25 microns. They are localized in the motor nuclei of the ventral horns among alpha and beta motor neurons. Gamma motor neurons provide motor innervation to 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 ensures communication of spinal cord neurons with each other or with overlying parts of the central nervous system. Reflex function allows you to realize all motor reflexes of the body, reflexes of internal organs, the genitourinary system, thermoregulation, etc. The spinal cord's own reflex activity is carried out by segmental reflex arcs.
Let us introduce some important definitions. The minimum force of stimulation that causes a reflex is called threshold(43) (or threshold stimulus) of a given reflex. Every reflex has receptive field(52), i.e., a set of receptors whose irritation causes a reflex with the lowest threshold.
When studying movements, it is necessary to divide a complex reflex act into separate relatively simple reflexes. At the same time, it should be remembered that in natural conditions a separate reflex acts only as an element of complex activity.
Spinal reflexes are divided into:
Firstly, by receptors, the irritation of which causes the reflex:
A) proprioceptive (own) reflexes from the muscle itself and the formations associated with it. They have a simple 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 receptors of internal organs and are manifested in 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,
V) skin reflexes occur when skin receptors are irritated by environmental signals.
Secondly, by organ:
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 By flexion (55) we should understand a decrease in the angle of a given joint, and by extension it is an increase. Flexion reflexes are widely represented in human movements. Characteristic of these reflexes is the great strength they can develop. At the same time, they get tired quickly. Extensor reflexes are also widely represented in human movements. For example, these include reflexes for maintaining a vertical posture. These reflexes, unlike flexion ones, are much more resistant to fatigue. Indeed, we can walk and stand for a long time, but to perform long-term work, such as lifting a weight with one hand, our physical capabilities are much more limited.
The universal principle of reflex activity of the spinal cord is called common final path. The fact is that the ratio of the number of fibers in the afferent (dorsal roots) and efferent (anterior roots) pathways of the spinal cord is approximately 5:1. C. Sherrington figuratively compared this principle to a funnel, the wide part of which is made up of the afferent pathways of the dorsal roots, and the narrow part is made up of the 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 may compete to occupy the final path. This can be illustrated with the following example. Let's imagine that a dog is running away from danger and is bitten by a flea. In this example, two reflexes compete for the common final path - the muscles of the hind paw: one is the scratching reflex, and the other is the walking-running reflex. At some moments the scratching reflex may overpower, and the dog stops and begins to scratch, but then the walking-running reflex may take over again, and the dog will resume running.
As already indicated, when carrying out reflex activity, individual reflexes interact with each other, forming functional systems. One of the most important elements of a functional system is reverse afferentation, thanks to which the nerve centers seem to evaluate how the reaction is performed 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 phasic reflex is knee reflex, which occurs when there is a slight blow to the tendon of the muscle in the popliteal cap. The stretch reflex prevents excessive stretching of a muscle that seems to resist being stretched. This reflex occurs as a muscle response to stimulation of its receptors, so it is often referred to as own muscle reflex. Rapid stretching of the muscle, just a few millimeters by a mechanical blow to 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 muscle;
Spinal ganglion;
Posterior roots;
Posterior horns of the third lumbar segment;
Motor neurons of the anterior horns of the same segment;
Fibers of the quadriceps femoris muscle.
The implementation of this reflex would be impossible if the flexor muscles did not relax simultaneously with the contraction of the extensor muscles. Therefore, during the extensor reflex, the motor neurons of the flexor muscles are inhibited by Renshaw intercalary inhibitory cells (24) (reciprocal inhibition). Phasic reflexes are involved in the formation of walking. The stretch reflex is characteristic of all muscles, but in the extensor muscles, they are well expressed and easily evoked.
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 occur during prolonged stretching of muscles, their main purpose is to maintain posture. In a standing position, tonic contraction of the extensor muscles prevents flexion of the lower extremities under the influence of gravity and ensures maintaining an upright position. Tonic contraction of the back muscles ensures human posture. Tonic contraction of skeletal muscles is the background for the implementation of all motor acts carried out with the help of phasic muscle contractions. An example of a tonic stretch reflex is the intrinsic reflex of the gastrocnemius muscle. This is one of the main muscles that helps maintain a person’s upright posture.
Reflex responses are more complexly organized, expressed in coordinated flexion and extension of the muscles of the limbs. Examples are flexion reflexes aimed at avoiding various damaging influences(Fig. 4.1.) . The receptive field of the flexion reflex is quite complex and includes various receptor formations and afferent pathways of different speeds. The flexion reflex occurs when pain receptors in the skin, muscles and internal organs are irritated. The afferent fibers involved in these stimulations have a wide range of conduction velocities - from myelinated fibers of group A to unmyelinated fibers of group C. All the various afferent fibers, impulses along which lead to the development of the flexion reflex, are combined under the name afferents of the flexion reflex.
Flexion reflexes differ from intrinsic muscle reflexes not only in the large number of synaptic switches on the way to motor neurons, but also in the involvement of a number of muscles, the coordinated contraction of which determines the movement of the entire limb. Simultaneously with the excitation of 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 torso are involved in the reaction. When motor neurons on the opposite side of the body are activated, not flexion is observed, but extension of the muscles of the opposite limb is observed - the cross extensor reflex.
Posture reflexes. They are even more complex posture reflexes– redistribution of muscle tone that 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 frogs and mammals, which are characterized by a tucked position of the limbs (rabbit).
For most mammals and humans, the main importance for maintaining body position is not bending, but extensor reflex tone. At the level of the spinal cord, they play a particularly important role in the reflex regulation of extensor tone. cervical posture reflexes. Their receptors are found in the muscles of the neck. The reflex arc is polysynaptic and closes at the level of the 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 - those that occur 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 increases, as a result of which the forelimbs bend and the hind limbs extend. When the head is tilted upward (posteriorly), opposite reactions occur - the forelimbs extend 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 neck muscles and fascia covering the cervical spine. Under conditions of natural behavior, they increase the animal's chance of reaching food located above or below head level.
The posture reflexes of the upper limbs are lost in humans. Reflexes of the lower extremities are expressed not in flexion or extension, but in the redistribution of muscle tone, ensuring the preservation of natural posture.
Second group of cervical postural reflexes occurs from the same receptors, but only when turning the head to the right or left (Figure 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 posture, which can be disrupted due to a change in the position of the center of gravity after turning the head. The center of gravity shifts towards the rotation of the head - 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 repeated flexion and extension of the limbs. Examples include the scratching and stepping reflexes. Rhythmic reflexes are characterized by the coordinated work of the muscles of the limbs and torso, the correct alternation of flexion and extension of the limbs, along with tonic contraction of the adductor muscles, which establish the limb in a certain position to the skin surface.
Abdominal reflexes (upper, middle and lower) appear with streak irritation of the skin of the abdomen. Expressed in the contraction of the corresponding areas of the muscles of the abdominal wall. These are protective reflexes. To evoke the upper abdominal reflex, irritation is applied parallel to the lower ribs directly below them, the reflex arc 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 the lower abdominal reflex, irritation is applied parallel to the inguinal fold (next to it), the reflex arc closes at the level of the XI-XII thoracic segment.
Cremasteric (testicular) reflex is to abbreviate m. cremaster and raising the scrotum in response to stroke 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 is expressed in the contraction of the external sphincter of the rectum in response to a streak of irritation or a prick of the skin near the anus, the reflex arc closes at the level of the IV-V sacral segment.
Autonomic reflexes. In addition to the reflexes discussed above, which belong to the category of somatic, as 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 ventral 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 heart muscle. Autonomic reflexes of the spinal cord are carried out in response to irritation of internal organs and end with contraction of the smooth muscles of these organs.
Lecture 19. Particular physiology of the central nervous system
The spinal cord is a nerve cord about 45 cm long in men and about 42 cm in women. It has a segmental structure (31 - 33 segments) - each of its sections is associated with a specific metameric segment of the body. The spinal cord is anatomically divided into five sections: cervical thoracic lumbar sacral and coccygeal.
The total number of neurons in the spinal cord is close to 13 million. Most of them (97%) are interneurons, 3% are classified as efferent neurons.
Efferent neurons spinal cord, related to the somatic nervous system, are motor neurons. There are α- and γ-motoneurons. α-Motoneurons innervate extrafusal (working) muscle fibers of skeletal muscles, which have a high speed of excitation along axons (70-120 m/s, group A α).
γ -Motoneurons dispersed among α-motoneurons, they innervate the intrafusal muscle fibers of the muscle spindle (muscle receptor).
Their activity is regulated by messages from higher-lying parts of the central nervous system. Both types of motor neurons are involved in the α-γ coupling mechanism. Its essence is that when the contractile activity of intrafusal fibers changes under the influence of γ-motoneurons, the activity of muscle receptors changes. Impulse from muscle receptors activates α-moto-neurons of the “own” muscle and inhibits α-moto-neurons of the antagonist muscle.
In these reflexes, the role of the afferent link is especially important. Muscle spindles (muscle receptors) are located parallel to the skeletal muscle, their ends are attached to the connective tissue membrane of the bundle of extrafusal muscle fibers using tendon-like strips. The muscle receptor consists of several striated intrafusal muscle fibers surrounded by a connective tissue capsule. The ending of one afferent fiber wraps around the middle part of the muscle spindle several times.
Tendon receptors (Golgi receptors) are enclosed in a connective tissue capsule and are localized in skeletal muscle tendons near the tendon-muscle junction. The receptors are the unmyelinated endings of thick myelinated afferent fibers (approaching the Golgi receptor capsule, this fiber loses its myelin sheath and divides into several endings). Tendon receptors are attached sequentially relative to the skeletal muscle, which ensures their irritation when the tendon is pulled. Therefore, tendon receptors send information to the brain that the muscle is contracted (and the tendon is tense), and muscle receptors send information that the muscle is relaxed and lengthened. Impulses from tendon receptors inhibit the neurons of their center and excite the neurons of the antagonist center (in the flexor muscles this excitation is less pronounced).
In this way, skeletal muscle tone and motor reactions are regulated.
Afferent neurons of the somatic nervous system are localized in the spinal sensory nodes. They have T-shaped processes, one end of which is directed to the periphery and forms a receptor in the organs, and the other goes into the spinal cord through the dorsal root and forms a synapse with the upper plates of the gray matter of the spinal cord. The system of interneurons (interneurons) ensures the closure of the reflex at the segmental level or transmits impulses to the suprasegmental regions of the central nervous system.
Neurons of the sympathetic nervous system are also intercalary; located in the lateral horns of the thoracic, lumbar and partially cervical parts of the spinal cord. They are background active, their discharge frequency is 3-5 pulses/s. Parasympathetic neurons of the autonomic nervous system are also intercalary, localized in the sacral spinal cord and also background active.
The spinal cord contains the regulatory centers for most internal organs and skeletal muscles.
Myotatic and tendon reflexes of the somatic nervous system, elements of the stepping reflex, control of inspiratory and expiratory muscles are localized here.
The spinal centers of the sympathetic department of the autonomic nervous system control the pupillary reflex, regulate the activities of the heart, blood vessels, kidneys, and organs of the digestive system.
The spinal cord is characterized by a conductive function.
It is carried out using descending and ascending paths.
Afferent information enters the spinal cord through the dorsal roots, efferent impulses and regulation of the functions of various organs and tissues of the body are carried out through the anterior roots (Bell-Magendie law).
Each root consists of many nerve fibers. For example, the dorsal root of a cat includes 12 thousand, and the ventral root - 6 thousand nerve fibers.
All afferent inputs to the spinal cord carry information from three groups of receptors:
1) skin receptors - pain, temperature, touch, pressure, vibration receptors;
2) proprioceptors - muscle (muscle spindles), tendon (Golgi receptors), periosteum and joint membranes;
3) receptors of internal organs - visceral, or interoreceptors. reflexes.
In each segment of the spinal cord there are neurons that give rise to ascending projections to the higher structures of the nervous system. The structure of the Gaulle, Burdach, spinocerebellar and spinothalamic tracts is well covered in the anatomy course.
Spinal cord consists of 31-33 segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1-3 coccygeal.
Segment- this is a section of the spinal cord associated with one pair of anterior and a pair of posterior roots.
The posterior (dorsal) roots of the spinal cord are formed by the central processes of afferent sensory neurons. The bodies of these neurons are localized in the spinal and cranial nerve nodes (ganglia). The anterior (ventral) roots are formed by axons of efferent neurons.
According to Bell-Magendie law , the anterior roots are efferent - motor or autonomic, and the posterior roots are afferent sensitive.
On a cross section of the spinal cord, a centrally located Gray matter, which is formed by a cluster of nerve cells. It's bordered by white matter, which is formed by nerve fibers. Nerve fibers of the white matter form dorsal (posterior), lateral and ventral (anterior) cords of the spinal cord which contain the conductive tracts of the spinal cord. In the posterior cords there are ascending cords, in the anterior cords there are descending ones, and in the lateral cords there are both ascending and descending pathways.
In gray matter, there are dorsal (posterior) and ventral (anterior) horns. In addition, there are lateral horns in the thoracic, lumbar and sacral segments.
All gray matter neurons can be divided into three main groups:
1) interneurons located mainly in the dorsal horns of the spinal cord,
2) efferent motor neurons localized in the anterior horns,
3) efferent preganglionic neurons of the autonomic nervous system, located in the lateral and anterior horns of the spinal cord.
A segment of the spinal cord, together with innervated areas of the body, is called metamer . A group of muscles innervated by one segment of the spinal cord is called myotome . The area of skin from which sensory signals enter a specific segment of the spinal cord is called dermatome .
There are three main functions of the spinal cord:
1) reflex,
2) trophic,
3) conductor.
Reflex function spinal cord may be segmental And intersegmental. Reflex segmental function spinal cord consists in the direct regulatory influence of efferent neurons of the spinal cord on the effectors innervated by it when irritating the receptors of a certain dermatome.
Reflexes whose arc switches in the spinal cord are called spinal . The simplest spinal reflexes include tendon reflexes , which provide contraction of skeletal muscles when their proprioceptors are irritated due to rapid short-term stretching of the muscle (for example, when a tendon is struck with a neurological hammer). Spinal tendon reflexes are clinically important because... each of them closes in certain segments of the spinal cord. Therefore, by the nature of the reflex reaction one can judge the functional state of the corresponding segments of the spinal cord.
Depending on the location of the receptors and the nerve center in humans, elbow, knee and Achilles tendon spinal reflexes are distinguished.
Elbow flexion reflex occurs when there is a blow to the tendon of the biceps brachii muscle (in the area of the ulnar fossa) and manifests itself in flexion of the arm at the elbow joint. The nerve center of this reflex is localized in the 5-6 cervical segments of the spinal cord.
Elbow extensor reflex occurs when there is a blow to the tendon of the triceps brachii muscle (in the area of the ulnar fossa) and manifests itself in extension of the arm at the elbow joint. The nerve center of this reflex is localized in the 7-8 cervical segments of the spinal cord.
Knee reflex occurs when there is a blow to the quadriceps femoris tendon below the kneecap and manifests itself in extension of the leg at the knee joint. The nerve center of this reflex is localized in the 2-4 lumbar segments of the spinal cord.
Achilles reflex occurs when there is a blow to the heel tendon and manifests itself in flexion of the foot at the ankle joint. The nerve center of this reflex is localized in 1-2 sacral segments of the spinal cord.
There are two types of fibers in skeletal muscle - extrafusal And intrafusal which are connected in parallel. Intrafusal muscle fibers perform a sensory function. They consist of connective tissue capsule, in which proprioceptors are located, and peripheral contractile elements.
A sharp, quick blow to the muscle tendon leads to its tension. As a result, the connective tissue capsule of the intrafusal fiber is stretched and proprioceptors are irritated. Therefore, pulsed electrical activity of motor neurons localized in the anterior horns of the spinal cord occurs. The discharge activity of these neurons is the direct cause of the rapid contraction of extrafusal muscle fibers.
Diagram of the reflex arc of the spinal tendon reflex
1) intrafusal muscle fiber, 2) proprioceptor, 3) afferent sensory neuron, 4) spinal cord motor neuron, 5) extrafusal muscle fibers.
The total time of the spinal tendon reflex is short, because its reflex arc is monosynaptic. It includes rapidly adapting receptors, phasic a-motoneurons, and FF and FR type motor units.
Reflex intersegmental function spinal cord is the implementation of intersegmental integration of spinal reflexes, which is provided by intraspinal pathways connecting different segments of the spinal cord.
Trophic function The function of the spinal cord comes down to the regulation of metabolism and the provision of nutrition to those organs and tissues that are innervated by the neurons of the spinal cord. It is associated with the non-pulse activity of neurons capable of synthesizing many trophotropic biologically active substances. These substances slowly move into the nerve endings, from where they are released into the surrounding tissue.
Conductor function The spinal cord is to provide two-way connections between the spinal cord and the brain. It is provided by its ascending and descending pathways - groups of nerve fibers.
There are three main groups of ascending pathways:
1) Goll and Burdakh,
2) spinothalamic,
3) spinocerebellar.
The paths of Gaulle and Burdach are conductors of skin-mechanical sensitivity from tactile receptors and proprioceptors to the sensory zones of the posterior central gyrus of the cerebral cortex. The Gaulle path carries information from the lower part of the body, and the Burdach path carries information from the upper part.
Spinothalamic tract is a conductor of tactile, temperature and pain sensitivity. This pathway ensures the transmission of information about the quality of the stimulus to the posterior central gyrus.
Spinocerebellar tracts carry information from tactile receptors, as well as proprioceptors of muscles, tendons and joints to the cerebellar cortex.
Descending pathways form pyramidal And extrapyramidal systems. Pyramid system includes pyramidal corticospinal tract. It is formed by the axons of large pyramidal neurons ( Betz cells), which are located in the motor (motor) zone of the precentral gyrus of the cerebral cortex.
In humans, the pyramidal tract has a direct triggering activating effect on spinal motor neurons innervating the flexor (flexor) muscles of the distal limbs. Thanks to this tract, voluntary conscious regulation of precise phasic movements is ensured.
Extrapyramidal system includes:
1) rubrospinal tract,
2) reticulospinal tract,
3) vestibulospinal tracts.
Rubrospinal tract formed by axons of neurons in the red nucleus of the midbrain, activating spinal flexor motor neurons. Reticulospinal tract is formed by the axons of neurons of the reticular formation of the hindbrain, which have both an activating and an inhibitory effect on flexor motor neurons. Vestibulospinal tracts are formed by axons of neurons of the vestibular nuclei of Deiters, Schwalbe and Bekhterev, which are located in the hindbrain. These pathways have an activating effect on spinal extensor motor neurons.
An animal whose spinal cord is separated from its brain is called spinal. Immediately after injury or separation of the spinal cord from the brain, spinal shock - a reaction of the body, which manifests itself in a sharp drop in excitability and inhibition of reflex activity or areflexia.
The main mechanisms of spinal shock (according to Sherrington) are:
1) elimination of descending activating influences entering the spinal cord from the upper parts of the central nervous system,
2) activation of intraspinal inhibitory processes.
There are two main factors that determine the severity and duration of spinal shock:
1) level of organization of the body (in a frog, spinal shock lasts 1-2 minutes, and in humans it lasts months and years),
2) level of spinal cord damage (the higher the level of damage, the more severe and prolonged the spinal shock).
The structure of reflex arcs of spinal reflexes. The role of sensory, intermediate and motor neurons. General principles of coordination of nerve centers at the level of the spinal cord. Types of spinal reflexes.
Reflex arcs- These are chains consisting of nerve cells.
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). Example the simplest reflex can serve knee reflex, which occurs in response to a short-term stretch of the quadriceps femoris muscle by a light blow to its tendon below the kneecap
(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 neuron body along the axon, the nerve impulse through the sensitive roots of the spinal nerves are sent to the spinal cord, where they form synapses with the bodies of effector neurons. At each interneuron synapse, with the help of biologically active substances (mediators), impulse transmission occurs. The axon of the effector neuron leaves the spinal cord as part of the anterior roots of the spinal nerves (motor or secretory nerve fibers) and is directed to the working organ, causing muscle contraction and increased (inhibited) gland secretion.)
More complex reflex arcs have one or more interneurons.
(The body of the interneuron in three-neuron reflex arcs is located in the gray matter of the posterior columns (horns) of the spinal cord and is in contact with the axon of the sensory neuron that arrives as part of the posterior (sensitive) roots of the spinal nerves. The axons of the interneurons are directed to the anterior columns (horns), where the bodies are located effector cells. The axons of effector cells are directed to muscles, glands, influencing their function. The nervous system has many complex multi-neuron reflex arcs, which have several interneurons located in the gray matter of the spinal cord and brain.)
Intersegmental reflex connections. In the spinal cord, in addition to the reflex arcs described above, limited by one or several segments, ascending and descending intersegmental reflex pathways operate. The interneurons in them are the so-called propriospinal neurons , the bodies of which are located in the gray matter of the spinal cord, and the axons ascend or descend at various distances in the composition propriospinal tracts white matter, never leaving the spinal cord.
Intersegmental reflexes and these programs facilitate the coordination of movements initiated at different levels of the spinal cord, particularly the forelimbs, hindlimbs, limbs, and neck.
Types of neurons.
Sensory (sensitive) neurons receive and transmit impulses from receptors “to the center”, i.e. central nervous system. That is, through them the signals go from the periphery to the center.
Motor (motor) neurons. They carry signals coming from the brain or spinal cord to the executive organs, which are muscles, glands, etc. in this case, the signals go from the center to the periphery.
Well, intermediate (intercalary) neurons receive signals from sensory neurons and send these impulses further to other intermediate neurons, or directly to motor neurons.
Principles of coordination activity of the central nervous system.
Coordination is ensured by selective excitation of some centers and inhibition of others. Coordination is the unification of the reflex activity of the central nervous system into a single whole, which ensures the implementation of all functions of the body. The following basic principles of coordination are distinguished:
1. The principle of irradiation of excitations. Neurons of different centers are interconnected by interneurons, so impulses arriving during strong and prolonged stimulation of receptors can cause excitation not only of the neurons of the center of a given reflex, but also of other neurons. For example, if you irritate one of the hind legs of a spinal frog, it contracts (defensive reflex); if the irritation is increased, then both hind legs and even the front legs contract.
2. The principle of a common final path. Impulses arriving in the central nervous system through different afferent fibers can converge to the same intercalary, or efferent, neurons. Sherrington called this phenomenon the “common final path principle.”
For example, motor neurons that innervate the respiratory muscles are involved in sneezing, coughing, etc. On the motor neurons of the anterior horns of the spinal cord, innervating the muscles of the limb, fibers of the pyramidal tract, extrapyramidal tracts, from the cerebellum, reticular formation and other structures end. The motor neuron, which provides various reflex reactions, is considered as their common final path.
3. The principle of dominance. It was discovered by A.A. Ukhtomsky, who discovered that irritation of the afferent nerve (or cortical center), which usually leads to contraction of the muscles of the limbs when the animal's intestines are full, causes an act of defecation. In this situation, the reflex excitation of the defecation center suppresses and inhibits the motor centers, and the defecation center begins to react to signals that are foreign to it. A.A. Ukhtomsky believed that at every given moment of life a defining (dominant) focus of excitation arises, subordinating the activity of the entire nervous system and determining the nature of the adaptive reaction. Excitations from various areas of the central nervous system converge to the dominant focus, and the ability of other centers to respond to signals coming to them is inhibited. Under natural conditions of existence, dominant excitation can cover entire systems of reflexes, resulting in food, defensive, sexual and other forms of activity. The dominant excitation center has a number of properties:
1) its neurons are characterized by high excitability, which promotes the convergence of excitations from other centers to them;
2) its neurons are able to summarize incoming excitations;
3) excitement is characterized by persistence and inertia, i.e. the ability to persist even when the stimulus that caused the formation of the dominant has ceased to act.
4. Feedback principle. The processes occurring in the central nervous system cannot be coordinated if there is no feedback, i.e. data on the results of function management. The connection between a system's output and its input with a positive gain is called positive feedback, and with a negative gain is called negative feedback. Positive feedback is mainly characteristic of pathological situations.
Negative feedback ensures the stability of the system (its ability to return to its original state). There are fast (nervous) and slow (humoral) feedbacks. Feedback mechanisms ensure the maintenance of all homeostasis constants.
5. The principle of reciprocity. It reflects the nature of the relationship between the centers responsible for the implementation of opposite functions (inhalation and exhalation, flexion and extension of the limbs), and lies in the fact that the neurons of one center, when excited, inhibit the neurons of the other and vice versa.
6. The principle of subordination(subordination). The main trend in the evolution of the nervous system is manifested in the concentration of the main functions in the higher parts of the central nervous system - cephalization of the functions of the nervous system. There are hierarchical relationships in the central nervous system - the highest center of regulation is the cerebral cortex, the basal ganglia, middle, medulla and spinal cord obey its commands.
7. Function compensation principle. The central nervous system has a huge compensatory capacity, i.e. can restore some functions even after the destruction of a significant part of the neurons that form the nerve center. If individual centers are damaged, their functions can transfer to other brain structures, which is carried out with the obligatory participation of the cerebral cortex.
Types of spinal reflexes.
Ch. Sherrington (1906) established the basic patterns of his reflex activity and identified the main types of reflexes he performs.
Actually muscle reflexes (tonic reflexes) occur when the stretch receptors of muscle fibers and tendon receptors are irritated. They manifest themselves in prolonged muscle tension when they are stretched.
Defensive reflexes are represented by a large group of flexion reflexes that protect the body from the damaging effects of excessively strong and life-threatening stimuli.
Rhythmic reflexes manifest themselves in the correct alternation of opposite movements (flexion and extension), combined with tonic contraction of certain muscle groups (motor reactions of scratching and stepping).
Position reflexes (postural) are aimed at long-term maintenance of contraction of muscle groups that give the body posture and position in space.
The consequence of a transverse section between the medulla oblongata and the spinal cord is spinal shock. It is manifested by a sharp drop in excitability and inhibition of the reflex functions of all nerve centers located below the site of transection
Spinal cord. The spinal canal contains the spinal cord, which is conventionally divided into five sections: cervical, thoracic, lumbar, sacral and coccygeal.
31 pairs of spinal nerve roots arise from the SC. The SM has a segmental structure. A segment is considered to be a segment of CM corresponding to two pairs of roots. There are 8 segments in the cervical part, 12 in the thoracic part, 5 in the lumbar part, 5 in the sacral part, and from one to three in the coccygeal part.
The central part of the spinal cord contains gray matter. When cut, it looks like a butterfly or the letter H. The gray matter consists mainly of nerve cells and forms protrusions - the posterior, anterior and lateral horns. The anterior horns contain effector cells (motoneurons), the axons of which innervate skeletal muscles; in the lateral horns there are neurons of the autonomic nervous system.
Surrounding the gray matter is the white matter of the spinal cord. It is formed by nerve fibers of the ascending and descending tracts that connect different parts of the spinal cord with each other, as well as the spinal cord with the brain.
The white matter consists of 3 types of nerve fibers:
Motor - descending
Sensitive - ascending
Commissural - connects the 2 halves of the brain.
All spinal nerves are mixed, because formed from the fusion of the sensory (posterior) and motor (anterior) roots. On the sensory root, before its merger with the motor root, there is a spinal ganglion, in which there are sensory neurons, the dendrites of which come from the periphery, and the axon enters through the dorsal roots into the SC. The anterior root is formed by axons of motor neurons of the anterior horns of the SC.
Functions of the spinal cord:
1. Reflex – consists in the fact that reflex arcs of motor and autonomic reflexes are closed at different levels of the SC.
2. Conductive – ascending and descending pathways pass through the spinal cord, which connect all parts of the spinal cord and brain:
Ascending, or sensory, pathways pass in the posterior cord from tactile, temperature receptors, proprioceptors and pain receptors to various parts of the spinal cord, the cerebellum, the brain stem, and the CGM;
Descending pathways that run in the lateral and anterior cords connect the cortex, brainstem, and cerebellum with motor neurons of the SC.
Reflex is the body's response to an irritant. The set of formations necessary for the implementation of the reflex is called a reflex arc. Any reflex arc consists of afferent, central and efferent parts.
Structural and functional elements of the somatic reflex arc:
Receptors are specialized formations that perceive the energy of stimulation and transform it into the energy of nervous excitation.
Afferent neurons, the processes of which connect receptors with nerve centers, provide centripetal conduction of excitation.
Nerve centers are a collection of nerve cells located at different levels of the central nervous system and involved in the implementation of a certain type of reflex. Depending on the level of location of the nerve centers, reflexes are distinguished: spinal (nerve centers are located in segments of the spinal cord), bulbar (in the medulla oblongata), mesencephalic (in the structures of the midbrain), diencephalic (in the structures of the diencephalon), cortical (in various areas of the cerebral cortex). brain).
Efferent neurons are nerve cells from which excitation spreads centrifugally from the central nervous system to the periphery, to the working organs.
Effectors, or executive organs, are muscles, glands, and internal organs involved in reflex activity.
Types of spinal reflexes.
Most motor reflexes are carried out with the participation of spinal cord motor neurons.
Muscle reflexes themselves (tonic reflexes) occur when stretch receptors in muscle fibers and tendon receptors are stimulated. They manifest themselves in prolonged muscle tension when they are stretched.
Protective reflexes are represented by a large group of flexion reflexes that protect the body from the damaging effects of excessively strong and life-threatening stimuli.
Rhythmic reflexes are manifested in the correct alternation of opposite movements (flexion and extension), combined with tonic contraction of certain muscle groups (motor reactions of scratching and stepping).
Positional reflexes (postural) are aimed at long-term maintenance of contraction of muscle groups that give the body posture and position in space.
The consequence of a transverse section between the medulla oblongata and the spinal cord is spinal shock. It is manifested by a sharp drop in excitability and inhibition of the reflex functions of all nerve centers located below the site of transection.
