Extensor reflex. Crossed extensor reflex

Approximately 0.2-0.5 seconds after the stimulus stimulates reflex flexion in one limb, the opposite limb begins to extend. This is called the crossed extensor reflex. Extension of the contralateral limb may push the entire body away from the object causing the painful stimulus in the withdrawn limb.

Nervous mechanism crossed extensor reflex. The right side of the figure shows the neural circuit responsible for the crossed extensor reflex, demonstrating that signals from sensory nerves travel to the opposite side of the spinal cord to excite the extensor muscles. Because the cross-extensor reflex typically does not begin until 200 to 500 ms after the onset of the noxious stimulus, many interneurons are recruited into the circuit between the primary sensory neuron and the motor neurons on the contralateral side of the spinal cord responsible for cross-extension.

After removal of the painful stimulus crossed extensor reflex has an even longer aftereffect than the flexion reflex. This long-lasting aftereffect is believed to result from the function of reverberant circuits among the interneurons.

The picture shows a typical myogram, recorded from the muscle involved in the crossed extensor reflex. The myogram demonstrates a relatively long latency period before the onset of the reflex and a long aftereffect after the end of the stimulus. A long aftereffect is useful in keeping the painfully stimulated area of ​​the body at a distance from the pathogenic agent until other nervous reactions lead to the removal of the entire body from the irritant.

Reciprocal inhibition and reciprocal innervation

In previous sections emphasized several times that excitation of one muscle group is often accompanied by inhibition of another muscle group. For example, when the stretch reflex excites one muscle, the antagonist muscle is often simultaneously inhibited. This is a phenomenon of reciprocal inhibition; the neural circuit that provides this reciprocal connection is called reciprocal innervation. Similar reciprocal connections often exist between muscles on two sides of the body, such as the flexor and extensor muscle reflexes outlined earlier.

The picture shows a typical example of reciprocal inhibition. In this case, a moderate but prolonged flexion reflex is excited in one limb of the body; Against the background of this reflex, a stronger flexion reflex is excited in the limb on the other side of the body. This stronger reflex sends reciprocal inhibitory signals to the first limb and reduces the degree of flexion. Finally, removing the stronger reflex allows the primary reflex to regain its previous intensity.

Limb reflexes. This group of reflexes is studied most often in clinical practice.

▓ Flexion reflexes. Flexion reflexes are divided into phasic and tonic.

▒ ^ Phasic reflexes- this is a single flexion of a limb with a single irritation of the skin or proprioceptors. Simultaneously with the excitation of the motor neurons of the flexor muscles, reciprocal inhibition of the motor neurons of the extensor muscles occurs. Reflexes arising from skin receptors are polysynaptic and have a protective value. Reflexes arising from proprioceptors can be monosynaptic and polysynaptic. Phasic reflexes from proprioceptors are involved in the formation of the act of walking. Based on the severity of phasic flexion and extension reflexes, the state of excitability of the central nervous system and its possible disorders are determined.

The following flexion phase reflexes are examined in the clinic: elbow and Achilles (proprioceptive reflexes) and plantar reflex (cutaneous). The elbow reflex is expressed in the flexion of the arm at the elbow joint and occurs when the m. tendon is struck with a reflex hammer. biceps brachii (when invoking the reflex, the arm should be slightly bent at the elbow joint), its arc closes in the 5-6th cervical segments of the spinal cord (C5 - C6). The Achilles reflex is expressed in plantar flexion of the foot as a result of contraction of the triceps muscle of the leg; it occurs when the Achilles tendon is struck with a hammer; the reflex arc closes at the level of the sacral segments (S1 - S2). Plantar reflex - flexion of the foot and toes with stroke stimulation of the sole, the reflex arc closes at the level of S1 - S2.

▒ ^ Tonic flexion, as well as extensor reflexes occur during prolonged muscle stretching; their main purpose is to maintain 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.

▓ ^ Extensor reflexes, like flexion, are phasic and tonic, arise from the proprioceptors of the extensor muscles, and are monosynaptic. Simultaneously with the flexion reflex, a cross-extensor reflex of the other limb occurs.

▒ ^ Phasic reflexes occur in response to a single irritation of muscle receptors. For example, when the quadriceps tendon is struck below the kneecap, a knee extensor reflex occurs due to contraction of the quadriceps femoris muscle. During the extensor reflex, the motor neurons of the flexor muscles are inhibited by Renshaw intercalary inhibitory cells (reciprocal inhibition). The reflex arc of the knee reflex closes in the second - fourth lumbar segments (L2 - L4). Phasic extensor reflexes are involved in the formation of walking.

▒ ^ Tonic extensor reflexes represent a prolonged contraction of the extensor muscles during prolonged stretching of the tendons. Their role is to maintain the pose. In a standing position, tonic contraction of the extensor muscles prevents flexion of the lower limbs and ensures maintaining an upright position. Tonic contraction of the back muscles ensures human posture. Tonic stretch reflexes of muscles (flexors and extensors) are also called myotatic.

▓ ^ Posture reflexes- redistribution of muscle tone that occurs when the position of the body or its individual parts changes. Posture reflexes are carried out with the participation of various parts of the central nervous system. At the level of the spinal cord, cervical posture reflexes are closed. There are two groups of these reflexes - those that occur when tilting and when turning the head.

▒ ^ First group of cervical postural reflexes exists only in animals and occurs when the head is tilted down (anteriorly). 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 natural conditions, 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. 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.

▓ Rhythmic reflexes - repeated repeated flexion and extension of the limbs. Examples include the scratching and stepping reflexes.

Pathological reflexes appear when the pyramidal tract is damaged, when spinal automatisms are disrupted. Pathological reflexes, depending on the reflex response, are divided into extension and flexion.

Extensor pathological reflexes in the lower extremities. The most important is the Babinski reflex - extension of the first toe when the skin of the outer edge of the sole is irritated by strokes; in children under 2–2.5 years old it is a physiological reflex. Oppenheim reflex - extension of the first toe in response to running the fingers along the crest of the tibia down to the ankle joint. Gordon's reflex - slow extension of the first toe and fan-shaped divergence of the other toes when the calf muscles are compressed. Schaefer reflex - extension of the first toe when the heel tendon is compressed.

Flexion pathological reflexes in the lower extremities. The most important reflex is the Rossolimo reflex - flexion of the toes during a quick tangential blow to the pads of the toes. Ankylosing spondylitis-Mendelian reflex - flexion of the toes when struck with a hammer on its dorsal surface. The Zhukovsky reflex is the flexion of the toes when a hammer hits the plantar surface directly under the toes. Ankylosing spondylitis reflex - flexion of the toes when hitting the plantar surface of the heel with a hammer. It should be borne in mind that the Babinski reflex appears with acute damage to the pyramidal system, for example with hemiplegia in the case of cerebral stroke, and the Rossolimo reflex is a later manifestation of spastic paralysis or paresis.

Flexion pathological reflexes in the upper limbs. The Tremner reflex is the flexion of the fingers in response to rapid tangential stimulation with the fingers examining the palmar surface of the terminal phalanges of the patient’s II-IV fingers. The Jacobson-Weasel reflex is a combined flexion of the forearm and fingers in response to a blow with a hammer on the styloid process of the radius. The Zhukovsky reflex is the flexion of the fingers of the hand when hitting the palmar surface with a hammer. Carpal-digital ankylosing spondylitis reflex - flexion of the fingers when percussing the back of the hand with a hammer.

Pathological protective, or spinal automatism, reflexes on the upper and lower extremities - involuntary shortening or lengthening of a paralyzed limb during an injection, pinching, cooling with ether or proprioceptive stimulation according to the Bekhterev-Marie-Foy method, when the examiner performs a sharp active flexion of the toes. Protective reflexes are often of a flexion nature (involuntary flexion of the leg at the ankle, knee and hip joints). The extensor protective reflex is characterized by involuntary extension of the leg at the hip and knee joints and plantar flexion of the foot. Cross protective reflexes - flexion of the irritated leg and extension of the other - are usually observed with combined damage to the pyramidal and extrapyramidal tracts, mainly at the level of the spinal cord. When describing protective reflexes, the form of the reflex response, the reflexogenic zone, is noted. the area of ​​evocation of the reflex and the intensity of the stimulus.

Cervical tonic reflexes occur in response to irritations associated with changes in the position of the head in relation to the body.

Magnus-Klein reflex - when the head is turned, the extensor tone in the muscles of the arm and leg, towards which the head is turned with the chin, increases, and the flexor tone in the muscles of the opposite limbs; flexion of the head causes an increase in flexor tone, and extension of the head - extensor tone in the muscles of the limbs.

Gordon's reflex - holding the lower leg in the extension position while inducing the knee reflex. Foot phenomenon (Westphalian) – “freezing” of the foot during passive dorsiflexion. The Foix-Thevenard tibia phenomenon is incomplete extension of the tibia at the knee joint in a patient lying on his stomach after the tibia has been held in extreme flexion for some time; manifestation of extrapyramidal rigidity.

Janiszewski's grasp reflex on the upper extremities - involuntary grasping of objects in contact with the palm; on the lower extremities - increased flexion of the fingers and toes when moving or other irritation of the sole. The distant grasping reflex is an attempt to grasp an object shown at a distance. It is observed with damage to the frontal lobe.

An expression of a sharp increase in tendon reflexes is clonus, which manifests itself as a series of rapid rhythmic contractions of a muscle or group of muscles in response to their stretching. Foot clonus is caused by the patient lying on his back. The examiner bends the patient's leg at the hip and knee joints, holds it with one hand, and with the other grabs the foot and, after maximum plantar flexion, jerks the foot into dorsiflexion. In response, rhythmic clonic movements of the foot occur while the heel tendon is stretched. Clonus of the patella is caused by a patient lying on his back with straightened legs: fingers I and II grasp the apex of the patella, pull it up, then sharply shift it in the distal direction and hold it in this position; in response, there is a series of rhythmic contractions and relaxations of the quadriceps femoris muscle and twitching of the patella.

Synkinesis is a reflex friendly movement of a limb or other part of the body, accompanying the voluntary movement of another limb (part of the body). Pathological synkinesis is divided into global, imitation and coordinator.

Global, or spastic, is called pathological synkinesis in the form of increased flexion contracture in a paralyzed arm and extension contracture in a paralyzed leg when trying to move paralyzed limbs or during active movements with healthy limbs, tension in the muscles of the trunk and neck, when coughing or sneezing. Imitative synkinesis is the involuntary repetition by paralyzed limbs of voluntary movements of healthy limbs on the other side of the body. Coordinator synkinesis manifests itself in the form of additional movements performed by paretic limbs in the process of a complex, purposeful motor act.

The movement of a limb in any joint requires the coordinated activity of various muscles acting on this joint. The contraction of one muscle group is coordinated with their relaxation antagonists(muscles with opposite action), which eliminates the opposition of mutually antagonistic muscle groups to each other.

Consider the activity of two muscles, A And IN, causing opposite movements of the limb relative to the joint (Fig. 8–36). When a muscle A stretched, its 1a afferents reflexively activate alpha motor neurons, causing its contraction. At the same time, the branches of afferents 1a of this muscle also activate inhibitory interneurons, the processes of which end on the alpha motor neurons of the muscle IN. Thus, a muscle strain A, causing its reflex contraction, simultaneously leads to relaxation of the antagonist muscle. Conversely, a muscle strain IN causes a myotatic reflex in it and reciprocally inhibits the muscle stretch reflex A. If such reciprocal inhibition did not exist, stretching of one muscle under the influence of contraction of its antagonist and the concomitant activation of its 1a afferents would cause an opposing reflex contraction of the stretched muscle.

In vertebrates, neuronal inhibitory circuits also play an important role in the muscular coordination of movements of different limbs (Fig. 8–37). This is especially pronounced in decerebrate animals. Decerebration(transection of the brain stem above the respiratory centers of the medulla oblongata, severing the connections between the forebrain and the spinal cord) leads to strengthening of spinal reflexes, since their inhibition by the brain ceases. Painful irritation of the paw A leads to its reflex withdrawal (flexion). Such flexion reflex is accompanied by inhibition of motor neurons innervating antagonistic muscles of the same paw, and, in addition, reflex extension of the contralateral limb. This reflex, called crossed extensor reflex arises as a result of the fact that simultaneously with the excitation of “flexor” motor neurons and inhibition of “extensor” neurons innervating the paw A, inhibition of “flexor” motor neurons and excitation of “extensor” neurons innervating the paw occur B(Fig. 8–37). It is obvious that the flexion and cross-extensor reflexes are adaptively interrelated. For example, if an animal accidentally steps on a sharp object with one paw, reflexively withdraws it, the opposite paw, due to the cross-extensor reflex, instantly straightens and takes on the entire weight of the body.

So far it has been emphasized functional significance of muscle spindles and Golgi tendon organs in the spinal regulation of motor function, but these two sensory organs also inform higher motor control centers about instantaneous changes occurring in the muscles. For example, the posterior spinocerebellar tracts transmit immediate information from both muscle spindles and Golgi tendon organs directly to the cerebellum at speeds reaching 120 m/sec, the fastest conduction speed of any part of the brain or spinal cord.

Additional paths carry the same information to the reticular areas of the brain stem and, to a lesser extent, directly to the motor areas of the cerebral cortex. Information from these receptors is critical for the feedback regulation of motor signals emanating from all these areas.

In spinal or decerebrate animal With almost any type of irritation of the skin of a limb, the flexor muscles contract, which leads to the withdrawal of this limb from the irritating object. This is called the flexion reflex.

Especially powerful flexion reflex in its classical form, it occurs when pain receptors are stimulated, for example by a pinprick, exposure to heat or injury, and is therefore also called the nociceptive reflex, or simply the pain reflex. Stimulation of touch receptors can also cause a flexion reflex, which is weaker and shorter lasting.

If painful effects if not a limb is affected, but some other part of the body, it will also withdraw from the stimulus, but the reflex may not be limited to the involvement of the flexor muscles, although the same type of reflex is based, therefore many manifestations of such reflexes in various areas of the body are called withdrawal reflexes.

Neural mechanism of the flexion reflex. The left side of the figure shows the neural pathways of the flexion reflex. In this case, the painful stimulus acts on the hand; As a result, the shoulder flexor muscles are excited, and when they contract, the hand is withdrawn from the painful stimulus.

Ways for excitation of the flexion reflex but do not go directly to the anterior motor neurons, but first approach the pool of interneurons of the spinal cord and only secondarily to the motor neurons. The shortest possible circuit is a three- or four-neuron path; however, most reflex signals pass through a larger number of neurons and involve the following main types of circuits: (1) divergent circuits, which promote the spread of the reflex to the muscles necessary for withdrawal; (2) circuits that inhibit antagonistic muscles, called reciprocal inhibition circuits (3) circuits to produce an aftereffect that lasts a fraction of a second after the stimulus ceases.

The figure shows a typical myogram, recorded from the flexor muscle during the flexion reflex. Within a few milliseconds of the start of stimulation of the pain nerve, a flexion reflex appears. Then, over the next few seconds, the reflex begins to tire, which is characteristic of essentially all complex integrative reflexes of the spinal cord. After the stimulus ends, the muscle contraction curve returns to the main line, but due to the aftereffect, this occurs after milliseconds. The duration of the aftereffect depends on the intensity of the sensory stimulus that caused the reflex; a weak tactile stimulus causes virtually no aftereffect, but in response to a strong painful stimulus, the aftereffect may last a second or more.

Aftereffect, which develops with the flexion reflex, is almost certainly related to the function of both types of circuits with a long discharge at the output. Electrophysiological studies show that the initial part of the aftereffect, lasting 6-8 ms, is the result of repeated pulse discharges of the excited interneurons themselves. In addition, the prolonged aftereffect after strong painful stimuli is undoubtedly associated with the inclusion of recurrent pathways, which initiate the repeated generation of impulses in the reverberant circuit of interneurons. They, in turn, conduct impulses to the anterior motor neurons, sometimes within a few seconds after the incoming sensory signal has ceased.

Thus, it is organized so as to withdraw the part of the body that is subject to pain or other irritation from the damaging stimulus. Moreover, due to the aftereffect, the reflex can keep the irritated part at a distance from the stimulus for 0.1-3 seconds after the cessation of its action. During this time, other reflexes and actions of the central nervous system can remove the entire body from the painful stimulus.

Structure of the withdrawal reflex. The set of muscles involved in withdrawal when the flexion reflex is stimulated depends on the sensory nerve stimulated. So, if a painful stimulus acts on the inside of the arm, not only the flexor muscles contract, but also the abductor muscles to pull the arm outward. In other words, the integrative centers of the spinal cord cause muscle contractions that can effectively remove the painful body part from the pain-causing object. This principle, called the principle of local sign, is applicable to any part of the body, but it is especially pronounced in the limbs due to their highly developed flexion reflexes.