When muscle spindle length changes suddenly, the primary ending (but not the secondary) is stimulated especially powerfully. This excessive excitation of the primary ending is called dynamic response, which means an extremely active reaction of the primary ending to a high rate of change in the length of the spindle. Even when the length of the spindle increases by only a fraction of a micrometer, and this increase occurs within a fraction of a second, the primary receptor transmits an enormous amount of additional impulses along large sensory nerve fibers with a diameter of 17 microns, but only as long as the length actually increases. Once the length increase stops, this extra surge of pulsed discharge returns to a much lower level than the static discharge still present in the response.

Vice versa, when shortening the spindle the opposite change occurs. Thus, the primary ending sends extremely strong, positive or negative, signals to the spinal cord, informing it of any change in the length of the muscle spindle.

Regulation of the intensity of static and dynamic responses by gamma motor neurons. The gamma motor nerves to the muscle spindle can be divided into two types: gamma dynamic (gamma-d) and gamma static (gamma-s). The first of them excite mainly intrafusal fibers with a nuclear bag, and the second excite mainly intrafusal fibers with a nuclear chain. When gamma-d fibers excite nuclear bag fibers, the dynamic response of the muscle spindle becomes extremely enhanced, while the static response remains almost unchanged.

Vice versa, gamma-s stimulation fibers that excite muscle fibers with a nuclear chain enhances the static response, with only a minor effect on the dynamic response.

Continuous discharge of muscle spindles under normal conditions. Normally, especially against the background of some degree of excitation of gamma-efferent fibers, in sensory nerve fibers muscle spindles constantly generate impulses. Stretching the muscle spindles increases the frequency of impulses, while shortening the spindles reduces it. Thus, the spindles can send positive signals to the spinal cord, i.e. an increased number of impulses, indicating a muscle stretch, or negative signals, i.e. the number of pulses is below normal, indicating that the muscle is not stretched.

Muscle stretch reflex

The simplest manifestation muscle spindle functions is a muscle stretch reflex. Whenever a muscle is suddenly stretched, the excitation of the spindles causes a reflex contraction of the large muscle fibers stretched muscle and closely related synergistic muscles.

Neural circuit of the stretch reflex. The figure shows the main contour of the muscle spindle stretch reflex. It can be seen that the type 1a proprioceptive nerve fiber emanating from the muscle spindle enters the posterior root spinal cord. Then a branch of this fiber goes directly to the anterior horn of the gray matter of the spinal cord and synaptically connects with the anterior motor neurons, which send motor nerve fibers to the same muscle from which the muscle spindle fibers originate. Thus, there is a monosynaptic pathway that allows the reflex signal to return with the shortest delay back to the muscle after spindle excitation. Most type II fibers from muscle spindles terminate in many interneurons of the gray matter, and their axons conduct signals to the anterior motor neurons with a delay or perform other functions.

Stretch reflex - a system for regulating muscle length

An isolated muscle preparation has elasticity, that is, it stretches when a force is applied to it (Fig. 15.3). The relationship between mouse tension Τ and its length L is described by the well-known curve of dependence of length on voltage in calm state(see ch. 4).

The same experiment can be done with a muscle in situ (in a living organism). For this, it is especially convenient to use the extensor muscle of a decerebrated animal. In conditions where the brainstem is cut at the level of the midbrain and there is no connection with the brain (see Chapter 5), this

the muscle resists the applied external force more - it becomes less pliable (more rigid). In this situation, an increase in stress ΔΤ causes a much smaller increase in length, AL r , than in the case isolated muscle(Fig. 15.3). If the dorsal or ventral roots of the spinal segment, from which this muscle is innervated, are cut, then this increased resistance disappears, and the curve of dependence of the length of the muscle at rest on the external load becomes similar to the same curve for an isolated muscle.

When the nerve fibers of the spinal cord are intact, the spinal cord-muscle system apparently acts in such a way as to counteract muscle lengthening by reflex contraction (reflex tone). One could also say that the length of the muscle is kept approximately constant, so that the stretch reflex is reduced to a system of regulation of the length of the muscle. The elements of this control scheme are as follows (Fig. 15.2):

336 PART IV. PROCESSES OF NERVOUS AND HUMORAL REGULATION

Try to make the same list for other control systems (for example, to regulate body temperature, arterial blood pressure, respiratory system), i.e. try to find anatomical and physiological equivalents for each of these technical terms. In doing this, keep in mind that there are several different effectors within most biological regulatory systems.

Functional analysis of the control system. In order to measure the transfer characteristics of individual components in a circuit, open the circuit at some point to eliminate feedback. The stretch reflex can be studied in the absence of feedback, if you cut the dorsal or ventral roots or temporarily block the conduction of nerve fibers by cooling them.

The transfer characteristics of the sensor, controller and effector are measured in open circuit. The dynamic properties of the control circuit and its elements, i.e., their behavior during a change in the controlled variable under a perturbing action and immediately after it, can be determined by the reaction to a step action. The corresponding procedure is discussed in the next section. The stationary characteristics of a given circuit and its components are described by characteristic curves, or functions, each of which reflects the relationship between an input variable and an output variable. In the case of the stretch reflex, the characteristic curve for sensor function relates the length of the muscle L to the discharge frequency in the afferent fibers 1a that come from the muscle spindle, F ia . The characteristic curve for the effector shows a gradual increase in the force of contraction of the extrafusal muscles with an increase in the frequency of F a discharges in the Aa-axon of the motor neuron. Sequential combination of characteristic curves for individual circuit elements allows you to get general characteristics control, i.e. the relationship between muscle length and strength muscle contraction(see, for example, the red curve in Figure 15.3). The smaller the slope of the control characteristic, the closer to

a constant value is the length of the muscle and, therefore, the more accurate the regulation.

Before proceeding to the further presentation, it is necessary to determine at a qualitative level the polarity of the control circuit: when transmitted through the sensor (muscle spindle), changes are carried out in the same direction, i.e., an increase in the length L causes an increase in the frequency of discharges F Ia. The same is true for transmission through the controller (α-motor neuron) when converting the frequency F la into the frequency F a of the discharges of α-motor neurons. However, when transmitted through the effector (extrafusal muscles), the changes occur in opposite directions: an increase in Fn causes a decrease in L. This is where the sign change occurs, which is necessary for the formation of negative feedback in the control system.

Each skeletal muscle responds to a rapid stretch with a contraction. The arc of this reflex is simplified. It consists of two links: afferent and efferent. As mentioned above, in muscles and tendons there are proprioceptors excited by stretching. This excitation through the cell of the intervertebral node along the radicular fiber, which enters the spinal cord as part of the posterior columns, is transmitted through reflex collaterals to the cells of the anterior horns of the spinal cord. Excitation simultaneously reaches many peripheral motor neurons, however, that muscle, in the receptor of which the excitation began, is excited to the maximum extent. Weaker excitation is transmitted to neighboring peripheral motor neurons, increasing their tonic tension. However, this irritation of excitation in the neighborhood does not proceed evenly, but mainly through those neurons that make up one or another functional system, for example, a step reflex. As you know, the functional system of the step reflex consists of successive phases: flexion in three joints - the hip, knee and ankle, and then extension in them.

Therefore, if you cause excitation in the quadriceps muscle, as is done when evoking a knee jerk, then simultaneously with its contraction (extension of the lower leg), the tone in the extensors of the thigh and foot increases.

Each muscle responds to stretch, but the sensitivity of a particular muscle to stretch is not the same. It is determined both by the structure of the joints and the physiological load on different muscles. In humans, all the weight in an upright position falls on the extensors of the legs. The preservation of the vertical position depends on their resistance to the action of gravity, therefore the myotatic reflex is most pronounced in relation to the patellar tendon and the Achilles tendon. In order to judge the severity of myotatic reflexes on the legs, knee reflexes and tendon reflexes are examined.

On the hands, which in practice are most loaded with flexion and extension movements in elbow joint, while shoulder joint more is in a fixed state due to the simultaneous contraction of antagonistically acting muscles, tendon reflexes from the tendons of the biceps and triceps are higher.

Examination of tendon reflexes. To study tendon reflexes, they do not use muscle stretching, which would correspond to a more natural stimulus, but mechanical irritation, striking the tendon. In this case, the tendon is brought into a state of moderate stretching. Reflexes are examined in various ways.

Biceps reflex, flexion-elbow reflex. The most convenient and reliable is the following method of evoking this reflex. The examiner puts the arm of the examinee, bent at an obtuse angle in the elbow joint, on his left forearm and, covering the elbow joint, puts the thumb of his left hand on the tendon of the biceps muscle. With a short blow hammer on the nail phalanx thumb cause contraction of the biceps brachii and rapid flexion of the forearm. This method of evoking a reflex is convenient in that the reflex tension of the tendon is felt even if the reflex is reduced and the movement of the hand is imperceptible to the eye.

Triceps reflex, extensor-elbow reflex. This reflex is elicited by a direct hammer blow on the short triceps tendon at its very point of attachment to the olecranon. The subject's arm should be bent at the elbow joint at an obtuse angle. The response is contraction of the triceps muscle and extension of the forearm.

Periosteal or carporadial reflex caused by hammer blow on the styloid process of the radius. In this case, there is a reduction in m. brachio-radialis and flexion of the forearm. If the blow is delivered in the pronation position, simultaneously with the flexion of the forearm, a supination movement occurs. If this is done in the supination position, a pronation movement is added.

Of the other periosteal reflexes that are important for practice, one can name scapular-brachial Bechterew's reflex. It is caused by a hammer blow on the inner edge of the scapula. In response, the shoulder is adducted and rotated outward.

Patella reflex, patellar reflex. To induce a knee jerk, different techniques are used. The following is most consistent with physiological relationships. The patient sits on the edge of a chair or bed, legs forward so that the thigh and lower leg form an obtuse angle. A blow of the hammer on the tendon of the quadriceps muscle below the kneecap causes the extension of the lower leg. If at the same time there is no obvious motor effect, the subject is offered, without changing the initial position, to press the toes on the floor. At the same time, the leg extensors are tense in three joints, including the knee joint, which helps to identify the reflex.

In the supine position, the knee jerk is best examined as follows. The patient lies on his back. The researcher brings his left hand under the knee fossae, slightly bending the patient's legs in the knee joints. In this position, they hit the tendon with a hammer. You can cause a reflex by striking with a hammer on your own finger, which gropes for the tendon of the quadriceps muscle.

When the knee jerk is elevated, it can be induced not only from the quadriceps tendon, but also from the tibia. Blows on it cause a weaker response movement, the greater the distance from the knee joint they are applied. Thus, the expansion of the reflexogenic zone of the knee jerk is judged.

Achilles reflex. To study the reflex from the Achilles tendon, the most convenient is the following: the patient is asked to kneel on a chair so that the feet hang freely over the edge of the chair. Striking a slightly stretched tendon, plantar flexion of the foot is observed. In a patient lying on his stomach (if such a position is possible), bend his legs in knee joint at a right angle and, slightly pulling the Achilles tendon, hit it with a hammer. This essentially reproduces the same position as in the study on the chair.

In the supine position on the back, it is less convenient to examine the reflex. However, by bending the leg at the knee joint and holding the foot by the fingers, it is possible to hit the tendon from below and cause a reflex.

Violation of tendon reflexes. The height of the tendon reflexes varies individually. There are people who fail to cause a single tendon reflex, and such areflexia is detected from childhood.

As a rule, the listed tendon reflexes are caused in healthy people. However, they have a normal reflex tone skeletal muscle. This tone, i.e., some resistance to muscle stretching, exists as a manifestation of a constantly acting myotatic reflex.

But the reflex arc of the myotatic reflex can be interrupted in one place or another, and then the tendon reflex disappears. If this break occurs in the afferent part of the arch, for example, in the posterior root, then tendon reflexes disappear and the reflex tone drops, some, and sometimes significant looseness of the joints occurs, and proprioceptive and other types of sensitivity are disturbed. If this break occurs in the efferent part of the arc, for example, when cells of the anterior horns of the spinal cord or anterior roots die, then simultaneously with the disappearance of tendon reflexes and a decrease in reflex tone, paralysis and muscle atrophy occur. If a break in the afferent and efferent parts of the reflex arc occurs simultaneously, as happens with damage to the mixed nerve, then along with the disappearance of the corresponding reflexes, muscle atrophy occurs and all types of sensitivity are disturbed in the areas of the affected nerves.

Tendon reflexes increase when the connection of the spinal reflex mechanisms with the brain is disrupted and the inhibitory influence falls out, primarily from the cerebral cortex through the pyramidal pathways. With an increase in tendon reflexes due to damage to the pyramidal tracts, their reflexogenic zone usually expands, and the reflex muscle tone increases. The selective distribution of muscle hypertension is characteristic: on the arms, the tone increases in the flexors and pronators of the forearm, the flexors of the hand and fingers, the muscles that lower the shoulder downwards and rotate it inward, on the legs - in the extensor and adductor muscles of the thigh, calf extensors, plantar flexors of the foot.

When the increase in tendon reflexes reaches a large degree, there is a so-called clonus of the foot and clonus of the patella.

Clonus is a repeated rhythmic contraction of a muscle in response to its stretching. Sometimes clonus of the foot can be observed when the patient, sitting on a chair or bed, presses the toes of the foot on the floor. This causes a sharp myotatic reflex, followed by some reverse movement due to the tension of the flexors, and as the patient continues to press the floor, the reflex from the Achilles tendon is repeated. The alternation of extensor and flexion movements of the foot (foot clonus) continues until the patient stops stepping on his toes and puts his foot on his heel. In this position, the patient can accelerate the detection of clonus of the foot by pressing jerkily on the knee and holding it in this position. Usually, to detect clonus of the foot in a patient lying on his back, with the left hand, brought under his knee, they bend the leg at the knee joint at an obtuse angle, and with a sharp right push, the foot is dorsiflexed, trying to keep it in this position.

Similarly to the clonus of the foot, sometimes it is possible to cause a clonus of the hand by making a dorsiflexion of it with an energetic push.

Clonus of the patella. With the patient lying on his back, with his left hand, brought under the knee, the leg is slightly bent at the knee joint. Thumb and forefinger right hand they grab the patella along its upper edge and jerk it in the distal direction, trying to keep it in this position. In this case, there are multiple contractions of the quadriceps muscle and the rhythmic movement of the patella.

An increase in tendon reflexes, as well as clonuses, can be observed in patients with functional diseases. nervous system- neuroses, asthenic conditions. However, unlike hyperreflexia and clonuses, which are caused by damage to the pyramidal tracts and are true "pyramidal signs", functional clonuses are characterized by inconstancy, rapid exhaustion, a general emotional reaction when they are called, and their uniform distribution on both sides. The presence or absence of other organic signs with them (pathological reflexes, changes in tone) makes it possible to more accurately determine their nature.

Simultaneously with an increase in tendon reflexes, the appearance of clonuses, an increase in reflex muscle tone when the spinal centers are isolated from the parts of the brain located above, a number of pathological reflexes arise. Read more about them in the article.

Stretch reflex in the clinic. Clonus of muscle spindles. In a clinical examination of a patient, the physician tests a variety of stretch reflexes to determine the degree of background or "tonic" excitation being transmitted from the brain to the spinal cord. This reflex is called as follows.

Knee and other muscle reflexes. In the clinic, excitation of the knee and other muscle reflexes is usually used to determine the sensitivity of stretch reflexes. To call a knee jerk, they hit the tendon of the patella with a neurological hammer; this instantly stretches the quadriceps femoris muscle, causing a dynamic stretch reflex, as a result of which lower limb jerks forward.

Similar reflexes can be obtained from almost any muscle in the body when struck either on their tendon or on the abdomen of the muscle itself. In other words, all that is required to elicit a dynamic stretch reflex is a sudden stretch of the muscle spindles. Neurologists use muscle reflexes to assess the degree of relief of the spinal centers. When a large number of facilitating impulses are transmitted from the upper regions of the central nervous system to the spinal cord, muscle reflexes are greatly enhanced. Conversely, if the facilitating impulses are suppressed or do not work, muscle reflexes are markedly weakened or absent.

These reflexes are more often used to determine the presence or absence of muscle spasticity in lesions of the motor areas of the brain or diseases that excite the bulboreticular facilitating area of ​​the brainstem. Usually, extensive lesions of the motor areas of the cerebral cortex, in contrast to the underlying motor regulatory areas (especially lesions associated with strokes or brain tumors), are accompanied by excessively enhanced muscle reflexes on the opposite side of the body.

Clonus - oscillation of muscle reflexes. Under certain conditions, muscle reflexes can oscillate. This phenomenon is called clonus. Oscillation is most easily explained using the example of ankle clonus. If a person standing on the tips of his toes (on tiptoe) suddenly stands on his entire foot, stretching the calf muscle, impulses from the muscle spindles are transmitted to the spinal cord. At the same time, the stretched muscle is reflexively excited and again raises the body. After a fraction of a second, the reflex contraction of the muscle stops, and the body "falls" on the foot again, thus stretching the spindles a second time. The dynamic stretch reflex lifts the body, but it also stops after a fraction of a second, and the body falls again, starting a new cycle. Often the calf stretch reflex continues to oscillate for a long time. This is the clonus.

Clonus usually develops only if the stretch reflex is strongly sensitized by facilitating impulses from the brain. For example, in a decerebrated animal, in which stretch reflexes are greatly facilitated, clonus develops easily. To determine the degree of relief of the spinal cord, neurologists test the patient for clonus by suddenly stretching the muscle and using a constant stretching force. If clonus develops, the degree of relief is undoubtedly high.

Let's consider some simple examples of the functioning of the motor analyzer with the participation of muscle spindles and Golgi receptors. In the formation of the myotatic reflex, or the reflex to muscle stretch (Fig. 15.5), afferent neurons take part, forming per-

Rice. 15.5.

A In the initial "given" state, a load of small mass (/) is held by the extrafusal fibers of the muscle B of the nerve fibers that form the afferent endings. only rare spontaneous action potentials are recorded. B.With an increase in the weight of the load (2 > 1), the muscle with muscle spindles is stretched. In afferent fibers, the frequency of action potentials increases, which enter through synaptic contacts on a-motoneurons (shown by an arrow in the direction from the muscle spindle) and excite them. From a-motoneurons, action potentials are directed to extrafusal muscle fibers (arrows towards the muscle) and through synaptic contacts cause muscle contraction.IN. Muscle contraction did not occur to a predetermined length. The elimination of the "mistake" is carried out with the help of fusimotor gamma neurons, which form motor endings on the intrafusal muscle fibers of the spindles. G.The muscle returns to the target length

primary afferent endings, and osmotoneurons, which provide motor innervation of extrafusal muscle fibers. When a muscle is stretched, muscle spindles are also stretched, which is accompanied by an increase in the frequency of action potentials in afferent fibers. Since afferent neurons are synaptically connected in the CNS with a-motor neurons, the frequency of action potentials also increases in the latter. Spreading along efferent fibers, action potentials through synaptic endings cause contraction - shortening of the length of the muscle. Reducing the stretching effect on the intrafusal fibers reduces the frequency of action potentials in the afferent nerve fibers, and the system returns to a state close to the original. However, this system does not provide a complete restoration of the original length. The remaining small difference between the original length of the muscle (before stretching) and the length after reflex contraction (called an error) cannot be determined by the system. This would require a feedback link, i.e., a motor neuron with an unlimitedly high sensitivity. The so-called fusimotor system, which includes intrafusal muscle fibers and fusimotor (y) motoneurons, which form motor synapses on intrafusal muscle fibers, contributes to the return of muscles to the original "given" length. Activation of this system by action potentials from the motor centers of the analyzer causes contraction of the end sections of the spindle and thereby stretching of the central non-contracting section where the afferent primary endings are located. This will lead to an additional increase in the frequency of action potentials in the afferent neuron, which will be perceived by the α-motor neuron, followed by sending efferent action potentials to the synaptic endings of the extrafusal fibers. As a result of this, an additional contraction will occur in the muscle and the original length will be reached.

From the foregoing, it becomes clear that the myotatic reflex serves to maintain a constant length of the muscle with changes in the load acting on it. This mechanism in animals, as, apparently, in humans, is carried out without the control of consciousness and plays a decisive role in maintaining posture. The extensor muscles responsible for the position of the body in space must have a certain predetermined length and, in contrast to gravity, keep the limbs of the animal in a straightened state.

Golgi tendon receptors are connected to muscle fibers in series (not in parallel like muscle spindles), so they should respond to changes in muscle tension, not length. It was found that through inhibitory interneurons, afferent impulses from the Golgi receptors affect a-motoneurons, reducing their level of activity. This, for example, can manifest itself in a decrease in the frequency of action potentials sent to the synapses of extrafusal muscle fibers, which prevents excessive muscle tension. It is also assumed that signaling by tendon receptors about muscle tension to α-motor neurons contributes to the correction of inaccuracies in the regulation of muscle length by myotatic reflexes.