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 a certain degree of excitation of gamma efferent fibers, impulses constantly arise in the sensory nerve fibers of the muscle spindles. 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 of the stretched muscle and the synergistic muscles closely associated with it.
Neural circuit of the stretch reflex. The figure shows the main contour of the muscle spindle stretch reflex. It can be seen that the proprioceptive nerve fiber type 1a, emanating from the muscle spindle, enters the posterior root of the 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.
Muscle spindles and Golgi tendon organs are involved in the implementation of stretch reflexes that occur in response to a sharp muscle stretch. At the same time, excitation of muscle receptors causes a reflex contraction of both this and synergistic muscles. On fig. 13–5 show reflex arcs of stretch reflexes, both monosynaptic (I) and polysynaptic (II).
Rice.13 –5 .reflexessprainsmonosynaptic(I, from muscle spindles, leads to contraction of the same muscle) Andpolysynaptic(II)
Monosynapticarc. I-proprioceptive nerve fibers extending from the muscle spindle enter the posterior root of the spinal cord and immediately go to the anterior horn, where they form synapses with -motor neurons that send signals to the muscle.
polysynapticarc additionally includes an intercalary neuron. On fig. 13–5(II) shows the arc of the inhibitory reflex that occurs when the Golgi tendon receptors are stretched.
DynamicAndstaticreflexessprains. There are dynamic and static components of the stretch reflex.
Dynamicreflexsprains is carried out with an unexpected rapid lengthening of the muscle, which leads to an equally rapid contraction. It's obvious that functionreflexdirectedagainstunexpectedchangesVlengthmuscles,because themuscleshrinking,overcomingstretching.
Staticreflexsprains. The dynamic stretch reflex occurs in a fraction of a second. After the muscle is stretched to its new length, a weak static stretch reflex follows. Its importance lies in the fact that it continues for as long as the length of the muscles is changed. Hence,functionstaticreflexsprainsAlsodirectedagainstforces,defiantexcessoriginallengthmuscles.
Signals to skeletal muscles from the spinal cord are usually discrete in nature (eg, increase intensity for a few milliseconds, change intensity level, decrease force of contraction, etc.). The fact that normally even the fastest movements are carried out smoothly is precisely due to the presence of the dynamic and static components of the stretch reflexes. In other words, dynamicAndstaticComponentsreflexsprains-regulatorssmoothnesscuts.
Participation of muscle spindles in voluntary movements
Signals from the motor cortex and other areas of the brain, coming to the ‑motor neurons of the spinal cord, simultaneously excite and ‑motor neurons (the phenomenon of coactivation of – and ‑motor neurons). 31% of efferents to skeletal muscles are type A nerve fibers. As a result ateveryonemuscularreductiongoing onsimultaneousreductionAndextra–AndintrafusalMV.
The -efferent system is activated by impulses coming from the bulboreticular activating formation of the brain stem, and indirectly by impulses coming into the bulboreticular activating formation from the cerebellum, basal ganglia, and cerebral cortex. This is because the bulboreticular activating formation is directly related to antigravity contractions, and antigravity muscles have the highest concentration of muscle spindles. That's why–efferentdampingmechanismespeciallyappearsintimewalkAndrunning.
OtherimportantfunctionsystemsmuscularspindleisVstabilizationprovisionsbodyintimetensemuscularactivities. The physiological mechanism of this effect is that during dynamic work (simultaneously with the contraction of the flexor muscle groups), the stretch reflexes of the extensor group muscles are enhanced. Any increase in contraction on one side of the joint is damped by an increase in stretch reflexes on the other side. As a result, stabilization of the position of the joint is achieved.
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 os-motoneurons, 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) motor neurons, 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 muscle length 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.
Mechanisms of excitation of proprioreceptors
The value of proprioreceptors
Information in the central nervous system about muscle tone and body position in space comes from the vestibular apparatus, eyes and muscle-articular receptors (own receptors or proprioceptors) of skeletal muscles, tendons, ligaments, joint capsules. Complex motor acts are coordinated with the help of proprioreceptors (mechanoreceptors) - muscle spindles located in skeletal muscles, and Golgi bodies located in tendons.
The central nervous system receives information from proprioreceptors along the spinothalamic and spinocerebellar pathways of deep sensitivity about the intensity and consistency of contractions of individual muscles and muscle groups, changes in movements in the joints under different loads. The cortical zone of the proprioceptive analyzer is located in the precentral gyrus of the frontal lobe. Analyzing the information received from proprioreceptors, the central nervous system sends response motor impulses to the muscles, expediently changing the nature of movements. Thanks to proprioceptors, a person, even without the help of vision, is completely oriented in the position of his body and its parts in space, is aware of the direction of movement, the degree of muscle tension necessary to perform the movement and maintain the posture.
Muscle spindles are located in the thickness of the skeletal muscles parallel to the muscle fibers. Their number in each muscle depends both on its size and on the function performed. At one end they are attached to the muscle, the other - to the tendon. Excitation in them occurs when the muscle fibers, tendons are lengthened during relaxation or passive stretching of the muscles. Muscle spindles are stretch receptors. In them, when the muscle is stretched, the frequency of nerve impulses increases. With isotonic muscle contraction, the frequency of impulses decreases or stops. Golgi tendon bodies, on the contrary, are stretched and excited during muscle contraction. Impulses from them along the afferent nerve fibers enter the central nervous system. Thus, muscle spindles register changes in muscle length, and tendon receptors - its tension (tone).
Impulses from the muscle spindles, when the muscle is stretched, arrive at the motor neurons of the spinal cord, as a result, the muscle contracts. This is the simplest example of a reflex arc that includes one synapse (monosynaptic). The most famous monosynaptic reflex is the knee reflex. These reflexes regulate the length of the muscle. The mechanism is important for the muscles that support the posture when running, walking.
During muscle contraction, tendon receptors are excited with simultaneous inhibition of motor neurons of the spinal cord of the same side. The weakening of muscle tone activates motor neurons. Thus, a constantly high muscle tone is maintained along the reflex arcs of the tendon receptors.
Each movement requires the coordinated action of several muscles: in order to take a pencil in your hand, several muscles must be involved, some of which must contract and others relax. Jointly acting muscles, i.e. contracting or relaxing at the same time are called synergists, in contrast to the opposing antagonist muscles. With any motor reflex of contraction and relaxation, synergists and antagonists are perfectly coordinated with each other.
In response to muscle stretching by an external force, the receptors of muscle spindles that react only to changes in length are excited ( stretch receptors) (Fig. 7.2), which are associated with a special type of small intrafusal muscle fibers.
From these receptors, excitation is transmitted through a sensitive neuron to the spinal cord, where the end of the axon is divided into several branches. Some branches of the axon form synapses with the motor neurons of the extensor muscles and excite them, which leads to muscle contraction: here is a monosynaptic reflex - its arc is formed by only two neurons. At the same time, the other branches of the afferent axon activate the activity of inhibitory interneurons of the spinal cord, which immediately suppress the activity of motor neurons for antagonist muscles, i.e. flexors. Thus, muscle stretching causes excitation of the motor neurons of the synergistic muscles and reciprocally inhibits the motor neurons of the antagonist muscles (Fig. 7.3).
The force with which a muscle resists a change in its length can be defined as muscle tone. It allows you to maintain a certain position of the body (posture). The force of gravity is aimed at stretching the extensor muscles, and their response reflex contraction counteracts this. If the stretching of the extensors increases, for example, when a heavy load falls on the shoulders, then the contraction increases - the muscles do not allow themselves to be stretched and due to this the posture is maintained. When the body deviates forward, backward or to the side, certain muscles are stretched, and a reflex increase in their tone maintains the necessary position of the body.
According to the same principle, reflex regulation of the length of the flexor muscles is carried out. With any bending of the arm or leg, a load is lifted, which may be the arm or leg itself, but any load is an external force seeking to stretch the muscles. The reciprocal contraction is regulated reflexively depending on the size of the load.
tendon reflexes can be induced by lightly striking with a neurological hammer on the tendon of a more or less relaxed muscle. From a blow to the tendon, such a muscle is stretched and immediately reflexively contracts.
Reflex sequence: Stretching a muscle causes it to contract.
The arc of the knee jerk (from the tendon of the quadriceps femoris):
Intramuscular stretch receptor (in the intrafusal muscle spindle);
Sensitive neuron (body - in the spinal ganglion);
Alpha motor neuron (body - in the anterior horns of the spinal cord);
Skeletal muscle (quadriceps femoris).
Thus, in the arc of this reflex (Fig. 7.4) only two neurons participate and, accordingly, there is one synapse; hence the name "monosynaptic stretch reflex". In addition, the circuit of reciprocal inhibition is connected with the arc of the reflex, due to which the contraction of the muscle is accompanied by the relaxation of its antagonist. Monosynaptic tendon reflexes can be obtained on any muscle group, regardless of whether they are flexors or extensors. All tendon reflexes occur when the muscle is stretched (therefore, they are stretch reflexes) and the excitation of intrafusal muscle spindle receptors. Any movement associated with muscle contraction requires the activation of not only alpha, but also gamma motor neurons.
: since as a result of this reflex, stretching (that is, lengthening) of the muscle leads to its contraction (that is, shortening), it is aimed at maintaining the constancy of the muscle length. Therefore, this reflex
It is an element of any movements that require the constancy of the length of the muscles, that is, holding the posture;
Makes movements smoother, as it prevents sudden changes in muscle length.
These two functions are extremely important, which is why myotatic reflexes are the most common reflexes in the spinal cord.
Voltage reflexes
In addition to the length in the working muscles, another parameter is reflexively regulated: tension. When a person begins to lift a load, the tension in the muscles increases to such a value that this load can be torn off the floor, but no more: to lift 10 kg, you do not need to strain your muscles, as for lifting 20 kg. In proportion to the increase in tension, impulses from tendon proprioceptors, which are called Golgi receptors (tension receptors). These are unmyelinated endings of the afferent neuron, located between the collagen bundles of tendon fibers connected to the extrafusal muscle fibers. With increasing tension in the muscle, such fibers stretch and squeeze the Golgi receptors. Increasing in frequency impulses are conducted from them along the axon of the afferent neuron to the spinal cord and transmitted to the inhibitory interneuron, which does not allow the motor neuron to be excited more than necessary (Fig. 7.5).
Reflex sequence: muscle tension leads to its relaxation. Arc reflex:
Tension receptor inside the tendon (Golgi tendon organ);
Sensitive neuron;
Intercalary inhibitory neuron;
Alpha motor neuron;
Skeletal muscle.
The physiological meaning of the reflex: thanks to this reflex, muscle tension leads to its relaxation (it is possible to stretch the tendon and cause the activation of the receptor only when the muscle is tense). Therefore, it is aimed at maintaining the constancy of muscle tension, therefore:
It is an element of any movements that require constancy of muscle tension, that is, holding a posture (for example, a vertical position that requires a sufficiently pronounced tension of the extensor muscles);
Prevents sudden muscle tension that can lead to injury.
Muscle length and tension are interdependent. If, for example, the outstretched arm relieves muscle tension, then the irritation of the Golgi receptors will decrease, and gravity will begin to lower the arm. This will lead to muscle stretching, an increase in the excitation of intrafusal receptors and the corresponding activation of motor neurons. As a result, muscle contraction will occur and the arm will return to its previous position.