Motor unit

Motor units of one muscle may be different. Depending on the speed of reduction, motor units are divided into slow / slow (S-ME) and fast / fast (F-ME). And F-ME in turn is divided by resistance to fatigue into fast-burning / fast-fatigable (FF-ME) and resistant to fatigue / fast-fatigue-resistant (FR-ME).

Internally, these ME motor neurons are divided accordingly. There are S-motoneurons (S-MH), FF-motoneurons (F-MH) and FR -motoneurons (FR-MH) S-ME are characterized by a high protein content of myoglobin, which is able to bind oxygen (O2). Muscles, mainly consisting of IU of this type, for their dark red color are called red. Red muscles perform the function of maintaining a person’s posture. The ultimate fatigue of such muscles occurs very slowly, and the restoration of functions happens the other way around, very quickly.

This ability is caused by the presence of myoglobin and a large number of mitochondria . ME of red muscles, as a rule, contain a large amount of muscle fibers. FF-ME consists of muscles capable of performing rapid contractions without noticeable fatigue. FF-ME fibers contain a large number of mitochondria and are capable of forming ATP by oxidative phosphorylation.

As a rule, the number of fibers in FF-ME is less than in S-ME. FR-ME fibers are characterized by a lower content of mitochondria than in FF-ME, and also by the fact that ATP in them is formed due to glycolysis . They lack myoglobin , so the muscles consisting of IUs of this type are called white. White muscles develop a strong and rapid contraction, but they get tired pretty quickly.

When the muscles are stretched, S-MH is excited first, but the frequency of their discharge is small (5-10 pulses / s). As the muscle is stretched, the frequency of S-MH pulses increases and reaches 40-50 pulses / s. With this stretch, FR-MH is included in the work. When connecting one FR-ME, the power increases by about 10 times. If the stretching continues, FF-MH begins to impulse, which means that FF-ME are connected, each of which gives an increase in strength by a factor of 4-5. The dependence of the force that a muscle develops on the degree of its stretching or on the length is called the muscle characteristic, which can be displayed on a graph as a curve.

Motor neurons innervating one muscle constitute a common motor neuron pool. In one motor neuron pool there can be motor neurons of different sizes. Large motor neurons with thick axons and many collaterals that interact with a large number of muscle fibers are contained in large IUs. Such MEs are characterized by a high rate of excitation, while they have a low excitability and generate a high frequency of nerve impulses (20–50 imp. / S).

Smaller IUs include small-sized MNs, which have slow-conducting thin axons and interact with a small number of muscle fibers. Large MN is excited only with large loads on the muscle, and small MN can be included in the work with a small muscular effort.

Increasing the load causes excitation of various types of MN in accordance with their size. The order of involvement of new MNs is thus, as a rule, the same for almost any kind of contraction: first, a smaller MN is involved in the process, then a larger one. Edward Henneman described this pattern in 1956 as the “principle of magnitude”.

Even before Heneman a number of scientists described some of the provisions of this pattern. In particular, Denny-Brown and Adrian Bronk, investigated the principles of the muscular unit. In 1929, they suggested that there are two ways in which the nervous system can control muscle contraction:

• pulse frequency increase
• an increase in the number of MNs involved in the process.

In 1938, Denny-Brown and Pennibecker provided the basic principles of the magnitude principle with respect to IU, unlike Henneman, who initially spoke only about MN, relating the order of their involvement in work with their sizes.

From the physical point of view, the Hennemann principle can be explained by the fact that different MNs have different input resistances. Small PLs have less membrane area, and therefore higher input resistance.

An interesting way to measure the input resistance. It is measured as follows: a microelectrode is introduced into a cell, a current is passed through it and it is looked at how much the membrane potential has changed. Identical synapses in different MNs create the same synaptic currents, since the resistance of the synapse itself is much greater than the input resistance of the MN. For small MNs, these currents will cause a greater shift in membrane potential and stronger membrane depolarization. Thus, at first smaller MNs are included in the work.

Another hypothesis, which was actively discussed by researchers, explained the "principle of magnitude," in connection with aI afferents. It was assumed that aI afferents give more synaptic endings on small MNs and these endings lie closer to the soma, and hence the efficiency of synapses is higher. In the process of researching this idea, a lot of quantitative data was obtained on the relationship between muscle afferents and MN. It turned out that a single afferent aI gives about 10 branches in the pool of MN of a given muscle, and each collateral forms up to 200 synapses, that is, the total afferent aI gives up to 2000 synapses. For example, in the MN the triceps pool is 500-700 MN. One MN on average has 2-4 synapses from one afferent fiber a I. Only one collateral is in contact with one MN. Mendell and Henneman showed that one afferent gives its endings quite diffusely throughout the entire pool, ending with 90% of all its MN and 50% of the synergist's muscles. Thus, it can be assumed that the input from muscle afferents is distributed fairly uniformly over the MN, so that the proper properties of the MN determine the order of their involvement.

Initially it was assumed that the principle of magnitude works with increasing isometric contraction. Isometric contraction is muscle contraction without changing its length. Muscle contracted isometrically when performing static work. (For example, you push your shoulder against a wall, but you cannot move it.) But muscle contraction is not always isometric (there is still isotonic and auxonic). Even in those muscles, by the example of which it is possible to study isometric contraction, the same motor unit may have different thresholds for activating (engaging) its next link for flexion and extension. Such comments, which are often referred to as “tasks of a specific motor neuron response” (Ericksson et al., 1984), described the action of a human chewing muscle (English, 1985) and confirmed that not all motor units are involved during muscle contraction.

The idea that there are separate groups of motoneurons that respond differently to any directional movement is an exception to the “principle of magnitude” for both the muscles of the jaw and the muscles of the limbs. Until some time, it was assumed that, depending on the type of movement, one or another motor unit is activated, but later it was proved that these are actually two muscles with two motor neuron pools, that is, this muscle is in the process of being divided into two different ones. A very convincing example was given by New Zealand neurologist Derek Denny-Brown, who in 1949 showed that with the “grasping” movement of the hand in the flexor profundus digitorum muscle, the motor units are switched on in one order, and with the “flexion” movement - in a different order.

Also, a series of experiments were conducted on intact and decerebrirized cats. First, in 1970, a group of scientists (Burke, Yankovskaya, Ten Bruggenkate) discovered during their research that FF and FR motor neurons can be polysynaptically excited by inputs from low-threshold skin afferents, and S-motor neurons can be inhibited by the same inputs. At the same time, the excitatory postsynaptic potential (ITSP) on F-motoneurons is disynaptic, and on S-motoneurons it is trisynaptic (Illert et al. 1976). In 1982, it was shown that this effect is caused by the motor cortex and red core (see Brain ) (Burke, 1982)

Then a reflex was opened, which is an example of a situation when it is necessary to turn on only fast motor units, without using slow ones. This is the natural spinal reflex "shaking paws." This reflex persists in spinal cats , and is naturally present in individuals with an intact nervous system . "Shaking" is observed in an intact cat, when it comes paw into the water. Therefore, low-threshold skin afferent paws are responsible for starting this reflex. The soleus is not involved in this reflex (“slow” leg muscle, see Leg , reduction time 80 ms), but only gastrocnemius (“fast” leg muscle, contraction time 20-25 ms) works. The frequency of shaking off the paws is very high, so (10-12 Hz), which excludes the possibility of engaging slow motor units.

In 1980, D.Smith (D.Smith et al. J. Physiol. 1980) in his work “Fast ankle extensors during shaking paws: their selective involvement” described the study of the solius and gastrocnemius of a cat in three states (standing, walking, jumping ). It turned out that both of these muscles work together in all three cases. The same has been shown for fast and slow extensor muscles of the forelimbs. It turned out that despite the fact that solius is a slow muscle, it does not interfere with fast movements (gallop, jumping). Again, Smith and others tried to find a movement that was too fast for a solius. Solius is able to develop an effort for 80 ms (the time of joint extension when jumping 1 m = 130–150 ms). Smith also found that the frequency of shaking the paw is close in frequency to tremor (10-13 times per second), therefore, with such rapid movement, the solius is silent. However, if the frequency of carding approaches 120-150 ms, the solius works (!), As in the jump.

In 1999, scientists from Atlanta (Kop and Sokolov) proved that for the medial and lateral heads of the gastrocnemius , the Hennemann principle of magnitude is quite applicable and greatly facilitates the coordination of the work of various muscles. At the same time, they explored the application of this principle to muscle motor neuron pools that are coactivating (such a movement in which the antagonist muscle or muscle group is still partially active when the initial muscle activity is exhausted. For example, the triceps are partially active when the biceps performs any maneuvers, and the quadriceps is partially activated when gamstring is active) in this particular movement.

• Human physiology edited by R Schmidt and G. Tevs 3rd edition Moscow "WORLD" 2004
• Human physiology, edited by V. M. Pokrovsky, G. F. Korotko
• Fundamentals of Psychophysiology: Textbook / Ed. ed. Yu. I. Alexandrov. - M .: INFRA-M, 1997.

• http://sportzal.com/post/839/
• http://medbiol.ru/medbiol/har/003a8db7.htm