Associate Professor, PhD A.M. Trofimov
Yelets State University of Ivan Bunin, Yelets

Keywords: motor neurons, motor centres, volitional action, notion of movement, motor program.

Introduction. Since mid-twentieth century to the present time the main idea of the movement synthesis mechanism has been the one about the existence of a certain mechanism of program management. According to a number of scientists, there are motor programs present in brain structures that are activated at certain moments by control centers resulting in movement [8, 9, 10, 11]. However, it should be noted that the researchers failed to answer a number of quite important questions. In particular, the ability of the human central nervous system to select appropriate movement options, start and finish them at the right time depending on existing needs and the current situation is hardly explained. The mechanism of the voluntary regulation of muscle contraction strength and movement speed is unclear. Abilities of a person to construct complex movements, repeat movements of other people and move being guided by their verbal instructions, revise the structure of movements in the process of training sessions remain unexplained. We are trying to answer these questions in the present study.      

Objective of the study was to clarify the structure of the nervous-muscular mechanism of motor activity control.

Results and discussion. To study what happens in the body when a person moves, we should start with studying what all movements have in common. In this regard, it makes sense to start with the structural features of the human musculoskeletal system. 

Segmented structure of a human body is its special feature. It is the movements of the body segments that enable a person to move himself and external objects [6]. Movements of the body segments are initiated by contractions of the muscles attached to them. Leverage mechanisms are necessary for organization of movements. Each mechanism includes two body segments (not necessarily adjacent), a muscle connecting them and a joint. Upon muscle contraction the leverage mechanism makes each of the segments rotate in the joint. Each muscle associated with the segment pulls it in its direction depending on the point of the muscle attachment. If the muscles are toughened separately, the number of the segment’s movement options from the same starting position will be equal to the number of these muscles. Simultaneous contractions of several muscles enable the segment to rotate in any plane (provided the joint structure allows for it). The direction of such movements will be determined by the combination of muscle toughening. The vector law of the composition of forces will be valid in these cases [7].

The segmented structure of the human body allows it to perform not only simple rotational, but also complex movements. The structure of a complex movement may include a set of individual translational movements (each consisting of 2-3 simple rotations), a set of rotations of individual segments, pronation and supination of individual segments and a set of movements of the moving body parts. 

Movements associated with changes in the position of two or more body segments, as well as movements of the segments in multi-axis joints in intermediate planes are initiated by contractions of two or more muscles. Furthermore, additional groups of muscles can be engaged in the movement initiation to fix the body parts in the predetermined positions and prevent their unauthorized oscillations. Changes in joints’ angles and therefore in positions of certain body segments at each moment of the movement depend on one factor, namely, the force with which each of the muscles pulls its associated body part.
The stronger the muscle contraction, the faster the rotational movement, and the greater the position change for the segment. If body segments are set in motion from one and the same position by means of contractions of the same muscles, but with different force, then the overall structure of the segments’ positional changes will look different. Joints’ angles changes composition will be different each time throughout the movement [7].

Thus, control over the intensity of muscle contraction becomes the fundamental element of any movement performance control. What is the mechanism of muscle contraction intensity control? To answer this question, let us study the neuromuscular system structure. 

Muscle fiber is a structural and functional unit of the skeletal muscles. Skeletal muscles are connected to the spinal motor neurons of the ventral horns of the spinal cord. The function of the motor neurons is neural excitation transmission from the cortex motor division to the muscle fibers. Each motor neuron innervates a group of muscle fibers. The fibers of such a group are associated with only one motor neuron. A group of muscle fibers with the innervating motor neuron is called a motor unit [8, 3]. Motor neurons that innervate the fibers of one muscle form a group of motor neurons.

Impulsivity is a characteristic of the neural excitation transmission, i.e. nervous energy is transmitted in short pulses with particular frequencies in different cases. This applies to both the excitation transmission from the motor center to the motor neurons and the subsequent excitation transmission from the motor neurons to thousands of muscle fibers [8, 9]. The frequency of the nerve impulses transmitted from the motor center to the motor neurons is determined by the excitation of the motor center itself. Excitation increase determines the frequency increase [7]. Upon receiving the signal from the motor center, the excitation of spinal motor neurons begins and they start sending impulses to the muscle fibers. The muscle fiber that receives the signal starts contracting. Similar to the case of motor centers, the frequency of the impulses transmission by the motor neurons toward the muscle fibers is determined by the intensity of its excitation. Therefore, the frequency of impulses transmission is sufficient to arrange tetanic contraction of the muscle fibers only upon reaching a certain excitation intensity. We called the state of a motor neuron in which its impulses allow muscle fibers to contract in the tetanic mode a state of operational excitation.    

Simultaneous excitation of several motor centers causes simultaneous work of several muscles, which occurs when performing complex movements.

A characteristic of the spinal motor neurons is their excitation dependency on the frequency of received impulses. The higher the frequency, the more intensive the excitation [8, 3]. Due to this, exceeding the frequency of certain impulses received is a condition for achieving the state of operational excitation. We called the frequency at which a motor neuron becomes operational a threshold frequency of operational excitation. The threshold value is determined by the size of the motor neuron. The bigger the motor neuron, the higher the threshold. The structure of any group of motor neurons includes motor neurons of different sizes with different threshold frequencies of operational excitation [5]. That is why a change in the number of motor units actually engaged in movements takes place when the excitation of the muscle’s motor center occurs. More intensive excitation provokes higher frequency with which the motor center sends impulses to motor neurons, and the more of them are able to fully stimulate the contraction of fibers, and the greater the number of motor units that start working. The number of contracting motor units determines the strength of muscle contraction as a whole. 

As can be seen, the presented model of muscle contraction lacks the last component, namely, the excitation mechanism of a motor center. In our opinion, excitation of a motor center results from an act of will. Its content is a conscious impact of an act of will on randomly chosen motor centers. Need for some physical effort is a prerequisite for manifestation of the will in relation to the motor centers. The intensity of the impact of a volitional action on a motor center determines the intensity of the excitation of the latter and, as a result, the frequency of the nerve impulses transmission from the motor center to spinal motor neurons.       

A person can manipulate the intensity of the impact of a volitional action on his/her own, thus controlling the strength of muscle contraction (the so-called intramuscular coordination).

The mechanism of a consciously organized energy impact on the motor centres of the cortex manifests itself in the human ability to initiate a contraction of any striated muscle of the body (with the exception of the cardiac muscle) and change its strength in any direction at his/her own will, at any time, regardless of any external factors.

A person can remember the power of volitional actions on motor centers and link these notions of the kinesthetic structure of movements. By remembering the power of a volitional action a person remembers the force applied when performing a particular motor action, and recalls it as needed. The kinesthetic accuracy of the movement performance depends on the accuracy of determining the motor centers in the motor cortex as well as on how accurate the impact on them will be strength-wise.  

The basis of motor training is the trial and error method. The first attempt to perform a movement is a trial when a student determines the strength of impacting the motor centers either approximately or randomly. Naturally, the chance to make a correct movement in such conditions is very small, hence it is followed by the correction phase. All the characteristics of the performed movement become reference points in preparing for the next attempt (on the condition that the student remembered something of what he had done). Next, imagining what should be done and what has been done, the student extrapolates in his mind’s eye and corrects the strength of volitional actions aimed at a particular motor center. Involuntary formation of the notions of the impact powers ratio (a notion of the impact powers composition) takes place simultaneously with the formation of the notions of the volitional actions’ impact on particular motor centers, i.e. there is a process similar to memorizing the ratio of values perceived by the visual sensory system, for example, the heights ratio of two observed objects.      

The criterion of the motor skill formation is association of the notion of the kinesthetic structure of movements with the notion of the muscles’ motor centres, the initiators of contractions of the right muscles. In addition, there should be a notion of the strength of volitional actions and their impact on each motor center, as the segments rotation speed, and therefore, the values of the joint angle variations during one and the same period of time are determined by the impact forces.  

A status of a motor program or a program of movement control can be assigned to the notion of movement, which includes a notion of its kinesthetic structure; a notion of the composition of motor centers activating the relevant muscles; a notion of the composition of volitional actions aimed at the motor centers; a notion of movement speed (when needed); a notion of the absolute power of a volitional action (when needed). In addition to the above mentioned elements the program of movement control may be supplemented by other components. 

Conclusion. In our opinion, the present study materials open up new areas and possibilities of experimental research of the central nervous system, first of all using technical means.  

References

1. Kots Y.M. Organizatsiya proizvol'nogo dvizheniya (Voluntary movement organization) / Y.M. Kots. – Moscow, 1975. – 224 p.
2. Pearson, R.S. Spinal'nye mekhanizmy upravleniya myshechnym sokrashcheniem (Spinal control mechanisms of muscle contraction) / R.S. Pearson. – Moscow: Nauka, 1985. – 183 p.
3. Popov G.I. Biomekhanika dvigatel'noy deyatel'nosti (Biomechanics of motor activity): Textbook for university students / G.I. Popov, A.V. Samsonova.– Moscow: Akademiya, 2011.– 320
4. Trofimov A.M. Teoriya psikhicheskogo obraza i assotsiatsiy (Theory of mental images and associations): Monograph / A.M. Trofimov. – Yelets: EGU im. I.A. Bunina (Bunin ESU), 2010 – 526 p.
5. Feldman, A.G. Mekhanicheskie svoystva skeletnoy myshtsy i ikh regulyatsiya nervnoy sistemoy (Mechanical properties of skeletal muscles and their regulation via nervous system) / A.G. Feldman // Fiziologiya dvizheniy (Movement physiology). – Leningrad: Nauka, 1976. – P. 38-68.
6. Feldman A.G. Tsentral'nye i reflektornye mekhanizmy upravleniya dvizheniyami (Central and reflex movement control mechanisms) / A.G. Feldman. – Moscow: Nauka, 1979. – 184 p.
7. Shapkov Y.T. Upravlenie aktivnostyu dvigatel'nykh edinits kak osnova koordinatsii dvizheniya (Motor unit activity control as a basis of movement coordination) / Y.T. Shapkov // Upravlenie dvizheniyami (Movement control) / Ed. by A.A. Minkin, G. Pik. – Moscow: Nauka, 1990. – P. 64-72.
8. Sherrington, Ch. Integrativnaya deyatel'nost' nervnoy sistemy (Integrative activity of nervous system) / Ch. Sherrington. – Leningrad: Nauka, 1969. – 390 p.

Corresponding author: amt59@yandex.ru

Abstract 

Objective of the study was to clarify the structure of the nervous-muscular mechanism of motor activity control. The article explores the problem of muscular control by the central nervous system in a free movement process. The author believes that it is individual consciousness that controls movements, and it is the key difference of the proposed model from the control mechanisms reported by other researchers. In the author’s opinion, it is the individual will or nervous energy focused on and exciting the muscular motor centres that should be considered the core instrument of human consciousness that controls muscular system operations. The finding is largely based on the detailed comparisons of the kinaesthesia of different movements of the human body and identification of the general conditions providing for every movement being accurate, dominated by the right composition of the joint angle variations. The right composition may be achieved only through the muscular contraction forces being duly controlled to move the relevant bodily segments. The article also describes the neuro-muscular mechanism that enables a person to consciously control the muscular system operations by activating the relevant muscles and controlling their contraction strength. Finally, the article gives an account of the consciousness performance sequence in a motor skill mastering process.