7.1. Motor programming
Biological role of movement
The behavior of organisms through interaction with the environment ensures the satisfaction of their principal requirements. In the course of evolution, motor reactions have significantly progressed: from the movement of protoplasmic pseudopods, fins, wings to complex movements of mammals and humans, including locomotion, manipulation, facial expressions, gestures, speech, writing, etc.
From the perspective of the functional system theory, movement acts as an external link of self-regulation in functional systems of nutrition, maintenance of osmotic pressure, and thermoregulation. At the same time, motor action has a systemic structure, as a part of the general architectonics of purposeful activity at the stage of efferent synthesis, when the future action is already set as a central process but is not yet implemented in the muscle work.
Efferent synthesis
The stage of efferent synthesis is formed under the influence of the afferent synthesis processes, decision-making, and anticipation of a useful result - the acceptor stage of action results. At the stage of efferent synthesis, the issues of creating an adequate action program and developing ways to implement it at the level of the executive mechanisms in each functional system are solved. The most important component of executive programs is a variety of movements - setting a pose, moving the body in space, maintaining body parts in a fixed position, manipulations. The movements themselves are accompanied by autonomic and endocrine reactions.
From a physiological point of view, efferent synthesis is the temporary organization of excitation and inhibition processes in the CNS, addressed to muscles, glands, and other tissues. Efferent synthesis ends with the formation of a general effector integral, which includes somatic, autonomic, and endocrine components of holistic behavioral activity.
Flexibility of effector functions
Various physiological functions - respiration, blood circulation, digestion, excretion, and others -may be involved in different functional systems as executive mechanisms. For example, heart activity supports the proper blood pressure, constancy of blood gas composition, and osmotic pressure. The choice of executive mechanisms in a specific functional system is carried out with the participation of memory mechanisms - the experience of using various physiological functions in a similar situation in the past. Necessary information is extracted from memory with the participation of dominant motivation and environmental factors. The processes of efferent synthesis are constantly dynamically rearranged under the control of reverse afferentation from the results of behavioral activity.
Muscle sense
I.M. Sechenov also pointed out that the movement is always a unity of "feeling and action". Muscle sense occurs based on the impulses entering the CNS from the receptors of the musculoskeletal system. This impulse allows the CNS to correct the process of performing a movement. Input from the muscles is a part of reverse afferentation about the parameters of the action stage and final results. Each dynamic program of efferent synthesis is evaluated by the acceptor apparatus of the action results.
General characteristics of muscle activity
Muscle activity provides posture, locomotion (walking, running, swimming); communication (writing, speech, gestures, mimicry); manipulation (handling objects). Motor functions are realized by three main processes: maintaining the muscle tone, establishing an adequate posture, and performing a specific voluntary movement.
Muscle tone as the basis of movement
Muscle tone is a state, in which a certain level of muscle activity is maintained for a prolonged period of time. Muscle tone is determined by the activity of individual muscle fibers when they receive impulses through motor fibers from α-motor neurons of the spinal cord.
Motor unit
Each individual α-motor neuron of the spinal cord innervates a strictly defined number of muscle fibers. The structure of the "motor neuron-muscle fiber" has been termed a motor unit. The number of muscle fibers combined into motor units can be different (Fig. 7.1). In the external eye muscles, one motor neuron innervates 3-4 muscle fibers (innervation index 1:3), and in the latissimus dorsi - up to 300 muscle fibers (innervation index 1:300). The more precise is the movement performed by the muscle, the more motor units it contains. There are three types of motor units: S - slow, fatigue-resistant, and consisting of type I muscle fibers; FF - fast, fatiguable, consisting of type IIB muscle fibers; FR - fast, fatigue-resistant, consisting of type IIA fibers.
Muscle unit
The set of muscle fibers of one motor unit is called a muscle unit. The fibers of each muscle unit belong to the same histochemical type: I, IIB, IIA.
Motor neuron pool
The total number of motor neurons innervating one muscle makes up a neural pool. The maintenance of muscle tone is achieved by the predominant activity of the so-called red muscle fibers containing a large amount of myoglobin with a low excitation propagation rate and contraction development. In humans, the extensor muscles that support vertical body position and work against gravitational forces contain more of these fibers.
Specific receptors are engaged in muscle tone: of the muscle spindle, tendon organs, as well as receptors located in the articular membrane.
Muscular spindles
Muscle spindles consist of a connective tissue capsule with intrafusal muscle fibers enclosed in it, where the processes of γ-motor neurons are attached (Fig. 7.2). The length of a muscle spindle is on average 10 mm, and the diameter is about 100 microns. There are two types of sensory endings in the spindles: afferents of group Ia with a conduction velocity up to 120 m/s and group II with a velocity up to 70 m/s. These sensory endings are classified as stretching receptors. The more spindles a muscle contains, the more delicate and accurate are the performed movements. Muscle spindles are attached to a muscle tendon at one end, and to a muscle fascia at the other end, i.e., parallel to the operating (extrafusal) muscle fibers. The motor function of muscle spindles is also provided by two types of nerve fibers: static γ-efferents ending on each nuclear bag fiber and excited under static loads, and dynamic γ - efferents ending on nuclear chain fibers, which are excited under dynamic loads. When the dynamic γ-motor neuron is activated, the dynamic responses of group Ia afferents secondarily increase, and when the static γ-motor neuron is activated, the static responses of both groups - Ia and II - increase.