Coordination of Movements

COORDINATION OF MOVEMENTS (from Latin coordinatio - ordering), the harmonization of impulses of individual muscle groups in time and space, directed toward achieving a specific motor effect. The established fact of participation in C. m. of almost all departments of the central nervous system indicates the extreme complexity of the organism's coordinative activity. In essence, it can be said that everything that distinguishes any simple or complex movement of a person from the twitching of a dissected frog muscle is exhaustively reduced to C. m._:-Each primary motor impulse sent to the muscle of a healthy organism from spinal cord cells is already coordinated in itself, proportional to the stimulus that caused this movement. If a person intends to lift a ball, then a feather, then a hundredweight weight, he sends completely different impulses to the muscles, depending on the representation of the resistance that each of these movements will have to encounter. Such primary C. m. (or C. of the primary impulse) in the vast majority of cases is carried out by the lower spinal departments of the central nervous system; this is confirmed by the fact that, for example, in a decapitated frog, the movements of the paw during the "wiping reflex" are strictly coordinated with the area of skin subjected to irritation and immediately change if one begins to irritate any other area of the body surface.-Even this primary C. m. turns out to be a very complex set of mechanisms. If one concentrates attention on the simplest joint with one degree of freedom (see Movements), it turns out that with any movement, simultaneously with the excitation of one muscle, there occurs a synergistic excitation of the synergists of this muscle and inhibition of its antagonists. This phenomenon, named reciprocal (i.e., mutually reciprocal) innervation, was noted by Bell as early as 1836 and was thoroughly studied by Sherrington at the beginning of the 20th century. Sherrington showed that in mammals (cats) this phenomenon is purely of spinal origin. In joints richer in mobility, reciprocal innervation turns into a more complex process of redistribution of tensions among all the muscles of a given joint. Sherrington also showed that the excitation of the flexors of one animal's leg is accompanied by simultaneous inhibition of the extensors of the other leg; moreover, the flexion reflex of one leg, caused by irritation from the periphery of the body, can be inhibited by the excitation of the extensors of the opposite leg. It is very likely that in these phenomena of bilateral mutual reciprocity we are dealing with the simplest components of the walking mechanism (see). A much greater role in C. m. is played by secondary impulses (secondary C.). No movement can be completely proportional from the very beginning; the very course of a movement already begun is accompanied by continuous corrective (correcting) impulses sent by the central nervous system to all muscles directly or indirectly involved in the movement. Secondary C. m. is a reflex process. In the formation of the centripetal arc of this reflex, most of the body's sensory apparatuses participate, first of all the proprioceptive apparatuses (end organs in tendons, muscles and joint capsules; pathway - posterior columns of the spinal cord), then the organs of deep and superficial skin sensitivity (pathways - tractus spino-thalamicus, tractus spino-cerebellaris), labyrinths and eyes. The central apparatuses of secondary C. m. lie above the spinal cord; they include the cerebellum, optic thalami, nuclei of the midbrain, and frontal lobes of the cerebrum. The proprioceptive reflex is disrupted or completely disappears with atrophy of the posterior columns of the spinal cord (Goll's bundle, Burdach's bundle), which leads to the development of ataxia (see). The role of this reflex is best clarified by the analysis of the phenomena of falling out in tabetic ataxia and in normal conditions amounts to the implementation of eupotia (correct hitting of the target point) and eumetry (correct measure of movements). Thus, this mechanism in essence regulates spatial C. m.; with its disruption, movements become excessive (dysmetria), lose accuracy (distopia) and confidence. Secondary C. m., associated with the conductivity of the lateral cerebellar pathway of the spinal cord, and in the brain - with the cerebellum, pons, labyrinths and frontal lobes, is much more complex, and its significance is best understood at present for the mechanisms of maintaining equilibrium. In diseases of the cerebellum, kinesthetic feeling (i.e., the sensation of position, direction and force of movement) usually does not suffer, but standing (Romberg's symptom) and gait are affected. The latter becomes unstable, there is a tendency to fall (astasia), difficulty in changing the direction of walking, etc. The normal C. m. of other parts of the body is also disrupted: eyes (nystagmus), hands (adiadochokinesia). In monkeys, with complete removal of the cerebellum, phenomena of disruption of the rhythm of gait (movements of the limbs in an abnormal sequence and with abnormal pauses, clumsy, jumping and intermittent running) have been observed, which forces one to attribute to the cerebellar apparatus a role in the implementation of temporal or rhythmic C. m.-rhythm.-The vestibular labyrinths play a very important role in the implementation of secondary C. m. together with the cerebellum. In addition to their main role as sense organs determining the direction of gravity and acceleration of the body in space and ensuring the maintenance of equilibrium, the labyrinths also take part, although not fully clarified, in maintaining the tone of neck muscles and extensor muscles of the limbs. Damage to the labyrinths leads to loss of equilibrium, dizziness, persistent spinning in place, etc.-The highest, most refined C. m. is carried out with the closest participation of the cerebral cortex. Occipital lobes make possible visual C., frontal lobes (in an as yet unexplained manner) influence the dexterity and grace of movements. Damage to the cortex of the left central sulcus and corpus callosum can lead to the development of apraxia (see), i.e., to a disorder of the ability to perform complex, purposeful sequences of movements. Among specific, local disorders of C. m. associated with damage to the cerebral cortex, aphasia and agraphia (see), paraphasia and paragrapia should be mentioned.