Phasic Contractions; Reflex and Volitional Movements

Phasic contractions result in movement; they may be either reflex or volitional in origin. A reflex movement is one which occurs as the result of the stimulation of receptors located at or near the surface of the body. Nerve impulses resulting from stimulation of the receptors travel over sensory nerve fibers to the spinal cord. Here the impulses are transmitted either directly or by way of intermediate neurones to the motor nerve cells. Tire central processes of the sensory nerve fibers terminate as tiny knobs or "end-feet" in contact with the dendrites and eell body of the motor neurone. This junction is called a synapse; its very great susceptibility to adverse conditions, including fatigue, is of particular significance to students of exercise physiology. The motor nerve cell discharges over the motor nerve fiber to the muscle where the impulses must cross another type of junction, the motor end plate, which is a specialized mass of tissue intervening between the nerve fiber and the sarcoplasm of the muscle fiber, lake the synapse, but to a lesser degree, the motor end plate is readily rendered non-functional by fatigue and the action of certain drugs. The simple reflex arc as here analyzed consists of the following structures: receptor, sensory nerve fiber, synapse, motor nerve cell and fiber, motor end plate, muscle fiber. The reflexes which utilize this simple type of circuit are called spinal reflexes, since they are mediated through the spinal cord without the intervention of higher centers.

Spinal reflex activity is primitive and unlearned in spite of its seeming purposiveness. The character of the reflex response to stimulation of any particular area of the body surface is determined largely by the type of receptor stimulated. Thus, painful stimulation of the sole of the foot causes flexion of the leg which withdraws the foot from contact with the injurious agent. On the other hand, gentle pressure applied to the same point elicits extension of the leg due to stimulation of pressure receptors just under the skin (this "extensor thrust" reflex is an integral part of the whole reflex mechanism of walking, and is normally operative on contact of the sole of the foot with the ground).

All reflexes are not of this simple spinal type. Some involve higher brain centers (for example, the instantaneous turning of the head toward the source of a sudden, loud noise). Despite their greater complexity of nervous pathways, however, the general principles are the same as for the simpler types: a reflex is an invariable, predictable response to stimulation of a particular type of receptor, and the response always accomplishes a useful purpose which is related to the nature of the stimulus.

Volitional movements are initiated by impulses which are discharged from certain areas of the cerebral cortex. Cortical (volitional) activity differs from reflex activity in several respects, one of the most important of which is that, unlike spinal activity, it is unpredictable. This results from the fact that it is determined not only by the nature of the immediate stimulus (if any) lint also by the stored memories of past experiences.

The region of the cerebral cortex which discharges the impulses witch bring about voluntary movements is called the motor area; it is a narrow strip of tissue located just in front of the central fissure. The motor impulses arise in the large pyramidal cells and are transmitted along a bundle of nerve fibers, the pyramidal tract, down through the brain stem and spinal cord to terminate by synapsing with ventral horn (motor) cells of the spinal cord. Most of the pyramidal tibet's cross to the opposite side in passing through the brain stem, so that the motor area on the left side controls the muscles on the right side of the body, and vice versa.

Electrical stimulation of the motor area reveals that the muscles of each region of the body have their own distinct controlling areas; stimulation of the arm area o the cortex, for example, results in contraction of the arm muscles of the opposite side of the body. The size of the cortical area which controls the activity of a given group of muscles is determined not by the size of the muscle group, but rather by the complexity of its activity. For example, the cortical area which controls tinge/' movements is much larger than the cortical area for the entire trunk musculature. Another very significant fact revealed by electrical stimulation of the motor area is that this area controls the contractions of single, discrete muscles or small groups of muscles---quite different from the complex group contractions which characterize voluntary activity. If, however, the region just in front of the motor area (known as the "pre-motor area") is stimulated, complex group movements (flexion and extension, pronation, and supination) are elicited. Since this type of response cannot be obtained if the connections between the motor and the pre-motor areas are cut, it is evident that impulses originating in the pre-motor area travel to the motor area and there stimulate the pyramidal cells. What is the significance of this indirect method of control? The answer has been obtained from clinical eases in which the pre-motor area has been destroyed. There is no paralysis of voluntary muscles, but the ability to execute complex, learned types of skilled activities, such as playing the piano, is lost. We are now in a position to analyze the way in which motor skills are acquired by practice. In the beginning, performance of the component parts of the activity requires constant attention; it is controlled by the motor area. Gradually, smoothness and accuracy of performance are developed and constant attention is no longer required; control of the activity has now been transferred to the pre-motor area.

While the pre-motor area of the cerebral cortex thus initiates the muscle contractions which make up skilled activities and ensures that they are performed in proper sequence, other portions of the nervous system are required to adjust the strength, duration and range of muscle movements. The cerebellum, through its connections with the motor areas or' the cortex on the one hand, and with the proprioceptors of the muscles and joints on the other, is the key structure in this coordination. As a muscle begins to contract its muscle spindles and tendon organs are stimulated, and some of the impulses are transmitted up the spinal cord to the cerebellum. In this way the cerebellum is kept constantly informed of the strength of muscle contractions and of the range of movement at the joints. Through its connections with the motor arcas of the cortex, it is then able to increase or decrease cortical motor activity and thus adjust the strength, dural ion and range of muscle movements to the requirements of the act. With training visual memory may largely supplant tile more primitive cerebral mechanism.

Adjustment of the range or extent of movement necessary to accomplish a given act is largely a matter of experience. We learn to correlate our visual impressions of the necessary range of movemerit with the corresponding proprioceptive impulses from the muscle spindles, so that eventually we are able to make this adjustment in the absence of visual stimulation. For example, in learning to type, we must guide the movement of our fingers to the appropriate keys by sight. With practice proprioceptive impulses are sufficient to guide our movements. If the cerebellum is injured proprioceptive information is faulty, and our tendency is to over-reach or underreach the keys; constant visual guidance becomes necessary.

There is evidence that the sensory receptors in the muscles and joints may suffer fatigue in exhausting exercise. This may partially account for the faulty neuromuscular coordination which is often associated with extreme fatigue.