Central Nervous Control Mechanism

The whole issue of the nervous control within muscle is highly sophisticated and too complex to explain in any real detail within the scope of this book. The following is a much simplified outline. The nervous system regulates the tension and length of muscles by influencing the contractile element. In muscle, nerve cells that receive signals are called afferent or sensory neurons and those which send signals are motor-neurons or efferent neurons.

These signals are communicated via nerve fibres (axons) and there is a link between the cells sending or receiving signals either directly or through other neurons. The cell bodies are located in the spinal cord or brain-stem. Neurons which cause contraction in muscle are called alpha-motor-neurons, while others inhibit or block the alpha-motor-neurons causing a relaxation in the muscle. When motor-neurons in one muscle are stimulated, the communication processes between muscles ensure that the motor-neurons of the muscle working in opposition to it (the antagonist) are inhibited. This allows limbs to move.

Throughout muscle are embedded special fibres that contract only at their ends and are called muscle spindles. In the central part are stretch receptors of which there are two types, one responding to the extent of a stretch (static) and the other to both the extent and speed of the stretch (dynamic) and can be likened to a strain gauge. Muscle spindles in a stretched state send signals to the alpha-motor-neuroes causing the muscle to contract. This in turn reduces the stimulation of the muscle spindles and slows the speed of the stretch. In other words the feedback from the muscle spindle regulates the amount of stretch. Permanently stretching a muscle allows the point at which contraction begins to slow the extension of the muscle to be adjusted. The muscle will reach a greater length before this protection mechanism sets in. The result is a potentially faster and more flexible limb.

Gamma-motor-neurons regulate the tension of muscle spindles. They are located in the spinal cord close to, and are more sensitive than, the alpha-motor-neurons. If the muscle is contracted and hence the spindle shortened, the tension is reduced within the spindle. The spindle can internally readjust its length and restore the tension and still be able to detect further changes in the muscle length. It is this very feature that allows a weight to be moved by muscular contraction. As the muscle contracts the spindle shortens thereby reducing the tension. Resetting the length and tension allows further shortening of the muscle (contraction) at the same original tension. The effect is a fixed weight supported by different lengths of muscle or an isotonic (constant tension) contraction.

Conversely, different weights may be held isometrically (constant length). An increase in weight results in increased tension within the muscle which would cause it to lengthen under excessive load were it not for the muscle spindle correspondingly shortening. The alpha-motor-neurons impulse initiates a contraction to compensate. The result is that the length of the muscle remains the same. Golgi organs are located in tendons and monitor tension much in the same way that muscle spindles do within muscle. The contracting muscle pulls on the tendon causing Golgi organs to fire impulses proportional to the force of contraction. This inhibits the flow of impulses from alpha-motor-neurons (responsible for contraction) to this muscle. The greater the tension in the tendon then the greater the inhibition to further contraction. This is the mechanism upon which isometric stretching is based. Some Golgi organs have high thresholds and act as a safety feature in case of excessive contraction or excessive stretching. Without this feature, if the pull of contraction is too great there is a risk of tearing either the muscle or the tendon. The same applies to over stretching - the tendon could rupture. It is at full length where further lengthening causes the muscle to be so stretched that impulses are sent to the alpha-motor-neurons to contract and oppose the stretch. Golgi organs monitor the tension in the tendon to influence muscle tension. The lower threshold Golgi organs relay information about the tension of a tendon to the muscle motor centres, both alpha and gamma so forming a feedback loop to control the muscle/tendon structure. If the tension in the unloaded muscle is reduced to below the normal conscious resting level, fewer impulses are sent from the stretch receptors and the muscle responds by shortening in an attempt to re-establish the tension. This accounts for muscular stiffness when awakening.

Various other receptors (propioceptors) around the body which respond to pressure, temperature changes, speed, motion and the position of a limb in conjunction with the muscle spindle and Golgi organs, influence reflexive reactions to changes in the body position and tension, all of which are under involuntary (subconscious) control. This reflex reaction is a very complex interrelated response and usually the whole body responds to any stimulus. It is the reason that, for example, the side splits (body to the front, legs out to the side) is not easily performed even when adductor and abductor muscles are well stretched and relaxed. There is no physical connection between your legs by muscle or ligament, so why is it so difficult? Reflexes are arranged that under normal circumstances they perform a useful function. It would be impossible to walk properly without some form of control to keep your legs from sliding apart! Any outward movement of the legs is met with the reflexive response of tensioning the adductor muscles to counter the abnormal walking movement. Posture is maintained. Training can overcome reflexive regulation of muscular tension (involuntary control).

It is very important to realise that the brain remembers the most often performed actions. In any dynamic process like leg exercises be they kicks or stretches, continuing when tired will compromise the extent of the stretch. What this means is that such exercises should only be done until fatigue begins. The maximum range must be the last action. If the height or extent of stretch begins to diminish through tiredness then the remembered muscle length will be shorter. Such exercises can be done several times in any one day but do them only when the muscles are fresh. It is the same reason that you should not learn errors - they become too well established and difficult to undo.