SOME FACTS ABOUT MUSCLE ACTION
1. We have seen that a muscle fibre requires a stimulus for contraction to occur. Stimuli below a threshold level do not cause contraction. When a stimulus above the threshold strength is applied the muscle contracts to its full extent. This is the all or none law.
2. It is very important to understand that the all or none law applies only to individual muscle fibres and not to the muscle as a whole. The force exerted by a muscle depends on the number of muscle fibres that are in contraction at a particular moment. This allows for a graded strength of muscle action depending upon need. A corollary of this is that large muscles with greater number of fibres can exert more power than smaller muscles. The concept of motor units has been already discussed. Large muscles that are required to act with great strength have larger motor units.
3. If a muscle fibre is stimulated repeatedly it fails to contract after some time. This is musclefatigue. Fatigue is caused by exhaustion of acetyl choline in the motor end plate, lack of oxygen andnutrients, and accumulation of lactic acid.
4. In a resting muscle some fibres are always in a state of contraction. After some time other fibres take over this function so that fatigue does not occur. This partial contraction gives the muscle its normal state of firmness. This is called muscle tone. If this tone was not present the body would collapse. Muscle tone is controlled by various reflexes dependant on impulses arising in muscle spindles and tendon end organs. These have already been described.
5. When the nerve supply of a muscle is interrupted, tone is lost and the muscle becomes flaccid. Partial loss of tone is calledhypotonia. In upper motor neuron lesions the tone is exaggerated (hypertonia) and the muscle becomes rigid.
6. The capacity of a muscle to maintain activity over a period of time is called endurance. This depends mainly on availability of glycogen. It is increased by training, as in atheletes. In most persons endurance reaches a maximum by the age of twenty, and declines after the age is fifty.
7. Apart from production of muscle contraction, nerve supply has a trophic effect. This maintains the integrity of the muscle. Denervation of a muscle leads to atrophy.
The functional status of a muscle is also influenced by activity (or the lack of it). If a normal limb is immobilised (e.g. by a plater cast) there is some degree of atrophy and the muscles become weak. Strength can be regained by suitable exercises (physiotherapy). Similarly, mormal muscles can hypertrophy and become stronger by exercise (as in atheletes).
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