Active tension in a muscle depends upon ?

Ans. is 'b' i.e., Number of motor units recruitedA motor neuron (nerve fiber) with all its peripheral branches and muscle fibers innervated by it is called motor unit. The motor-unit obey all or none law, i.e., when the nerve fiber (neuron) of the motor unit is stimulated, all the fibers of motor unit will either contract maximally or not contract at all, depending upon whether the stimulus is of threshold or subthreshold intensity. Since the unit of activation is nerve fiber, the unit of contraction is a motor unit.Muscles which are concerned with fine graded and precise movement (e.g., fingers and eye ball) have only 3-6 fibers in a motor unit. On the other hand, muscles for gross movements (e.g., leg) contain 600-1000 muscle fibers per unit. If an isolated muscle is stimulated the contractile response depends on the strength of stimulus. The dependence of the strength of contractile response on the strength of stimulus seem at first to violate the all or none followed by excitable tissue. But in fact skeletal muscle also follows all or none low at cellular level, i.e., at motor unit level (read above : motor unit). Each motor unit will either contracts with full force or not at all. But the number of motor units contracting can be altered. The larger the number of motor units which contract, stronger is the contraction of muscle (whole muslce).So, with increasing strength of stimulus, it is the number of contracting motor units which increases and resulting in increased strength of contraction.