Answer
Recruitment or multiple motor unit summation
A motor unit is a single somatic motor neuron and all the muscle fibers it innervates. Some motor units are small ( 1:5) and some ar large (1: 1000)- neuron to muscle fibers. The muscle fibers of specific a motor unit are not necessarily clustered together, but are mingled with fibers of other motor units within a muscle.
A single stimulus at threshold may cause a weak twitch in a muscle. However, if the strength of the stimulus is increased ( still above threshold) a stronger twitch will result. This happens because stronger stimuli stimulate more and more nerve fibers in the motor nerve to fire ; this results in the contraction of more and more motor units. This process of causing increasing numbers of motor units to contract is called recruitment , also known as multiple motor unit (MMU) summation. This is one way in which the strength of muscle contractions are varied.
Work Step by Step
Application of a subthreshold stimulus to a muscle produces no twitch at all . A stimulus that is just at threshold may produce just a weak twitch. However, if stronger and stronger stimuli of greater voltages are applied they will induce stronger and stronger contractions. This happens because higher voltages excite more and more nerve fibers in a motor nerve, and therefore stimulate more and more motor units to contract.
High frequency stimuli can also produce stronger contractions. If the stimulus frequency is low, there will be a twitch for each stimulus, and it will subside before the next stimulus arrives. However, if the stimulus is higher in frequency (20-40 stim/sec), each new stimulus arrives before the contraction from the previous one has fully relaxed. So each stimulus, potentiated by the previous one, causes each succeeding twitch to achieve higher and higher tension. This is temporal summation
Wave summation happens when the muscle has time to relax only partially between stimuli. In such a case the myograph of the muscle contraction is a line of sustained fluctuating contraction. Such a line graph shows incomplete ( partially fused) tetanus.
Complete tetanus.
In laboratory preparations of frogs muscles,the stimuli can be applied at such a fast rate that the muscle does not have time to relax at all. In such a case the line graph shows one straight fused contraction with no relaxation between contractions. This is complete fused tetanus.
The nervous system cannot normally induce complete tetanus. This is just as well, since complete tetanus is injurious to muscles and associated tissues
It should be noted that all the motor units in a muscle do not normally contract together, Especially in large postural muscles, they take turns-- when one set becomes fatigued, others take over, This resists fatigue and enables the maintenance of postures, for example ,standing at attention, for long periods. This changing of shifts also facilitates smooth muscular movements.
