In: Anatomy and Physiology
Recruitment- Recruitment of the motor unit refers to the activation of additional motor units to accomplish an increase in contractile strength in a muscle.
Summation- Summation is the additive effect of several electrical impulses on a neuromuscular junction, the junction between a nerve cell and a muscle cell.
There is a wide range of motor units within many skeletal muscles that gives the nervous system a wide range of control over the muscle. The small motor units in the muscle have smaller, lower-threshold motor neurons that are more excitable, firing first to their skeletal muscle fibers. Activation of these smaller motor units, results in a relatively small degree of contractile strength generated in the muscle. As more strength is needed, larger motor units, with bigger, higher-threshold motor neurons are enlisted to activate larger muscle fibers. This increasing activation of motor units produces an increase in muscle contraction known as Recruitment. As more motor units are recruited, the muscle contraction becomes progressively stronger.
Summation also results in greater contraction of the motor unit because if the fibers are stimulated while a previous twitch is still occurring, the second twitch will be stronger. The excitation-contraction coupling effects of successive motor neuron signaling is summed or added together in summation.