Recruitment ofMotor Neurons

A single muscle typically receives inputs from hundreds of motor neurons. Thus, tension in the muscle can be increased by increasing the number of these motor neurons that are firing action potentials; the tension produced by activating individual motor units sums to produce the total tension in the muscle. A simplified example is shown in Figure ll-4. The increase in the number of active motor neurons is called recruitment of motor neurons and is an important physiological means of controlling muscle tension. When motor neurons are recruited into action during naturally occurring motor behavior, such as locomotion or lifting loads, the order of recruitment is determined by the size of the motor unit. As the tension in a muscle is increased, starting from the relaxed state, motor units containing a small number of muscle fibers are the first to be recruited; larger motor units are recruited later. Thus, when there is little

Time (msec)

Stimulate action potentials

Figure 11-4 A simple muscle consisting of four motor units of varying size. The graph (bottom) shows isometric tension in response to simultaneous action potentials (at the arrow) in various combinations of the motor neurons.

activity in the pool of motor neurons controlling a muscle and the tension in the muscle is low, small motor units are recruited to produce an increase in tension. This insures that the added increments of tension are small and prevents large jerky increases in tension when the tension is small. As tension increases, however, further increases in tension must be larger in order to make a significant difference; thus, larger motor units are added, resulting in larger increments of tension when the background tension is already high. This

Fast fiber

Slow fiber

Figure 11-5 Comparison of the speed of development of isometric tension in fast and slow muscle fibers.

Fast fiber

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0 50 100 150 200 250 Time after activation (msec)

0 50 100 150 200 250 Time after activation (msec)

behavior is referred to as the size principle in motor neuron recruitment. In Figure 11-4, for example, it would be expected that tension would be increased by adding the smaller motor units (numbers 1 and 2) first and the largest unit (number 4) last.

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