The Motor Unit

A single motor neuron makes synaptic contact with a number of muscle fibers. The actual number varies considerably from one muscle to another and from one motor neuron to another within the same muscle; a single motor neuron may contact as few as 10 -20 muscle fibers or more than 1000. However, in mammals, a single muscle fiber normally receives synaptic contact from only one motor neuron. Therefore, a single motor neuron and the muscle fibers to which it is connected form a basic unit of motor organization called the motor unit. A schematic diagram of the organization of a motor unit is shown in Figure 11-1. Recall from Chapter 8 that the synapse between a motor neuron and a muscle fiber is a one-for-one synapse that is, a single presynaptic action potential produces a single postsynaptic action potential and hence a single twitch of the muscle cell. This means, then, that all the muscle cells in a motor unit contract together and that the fundamental unit of contraction of the whole muscle will not be the contraction of a single muscle fiber, but the contraction produced by all the muscle cells in a motor unit.

Gradation in the overall strength with which a particular muscle contracts is under control of the nervous system. There are two basic ways the nervous system uses to accomplish this task: (1) variation in the total number of motor neurons activated, and hence in the total number of motor units contracting;

Figure 11-1 Schematic illustration of the innervation of a small number of muscle fibers in a muscle. The shaded muscle fibers form part of the motor unit of motor neuron I and the unshaded fibers form part of the motor unit of motor neuron 2.

and (2) variation in the frequency of action potentials in the motor neuron of a single motor unit. The greater the number of motor units activated, the greater the strength of contraction; similarly, within limits, the greater the rate of action potentials within a motor unit, the greater the strength of the resulting summed contraction. We will consider each ofthese mechanisms in turn below.

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