ANY ACTION—ASCENDING A FLIGHT of stairs, typing on a keyboard, even holding a pose—requires coordinating the movement of body parts. This is accomplished by the interaction of the nervous system with muscle. The role of the nervous system is to activate the muscles that provide the forces needed to move in a particular way. This is not a simple task. Not only must the nervous system decide which muscles to activate, how much to activate them, and the sequence in which they must be activated in order to move one part of the body, but it must also control the influence of the resultant muscle forces on other body parts and maintain the required posture.
This chapter examines how the nervous system controls muscle force and how the force exerted by a limb depends on muscle structure. We also describe how muscle activation changes to perform different types of movement.
The Motor Unit Is the Elementary Unit of Motor Control
A Motor Unit Consists of a Motor Neuron and Multiple Muscle Fibers
The nervous system controls muscle force with signals sent from motor neurons in the spinal cord or brain stem to the muscle fibers. A motor neuron and the muscle fibers it innervates are known as a motor unit, the basic functional unit by which the nervous system controls movement, a concept proposed by Charles Sherrington in 1925.
A typical muscle is controlled by a few hundred motor neurons whose cell bodies are clustered in a motor nucleus in the spinal cord or brain stem. The axon of each motor neuron exits the spinal cord through the ventral root or through a cranial nerve in the brain stem and runs in a peripheral nerve to the muscle. When the axon reaches the muscle, it branches and innervates from a few to several thousand muscle fibers (Figure 31–1).
A typical muscle consists of many thousands of muscle fibers working in parallel and organized into a smaller number of motor units. A motor unit comprises a motor neuron and the muscle fibers it innervates, illustrated here by motor neuron A1. The motor neurons innervating one muscle are usually clustered into an elongated motor nucleus that may extend over one to four segments within the ventral spinal cord. The axons from a motor nucleus exit the spinal cord in several ventral roots and peripheral nerves but are collected into one nerve bundle near the target muscle. In the figure, motor nucleus A includes all those motor neurons innervating muscle A; likewise, motor nucleus B includes all the motor neurons that innervate muscle B. The extensively branched dendrites of each motor neuron (not shown in the figure) tend to intermingle with those of motor neurons from other nuclei.