34: The Motor Unit and Muscle Action

Introduction

• The Motor Unit Is the Elementary Unit of Motor Control

  • A Motor Unit Consists of a Motor Neuron and Multiple Muscle Fibers

  • The Properties of Motor Units Vary

  • Physical Activity Can Alter Motor Unit Properties

  • Muscle Force Is Controlled by the Recruitment and Discharge Rate of Motor Units

  • The Input–Output Properties of Motor Neurons Are Modified by Input from the Brain Stem

• Muscle Force Depends on the Structure of Muscle

  • The Sarcomere Contains the Contractile Proteins

  • Noncontractile Elements Provide Essential Structural Support

  • Contractile Force Depends on Muscle Fiber Activation, Length, and Velocity

  • Muscle Torque Depends on Musculoskeletal Geometry

• Different Movements Require Different Activation Strategies

  • Contraction Velocity Can Vary in Magnitude and Direction

  • Movements Involve the Coordination of Many Muscles

  • Muscle Work Depends on the Pattern of Activation

• An Overall View

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 just those muscles that will exert the force needed to move in a particular way. This is not a simple task: Not only must the nervous system decide which muscles to activate and how much to activate them in order to move one part of the body, but it must also control muscle forces on other body parts and maintain 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 differs with 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 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 (Figure 34–1). 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.