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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
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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.
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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.
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The Motor Unit Is the Elementary Unit of Motor Control
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A Motor Unit Consists of a Motor Neuron and Multiple Muscle Fibers
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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.
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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.
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