Neural Control of Skeletal Muscles

Skeletal muscles contain stretch receptors called muscle spindles that stimulate the production of impulses in sensory neurons when a muscle is stretched.These sensory neurons can synapse with alpha motoneurons, which stimulate the muscle to contract in response to the stretch. Other motor neurons, called gamma motoneurons, stimulate the tightening of the spindles and thus increase their sensitivity.

Motor neurons in the spinal cord, or lower motor neurons (often shortened to motoneurons), are those previously described that have cell bodies in the spinal cord and axons within nerves that stimulate muscle contraction (table 12.6). The activity of these neurons is influenced by (1) sensory feedback from the muscles and tendons and (2) facilitory and inhibitory effects from upper motor neurons in the brain that contribute axons to descending motor tracts. Lower motor neurons are thus said to be the final common pathway by which sensory stimuli and higher brain centers exert control over skeletal movements.

Fox: Human Physiology, Eighth Edition

1 12. Muscle: Mechanisms of 1 Text 1 1 © The McGraw-Hill 1 Contraction and Neural Companies, 2003 Control

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Chapter Twelve

Table 12.6 A Partial Listing of Terms Used to Describe the Neural Control of Skeletal Muscles

Term

Description

1. Lower motoneurons

2. Higher motoneurons

3. Alpha motoneurons

4. Gamma motoneurons

5. Agonist/antagonist

6. Synergist

7. Ipsilateral/contralateral

8. Afferent/efferent

Neurons whose axons innervate skeletal muscles—also called the "final common pathway" in the control of skeletal muscles Neurons in the brain that are involved in the control of skeletal movements and that act by facilitating or inhibiting (usually by way of interneurons) the activity of the lower motoneurons Lower motoneurons whose fibers innervate ordinary (extrafusal) muscle fibers Lower motoneurons whose fibers innervate the muscle spindle fibers (intrafusal fibers) A pair of muscles or muscle groups that insert on the same bone, the agonist being the muscle of reference A muscle whose action facilitates the action of the agonist

Ipsilateral—located on the same side, or the side of reference; contralateral—located on the opposite side Afferent neurons—sensory; efferent neurons—motor

ift The disease known as amyotrophic lateral sclerosis (ALS) involves degeneration of the lower motor ^ neurons, leading to skeletal muscle atrophy and paralysis. This disease is sometimes called Lou Gehrig's disease, after the baseball player who suffered from it, and also includes the famous physicist Steven Hawking among its victims. Scientists have recently learned that the inherited form of this disease is caused by a defect in the gene for a specific enzyme— superoxide dismutase. This enzyme is responsible for eliminating superoxide free radicals, which are highly toxic products that can damage the motor neurons. The mutant gene produces an enzyme that has a different, and in fact destructive, action.

The cell bodies of lower motor neurons are located in the ventral horn of the gray matter of the spinal cord (chapter 8). Axons from these cell bodies leave the ventral side of the spinal cord to form the ventral roots of spinal nerves (see fig. 8.24). The dorsal roots of spinal nerves contain sensory fibers whose cell bodies are located in the dorsal root ganglia. Both sensory (afferent) and motor (efferent) fibers join in a common connective tissue sheath to form the spinal nerves at each segment of the spinal cord. In the lumbar region there are about 12,000 sensory and 6,000 motor fibers per spinal nerve.

About 375,000 cell bodies have been counted in a lumbar segment—a number far larger than can be accounted for by the number of motor neurons. Most of these neurons do not contribute fibers to the spinal nerve. Rather, they serve as interneu-rons, whose fibers conduct impulses up, down, and across the central nervous system. Those fibers that conduct impulses to higher spinal cord segments and the brain form ascending tracts, and those that conduct to lower spinal segments contribute to descending tracts. Those fibers that cross the midline of the CNS to synapse on the opposite side are part of commissural tracts. Interneurons can thus conduct impulses up and down on the same, or ipsilateral, side, and can affect neurons on the opposite, or contralateral, side of the central nervous system.

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Responses

  • annunziata
    What is the final common pathway to the skeletal muscles?
    8 years ago
  • Joel
    Is final common pathway for skeletal muscle contraction?
    8 years ago
  • ruby
    What is the neural control of the skeletal muscle contraction?
    8 years ago
  • tito
    How do motor neurons stimulate skeletal muscle?
    8 years ago
  • Yonatan
    What is neural control physiology?
    7 years ago
  • bandobras brandybuck
    What neurons stimulate skeletal muscles?
    7 years ago
  • darryl
    What is involved in nuero control of muscles?
    6 years ago
  • RIHANNA WATSON
    How does the brain control skeletal muscles?
    1 year ago
  • CORY
    What is the structure that allows neurons to control skeletal muscles?
    7 months ago

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