Reciprocal Innervation and the Crossed Extensor Reflex

In the knee-jerk and other stretch reflexes, the sensory neuron that stimulates the motor neuron of a muscle also stimulates interneu-rons within the spinal cord via collateral branches. These interneu-rons inhibit the motor neurons of antagonist muscles via inhibitory postsynaptic potentials (IPSPs). This dual stimulatory and inhibitory activity is called reciprocal innervation (fig. 12.29).

When a limb is flexed, for example, the antagonistic extensor muscles are passively stretched. Extension of a limb similarly stretches the antagonistic flexor muscles. If the monosynaptic stretch reflexes were not inhibited, reflex contraction of the

Ipsps Inhibitory Postsynaptic Potentials

Spinal cord

Sensory neuron

Sensory neuron

Diagram Golgi Tendon Reflex

Golgi tendon organ

Golgi tendon organ

■ Figure 12.28 The action of the Golgi tendon organ. An increase in muscle tension stimulates the activity of sensory nerve endings in the Golgi tendon organ. This sensory input stimulates an interneuron, which in turn inhibits the activity of a motor neuron innervating that muscle. This is therefore a disynaptic reflex.

352 Chapter Twelve

352 Chapter Twelve

Reciprocal Innervation

■ Figure 12.29 A diagram of reciprocal innervation. Afferent impulses from muscle spindles stimulates alpha motoneurons to the agonists muscle (the extensor) directly, but (via an inhibitory interneuron) they inhibit activity in the alpha motoneuron to the antagonist muscle.

antagonistic muscles would always interfere with the intended movement. Fortunately, whenever the "intended," or agonist muscles, are stimulated to contract, the alpha and gamma motoneurons that stimulate the antagonist muscles are inhibited.

The stretch reflex, with its reciprocal innervations, involves the muscles of one limb only and is controlled by only one segment of the spinal cord. More complex reflexes involve muscles controlled by numerous spinal cord segments and affect muscles on the contralateral side of the cord. Such reflexes involve double reciprocal innervation of muscles.

Double reciprocal innervation is illustrated by the crossed-extensor reflex. If you step on a tack with your right foot, for example, this foot is withdrawn by contraction of the flexors and relaxation of the extensors of your right leg. The contralateral left leg, by contrast, extends to help support your body during this withdrawal reflex. The extensors of your left leg contract while its flexors relax. These events are illustrated in figure 12.30.

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Responses

  • laura eisenhower
    What are the steps of muscle innervations?
    8 years ago
  • Gerda
    How use reciprocal innervation to stretch?
    8 years ago
  • anna
    How is reciprocal innervation important for human activity?
    8 years ago
  • SARA
    What is tendon reflex reciprocal innervation?
    8 years ago
  • dewitt
    What stimulates tendon reflexes?
    7 years ago
  • Fredegar Lightfoot
    How reciprocal inhibition stepping on a tack?
    7 years ago
  • reima
    What is the significance of reciprocal innervation and double innervationn a muscle reflex?
    6 years ago
  • ibrahim
    How do reciprocal innervation and the crossed extensor reflex assist the withdrawal reflex?
    5 years ago
  • nestore
    Why must the crossed extension reflex accompany a withdrawal reflex of the leg?
    11 months ago
  • kian
    What happens when you suffer a condition that reverses a crossed extensor reflex?
    10 months ago
  • STEPHEN HANRAHAN
    What is recripical innervation?
    6 months ago
  • Poppy
    What is the role for reciprocal innervation in stretch reflexes?
    5 months ago
  • aldo pisani
    Does A stretch reflex exhibits reciprocal innervation?
    3 months ago
  • Franco
    How does cross extensor reflex compare to reciprocal innervation?
    7 days ago

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