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SPINAL CORD IIReflexes of the

Spinal Cord

C.R. Houser

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Reflexes to be discussed -

• Stretch Reflex (Deep Tendon Reflex or Myotatic Reflex)

• Golgi Tendon Organ Reflex

• Flexor Withdrawal Reflex and Crossed Extension Reflex

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Muscle Spindle

Nolte 9-13

Key to “Deep Tendon” (Stretch) Reflexes

Emphasis on:• Nuclear bag fiber

• Primary (Ia) afferent (sensory) innervation

• Gamma motor innervation

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Dorsal Root Gang.

Extensor Muscle

Flexor Muscle

α motor

Dorsal Nuc. Of Clarke (C8-L2)

To Antagonist

Synergist

γ motor

Muscle Spindle

D. Spino-Cerebell. Tract

Stretch Reflex and Gamma Loop

Influences from: 1. Descending motor systems; and 2. Cutaneous afferents to Gamma and Alpha motor neurons

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Muscle Spindle Responses

Purves, 16.11

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Alpha and Gamma motor neuron coactivation is common.

Thus the muscle spindles remain sensitive even during muscle contraction.

Purves, 16.11

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Roles of the Muscle Spindle• Participates in automatic adjustments

of the body (i.e. to maintain posture).

• Compensates for changes in load during motor activity.

• Contributes to normal muscle tone.

• Involved in sense of limb position and movement.

8Purves, Fig. 16.10

Muscle spindle helps compensate for changes in load.

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Summary of Muscle Spindle Function and the Stretch Reflex

• Muscle spindles are located in parallel with regular muscle fibers.

• Primary afferents respond to stretch of the muscle and contraction of the ends of the spindle.

• Stimulation (stretch) of primary afferents of the spindle leads to facilitation of motor neurons to the same muscle.

• Gamma motor neurons receive input from cutaneous afferents and descending motor systems.

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Golgi Tendon Organ Reflex

Nolte 9-15 Purves 16.13

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Golgi Tendon Organ Responses

Purves, 16.12

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Roles of the Golgi Tendon Organ

• Provides negative feedback to regulate muscle tension.

• Helps maintain steady level of force.

• Contributes to fine adjustments in the force of contraction.

• Prevents muscles from generating excessive tension.

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Comparison of Muscle Spindle and Golgi Tendon Organ Activity

Purves, 16.12

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Comparison of Muscle Spindle and Golgi Tendon Organ Activity

Purves, 16.12

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Flexor Withdrawal Reflex

Crossed Extension Reflex

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Central Pattern Generators

Local circuits in the spinal cord can control complex, rhythmic patterns of movement, such as those in locomotion.

Purves 16.15

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We’ll continue next week with:

• Brainstem – General Organization

• Major Motor and Sensory pathways of the Spinal Cord

• Apply information – Spinal Cord Injuries