1 SPINAL CORD II Reflexes of the Spinal Cord C.R. Houser

2 Reflexes to be discussed - Stretch Reflex (Deep Tendon Reflex or Myotatic Reflex) Golgi Tendon Organ Reflex Flexor Withdrawal Reflex and Crossed Extension Reflex

3 Muscle Spindle Nolte 9-13 Key to “Deep Tendon” (Stretch) Reflexes Emphasis on: Nuclear bag fiber Primary (Ia) afferent (sensory) innervation Gamma motor innervation

4 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

5 Muscle Spindle Responses Purves, 16.11

6 Alpha and Gamma motor neuron coactivation is common. Thus the muscle spindles remain sensitive even during muscle contraction. Purves, 16.11

7 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.

8 Purves, Fig Muscle spindle helps compensate for changes in load.

9 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.

10 Golgi Tendon Organ Reflex Nolte 9-15 Purves 16.13

11 Golgi Tendon Organ Responses Purves, 16.12

12 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.

13 Comparison of Muscle Spindle and Golgi Tendon Organ Activity Purves, 16.12

14 Comparison of Muscle Spindle and Golgi Tendon Organ Activity Purves, 16.12

15 Flexor Withdrawal Reflex Crossed Extension Reflex

16 Central Pattern Generators Local circuits in the spinal cord can control complex, rhythmic patterns of movement, such as those in locomotion. Purves 16.15

17 We’ll continue next week with: Brainstem – General Organization Major Motor and Sensory pathways of the Spinal Cord Apply information – Spinal Cord Injuries