Monday Jan. 16 Chapter 15 rm 357

1. Monday Jan. 16Chapter 15rm 357 • Threshold for activation • Examples of motor units • Motor unit recruitment • The muscle spindle • The influence of sensory activity on movement • Other sensory feedback affects motor performance 03-44-485

2. Threshold for activation • Small slow motor units have low threshold of activation • Are tonically active (posture) 03-44-485

3. Threshold of activation • For large, fast motor units • Reached for rapid movements with great force (eg. Jumping) 03-44-485

4. Examples of motor units • In the soleus (posture) • Small motor units (180) • In the gastrocnemus • Mixture of small and large motor units (up to 2000) • In extra-ocular muscle • Tiny motor units (3) 03-44-485

5. Motor unit recruitment • Progressive increase in muscle tension from motor units recruiting according to their size. • Weak stimulus activates small motor units 03-44-485

6. Motor unit recruitment • Low threshold S units • --> FR units • --> FF units • The size principle: systematic relationship for orderly recruitment. • Fig. 15.6 03-44-485

7. Effect of action potential frequency • Contributes to muscle tension • Summation of muscle contractions • Unfused tetanus for normal conditions • Smooth contraction because of asynchronous firing of different motor neurons and tension averages. 03-44-485

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9. Muscle stretch reflexes • Response to muscle stretch • --> direct excitatory feedback to motorneurons for the stretched muscle • Help to regulate the degree of muscle contraction. 03-44-485

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11. Muscle spindle • 8 - 10 intrafusal fibers • Large sensory fibers (1 afferents) • Largest axons in the nerves. • Rapid conduction 03-44-485

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13. Muscle spindle • Rapid reflex adjustments • Afferent axons have mechanically gated ion channels around the spindle • Monosynaptic excitatory connections with motor neurons 03-44-485

14. Reciprocal innervation • Excitatory synapse on the muscle that was stretched • Local circuit neurons make inhibitory synapse on antagonist muscle • Rapid contraction of stimulated muscle • Relaxation of antagonist muscle. 03-44-485

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16. Muscle spindle • Unusual monosynaptic reflex • Responsible for muscle tone • A negative feedback loop to keep muscle at desired length. 03-44-485

17. Sensory activity and movement • The activity of the motor neuron can be adjusted by: • Upper motor neurons • Local reflex circuits • The motor neuron is the gain that adjusts functional requirements. • motor neuron works with  motor neurons during voluntary adjustments 03-44-485

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19. An example of motor neuron modulation of muscle spindle responses: • Activate  mn, but not mn • --> extrafusal muscle contracts • --> intrafusal muscle does not contract • --> spindle afferent (1a) is not stimulated 03-44-485

20. An example of motor neuron modulation of muscle spindle responses: • Both  and  mns are activated. • --> extrafusal and intrafusal muscles contract • --> 1a fiber keeps firing. • The  mns are needed for the 1a fibers to function and for the muscle spindle to function during changes in muscle length. 03-44-485

21. Another sensory feedback for locomotion • Golgi tendon organ • Encapsulated • Located at tendon/muscle junction • 1b sensory fiber • In series with muscle • Muscle contraction exerts force on tendon and on Golgi tendon organ 03-44-485

22. Golgi tendon organ • Sensitive to changes in tension 03-44-485

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