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Monday Jan. 16
Chapter 15
rm
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
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Threshold for activation
• Small slow motor units have low
threshold of activation
– Are tonically active (posture)
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Threshold of activation
• For large, fast motor units
– Reached for rapid movements with great
force (eg. Jumping)
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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)
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Motor unit recruitment
• Progressive increase in muscle tension
from motor units recruiting according to
their size.
• Weak stimulus activates small motor
units
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Motor unit recruitment
• Low threshold S units
• --> FR units
• --> FF units
• The size principle: systematic relationship for orderly
recruitment.
• Fig. 15.6
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Effect of action potential frequency
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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.
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Muscle stretch reflexes
• Response to muscle stretch
• --> direct excitatory feedback to
motorneurons for the stretched muscle
• Help to regulate the degree of muscle
contraction.
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Muscle spindle
• 8 - 10 intrafusal fibers
• Large sensory fibers (1
afferents)
• Largest axons in the
nerves.
• Rapid conduction
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Muscle spindle
• Rapid reflex adjustments
• Afferent axons have mechanically gated
ion channels around the spindle
• Monosynaptic excitatory connections
with motor neurons
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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.
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Muscle spindle
• Unusual
monosynaptic reflex
• Responsible for
muscle tone
• A negative feedback
loop to keep muscle
at desired length.
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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
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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
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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.
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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
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Golgi tendon
organ
• Sensitive to
changes in tension
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