Control of Body Movement
301
Action potentials
Intrafusal
fiber
Extrafusal muscle fiber
Time
Time
Stretch
Alpha motor neuron
action potentials
Contraction
Time
Contraction
Muscle stretch
(a)
Alpha-gamma coactivation
(c)
Extrafusal fiber contraction
Alpha motor neuron
action potentials
Gamma motor neuron
action potentials
(b)
Afferent
neuron
Figure 10–5
(a) Passive stretch of the muscle by an external load activates the spindle stretch receptors and causes an increased rate of action potentials in the
afferent nerve. (b) Contraction of the extrafusal fi bers removes tension on the stretch receptors and lowers the rate of action potential fi ring.
(c) Simultaneous activation of alpha and gamma motor neurons results in maintained stretch of the central region of intrafusal fi bers. Afferent
information about muscle length continues to reach the central nervous system.
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