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Chapter 12. Reflex & Voluntary Control of Posture & Movement

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A medical student was doing research in a neurophysiology laboratory that studied the role of γ-motor neurons in control of group Ia spindle sensitivity. As an initial project he recorded the group Ia muscle spindle afferent activity during stimulation of only α-motor neurons to the same muscle with that seen during simultaneous stimulation of α- and γ-motor neurons to that muscle. What is he likely to observe?

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A. During stimulation of only α-motor neurons, group Ia spindle afferent activity will be markedly increased; during stimulation of both α- and γ-motor neurons, group Ia spindle afferent activity will be suppressed.

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B. During stimulation of only α-motor neurons, group Ia spindle afferent activity will be inhibited; during stimulation of both α- and γ-motor neurons, group Ia spindle afferent activity will increase.

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C. During stimulation of only α-motor neurons, group Ia spindle afferent activity will be moderately reduced; during stimulation of both α- and γ-motor neurons, group Ia spindle afferent activity will be further reduced.

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D. During stimulation of only α-motor neurons, group Ia spindle afferent activity will be moderately increased; during stimulation of both α- and γ-motor neurons, group Ia spindle afferent activity will be markedly increased.

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E. During stimulation of only α-motor neurons, group Ia spindle afferent activity will be moderately increased; during stimulation of both α- and γ-motor neurons, group Ia spindle afferent activity will be further increased.

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The correct answer is B. The Ia spindle afferents characteristically stop firing when the muscle contracts during electrical stimulation of only the α-motor neurons to the extrafusal fibers because the muscle shortens and the spindle is unloaded (rules out options A, D, and E). Stimulation of γ-motor neurons cause the contractile ends of the intrafusal fibers to shorten and therefore stretches the nuclear bag portion of the spindles, deforming the endings, and initiating impulses in the Ia fibers (rules out options A and C).

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An MD/PhD candidate was interested in studying factors that control skeletal muscle force, including the inverse myotatic reflex. In her thesis proposal she outlines the fundamental elements of this reflex. What are these elements?

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A. Muscle spindles, group Ia spindle afferents, and α-motor neurons

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B. Muscle spindles, group II spindle afferents, spinal excitatory interneurons, and α-motor neurons

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C. Golgi tendon organ, group II tendon afferents, spinal excitatory interneurons, and α-motor neurons

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D. Golgi tendon organ, group Ib afferents, spinal inhibitory interneurons, and α-motor neurons

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