Which of the following excites the polar regions of the muscle - ProProfs Discuss
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Which of the following excites the polar regions of the muscle spindle?



A. Unmyelinated C fibers
B. 1A fibers
C. Gamma motor neurons
D. Alpha motor neurons
E. General visceral efferent fibers

This question is part of

USMLE step 1 Qs (10)
Asked by Dalston, Last updated: Apr 02, 2020

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2 Answers

L. Hawkes

L. Hawkes, Teacher, Memphis

Answered Feb 14, 2019

The correct answer to this question is C. A gamma motor neuron is a lower neuron that is a part of muscle contraction and is about 30% of the fibers that leave the spinal cord and go to the muscle. When gamma motor neurons are excited, they reset the spindle by stretching it, resulting in the lowering of the threshold for activation.

The firing of the gamma motor neuron will cause the fibers to contract. This is an important part of the muscle contraction because the mucle spindle's fibers will loosen. The amount of tension will determine if the neurons are functiong properly.

 

John Smith

John Smith

Answered Sep 08, 2016

Gamma motor neurons -gamma motor neurons innervate the polar regions of the muscle spindle and, when excited, cause resetting of the spindle by stretching it, resulting in a lowering of the threshold for activation of that receptor by an external force. unmyelinated c fibers mediate nociceptive sensations from the periphery to the spinal cord and thus do not relate to this question. 1a fibers arise from the nuclear region of the spindle and mediate spindle activity to the spinal cord and thus form the afferent limb of the monosynaptic stretch reflex. alpha motor neurons arise in the ventral horn of spinal cord and innervate extrafusal muscle fibers, causing movement of the limb when excited. it does not innervate the polar regions of the spindle. general visceral afferent fibers exit from the intermediolateral cell columns (at t1l3 for sympathetics and s2s4 for parasympathetics) of the spinal cord and innervate postganglionic neurons for these respective autonomic systems. such fibers, therefore, do not relate to muscle spindles, including their polar regions.
 

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