The muscles of the back contract to straighten so that you sit at a more erect posture and subsequently the muscles are suddenly stretched as you nod off. Which of the following best represents the sequence of events when you start to nod off?
(1) Muscles of the back are stretched.
(2) The muscle spindle is stretched.
(3) Action potentials in sensory neurons of the muscle spindles increase.
(4) Action potentials in alpha motor neurons of skeletal muscle fibers increase.
(5) Skeletal muscle of the back contract.

A) 1, 2, 3, 4, 5
B) 1, 5, 2, 3, 4
C) 1, 5, 3, 2, 4
D) 5, 2, 3, 4, 1
E) 4, 5, 3, 2, 1

Question

SaiAnni Hyatt · Accepted Answer

Final Answer : A, Explanation :  The sequence starts with the muscles of the back being stretched as you nod off, which then stretches the muscle spindle (a sensory receptor within the muscle). This stretching leads to an increase in action potentials in the sensory neurons of the muscle spindles. The increased action potentials in the sensory neurons then lead to an increase in action potentials in the alpha motor neurons of the skeletal muscle fibers, causing the skeletal muscles of the back to contract and straighten the posture.

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