Which of the following events accompanies the rapid voluntary flexion of the arm?
Correct Answer: An increase in the activity of the Ia afferent fibers from the triceps (the antagonist)
Description: The firing of the alpha motoneurons to the biceps muscle produces rapid flexion of the arm. Because the arm is rapidly flexed, the Ta afferent neurons innervating the muscle spindles in the biceps, which detect muscle length, will reduce their firing rate. The lb afferents, innervating the Golgi tendon organs (GTO) from the biceps, will detect the contractile activity of the biceps and increase their firing rate. The triceps are stretched during the arm flexion and so their la afferents will increase their firing rate. lb afferents do not respond when muscles are passively stretched, so their firing rate will not change.
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