What is the effect of muscle spindle activity on the agonist and antagonist muscles?

• A. Agonist inhibitory; antagonist facilitatory
• B. Agonist facilitatory; antagonist inhibitory
• C. Both inhibitory
• D. Both facilitatory

Answer: (B)

When a muscle is stretched, its muscle spindle detects that stretch and increases the firing of Ia sensory afferents. This signal enters the spinal cord and directly excites the alpha motor neurons of the same muscle, causing it to contract—the agonist becomes facilitatory. At the same time, the Ia input engages inhibitory interneurons that suppress the alpha motor neurons to the opposing muscle, the antagonist, reducing its activity. This reciprocal inhibition ensures the antagonist relaxes as the agonist contracts, allowing smooth adjustment to the stretch. In short, muscle spindle activity promotes facilitation of the agonist and inhibition of the antagonist.