Muscle spindles detect _____; when stimulated they cause _____ of the agonist and _____ of the antagonist.

• A. Compression; relaxation; excitation
• B. Stretch; inhibition; excitation
• C. Load; contraction; relaxation
• D. Stretch; excitation; inhibition

Answer: (D)

Muscle spindles sense stretch in the muscle. When the muscle length increases, the spindle activates Ia afferent fibers that enter the spinal cord and trigger a stretch reflex: they directly excite the same muscle’s alpha motor neurons, causing contraction to resist the stretch, and they recruit inhibitory interneurons that suppress the antagonist’s motor neurons, leading to relaxation of the opposing muscle. This combination explains why the correct statement pairs detection of stretch with excitation of the agonist and inhibition of the antagonist. It’s helpful to contrast this with Golgi tendon organs, which detect load/tension and produce the opposite effect on the muscles.