What do muscle spindles detect?

• A. Change in joint angle
• B. Change in tendon tension
• C. Change in bone density
• D. Change in muscle length and stretch

Answer: (D)

Muscle spindles are proprioceptors that monitor the length of a muscle and how quickly that length is changing. They are embedded in skeletal muscle as intrafusal fibers with sensory endings that respond when the muscle is stretched. When the muscle lengthens, the spindle is stretched and sends signals (through Ia and II afferents) to the spinal cord about both the current length and the rate of stretch. This information helps the nervous system adjust muscle length and tone, and it underlies the stretch reflex that resists sudden lengthening. So they detect changes in muscle length and stretch, not joint angle or tendon tension (the latter is sensed by Golgi tendon organs).