Answer
Muscle spindles are activated when central areas of the intrafusal fibers are stretched.
Work Step by Step
Muscle spindles are proprioceptors in skeletal muscles that monitor changes in muscle length-- they also function in stretch reflexes. Each spindle has several slowly adapting sensory nerve endings that wrap around an average of 6 special muscle fibers called intrafusal fibers.
The main function of the muscle spindle is to measure muscle length--the degree of stretch-- of muscle fibers. Sudden or prolonged stretch of the central region of intrafusal fibers stimulates sensory nerve endings with resultant generation of nerve impulses. These nerve impulses (APs) then propagate into the CNS.
