A specialized sensory mechanoreceptor of up to 20 muscle fibers with a wrapped neuritis that is stretch sensitive, embedded in muscle, and relaying information regarding muscle length and change of length (i.e., stretch). The muscle fibers within the spindle are referred to as intrafusal muscle fibers, while those that move the bones are extrafusal muscle fibers. The spindles themselves have both afferent and efferent connections, with the afferent fibers transferring information from activated stretch receptors to the nervous system. The intrafusal fibers receive input from their own pool of neurons (viz., gamma motoneurons), while extrafusal fibers are innervated by alpha motoneurons (see figure below for further details).
