This transverse section (HA12) is taken from the lumbrical muscle, one of the small, highly innervated, intrinsic muscles of the hand that is involved in fine movements of the fingers. Coordination of these movements is accomplished by large numbers of proprioceptors, called muscle spindles. Spindles monitor the state and rate of local muscle contraction. Each spindle is surrounded by a cellular sheath that is the perineurium. Inside the sheath are cross-sections of very small, special myofibers called intrafusal fibers.