A stretch receptor in vertebrate muscle. Muscle spindles are modified muscle cells that run parallel to normal muscle fibres; each consists of a capsule containing small striated muscle fibres (intrafusal fibres). Muscle spindles respond to stretching of the muscle by sending nerve impulses to the central nervous system via sensory nerves. These signals stimulate motor neurons to increase contraction of the muscle, thereby adjusting the muscle tone. The spindles thus play an important role in the subconscious maintenance of posture and movement. See also stretch reflex.