Morphology of certain sensory and motor endings in muscle spindles of the rabbit's diaphragm.

The morphological features of certain sensory and motor nerve endings in muscle spindles in the rabbit's diaphragm were examined using a variety of techniques employed in light microscopy. The nuclear accumulation in the equatorial region of the spindle is encased in a dense capsule penetrated by both sensory and motor fibers. The sensory fibers from a profuse internuclear network terminating in round and triangular buttons in direct contact with the sarcoplasmic membrane of the muscle fiber. Motor end plates trail and plate endings are distributed throughout the spindle save the equatorial region. Muscle spindle motor end plates typify those observed on extrafusal muscle fibers. The trail and plate endings are readily identified since the plate axons are short in length giving rise to claw and bulb type terminal. The terminals are directly applied to the muscle fiber. The elongated axon of the trail fiber terminates in either thin tapers or in brush-like constructions.