A reexamination of multiaxonal nerve endings innervating intrafusal muscle fibers of the Chinese hamster.

The multiaxonal nerve endings, consisting of a bundle of axons, were reexamined in the muscle spindles of the adult Chinese hamster. In addition to the frequent occurrence of the multiaxonal endings innervating the nuclear bag fibers in the equatorial region (Desaki and Uehara, 1988), these endings were also occasionally distributed not only in the equatorial region of the nuclear chain fibers, adjacent to annulospiral sensory endings, but also in the polar region of the same muscle fiber. They were derived from non-myelinated stem nerves becoming varicose along their course. Terminal axons composing the endings were almost naked, except at the initial muscle-contact portion, where they were surrounded by Schwann cell processes. They contained a significant number of clear synaptic vesicles and large granulated vesicles, and were closely apposed to each other and to the muscle cell surface without an interposing basal lamina. Some terminal axons formed synaptic contacts with each other and with the muscle cell surface. These structural features, including their wide distribution, suggest that the multiaxonal endings may belong to the autonomic nerves rather than the fusimotor nerves.