A study of factors influencing synapse formation by a foreign nerve in skeletal muscle of Rana pipiens.

The possible influence of the pathway taken by regenerating axons on the characteristics of the synapses they form was investigated by tying the proximal stump of the hypoglossal nerve (h.n.) to the distal stump of the cutaneus pectoris (c.p.) nerve. This technique allowed the axons from the h.n. to innervate the c.p. muscle via the neural tubes of the degenerated original nerve. The synapses formed by the inappropriate hypoglossal axons in the c.p. muscle after tying the two nerves together were more efficient than synapses formed by the h.n. in the c.p. muscle in a previous study, when the hypoglossal axons had grown across the muscle fibres in an apparently disorganized fashion, rather than within the neural tubes. However, the synapses formed by hypoglossal axons after tying the h.n. and c.p. nerves together were still abnormal compared to unoperated or self-reinnervated muscles. In a further experiment, the brachial nerve (b.n.), which originates from the same spinal segments as the nerve to the c.p. muscle was tied to the distal stump of the c.p. nerve. Synapses formed in the c.p. muscle by the regenerated brachial axons were more efficient than those formed by the h.n. in the c.p. muscle. These results suggest that both the pathway taken by regenerating axons and the segmental origin of the nerves may influence the characteristics of synapses formed by a foreign nerve in frog skeletal muscle.