The pattern of innervation of a polyneural muscle: axolotl iliotibialis

The pattern of innervation on individual iliotibialis muscle fibres from axolotl (Ambystoma mexicanum) has been investigated histologically and electrophysiologically. These polyneural fibres were found to be innervated on average at five end plate sites. The sites were distributed irregularly along each fibre. Average end plate length was found to be approximately 70 micrometer. Most end plates were separated by less than 1000 micrometer; 26% by less than 150 micrometer; the average separation was 516 micrometer. Advantage was taken of the dual innervation of the muscle to investigate the separation between synaptic terminals from different axons. Some individual fibres were found to be innervated by axons from two different spinal nerves. End plate sites on dually innervated fibres were located by ACh iontophoresis. 30% of such sites were found to be innervated by more than one axon terminal. The average separation of such sites was found to be 9 micrometer. Four different axons were found to innervate some individual muscle fibres. It is suggested that the unusual ability of axolotl muscle fibres to accept synaptic terminals from different axons at closely adjacent sites may be a major factor underlying selective reinnervation in this animal.