Motor innervation of striated oesophageal muscle. Part 2. Characteristics of the oesophagomotor fibres in the rat studied by implanting the vagus nerve into a skeletal muscle.

In 12 rats the right vagus nerve distal to its recurrent laryngeal branch was implanted into the inferior segment of the denervated sternohyoid muscle. One month after implantation the first signs of neuromuscular transmission at the vagal motor endings could be recorded. Two months after implantation the reinnervated muscles showed vigorous contractions on electrical stimulation of the vagus nerve. During the performance of propulsive waves of the oesophagus the implanted vagus nerve caused clonic to tetanic contractions of the sternohyoid muscle, thus proving the oesophagomotor genesis of the reinnervating nerve fibres. In addition, the vagus-innervated motor end-plates were shown to exhibit the same ultrastructural peculiarities as the original neuromuscular junctions of the oesophagus. In sections stained for cholinesterase it could be demonstrated that the oesophagomotor fibres had preferentially reinnervated the denervated motor end-plates. In many instances the subneural apparatus was not completely covered by the vagal axon terminals. Newly formed, ectopic vagal motor endings were few in number and confined to muscle fibres immediately adjacent to the site of nerve-implantation. Six months after implantation some of the vagal motor endings showed signs of degeneration. As in the oesophagus, the reinnervating oesophagomotor fibres proved to be unmyelinated, sometimes forming a plexus-like intramuscular network before terminating at motor end-plates. Myelinated vagal nerve fibres were also observed running between the skeletal muscle fibres, but they did not establish any demonstrable form of neuromuscular contacts. It was concluded that, in the rat, the myelinated fibres of the oesophageal nerves are afferent, whereas the oesophagomotor fibres, although supplying striated muscle, are unmyelinated.