Observations on neuromuscular connection between the vagus nerve and skeletal muscle

Abstract

The reinnervation of rat skeletal muscle fibres by the vagus nerve was studied. Vagal fibres in the rat can establish connections with regenerating muscle fibres and those muscle fibres supplied by the vagus become multiply innervated. The vagus nerve made connections with a normally innervated sternohyoid muscle when this muscle was rendered chemosensitive. Several endplates were formed by the vagus nerve on the same muscle fibre, and the structure of these differed from the endplates formed by the muscle's own nerve. If the muscle's own nerve and the vagus were cut and resutured into the sternohyoid muscle of the rat, many muscle fibres become innervated by their own nerve and by the vagus. Evidence is presented to show that each nerve established contacts at a separate site along the same muscle fibre. These connections were permanent, for even after 2 years following the operation the muscle was supplied by both nerves. Thus, in these experiments the muscle's own nerve did not ‘suppress’ the contacts made by an alien nerve. In rabbits the vagus and the accessory nerve were sutured into the sternomastoid muscle. Only a small proportion of the muscle became reinnervated by the vagus, whereas the accessory nerve reinnervated most of the muscle. Very few muscle fibres had become innervated by both nerves. The sternohyoid and sternomastoid are fast contracting muscles. When their fibres were reinnervated by the vagus nerve, their speed of contraction became significantly slower than those reinnervated by the original somatic motor nerve. The vagus nerve therefore determines the properties of the muscle fibres that it supplies. It is suggested that it is the slow tonic pattern of activity of the vagus nerves that determines the contractile speed of the muscles.