1. Histological and electrophysiological studies of the spinal nerves, nerve roots and muscles of the abdominal wall of the marine teleost Cottus scorpius have been undertaken to determine the extent and nature of polyneuronal innervation of the fast muscles.
2. Spinal nerves at proximal and distal levels, and the dorsal roots, contain axons in a single mixed population with a mean diameter of 2-4 µm., while the ventral roots contain axons in two diameter classes with means at 4-6 and 12-14 µm.
3. Between 8 and 22 distributed nerve terminations were counted on fifty-two teased intact single muscle fibres stained for acetylcholinesterase activity. The average distance between the terminals is 0.64 mm. (range 0.094-2.050 mm.).
4. The compound action potential of the nerve comprises two principal peaks with conduction velocities of 17.0-23.8 m./sec. and 1.5-12.2 m./sec. at 10-12° C.
5. Fast muscle fibres gave two types of electrical response--all-or-none spike potentials that are propagated with a conduction velocity of c. I.I m.7/sec. at 10-120° C., and quantized distributed junction potentials.
6. The electrical properties of the nerves and roots suggest that the fast muscles are innervated by a single class of fast axons and possibly by a few slow axons.
7. Simultaneous recordings of nerve and muscle activities were made at different stimulus intensities. In all cases muscle responses were correlated with the first peak of the compound action potential, and appeared with the same or only slightly different latencies.
8. Each muscle fibre is shown electrophysiologically to be polyneuronally innervated by 2-5 axons from a single spinal nerve, and to receive a similar axonic complement from each of four spinal nerves.
9. Polyneuronal innervation of the muscle fibres by 8-22 different axons in the absence of multiterminal innervation is postulated.
Selective innervation of types of fibres in developing rat muscle
