Different types of extrafusal muscle fibres in snake costocutaneous muscles.

1. Tonic and twitch muscle fibres were identified physiologically in m. costocutanei superiores and inferiores of garter snakes and grass snakes.2. Tonic fibres were multiterminally innervated and showed s.j.p.s in response to nerve stimulation. They did not show propagated A.P.s. They were innervated by motor axons with lower conduction velocities than those to twitch fibres, and often gave a contraction and developed tension in response to a single shock to the nerve. Intracellular square pulse analysis showed that C(m) = 1 muF/cm(2) and R(m) = 40,000 Omega cm(2).3. Twitch fibres showed a conducted action potential in response to nerve stimulation, and focal, as opposed to diffuse, innervation. They showed a variety of isometric twitch contraction times (times-to-peak of about 30-65 msec). Groups of similar motor units contained fibres of approximately similar contraction times. Slow twitch (and tonic) fibres often appeared silvery under dark field illumination, while faster twitch fibres appeared clear. No difference in C(m), R(m) or lambda was found between faster and slow twitch fibres. Values were approximately 3-4 muF/cm(2), 3000-4000 Omega cm(2) and 2 mm respectively.