Different types of extrafusal muscle fibres in snake costocutaneous muscles

Abstract

Su Y1. 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 Cm = 1 /LF/cm2 and Rm = 40,000 Q cm2.3. 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 Cm, Rm or A was found between faster and slow twitch fibres. Values were approximately 3-4 /F/cm2, 3000-4000 Q cm2 and 2 mm respectively. IMTRODUCTIONThe work described in this paper consists of physiological studies on snake muscle fibres. The main findings are that one type of fibre responds to a single shock to the nerve with a propagated action potential and an all-or-none twitch (excluding post-twitch and post-tetanic potentiation). A second type of fibre responds to a single shock with a non-propagated junctional potential and, at most, a small contracture. Evidence is pre-* Author's present address.

Characteristics of the acetylcholine‐operated channel at twitch and slow fibre neuromuscular junctions of the garter snake.

“…In addition, Ridge (1971) has described muscle membrane properties of both twitch and slow fibres in this preparation; he reported DC space constants of 1 8 and 4-4 mm respectively. As a consequence of the long space constant and small receptor area on the muscle, the postsynaptic membrane of these neuromuscular junctions should be effectively space clamped with the two micro-electrode voltage clamp method.…”

“…First, there are separate nerve fibre systems which innervate them (e.g. Ridge, 1971). Both systems of nerve fibres run together in the same nerve bundle up to the point where the bundle inserts into the whole muscle.…”

Metabolic capacity and myosin expression in single muscle fibres of the garter snake.

“…The correspondence of these anatomical features with physiological differences among snake fibre types has been shown previously (Ridge, 1971;Wilkinson & Lichtman, 1985). Fibre diameters varied along the length of fibres but were generally largest for F fibres, medium for S fibres, and smallest for T fibres, with some overlap between classes.…”

“…However, it is not year clear if such fibres comprise a significant portion of the adult fibre population in those animals. In contrast, the reptilian S fibres are a predominant (25 % of fibres in the transversus abdominis muscle; Wilkinson & Lichtman, 1985; see also Ridge, 1971) adult fibre type in which two myosin heavy chains are consistently co-expressed.…”

“…Slower twitch (and tonic) fibres contained refractile lipid droplets giving a 'rough' appearance when viewed with Nomarski optics; the droplets were absent from faster twitch fibres, giving a 'smooth' appearance (Ridge, 1971;Wilkinson & Lichtman, 1985;Wilkinson & Nemeth, 1989).…”