Properties of twitch motor units in snake costocutaneous muscle

1. Single motor units of twitch muscle fibres were studied in isolated nerve-muscle preparations of m. costocutanei inferiores from grass and garter snakes. Preparations were superfused with Ringer solution at a controlled temperature of 22 degrees C.2. The peak and time-to-peak tension (contraction time) were measured for isometric twitches of forty-seven whole muscles and 83 motor units. The sample of motor units was drawn from an estimated total population of 213-355 twitch units. Peak tetanic tensions were also measured. The measurements were made at muscle lengths at which the twitch tension was maximal, and this length was not always the same for whole muscle and unit twitches. In fifty-nine cases 1 and in twelve cases 2 motor units were isolated from each muscle.3. Whole muscle contraction times ranged from 22-61 msec (mean +/- S.D. = 40.3 +/- 9.8 msec) and those for units from 18-92 msec (mean +/- S.D. = 46.9 +/- 15.9 msec). The wide range for whole muscles is discussed.4. The percentage of the whole muscle tetanic tension contributed by each unit (unit size) was calculated. Contraction time was inversely related to unit size.5. Twitch-tetanus ratios were calculated and found not to be related to unit contraction time.6. The conduction velocities of axons innervating 23 motor units were calculated from latency measurements at two points along the length of the nerve. They ranged from 1.9 to 10.4 m/sec. Axon conduction velocity was inversely related to unit contraction time, and directly related to unit size.