An electrophysiological study of chelonian skeletal muscle

1. Membrane properties of fibres of the retractor capitis muscles of the tortoise, Testudo graeca, and of the terrapin, Pseudemys elegans scripta, have been investigated with electrophysiological techniques. The features studied were qualitatively and quantitatively similar in both.2. Fibres are electrically excitable and many extend through 90% of the length of the muscle. In the tortoise muscle conduction velocity was 0.8 m/sec in hibernating animals, and 1.3 m/sec in active animals.3. Resting potentials averaged -80 mV and action potentials ca. 115 mV. The critical membrane potential in the tortoise was -38.5 mV. Neurally initiated action potentials, recorded at the end-plate, were reduced in amplitude by 14 mV in both forms, and the end-plate ;step' from which the action potential arose was -39 mV. Resting potentials at the end-plate were not different from those recorded at non-junctional sites. The rise time of the action potential at non-junctional sites was 1.1 msec, and the time for repolarization to 50% was 2 msec in both forms. In hibernating tortoises the rise time was three times longer and the fall time doubled.4. The average calculated fibre diameter was ca. 50 mu in the tortoise muscle, and membrane constants were: lambda-1.5 mm; tau-34 msec; R(m)-4860 Omegacm(2); C(m)-7.9 muF/cm(2).5. Most of the superficial fibres in both tortoise and terrapin muscles were multiply innervated, but end-plates were focal rather than diffuse.6. Junctional membranes in both types of preparations were highly sensitive to iontophoretically applied ACh, but sensitivity fell off sharply with distance from the end-plate. The maximum ACh sensitivity averaged 18.8 mV/nC in the tortoise and 15.5 mV/nC in the terrapin. The distance over which this sensitivity fell to 1/10 was ca. 150 mu in the tortoise and ca. 100 mu in the terrapin. In non-junctional regions, ACh reactivity was not detectable in the terrapin muscle. Tortoise fibres were demonstrably reactive to ACh everywhere, but the sensitivity in the non-junctional membrane was 10(-3) to 10(-5) of the sensitivity of the junctional membrane.7. Miniature end-plate potentials (M.e.p.p.s.) could be recorded at junctional regions in either tortoise or terrapin muscles. They occurred at random intervals and had a normal amplitude distribution. The average amplitude was 0.78 mV in the tortoise and 0.54 mV in the terrapin. These amplitudes were approximately doubled with neostigmine, and were greatly decreased by (+)-tubocurarine. The average frequency was 0.2/sec in the tortoise and 0.4/sec in the terrapin. In the tortoise very low frequency m.e.p.p.s were encountered at a number of junctions (< 1/min). These did not show a skewed amplitude distribution and could be increased in frequency with hypertonic solutions. They were not associated with junctions at which transmission had failed.8. The quantum content of the normal end-plate potential was determined by two methods, and was between 100 and 200 units, for both tortoise and terrapin.9. Thus, the retractor capitis muscles of both terrapin and tortoise appear to be composed largely of ;twitch' fibres. The presence of a very low-level sensitivity to ACh in non-junctional sites of tortoise fibres is regarded as a quantitative rather than a qualitative difference from terrapin fibres. It is suggested that it reflects a condition in which the regulating influence of the motor nerve does not suppress the receptor density in the non-junctional membrane below the level of detectability.