An electrophysiological study of chelonian skeletal muscle

Abstract

SUMMARY1. 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 ,u in the tortoise muscle, and membrane constants were: A-1-5 mm; 7-34 msec; Qcm2; Cn7-7 9 1FF/cm2. 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 j / in the tortoise and ca. 100 /s in the terrapin. In non-junctional regions, ACh reactivity was not * Present address: Department of Physiology, School of Medicine, University of Virginia, Charlottesville, Virginia, U.S.A. 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, a...

Different types of extrafusal muscle fibres in snake costocutaneous muscles

“…It is possible that such intermediate fibres do occur in snake, but were missed in this study. Levine (1966) found in chelonian retractor capitis muscles that many fibres were multiterminally (though focally) innervated. These fibres had normal APs with overshoot, and Rm and Cm were respectively about 4860 Q cm2 and 79 ,uF/cm2, values more characteristic of twitch than tonic fibres.…”

“…Levine (1966) Fatt & Katz (1951; using square pulse analysis and intracellular recording). Mean values in frog were A = 2-4 mm (diameter = 137 is), Rm = 4100 Q cm2.…”

Histological and electrophysiological investigation of lizard skeletal muscle

“…The electrophysiological methods used were unsuitable for determining the number of inputs to a poly-axonally innervated excitable fibre. Even some of the fibres in which only one action potential was recorded may have received more than one input, but if the lowest threshold axon supplied the fibre at a point nearer the electrode than did the others, only a single potential would be seen because of spike collision (see also Levine, 1966). Estimates of passive properties might be useful in distinguishing these fibres from those with a single en plaque terminal and it is unfortunate that this was not possible in these experiments.…”

“…His findings suggested their presence in reptiles. Levine (1966) found that muscle fibres of the tortoise which exhibit a maintained contractile response to acetylcholine application are able to produce propagated action potentials. Similar findings by Ginsborg (1960) on avian muscle also suggested that responses to acetylcholine application, alone, were insufficient indicators of a muscle's functional properties.…”