Histological and electrophysiological investigation of lizard skeletal muscle

1. The patterns of innervation and electrical properties of muscle fibres in a skeletal muscle of the blue tongue lizard Tiliqua nigrolutea have been investigated.2. Gold impregnation of nerve terminals and staining of muscle fibre junctional areas for cholinesterase showed that there are two histological types of muscle fibre in scalenus muscles of the lizard: (a) those usually receiving single en plaque innervation, and (b) those that receive multiple en grappe terminations.3. In normal solution and in solutions to which small doses of curare were added, two types of subthreshold post-junctional response were recorded following nerve stimulation, (a) potentials with rapid rates of rise and a half-decay time of less than 10 msec and (b) responses with fast rise times and long half-decay times (50 msec or more).4. Fast time course subthreshold responses often gave rise to propagated action potentials. In curarized preparations ((+)-tubocurarine 0.4-1.0 mug/ml.) action potentials failed, giving way to junction potentials of decreasing amplitude, when stimulation was maintained at rates of 5/sec or more. The decay phases of fast time course potentials were closely approximated by error functions.5. Slow time course responses summated during repetitive stimulation, but action potentials were never produced. The decays of slow junction potentials were well fitted by exponentials. It is suggested that fibres in which they were recorded received distributed, en grappe innervation.6. Fibres in which fast time course junction potentials were recorded were excited by direct stimulation via an intracellular micro-electrode. They had apparent membrane resistance and capacity of about 4000 Omega. cm(2) and 7 muF/cm(2).7. Fibres exhibiting slow junction potentials could not be excited directly, even when depolarizing pulses were preceded by hyperpolarizations of 50 mV or more for more than 20 msec. Such fibres had an apparent membrane resistance of 31 x 10(3) Omega.cm(2) (mean) and a capacity of less than 3 muF/cm(2) (eight fibres).8. Curarized muscles developed tension in response to nerve stimulation at frequencies of 5/sec or more. The tension profile was smooth at even the lowest frequencies at which a response was elicited. Normal fusion frequency was around 50/sec.9. It is suggested that fibres exhibiting slow junction potentials were functionally similar to amphibian tonic fibres.10. Some fibres were found in which action potentials could be elicited by stimulation of one or more axons. Their subthreshold responses and passive properties were not determined and it remains uncertain to which of the two structural types they belong.