Electrophysiological identification of two types of fibres in rat extraocular muscles.

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Abstract
The synaptic potentials and electrical properties of rat inferior rectus muscles were examined in vitro. In most fibers the spontaneous synaptic activity consisted of typical miniature end-plate potentials which had a normal distribution of amplitudes and rather uniform time courses. Suprathreshold and maximal nerve stimulation evoked unitary end-plate potentials (epp). The synaptic activity of these fibers could be recorded only in the innervation zone of the muscle. These fibers were identified as being focally innervated. Focally innervated fibers gave action potentials upon direct and indirect stimulation. They had an effective resistance (Reff) of 1.62 .+-. 0.22 M.OMEGA. (mean .+-. SE, 22 fibers) and a time constant (.tau.m) of 3.8 .+-. 0.4 ms (21 fibers). Voltage-current curves in control saline were linear between membrane potentials of -50 to -140 mV. In a small number of fibers the spontaneous synaptic activity consisted of miniature small-nerve junction potentials which had a skewed distribution of amplitudes with predominance of smaller voltages and time courses with a wide range of variation. Nerve stimulation evoked composite small-nerve junction potentials (sjp) which could be resolved into unitary components by varying the strength of stimulation. Sjp had a higher threshold than epp. Synaptic potentials could be recorded outside the innervation zone, at various sites along the muscle length. These fibers were recognized as being multiply innervated with polyneuronal innervation. Multiply innervated fibers lacked action potentials, had a large Reff of 6.0 .+-. 1.1 M.OMEGA. (6 fibers) and a prolonged .tau.m of 29.8 .+-. 4.8 ms. Reff show a moderate decrease to hyperpolarization and a rather large decrease to depolarization which denote, respectively, the presence of anomalous and delayed rectification. Rat extraocular muscles contain at least 2 populations of muscle fibers that in terms of synaptic activity and electrical properties are comparable to twitch fibers of other mammalian muscles and to slow or tonic fibers of amphibians.