SUMMARY1. The ionic conductances underlying some of the electrophysiological properties of multiply innervated or tonic fibres of rat extraocular muscles were examined in vitro with double-barrelled micro-electrodes.2. Exposure of the muscle to a Cl-free saline did not change the effective resistance (Reff) of tonic fibres which was 5-14 + 0-45 MQ (n = 7) in control saline and 4-78 + 0-45 MQ (n = 12) in Cl-free saline (P > 0.1). In contrast, in singly innervated or twitch fibres Cl removal increased Reff from 1-77 + 0-21 MQ (n = 19) to 2-69 + 0-12 MD (n = 22) (P < 0.001).3. Tonic fibres with membrane potentials restored to -80 mV by injecting current responded to intracellular depolarizing pulses with a brief, slow response (slow peak potential) which added to the rising phase of the electrotonic potential. The slow peak potential began at a membrane potential of -40 to -35 mV and was graded. Increasing depolarizations evoked faster and larger responses which did not overshoot the zero level of membrane potential.4. The slow peak potential was not blocked by 10 /tM-D-600 hydrochloride but was markedly reduced by the absence of Na and by 10 /SM-tetrodotoxin. The response was broadened about five times by 25 mM-tetraethylammonium.5. Raising bath temperature from 21-25 TC to 37 TC reversibly depressed and shortened the slow peak potential but did not transform it into an action potential.6. It is concluded that the characteristic high Retf of tonic fibres results from a lack of a membrane conductance to Cl and that the slow peak potential involves the transient activation of Na and K channels which are pharmacologically similar to the respective channels of twitch fibres.

The effects of denervation by nerve section on the electrical properties of tonic and twitch fibres of rat extraocular muscles were examined. Normally innervated tonic fibres lack action potentials. Upon direct stimulation they generate graded, voltage‐dependent responses or slow peak potentials (s.p.p.s). However, one week after denervation the s.p.p.s are transformed into action potentials which are slower and broader than those of twitch fibres. The action potentials are Na dependent and partially resistant to blockade with 10(‐5) M‐tetrodotoxin and 10(‐6) M‐saxitoxin. Changing the holding potential of the fibres from ‐80 mV to more negative levels increases the maximal rate of rise of the action potential. This effect is not observed on the s.p.p.s of normally innervated fibres. Following denervation the resting potential of tonic and twitch fibres becomes about 10‐15 mV less negative. In denervated muscles stimulation with pulses of hyperpolarizing current evokes graded responses in tonic fibres and action potentials in twitch fibres. In normally innervated muscles, these anodal break responses are never observed in tonic fibres and are very rare in twitch fibres. By two weeks after nerve section, reinnervation is present. The action potentials of tonic fibres are still present but stronger stimulation is needed to evoke anodal break responses. By three weeks, direct stimulation of tonic fibres evokes normal s.p.p.s in about 25% of the studied fibres and action potentials in the rest. By four weeks, most tonic fibres have lost the action potential but small anodal break responses can be evoked in most. It is suggested that following denervation a new population of Na channels appears in tonic fibres. The properties of these channels are different from those of the channels normally present in innervated tonic fibres but they are in some ways similar to those of the channels which appear in twitch fibres following denervation.