Intracellular recordings from the swim musculature of the pteropod mollusc Clione limacina revealed two types of activity during spontaneous swimming movements. One group of muscle cells produced either excitatory junctional potentials (EJPs) or spike-like responses during both slow and fast swimming. The second group of muscle cells were either silent or produced extremely small EJPs during slow swimming, and EJPs or spike-like responses during fast swimming. Injections of horseradish peroxidase (HEP) into the two types of muscle cells indicated that the former are slow-twitch fibers while the latter are fast-twitch fibers. A comparison of the two muscle cell types indicated that the slow-twitch fibers are larger and have less negative resting potentials than the fast-twitch cells. Electrical coupling was detected between pairs of slow-twitch cells, but could not be demonstrated between pairs of fast-twitch cells or between mixed pairs of cells. Spike-like responses in both cell types exhibited non-synaptic facilitation in response to direct depolarizing stimulation. Both the maximum size and the initial rate of facilitation were frequency-dependent. Spike-like responses were followed by a depolarizing afterpotential that apparently summed to produce a baseline depolarization that may be involved in non-synaptic facilitation.

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