1. Intracellular recordings have been made from neurones lying in the myenteric plexus of guinea-pig duodenum; some aspects of their membrane properties have been studied by passing current through the intracellular electrode while recording changes in membrane potential.2. The current—voltage relationship was linear for small changes in membrane potential, input resistances ranging from 125 to 250 MΩ. Larger hyperpolarizing currents (causing changes of 20-40 mV) caused the input resistance to fall.3. Depolarizing currents of 1 to 10 × 10-10 A initiated action potentials with amplitudes of up to 95 mV.4. Two types of cell were distinguished when an action potential was initiated. In one group the action potential and undershoot had a form similar to that recorded from other mammalian ganglia. In the second group an action potential was followed by both an undershoot and a prolonged afterhyperpolarization which was associated with a decrease in cell resistance.5. The two groups of cells were further distinguished by their responses to transmural stimulation. Only those cells which did not show an afterhyperpolarization could be shown to receive a synaptic input.6. The mechanism by which each cell type generates an action potential was different. The action potentials recorded from cells which had a detectable synaptic input were abolished by tetrodotoxin. In contrast, those recorded from the other type of cells persisted in the presence of tetrodotoxin. Preliminary experiments suggest that during an action potential these cells become permeable to both sodium and calcium ions.7. Abolition of the calcium component of the action potential in cells which generated an afterhyperpolarization abolished this latter potential.8. The role of these two groups of cells is discussed.
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