1. An inhibitory potential of long duration has been analysed in intracellular recordings from mitral cells in the isolated turtle olfactory bulb preparation. 2. A single volley in the olfactory nerves or lateral olfactory tract elicited synaptic inhibition in mitral cells, followed by a long‐lasting hyperpolarization of the cell. This slow potential has been termed the Is component, to distinguish it from the earlier I1 and I2 components to the inhibitory post‐synaptic potentials (i.p.s.p.s). 3. The slow Is component was inhibitory, as shown by the interruption of spontaneous discharges, and the blockage of responses to injected current and incoming volleys. The duration of the hyperpolarization following a single volley usually lasted up to 5 sec, and occasionally longer. 4. The Is response decreased in amplitude with either depolarizing or hyperpolarizing current injection, without showing a reversal potential. This contrasted with the earlier I1 and I2 components, which showed reversal potentials characteristic of i.p.s.p.s. 5. Tests of membrane conductance showed that the conductance, which was increased during the preceding i.p.s.p., decayed to control values during the early part of the Is potential. 6. The Is response shares some characteristics with slow potentials reported in certain other vertebrate neurones. This type of response may be involved in longer‐term neuromodulatory control of the excitability of the mitral cell.
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