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1. In the central neurones that have so far been examined the passive electrical properties of the extrasynaptic membrane has been shown to be relatively constant in the subthreshold range. Consequently, excitatory synaptic potentials produced by chemical transmission tend to vary in amplitude with changes in membrane potential, decreasing with depolarization and increasing with hyperpolarization.

2. In the two symmetrical giant cells of the ventral metacerebrum of the snail, the EPSPs failed to show the expected alterations in amplitude with changes in membrane potential. Near the resting level the EPSP increased slightly with membrane depolarization and decreased slightly with hyperpolarization.

3. These paradoxical results were not attributable to a change in transmitter release since similar results were obtained when ACh, the putative transmitter, was released iontophoretically on to the cell membrane by means of an extracellular pipette.

4. Measurement of the current—voltage relation of the extrasynaptic membrane revealed two types of rectifying conductance changes. The first, an increase in conductance with depolarization, was turned on at a depolarization of about 15 mV. Its conductance change was similar to the delayed rectification familiar from studies of peripheral nerve and muscle. The second occurred on either side of the resting level, from about 15 mV hyperpolarization to about 10 mV depolarization, and manifested itself as a decrease in conductance with depolarization and an increase with hyperpolarization. By analogy to a similar phenomenon known to occur in skeletal muscle this second rectification has been termed anomalous rectification.

5. The average resistance at 25 mV hyperpolarization was 2·3 x 10⁶ , while at 10 mV depolarization it was 2·1 x 10⁷ , yielding an average rectification ratio of 10 for the anomalous conductance change.

6. The anomalous rectifying conductance seems to account for the paradoxical behaviour of the EPSP and ACh response to changes in membrane potential. Moreover, the finding that the sharpest change in the anomalous rectification curve occurred on either side of the resting level suggests that this rectification is functionally important as a postsynaptic determinant of synaptic efficacy. Several additional lines of evidence in support of this suggestion have been obtained.

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