TWO INHIBITORY MECHANISMS IN THE MAUTHNER NEURONS OF GOLDFISH

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Abstract
One of the inhibitory mechanisms studies, is of the familiar type in which changes in post-synaptic membrane permeability are brought about by a transmitter substance. Cl was specifically implicated. The other mechanism depends upon certain positive extracellular potential changes which hyperpolarize the Mauthner cell (M-cell) and which are generated by the presynaptic elements; an example of "electrical" inhibition. Both the chemical and electrical types of inhibition could be initiated in each of two ways: a) by activating the M-cell themselves (collateral inhibition) and b) by stimulating the contralateral eighth cranial nerve. The following evidence led to the conclusion that the second type of inhibition was, in fact, electrical: 1) The positive extracellular potential which underlay the inhibition was found, in studies of its spatial distribution , not to be generated by the M-cell. 2) The effect could be imitated by passing relatively small pulses of anodic current (0.1 - 0.3[mu]A) through an extracellular microelectrode near the cell; the level of extracellular positivity needed to block was then similar to that during the natural inhibition. 3) No appreciable change in membrane conductance was detected during the inhibitory action.