Postsynaptic Potentials of Limited Duration in Visual Neurones of a Locust

Abstract

ABSTRACT Large, second-order neurones of locust ocelli (‘L-neurones’) make both excitatory and inhibitory connections amongst each other. A single L-neurone can be presynaptic at both types of connection. At the excitatory connections, transmission can be maintained for long periods without decrement. In contrast, inhibitory postsynaptic potentials (IPSPs) never last for more than 15–35 ms. This paper examines mechanisms which could limit the duration of these IPSPs. An IPSP begins 4-5 ms after a presynaptic neurone starts to depolarize from its resting potential, and the time-to-peak is 7 ms. The amplitude of an IPSP depends both upon the amplitude of the peak presynaptic potential and upon the potential at which a presynaptic neurone is held before it is depolarized. The rate at which a postsynaptic neurone hyperpolarizes to produce an IPSP is proportional to the rate at which the presynaptic neurone depolarizes, independent of the potential from which the presynaptic depolarization starts. A maximum rate of postsynaptic hyperpolarization is reached when the presynaptic neurone depolarizes at 10 mV ms−1. Once an IPSP has occurred, both the amplitudes and the rates of hyperpolarization of subsequent IPSPs are depressed. The connection recovers its full ability to transmit over a period of 1-5 s. Larger IPSPs are followed by initially greater depression than smaller IPSPs. A connection can begin to recover from depression while the presynaptic neurone is held depolarized from resting. Transmission fails when a presynaptic neurone is depolarized by pulses shorter than 2 ms. The most likely reason why the duration of the IPSPs is limited is that calcium channels in the presynaptic terminal inactivate within 7 ms of first opening.