![]() ![]() L-neurones produce single spikes, or, rarely, short bursts of spikes when illumination is reduced ( Chappell & Dowling, 1972 Patterson & Goodman, 1974 Wilson, 1978 a) or when a pulse of hyperpolarizing current injected into an L-neurone ends ( Wilson, 1978 b). If either of these processes occurred rapidly, then slow changes in presynaptic potential would not be transmitted across the inhibitory connections, and fast depolarizing potentials, such as spikes, would be required to effect transmission. Two of the possible mechanisms which would limit the duration of these PSPs are that presynaptic transmitter becomes depleted, or that postsynaptic receptors desensitize. In contrast, at the inhibitory connections which L-neurones make amongst each other, the postsynaptic potential (PSP) has a duration limited to less than 35 ms, no matter for how long the presynaptic terminal is held depolarized ( Simmons, 1982 a). Transmission at the excitatory connections can be sustained for long periods without decrement. Because the normal resting potential of an L-neurone is depolarized above the threshold for release of transmitter at the excitatory connections, small hyperpolarizations, such as those that occur when ocellar illumination is increased, are communicated across these connections ( Simmons, 1981, 1982a). At the excitatory connections, small, slowly changing variations in presynaptic potential are transmitted to the postsynaptic neurone, as long as the presynaptic neurone is depolarized from a threshold potential. L-neurones make excitatory connections with some large brain neurones which descend in the nerve cord ( Simmons, 1981), and make both excitatory and inhibitory connections laterally amongst each other ( Simmons, 1982 a). A single L-neurone can make both types of output connection. The output connections which L-neurones make are of particular interest because two types of them, which have contrasting properties, have been found. 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. Transmission fails when a presynaptic neurone is depolarized by pulses shorter than 2 ms. ![]() Larger IPSPs are followed by initially greater depression than smaller IPSPs.Ī connection can begin to recover from depression while the presynaptic neurone is held depolarized from resting. The connection recovers its full ability to transmit over a period of 1-5 s. Once an IPSP has occurred, both the amplitudes and the rates of hyperpolarization of subsequent IPSPs are depressed. A maximum rate of postsynaptic hyperpolarization is reached when the presynaptic neurone depolarizes at 10 mV ms −1. 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. 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. This paper examines mechanisms which could limit the duration of these IPSPs.Īn IPSP begins 4-5 ms after a presynaptic neurone starts to depolarize from its resting potential, and the time-to-peak is 7 ms. In contrast, inhibitory postsynaptic potentials (IPSPs) never last for more than 15–35 ms. At the excitatory connections, transmission can be maintained for long periods without decrement. A single L-neurone can be presynaptic at both types of connection. Large, second-order neurones of locust ocelli (‘L-neurones’) make both excitatory and inhibitory connections amongst each other. ![]()
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