IDENTIFICATION OF DIRECTIONALLY SELECTIVE MOTION-DETECTING NEURONES IN THE LOCUST LOBULA AND THEIR SYNAPTIC CONNECTIONS WITH AN IDENTIFIED DESCENDING NEURONE

The anatomy and physiology of two directionally selective motion-detecting neurones in the locust are described. Both neurones had dendrites in the lobula, and projected to the ipsilateral protocerebrum. Their cell bodies were located on the posterio-dorsal junction of the optic lobe with the protocerebrum. The neurones were sensitive to horizontal motion of a visual stimulus. One neurone, LDSMD(F), had a preferred direction forwards over the ipsilateral eye, and a null direction backwards. The other neurone, LDSMD(B), had a preferred direction backwards over the ipsilateral eye 1. Motion in the preferred direction caused EPSPs and spikes in the LDSMD neurones. Motion in the null direction resulted in IPSPs 2. Both excitatory and inhibitory inputs were derived from the ipsilateral eye 3. The DSMD neurones responded to velocities of movement up to and beyond 270°s−1 4. The response of both LDSMD neurones showed no evidence of adaptation during maintained apparent or real movement 5. There was a delay of 60–80 ms between a single step of apparent movement, either the preferred or the null direction, and the start of the response 6. There was a monosynaptic, excitatory connection between the LDSMD(B) neurone and the protocerebral, descending DSMD neurone (PDDSMD) identified in the preceding paper (Rind, 1990). At resting membrane potential, a single presynaptic spike did not give rise to a spike in the postsynaptic neurone

Postsynaptic Potentials of Limited Duration in Visual Neurones of a Locust

Large, second-order neurones of locust ocelli (‘L-neurones’) make both excitatory and inhibitory connections amongst each other. A single Lneurone 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. 1. 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. 2. 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. 3. 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 10mV ms−1 4. 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. 5. A connection can begin to recover from depression while the presynaptic neurone is held depolarized from resting. 6. Transmission fails when a presynaptic neurone is depolarized by pulses shorter than 2 ms. 7. 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.