Synaptic transmission between second- and third-order neurones of a locust ocellus

Summary

• 1. This paper describes the operation of synapses which large, second-order neurones of each ocellus of a locust (L-neurones) make with a pair of large, identified descending neurones on either side of the brain (Fig. 1). Intracellular recordings were made from the axons of the L-neurones, and usually from the cell bodies of the third-order neurones.

• 2. The third-order neurones respond, in addition to changes in illumination, to currents of air which deflect wind sensitive hairs on the head (Fig. 2B).

• 3. Both L-neurones and the third-order neurones hyperpolarise when their ocellus is illuminated (Figs. 2, 3). The hyperpolarising responses of the third-order neurones saturate at a light intensity which is well within the range of intensities to which the L-neurones respond (Fig. 3). L-neurones and third-order ocellar neurones produce sharply rising regenerative responses when a bright light is switched off, and the third-order neurones spike at less intense changes in illumination than L-neurones do (Fig. 3A).

• 4. L-neurones make excitatory chemical synapses with the third-order ocellar neurones (Figs. 4, 5). In steady daylight illumination, L-neurones continually release transmitter onto the third-order neurones (Fig. 4). The hyperpolarising responses of the third-order neurones to increases in illumination are due to decreases in the rate of release of transmitter from the L-neurones.