Interactions made by large second-order neurones of the median ocellus of the locust

Summary

• 1. The median ocellus of a locust is connected with each lateral ocellus by a pair of neurones, called ML-neurones. A bilateral pair of M1 neurones also connect the median ocellus with the brain.

• 2. Each ML-neurone derives a similar type of input from neurones in each ocellus which it innervates, and signals from the two ocelli sum in the axon of the ML-neurone. The axon has a length of about 3.8 mm, which is of the order of twice the electrotonic length constant. Only hyperpolarising responses to increased ocellar illumination have been recorded.

• 3. Each ML-neurone makes a reciprocal inhibitory interaction with the other three ML-neurones. The duration for which these inhibitory interactions can sustain transmission is severely limited.

• 4. The ML-neurones make excitatory interactions with the left and right M1-neurones. These interactions are probably capable of maintaining transmission for long periods, but the interactions are relatively weak.

• 5. Some interactions have been found between large second-order ocellar neurones (L-neurones) and small neurones which produce trains of spikes in the ocellar nerves (S-neurones). L-neurones make excitatory interactions with S-neurones which are inhibited by light, and inhibitory interactions with S-neurones which are excited by light.