Structure of a tonically transmitting synapse between identified interneurone in the locust brain

The ultrastructure and distribution of synapses between large second-order and third-order neurones of a locust ocellus are described. Pairs of neurones, shown by physiological tests to be connected by a tonically transmitting, excitatory connection, have been injected with hexamminecobaltic ions, which allows their profiles to be recognised with electron microscopy. The synapses are made in the lateral ocellar tract, between the axons of some of the second-order neurones and a network of fine processes from the third-order neurone. A physiological connection is composed of several thousand discrete anatomical synapses. Each anatomical synapse is composed of a presynaptic density associated with a cloud of round, electron-lucent vesicles overlying an intercellular cleft. Parallel with the presynaptic density is a row of omega-shaped invaginations of the presynaptic plasmalemma. The synapses are diadic and are close to synapses from small, unidentified neuronal profiles onto the third-order neurone.