Integration and spike initiation in neuronal terminals.

Axoaxonal synapses impinge on the terminals of neurons and are thought to play a crucial role in the regulation of synaptic transmission. In the crayfish brain, the terminals of several identified interneurons exhibit both EPSPs and IPSPs in response to cephalic sensory stimulation. The terminal arbors extend into primary sensory neuropils near the terminals of primary afferent axons of the antennae and statocysts. The EPSPs arise monosynaptically from primary afferent stimulation and can elicit action potentials. Bidirectional conduction is routinely observed. Terminal IPSPs are transmitted via a polysynaptic pathway. Furthermore, two morphological classes of synaptic profiles impinge on the presynaptic terminals. These results indicate that the neuron terminal can function as a second, independent site of synaptic integration.