Convergence of multimodal sensory input onto higher-level neurons of the crayfish olfactory pathway

Intracellular electrophysiological studies of lateral protocerebral interne urons (LPIs) in the crayfish Procambarus clarkii have revealed convergence of multimodal sensory information onto these higher-level cells of the crus tacean central olfactory pathway. Antennular stimulation by odors or electr ical shocks generates excitatory-inhibitory sequences in some LPIs as does electrical or hydrodynamic stimulation of the antennae. Photic stimulation of the ipsilateral compound eye generates excitatory responses in LPIs, usu ally in the form of trains of impulse bursts that are timed to the peaks of the spontaneous oscillatory activity that characterizes these neurons. Foc al electrical stimulation of the olfactory lobe, the termination point of a ntennular afferent input, or the accessory lobe, where higher-level visual and tactile inputs converge, also generates brief excitation and a delayed, prolonged inhibition in LPIs. Both phases of this activity are thought to be transmitted to the lateral protocerebrum via deutocerebral projection ne urons, which have extensive dendritic arborizations in the olfactory lobe a nd the accessory lobe. The excitatory pathway is thought to synapse directl y with target LPIs, whereas the inhibitory pathway is probably indirect and mediated through GABAergic interneurons within the lateral protocerebrum. There is evidence that both presynaptic and postsynaptic inhibition suppres s activity in LPIs. Preliminary observations suggest that a small cluster o f neurons adjacent to the hemi-ellipsoid body are inhibitory to LPI activit y. Multimodal inhibitory and excitatory modulation of LPI activity may play a part in the contextual identification of odors in the crayfish olfactory system.