RESPONSE PROPERTIES OF HIGHER-LEVEL NEURONS IN THE CENTRAL OLFACTORY PATHWAY OF THE CRAYFISH

We have examined the electrical activity of interneurons within the hi gher levels of the crayfish olfactory system. In unstimulated isolated crayfish head preparations, local protocerebral interneurons (LPI) of the hemiellipsoid bodies generate periodic, low-frequency membrane de polarizations. The most reasonable explanation for these baseline fluc tuations, which were exhibited by all of the LPIs examined and which w ere reversibly abolished by either-tetrodotoxin or low-calcium saline solution, is that they reflect periodic synaptic drive from the axon t erminals of olfactory projection neurons. One-third of tested LPIs gen erated impulses in response to the odor stimuli we applied to the ante nnules. Those cells that did respond exhibited a brief excitatory post synaptic potential and one or two action potentials, even during prolo nged odor pulses. Many of the responding neurons also exhibited a dela yed impulse burst 1 or 2 s following the stimulus pulse. Most of the r esponding cells recovered their sensitivity to odors very slowly, exhi biting disadaptation periods of several minutes. The apparent refracto ry nature of individual LPIs to olfactory stimulation is attributed in part to a hypothesized selectivity of connections between projection neurons and protocerebral targets and in part to the electrical isolat ion of the recording electrode from many regions of the extensive LPI dendritic tree.