THE MECHANISM OF THE SHADOW REFLEX IN CIRRIPEDIA III. RHYTHMICAL PATTERNED ACTIVITY IN CENTRAL NEURONS AND ITS MODULATION BY SHADOWS

¹ Department of Biology, Reed College, Portland, Oregon 97202

The isolated barnacle central nervous system has been used to study neurons related to the shadow reflex and rhythmical behavior. The function of certain of the neurons has been ascertained by exploiting a preparation that included three pairs of cirri and the scutal adductor muscle which permitted simultaneous neuron-muscle fiber intracellular recording. The inclusion of the median photoreceptor in all of the preparations provided a means of assaying the effect of shadows on the system.

The absence of peripheral structures in the isolated preparation coupled with the observation that rhythmical bursting patterns will persist for hours is strong evidence for one or more spontaneous "oscillators" in the CNS that are capable of driving the basic fishing, pumping behavioral repertoire of the barnacle.

The principle finding reported is that there are two classes of bursters, identifiable by location, bursting pattern, and response to "light-off" that exhibit reciprocity. Evidence is presented to link the laterally located bursters (excited at "light-off") to withdrawal, the mid-line bursters (inhibited at "light-off") to extension, in the intact animal.

Evidence the bursters are driven and/or modulated by synaptic input is provided by observations on nonspiking bursters, by passing hyper- and depolarizing currents, and by treating the ganglion with high Mg²⁺, low CA²⁺ artificial sea water. In some of the bursters, synchronization is accomplished by a common antecedent interneuron.

Simultaneous neuron-muscle recordings and locating axons of impaled neurons in peripheral nerves has established some of the bursters as motor neurons.

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