Optical assessment of spatial patterning of strength-duration relationship for vagal responses in the early embryonic chick brainstem.

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The spatial patterning of strength-duration relationship for neural responses to vagus nerve stimulation in the early embryonic chick brainstem preparation was assessed by means of an optical method for monitoring neural activity using a voltage-sensitive dye. Intact brainstem, and brainstem slice preparations, together with the attached vagus nerve, were dissected from 6- to 8-day (d)-old chick embryos. They were stained with a voltage-sensitive merocyanine-rhodanine dye (NK2761). Application of depolarizing square current pulses to the vagus nerve fibers using a micro-suction electrode evoked voltage-related optical (absorption) responses in the brainstem. The optical responses were recorded simultaneously from 127 contiguous sites in both the intact and slice preparations, using a 12 x 12-element photodiode array. The amplitude of the evoked signals and the extent of the response area depended on the strength and duration of the stimulating current pulse. Thus, we determined the threshold current strength needed to evoke the fast spike-like optical signal, corresponding to the action potential, in response to vagal stimulation at a given duration, and we obtained strength-duration curves for 100-200 contiguous regions in the preparation. The strength-duration curves could be apparently fitted by the empirical formula of Weiss. From these curves we obtained the rheobasic currents as a possible indication of neural responsiveness to peripheral vagus stimulation, and we constructed the map of the spatial distribution of the rheobasic currents. The rheobasic current is smallest in the central region of the response area, and it is distributed in a layered pattern of increasing value. In accordance with such a pattern, the minimum rheobasic current decreased and the response area expanded developmentally. The results suggest that the ordered patterns of spatial distributions of the size of the axon, the specific membrane resistance, sodium channel activity, and other electrophysiological factors underlie the initial process of functional organization of the dorsal motor nucleus within the brainstem during early embryonic development.