Embryogenetic expression of glossopharyngeal and vagal excitability in the chick brainstem as revealed by voltage-sensitive dye recording

Using voltage-sensitive dye recording, we traced the ontogenetic expression of neural excitability related to the glossopharyngeal nerve (N. IX) and the vagus nerve (N. X) in the embryonic chick brainstem. At the 3.5-day embryonic stage, by averaging optical signals, we succeeded in recording very small action potential-related optical responses (ΔI/I < 10−4) induced by N. IX stimulation in the nucleus of the glossopharyngeal nerve (motor nucleus). This suggests that glossopharyngeal excitability in the motor nucleus is first generated no later than this developmental stage. On the other hand, action potential-related optical responses induced by N. X stimulation were first detected at the 4-day embryonic stage. Comparison with morphogenesis indicated that glossopharyngeal and vagal motoneurons acquire their excitability and send their axons to the periphery soon after they leave the proliferative pool.