Neural mechanism generating firing patterns in jaw motoneurons during the food-induced response in Aplysia kurodai

Summary

• 1. In semi-intact preparations food-induced firing patterns in two identified premotor neurons (MA1a and MA2) were explored. During the rhythmic response the membrane potential of the premotor neurons and the jaw motoneurons underwent synchronous depolarization. Then the premotor neurons and the jaw-opening motoneuron started firing almost simultaneously, while the jaw-closing motoneurons started firing after the maximum frequency of the burst of spikes in the premotor neurons.

• 2. When a large hyperpolarizing current pulse was passed into one of the premotor neurons at the expected time of its burst in order to block its spike activity, the initiation time of firing of the ipsilateral jaw-closing motoneurons was advanced during the cycle, and the slope of the rising phase of the depolarization in the motoneurons became steeper.

• 3. Repetitive electrical stimulation of the cerebralbuccal connective (c.b.c.) induced rhythmic membrane potential changes similar to those induced by food stimuli in the neurons studied. In the other two premotor neurons (MA1b and MA1c) similar firing patterns to those in MAla and MA2 were induced by c.b.c. stimulation.

• 4. When a large steady hyperpolarizing current was passed into one of the premotor neurons or when inhibitory effects of the premotor neurons on the jaw-closing motoneurons were removed by d-tubocurarine during the response induced by c.b.c. stimulation, the initiation time of firing of the ipsilateral jaw-closing motoneurons was advanced during each cycle. Moreover, the delay of firing of the jaw-closing motoneurons during each depolarization decreased with an increase in the intensity of hyperpolarizing current.

• 5. The present findings suggest that during the food-induced response the identified premotor neurons generate the delay of firing of the ipsilateral jaw-closing motoneurons during each depolarization.