Swim Initiation Neurons in Tritonia diomedea1

Abstract

Two groups of interneurons, Tr1 and DRI, have been identified in the escape swim circuit of the marine mollusc Tritonia diomedea that have important roles in behavioral initiation. DRI functions as a command neuron, receiving direct excitatory input from the afferent neurons, and in turn directly exciting the DSI neurons of the central pattern generator. DRI fires throughout the swim motor program, and activity in DRI is both necessary and sufficient for sensory input to elicit the swim motor program. Tr1 is an excitatory interneuron that fires briefly in response to sensory input and then remains silent during the motor program. Tr1 excites DRI with an excitatory connection that has fast and slow components and thus appears to have a role in converting brief afferent neuron activity to long-lasting firing in downstream circuit elements. These neurons complete the description of a continuous synaptic pathway from afferent to flexion neurons in the Tritonia swim circuit. Their identification should facilitate studies of motor program initiation, as well as of how various forms of experience, including simple forms of learning, act to influence neuronal decision-making processes.