FUNCTIONAL CONSEQUENCES OF COMPARTMENTALIZATION OF SYNAPTIC INPUT

Intra-axonal recordings of stomatogastric nerve axon 1 (SNAX1) indicat e that there are synaptic inputs onto the SNAX1 terminals in the stoma togastric ganglion (STG) of the crab Cancer borealis (Nusbaum et al., 1992b). To determine whether this synaptic input only influenced SNAX1 activity within the STG, we identified the SNAX1 soma in the commissu ral ganglion (CoG). We found that this neuron has a neuropilar arboriz ation in the CoG and also receives synaptic inputs in this ganglion. B ased on its soma location, we have renamed this neuron modulatory comm issural neuron 1 (MCN1). While intracellular stimulation of MCN1(soma) and MCN1(snax) has the same excitatory effects on the STG motor patte rns, MCN1 receives distinct synaptic inputs in the STG and CoG. Moreov er, the synaptic input that MCN1 receives within the STG compartmental izes its activity. Specifically, the lateral gastric (LG) neuron synap tically inhibits MCN1(snax)-initiated activity within the STG (Nusbaum et al., 1992b), and while LG did not inhibit MCN1(soma)-initiated act ivity in the CoG, it did inhibit these MCN1 impulses when they arrived in the STG. As a result, during MCN1(soma)-elicited gastric mill rhyt hms, MCN1(soma) is continually active in the CoG but its effects are r hythmically inhibited in the STG by LG neuron impulse bursts. One func tional consequence of this local control of MCN1 within the STG is tha t the LG neuron thereby controls the timing of the impulse bursts in o ther gastric mill neurons. Thus, local synaptic input can functionally compartmentalize the activity of a neuron with arbors in distinct reg ions of the nervous system.