Electrophysiological studies on the branchial ganglion in the opisthobranch molluscs (Aplysia andDolabella)

1. The branchial ganglion of the gills ofAplysia kurodai, A. juliana andDolabella auricularia were investigated, using electrophysiological techniques. 2. The branchio-ganglionic neurons (BGNs) generated spontaneous tonic impulses with superimposed pacemaker potentials and mainly periodic bursting impulses superimposed on slow sustained depolarizing potentials. 3. BGNs were synchronously active, coupled one to another through bidirectional electrical synapses. 4. Electrical stimulation of the branchial nerve elicited excitatory postsynaptic potentials (EPSPs), in BGNs, which showed facilitation and summation and were blocked with high Mg2+ sea water, indicating that the EPSPs were chemically mediated. 5. Electrical stimulation of a nerve extending posterior to the ganglion also produced EPSPs in BGNs. These EPSPs showed summation (but not facilitation), and were blocked with high Mg2+ sea water, indicating that the EPSPs were chemically mediated. 6. Tactile stimulation to the pinnule exerted excitatory transmission of EPSPs which caused BGNs to fire a burst of impulses, and resulting in a reflexive pinnule contraction. 7. Electrical stimulation applied to a nerve of the branchial ganglion produced inhibitory postsynaptic potentials (IPSPs) which showed summation and had a reversal potential at a level slightly higher than the resting membrane potential. The inhibitory transmission seems to be from the mechanoproprioceptor in the gill, since it was elicited by a spontaneous gill contraction or by perfusion of the branchial vasculature with sea water. 8. When either spontaneous bursts of the BGNs occurred, or bursts of BGNs were induced by intracellular current application, EJPs were evoked in the pinnule muscle. Thus, the peripheral nervous system including the branchial ganglion may contain motor neurons of the pinnule muscle. 9. The branchial ganglion had functions of the peripheral reflex center and the distributing center of central motor influence to the branchial muscle.