Some characteristics of synaptic potentials arising in the large cells of the lobster cardiac ganglion were investigated by means of intracellular recording and current injection.
1. The spontaneous burst was mainly composed of two kinds of synaptic potentials differing in various features, indicating that the large cells were innervated by two common presynaptic nerve fibers from pacemakers.
2. Antifacilitation was observed in large synaptic potentials, while it was not observed in small synaptic potentials.
3. Depolarization of the large cell accelerated the initiation of large synaptic potentials, andhyperpolarization impeded it. The electrotonic interaction between large cells and the presynaptic nerve cells, which produce large synaptic potentials, was rather strong.
4. Polarization of the large cell had less effect on the initiation of small synaptic potentials than on that of large synapticpotentials; the electrotonic interaction between large cells and the presynaptic nerve cells, which produce small synaptic potentials, was exceedingly weak.
5. A brief current pulse applied to the large cell evoked the slow potential, which was separated from a train of small synaptic potentials. The positive feedback of the slow potential was demonstrated well.
6. Repetitive small synaptic potentials had an acceleratory effect on the burst initiation. The rate of slow depolarization increased with development of small synaptic potentials.
7. Two types of presynaptic nerve cells exhibited differences in time courses of repetitive discharges; one produced a brief train of discharges, and the other an extensive train.
8. It was concluded that the large cells are controlled by two types of presynaptic nervecells; one evokes large synaptic potentials, initiating impulses, and the other induces small synaptic potentials, increasing excitability of the postsynaptic membrane. The latter controls the former. The large cell behaves as a pacemaker when presynaptic nerve cells become inactive or when their activities are delayed.