Semi-isolated preparations of the nine-celled cardiac ganglion of the crab, Portunus sanguinolentus, were studied electrophysiologically, using simultaneous recording from extracellular and two or three intracellular electrodes. Nine penetrations of small cells were achieved. 2. Three large (80 x 120 micron) cells lie near the anterior end of the 5-mm main trunk; two large and four small (less than 50 micron) cells at the posterior end. Large-cell axons pass along the main trunk and then exit to innervate cardiac muscle; small-cell axons do not leave the ganglion. 3. The semi-isolated ganglion produces spontaneous electrical activity organized into regularly patterned, rhythmic bursts of large- and small-cell impulses recurring at rates of 0.3-0.6/s and lasting 500-800 ms. Small impulse activity commences and ends each burst. Small cells fire trains during the burst, but impulses are not synchronized among them. Large-cell trains are synchronous, are at about one-half the frequency, and have fewer impulses than small-cell trains. 4. Intracellular recordings from small cells show a slow, pacemaker depolarization from a maximum membrane potential of -54 mV leading with only a slight inflection at ca. -50 mV to a depolarized plateau at ca. -40 mV; nonovershooting impulses are superimposed on this but cease before it repolarizes. Impulses, therefore, arise at a site distant from the soma and do not invade it. Deflections suggesting synaptic potentials are not seen. 5. Intracellular recordings from large cells show complex depolarizations corresponding to extracellularly recorded bursts. These represent excitatory postsynaptic potentials (EPSPs) corresponding with individual small-cell impulses, attenuated, non-overshooting spikes, and an underlying slow depolarization; usually no pacemaker depolarization is apparent between bursts. Chemically mediated transmission is probable for the EPSPs because they show delay, increase in amplitude with hyperpolarization, sometimes show facilitation, and are reduced in saline having one-third Ca, 3 x Mg. 6. EPSPs, impulses, and the slow depolarization occur synchronously among the large cells. Potentials recorded from posterior cells are attenuated and slower than those of the anterior cells. This is interpreted to reflect sites of occurrence more distant from the soma in the posterior than in the anterior cells. Impulses do not invade the somata. 7. Intracellular recordings from large-cell axons 4 mm from the soma show overshooting action potentials arising sharply from a base line. EPSPs are absent or highly attenuated and there is little underlying depolarization (less than 2 mV). 8. Current passing with electrodes intracellular to two cells has established directly that all large cells are electrotonically coupled and that an anterior cell and a small cell are coupled. Changes of burst rate during current passing into any large cell indicate that all large cells and small cells are electrotonically coupled. 9...
