ISOLATION AND CHARACTERIZATION OF SLOW, DEPOLARIZING RESPONSES OF CARDIAC GANGLION NEURONS IN THE CRAB, PORTUNUS-SANGUINOLENTUS

Tetrodotoxin-resistant, active responses to depolarization of the large cardiac ganglion cells were studied in semi-isolated preparations from the crab, Portunus sanguinolentus. About 20 min after introduction of TTX, small-cell impulses and resultant EPSPs in large cells cease, while rhythmic, spontaneous bursting of large cells continues. A pacemaker depolarization between bursts and slow depolarizations underlying the impulse bursts are prominent at this time. In the quiescent, TTX-perfused ganglion, injection of depolarizing current into any one of the large cells results in active responses. At current strengths of sufficient intensity and duration (e.g., 20 nA, 20 ms; 5 nA, 500 ms) to depolarize a large cell by ca. 10 mV from resting potential (-53 mV, avg), the graded responses become regenerative and of constant form, provided the stimulation rate is <0.15/s. responses poresponses have been termed 'driver potentials'. At more rapid rates, thresholds are increased and respones reduced. Driver potentials of anterior large cells reach peak amplitudes of ca. 20 mV (to -32 mV), have maximum rates of rise of 0.45 V/s and of fall of 0.2 V/s, and a duration of ca. 250 ms. The ability to elicit near-synchronous responses and the identity of amplitude and form of responses among anterior cells and of posterior cells, regardless of which cell receives depolarizing current, indicates that all undergo active responses and are stimulated by electrotonic spead of depolarization. Depolarization by steady current increases the absolute threshold, decreases the maximum depolarization of the peak, and slows rates of rise and fall. Hyperpolarization increases rates of rise and fall; the absolute value reached by the peak depolarization is unchanged. Hyperpolarization reduces the amplitudie of the rapid after potential relative to the displaced resting potential. Hyperpolarization current pulses imposed during the rise and peak of driver-potential responses are followed by redevelopment of a complete response. Sufficiently strong hyperpolarization can terminate a response. The current strength needed to terminate a response decreases the later during the response the pulse is given. Depolarizing pulses given near the peak of a response displace the potential less than hyperpolarizing pulses of the same strength; they result in more rapid repolarization. Given during the repolarizing phase, they alter the course of the response little.