SUIMMARY1. Action potentials resulting from direct stimulation can be recorded from the soma of the Aplysia giant neurone (located in the visceral ganglion) in sodium-free and in calcium-free external solutions. The neurones were impaled by internal micro-electrodes throughout the change of external solutions.2. Complete replacement of either sodium or calcium in the bathing medium with Tris results in only a partial reduction of spike overshoot. Simultaneous replacement of both sodium and calcium reversibly and quickly abolishes the spike.3. The sodium component of the spike in a calcium-free medium is blocked by tetrodotoxin; the dg e e n -dependent spike in sodium-free medium. Externally applied cobalt chloride blocks only the calcium-dependent component.4. In calcium-free media, the overshoot value varies with sodium concentration in the manner predicted for a sodium electrode. In sodium-free media, the membrane behaves like a calcium electrode.5. These results suggest that, during the normal action potential, both sodium and calcium act as carriers of the inward-directed current.

SUMMARY1. The hyperpolarization between bursts in the R 15 cell of Aplysia is accompanied by an increase in membrane slope conductance.2. The post-burst hyperpolarization can be observed with ouabain, lithium, or potassium-free solution if artificial inward current is applied. The hyperpolarization can be observed with dinitrophenol or cooling to 10°C, with no injected current. Thus, the hyperpolarization apparently is not due to the cyclic activity of an electrogenic pump.3. A reversal potential for the post-burst hyperpolarization can be demonstrated by passage of inward current during the inter-burst period. The reversal of direction of the potential depends on recent occurrence of a burst.4. The reversal potential varies with external potassium concentration, but not with chloride or sodium.5. The post-burst hyperpolarization is not blocked by external tetraethylammonium at a concentration which greatly prolongs the action potentials.6. During the onset of spike blockage by, and recovery from, calciumfree + tetrodotoxin saline, the bursts of action potentials appear to be driven by endogenous waves of membrane potential.7. The hyperpolarizing phase of the waves in calcium-free + tetrodotoxin medium is accompanied by an increased slope conductance.8. A reversal potential can be demonstrated for the hyperpolarization following a wave in calcium-free + tetrodotoxin medium by applying inward current during the interwave period.9. The waves in calcium-free + tetrodotoxin medium are blocked by ouabain but can be reinstated by artificial hyperpolarization.10. The post-burst hyperpolarization and the post-wave hyperpolarization appear to result from a periodic increase in membrane conductance, primarily to potassium ions. DOUGLAS JUNGE AND CATHY L. STEPHENS

SUMMARY1. Intracellular records from Aplysia giant (R 2) cell somata showed long lasting 4-10 mV hyperpolarizations after passage of outward current through a second intracellular electrode.2. An increase in membrane slope conductance occurred simultaneously with the post-stimulus hyperpolarization (PSH).3. Both the PSH and conductance-increase varied strongly with stimulus amplitude and duration.4. Both the PSH and the conductance increase occurred in Ca-free medium containing tetrodotoxin, when action-potential production was completely blocked.5. The PSH persisted in the presence of ouabain or DNP, with cooling, with removal of external K+, and in media where all the Na+ was replaced with Li+, suggesting that it was not due to the activity of an electrogenic pump.6. A reversal potential for the PSH was demonstrated by application of maintained inward current following the end of an outward-directed stimulus.7. The PSH reversal potential varied with [K]o, but not with [Cl]o or [Na]o, suggesting that the PSH was mainly due to an increase in K conductance.8. The PSH and the conductance increase were reduced strongly when all the Na+ was replaced with Tris, and only slightly when Na+ was replaced with sucrose.

Relations between the membrane potential and the tension associated with changes in membrane potential were analyzed in barnacle giant muscle fibers by using voltage clamp techniques. With a step change in membrane potential the tension reaches its final level with a time course which is expressed by the difference of two exponential functions. The time constants rl (0.2-0.4 sec at 23°C) and r2 (0.07-0.12 sec at 23°C) are independent of the new membrane potential at least for a relatively small membrane potential change while the final level of tension is a function of the potential. Decreasing the temperature increases both rl and T2 (Q~0 = -2 to -3 ) and the increase of the tonicity of the external medium increases rx but not r2. The final level of tension is related by an S-shaped curve to the membrane potential. The slope of the final tension-meml~rane potential curve increases with increasing external Ca concentration and is reduced when a small amount of transition metal ions is added to the medium. This suggests that the influx of Ca ions through the membrane is an important factor in the development of tension.The relation between muscle tension and membrane potential has been studied in frog muscle fibers (Hodgkin and Horowicz, 1960), in cardiac muscle fibers (Niedergerke, 1956 b), and in muscle fibers of a barnacle, Balanus nubilus Darwin (Hoyle and Smyth, 1963) by observing potassium contractures. Similar studies have also been done by altering the membrane potential with polarizing currents in crayfish muscle fibers (Orkland, 1962 a) and in barnacle muscle fibers (Edwards, Chichibu, and Hagiwara, 1964). Considerable information has been accumulated from these studies on the relation between the amplitude of muscle tension and the membrane potential. However, very little has been done to analyze how the time course of tension development is determined by changes of the membrane potential.For this type of analysis it is necessary to control the membrane potential of I57