1. The membrane response of the distal photoreceptors in the retina of the scallop Pectin irradians to the termination of a bright white light (off response) is shown to be composed of the decay of the hyperpolarizing receptor potential and an action potential with slow kinetics. 2. The action potential can be produced in darkness in the absence of external Na+ ions by membrane depolarization. 3. The action potential is maintained by replacement of external Ca2+ with Sr2+ or Ba2+, but not by Mg2+. In normal external Ca2+ (9mM), the action potential is abolished by the addition of the Ca2+ inhibitors, La3+, Co2+, and Mn2+ or the organic Ca2+ antagonist D‐600. 4. Elevated external Ca2+ concentrations increase the rate of rise and peak amplitude of the action potential as well as the rate of repolarization and after hyperpolarization, but decrease the duration. 5. The rate of rise and peak amplitude of the action potential are increased by the K+ antagonists tetraethylammonium (TEA) 4‐amino‐phyridine (4‐AP), Ba2+ and procaine. The antagonists have different effects on subsequent phases of the response, however. External TEA and Ba2+ increase the duration, but decrease the rate of repolarization and abolish the after hyperpolarization, whereas external 4‐AP and procaine increase the rate of repolarization, decrease the duration and increase the after hyperpolarization. 6. The ratio of the Ca2+ to K+ permeability (P Ca/P K) estimated from the constant field equation at the peak of the action potential in different external Ca2+ concentrations is close to 1. 7. The maximum rate of rise and the peak amplitude of the action potential are increased by membrane hyperpolarization and decreased by membrane depolarization. They are decreased by background light intensity relative to their value in the dark. 8. In normal ASW the action potential can be identified during the off response as a small overshoot of membrane potential relative to its value in the dark. 9. The rate of repolarization of the off response in normal ASW is reduced by agents or conditions which inhibit or reduce Ca2+ permeability changes, e.g. external Co2+ and La2+ or zero external Ca2+. 10. Our results suggest that a voltage‐dependent increase in membrane permeability to Ca2+ and to K+ ions modifies the repolarizing phase of the receptor potential.
1. Membrane currents from the bursting pace-maker neurone R-15 of Aplysia were measured under conditions designed to simulate membrane oscillations. Changes in the absorbance of the Ca(2+)-sensitive dye arsenazo III were used to monitor changes in the free intracellular Ca(2+) concentration, [Ca](i), under these conditions. In addition, changes in the extracellular K(+), concentration [K](o) were measured with K(+)-sensitive electrodes.2. In normal external ionic conditions the depolarizing phase of pace-maker activity was associated with a slow inward current and the hyperpolarizing phase with a slow outward current.3. In cells where the early inward Na(+) current was blocked by tetrodotoxin and outward K(+) currents were suppressed by intracellular EGTA and extracellular tetraethylammonium and 4-aminopyridine, the slow inward current was significantly larger in amplitude and was suppressed by removal of external Ca(2+) or the addition of external La(3+), but not by the removal of external Na(+).4. The slow inward current was increased when [Ca](o) was raised and decreased when it was reduced in the manner expected for current flow through a Ca(2+) channel. The selectivity of the slow inward current for divalent cations was [Formula: see text].5. The slow inward current was only slightly reduced by a 10 degrees C reduction in temperature.6. In normal external and internal ionic conditions changes in dye absorbance occurred when the membrane was depolarized with slow triangular voltage ramps or long depolarizing steps within the pace-maker oscillation range. The obsorbance change, and thus the increase in Ca(2+), [Ca](i), was well correlated with the appearance of the slow inward current. Moreover, the magnitude of the slow outward current was dependent upon the change in [Ca](i).7. The slow inward current and a substantial fraction of the outward current, as well as the change in [Ca](i), were reduced appreciably by the addition of La(3+) ions (3 mM) to the external medium.8. The increase in [Ca](i) during prolonged depolarization was not affected by external tetrodotoxin or by the removal of external Na(+), but was abolished by a Ca(2+)-free external medium containing EGTA. Nevertheless, significant changes occurred in [Ca](i) during depolarization in 0.1 mM-external Ca(2+).9. In normal external and internal ionic conditions extracellular K(+), [K](o), increased during the depolarizing phase of the pace-maker cycle and decayed during the hyperpolarizing phase.10. There was a measurable increase in [K](o) during small prolonged depolarizing steps which produced a net inward current, indicating that inward and outward currents overlap under normal conditions.11. In the absence of action potential discharge, [Ca](i) increased during the depolarizing phase and decreased during the hyperpolarizing phase of the membrane oscillation.12. It is proposed that pace-maker oscillations depend upon three separate but linked systems which include a voltage-dependent Ca(2+) current, the free intracellular Ca(2+) concentration and the Ca(2+)-activated K(+) current.
Arsenazo III was used to measure changes in the free intracellular calcium ion concentration during spontaneous bursting pacemaker activity in the Aplysia R15 neuron. Intracellular calcium increased during the burst, and this increase was sufficient to cause the hyperpolarization that followed. The results suggest that the interval between bursts is determined by the rate of subsequent decline of free intracellular calcium.
ABSTRACT The movement of ions through the extracellular space in the tissues surrounding the giant neuron (G cell) of the nudibranch mollusc, Anisodoris nobilis, was studied with anatomical and physiological techniques. The diffusion pathways in the gastro-oesophageal ganglion and nerve were identified anatomically in electron micrographs obtained from preparations which were first incubated in sea water containing lanthanum chloride and subsequently fixed in a basic solution of glutaraldehyde. A lanthanum precipitate was found extracellularly in the connective sheath, in the extracellular clefts between the glial cells and in the space between the glia and the G cell. The rate of diffusion of potassium was estimated from the rate of change of the G cell membrane potential following a change in the potassium concentration of the artificial sea water bathing the preparation. The average half-time for diffusion corresponds to an equivalent diffusion pathway of about 200 μ m. This value is sufficiently close to the average length (70–95 μ m) of the pathways identified with lanthanum to suggest that the restriction to diffusion is minor. The permeability of the extracellular space to potassium is high (5· 7× 10−1 cm/sec), and our calculations show that a difference larger than 2 mM, between the potassium concentration of the external solution and that of the fluid layer adjacent to the G cell membrane, cannot be maintained under most conditions.
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)