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PubMed1964

THE EFFECTS OF VARIOUS IONS ON RESTING AND SPIKE POTENTIALS OF BARNACLE MUSCLE FIBERS.

Effects of monovalent cations and some anions on the electrical properties of the barnacle muscle fiber membrane were studied when the intra- or extracellular concentrations of those ions were altered by longitudinal intra-cellular injection. The resting potential of the normal fiber decreases linearly with increase of logarithm of [K(+)](out) and the decrement for a tenfold increase in [K(+)](out) is 58 mv when the product, [K(+)](out) .[Cl(-)](out), is kept constant. It also decreases with decreasing [K(+)](in) but is always less than expected theoretically. The deviation becomes larger as [K(+)](in) increases and the resting potential finally starts to decrease with increasing [K(+)](in) for [K(+)](in) > 250 mM. When the internal K(+) concentration is decreased the overshoot of the spike potential increases and the time course of the spike potential becomes more prolonged. In substituting for the internal K(+), Na(+) and sucrose affect the resting and spike potentials similarly. Some organic cations (guanidine, choline, tris, and TMA) behave like sucrose while some other organic cations (TEA, TPA, and TBA) have a specific effect and prolong the spike potential if they are applied intracellularly or extracellularly. In all cases the active membrane potential increases linearly with the logarithm of [Ca(++)](out)/[K(+)](in) and the increment is about 29 mv for tenfold increase in this ratio. The fiber membrane is permeable to Cl(-) and other smaller anions (Br(-) and I(-)) but not to acetate(-) and larger anions (citrate(-), sulfate(-), and methanesulfonate(-)).

Acetates↗
PubMed1964

RELATION BETWEEN MEMBRANE POTENTIAL CHANGES AND TENSION IN BARNACLE MUSCLE FIBERS.

Constant current pulses have been applied to single muscle fibers of the barnacle, Balanus nubilus Darwin, with an axial metal electrode. The membrane potential change, which took place over a large part of the muscle fiber, was measured with a similar electrode. Depolarizing pulses, if the voltage was greater than threshold, produced tension. The size of the tension was a function of the magnitude and the duration of the depolarizing pulses. The latency between the onset of depolarization and tension can be only in part attributable to mechanical factors. AC stimulation produced tension, but 5 to 10 seconds were required for the steady-state level of the tension to be reached. Muscles were depolarized in elevated K and studied after the contracture had terminated. If not too depolarized, further depolarization produced tension. Termination of hyperpolarizing pulses also produced tension, which decayed quite slowly. Hyperpolarizing pulses reduced, or abolished, any preexisting tension. Thus, it appears that at certain values of the membrane potential tension is set up, but there is also a slow process of accommodation present.

Animals↗
PubMed1964

SPONTANEOUS SLOW POTENTIAL FLUCTUATIONS IN THE LIMULUS PHOTORECEPTOR.

Spontaneous, subthreshold fluctuations of membrane potential are recorded in the eccentric cell body or dendrite of the dark-adapted Limulus ommatidium. These slow potential fluctuations (SPF's) are random in amplitude and in time of occurrence. The relation between average frequency of SPF's and light intensity is linear for low light intensities and becomes a non-linear saturation for higher intensities. The occurrences of SPF's have Poisson statistics in the dark but are non-Poisson with light stimuli. Light-adapting the ommatidium greatly decreases the SPF amplitude, and it increases the average frequency of SPF's in the dark and in response to light (facilitatory action). The shape (time course of response) of the SPF does not change at different light intensities and it is the result of a concurrent and conterminous change in membrane resistance. The functional properties of the SPF's are analyzed in terms of a stochastic model based on the summation of random events in time ("shot effect").

Animals↗
PubMed1965

THE MECHANISM OF DISCHARGE PATTERN FORMATION IN CRAYFISH INTERNEURONS.

Excitatory and inhibitory processes which result in the generation of output impulses were analyzed in single crayfish interneurons by using intracellular recording and membrane polarizing techniques. Individual spikes which are initiated orthodromically in axon branches summate temporally and spatially to generate a main axon spike; temporally dispersed branch spikes often pace repetitive discharge of the main axon. Hyperpolarizing IPSP's sometimes suppress axonal discharge to most of these inputs, but in other cases may interact selectively with some of them. The IPSP's reverse their polarity at a hyperpolarized level of membrane potential; they sometimes exhibit two discrete time courses indicating two different input sources. Outward direct current at the main axon near branches causes repetitive discharges which may last, with optimal current intensities, for 1 to 15 seconds. The relation of discharge frequency to current intensity is linear for an early spike interval, but above 100 to 200 impulses/sec. it begins to show saturation. In one unit the current-frequency curve exhibited two linear portions, suggesting the presence of two spike-generating sites in the axon. Current threshold measurements, using test stimuli of different durations, showed that both accommodation and "early" or "residual" refractoriness contribute to the determination of discharge rate at different frequencies.

Animals↗
PubMed1965

ELECTRICAL AND MECHANICAL RESPONSES IN DEEP ABDOMINAL EXTENSOR MUSCLES OF CRAYFISH AND LOBSTER.

Electrical and mechanical studies have been made of the deep abdominal extensor muscles, medial (DEAM) and lateral (DEAL), of crayfish and lobster. The medial muscle responds to direct (intracellular) and indirect stimulation with a transient membrane depolarization which exhibits the properties of a propagated non-decremental action potential but does not overshoot the zero level. The amplitude is about 30 mv in crayfish and 50 mv in lobster. It is followed by a fast all-or-none twitch whose duration at 20 degrees C is 30 to 50 msec. and whose developed tension is 500 gm/cm(2) or about half the tetanic value. Membrane potential is K(+)-dependent and immersion in high K(+) induces a brief transient tension rise as in other twitch-type muscles. The action potential and twitch are normal even if all external Na(+) is replaced with sucrose but vary with external Ca(++), the action potential increasing 8 to 10 mv for a twofold increase in Ca(++). The lateral muscle (DEAL) is much slower and responds to intracellular stimulation only with an electrotonic or a local response. Mechanical responses and relaxation speeds are slow with minimal duration of contraction of 0.5 to 2 seconds. Immersion in high K solutions induces large maintained tensions. Sarcomere length in the fast DEAM is uniform and about 2 micro at rest, but in the DEAL speed is less and sarcomere length is greater averaging about 4.5 micro but with a mixed population of fibers.

Action Potentials↗