Restoration of membrane excitability of calcium-deficient muscles by anodal polarization.
TL;DR: The electrical activity of some muscles disappeared instantaneously in calcium-free Ringer, particularly in spring frogs, which seems to suggest that excitability depends on the properties of the membrane per se, which may be altered by the presence or absence of calcium ions.
Abstract: IT is well known that marked electrophysiological and biochemical disturbances occur in nerve and muscle immersed in calcium-free Ringer's solution1. In the present experiment, isolated frog (Rana pipiens) sartorius muscles were used. When they were soaked in calcium-free Ringer solution (112 mM sodium chloride ; 2 mM potassium chloride ; 2.4 mM sodium bicarbonate), initial hyper-excitability was followed by a loss of excitability. This effect of calcium deficiency was enhanced by adding ethylene-diaminetetraacetic acid to the medium. In calcium-free Ringer containing 4 mM ethylenediaminetetra-acetic acid, muscles twitch while the resting potential rapidly drops to 50–30 mV. within an hour. As usual, the electrical activity which ordinarily accompanies stimulation was completely but reversibly eliminated in the 60–120 min. following immersion. This loss of excitability cannot be explained simply by the reduction of the resting potential, since a number of fibres showed, transiently, a relatively high resting potential in spite of a loss of excitability2. Whether excitability was maintained depended on the susceptibility of individual fibres; even a seasonal variation being apparent. For example, the electrical activity of some muscles disappeared instantaneously in calcium-free Ringer, particularly in spring frogs. This seems to suggest that excitability depends on the properties of the membrane per se, which may be altered by the presence or absence of calcium ions.
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TL;DR: Conversion of graded responsiveness of lobster muscle fibers to all-or-none activity by alkali-earth and tetraethylammonium (TEA) ions appears to be due to a combination of effects.
Abstract: Conversion of graded responsiveness of lobster muscle fibers to all-or-none activity by alkali-earth and tetraethylammonium (TEA) ions appears to be due to a combination of effects. The membrane is hyperpolarized, its resistance is increased, and its sensitivity to external K+ is diminished, all effects which indicate diminished K+ conductance. While the spikes are prolonged, the conductance is higher throughout the response than it is in the resting membrane. Repetitive activity becomes prominent. These effects indicate maintained high conductance for an ion which causes depolarization. This is normally Na+, since its presence in low concentrations potentiates the effects of Ba++, but the alkali-earth ions and TEA can also carry inward charge. Ba++, Sr++, and TEA appear to be more effective than is Ca++ in its normal role, which is probably to depress K+ conductance and Na inactivation. Thus, conversion of graded to all-or-none responsiveness appears to occur because of the relative increase of depolarizing inward ion flux and decrease of repolarizing outward flux.
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TL;DR: The data indicate that the alkali-earth ions exert a complex of effects on the different action components of electrically excitable membrane, which can act to convert graded responsiveness to the all-or-none variety.
Abstract: Graded electrically excited responsiveness of Romalea muscle fibers is converted to all-or-none activity by Ba++, Sr++, or Ca++, the two former being much the more effective in this action. The change occurs with as little as 7 to 10 per cent of Na+ substituted by Ba++. The spikes now produced have overshoots and may be extremely prolonged, lasting many seconds. During the spike the membrane resistance is lower than in the resting fiber, but the resting resistance and time constant are considerably increased by the alkali-earth ions. The excitability is also increased, spikes arising neurogenically from spontaneous repetitive discharges in the axon as well as myogenically from spontaneous activity in the muscle fibers. Repetitive responses frequently occur on intracellular stimulation with a brief pulse. The data indicate that the alkali-earth ions exert a complex of effects on the different action components of electrically excitable membrane. They may be described in terms of the ionic theory as follows: The resting K+ conductance is diminished. The sodium inactivation process is also diminished, and sodium activation may be increased. Together these changes can act to convert graded responsiveness to the all-or-none variety. The alkali-earth ions can also to some degree carry inward positive charge during activity, since spikes are produced when Na+ is fully replaced with the divalent ions.
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TL;DR: This chapter describes the nervous system and nerve activity of insects and the development of the capillary microelectrode made it possible to record membrane resting and action potentials from a wide variety of excitable tissues.
Abstract: Publisher Summary This chapter describes the nervous system and nerve activity of insects. There are at least three aspects of morphology and histology of insect nerve, which should be carefully examined to interpret physiological experiments: (1) organization of the giant fiber system to which microelectrode techniques are applied, (2) fine structure of the axon membrane that is undoubtedly the important site of excitation, (3) histology and biochemistry of the perineural sheath that may act as a diffusion barrier for ions. It was not until the discovery of the overshoot of the membrane potential during excitation that the mechanism of action potential production could be interpreted on a physico-chemical basis. The measurement was made by introducing one electrode longitudinally into the axoplasm of a squid giant axon. The development of the capillary microelectrode made it possible to record membrane resting and action potentials from a wide variety of excitable tissues. Although the membrane potential reflects concentration gradients of ions across the membrane and selective permeability of the membrane, the energy that enables ions to be distributed with concentration gradients and makes the membrane semipermeable to ions is thought to be supplied by metabolism.
73 citations
TL;DR: The hypothesis that the striated muscle fibers of the brackish-water crab, Callinectes sapidus, are characterized by a Donnan-like distribution is strongly supported.
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