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16 recordsLinked to original sources
PubMed1961

Graded and all-or-none electrogenesis in arthropod muscle. I. The effects of alkali-earth cations on the neuromuscular system of Romalea microptera.

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.

Alkalies↗
PubMed1961

Graded and all-or-none electrogenesis in arthropod muscle. II. The effects of alkali-earth and onium ions on lobster muscle fibers.

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.

Alkalies↗