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 tau(1) (0.2-0.4 sec at 23 degrees C) and tau(2) (0.07-0.12 sec at 23 degrees 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 tau(1) and tau(2) (Q(10) = -2 to -3) and the increase of the tonicity of the external medium increases tau(1) but not tau(2). The final level of tension is related by an S-shaped curve to the membrane potential. The slope of the final tension-membrane 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.

Animals↗

PubMed1968

Sodium and calcium components of action potentials in the Aplysia giant neurone.

1. 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 drug has no effect on the calcium-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.

Action Potentials↗

PubMed1967

Multi-ionic action potentials in molluscan giant neurones.

Explore the source record for details and available documents.

Action Potentials↗

PubMedDate not supplied

Turns and averaging as static and dynamic measures of masseter EMG activity.

Surface EMG records from human masseter muscles were analyzed at various fractions of the maximum voluntary contraction (MVC). Frequencies of turns, or reversals of signal direction, and integrated or average rectified values were compared as quantitative measures of this type of data. Averaged signals were useful for within-study comparisons or for revealing dynamic changes in EMG during excitation or inhibition. However, averaged values depend strongly on signal amplitude (and thus on electrode characteristics and muscle size), and are not easily transferable between studies. Turns frequencies increased with contraction up to 75% MVC, and were less dependent on signal amplitude than were averages. Turns frequencies may thus be useful for quantitative comparisons between studies or between different muscles.

Arm↗