Membrane potential and threshold of single muscle fibers

Field Stimulation of 2-D Sheets of Excitable Tissue

This paper develops equations for the transmembrane potentials (Vm) that occur in two-dimensional (2-D) sheets of tissue in response to field stimulation from an electrode near but not on the surface of the tissue. Comparison of results with those for one dimension shows that an additional term is present in the 2-D equations that influences the evolution of Vm in the interval between the end of the stimulus and the active propagation that may follow. The results provide an analytical framework for understanding Vm in response to field stimulation in two dimensions, both during the tissue's critical linear phase and thereafter.

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.

Potassium and the recovery of arterial smooth muscle after cold storage

The influence of K on the performance of vascular smooth muscle was studied by observing the mechanical performance of the muscle under conditions in which the magnitudes of [K_i] and of the [K_i]:[K_o] ratio varied in opposite directions. During prolonged storage at 4°C the artery strips lost K and their ability to respond to stimuli. Subsequently they were transferred to recovery solutions of various [K_o] at 38°C. The initial rate of K_i reaccumulation and steady state [K_i] were greater in solutions of higher [K_o]. Conversely for any time during recovery, the greater [K_o], the smaller the [K_i]:[K_o] ratio. When the strip was placed in the warm recovery solution it first contracted and then relaxed. The initial contraction was not relatable to [K_o] of the recovery solution but the subsequent relaxation was greater in rate and magnitude as [K_o] was greater. As the muscles recovered further they went into tonic contracture. As the [K_o] in the recovery solutions was greater these contractures occurred after shorter recovery times, and attained greater amplitude at a faster rate. Solution-switching experiments indicated a dependence of responses to electrical shocks on both the [K_i]:[K_o] ratio and [K_i]. Conclusions drawn were: (a) increased [K_i] increases contractility, (b) increased [K_i] increases the rate of relaxation, (c) excitability is decreased by too high or low a [K_i]: [K_o] ratio, and (d) the extent of tonic shortening depends on the [K_i]:[K_o] ratio.

Succinylcholine and muscle excitability

Succinylcholine lowers the resting membrane potential taken with microelectrodes similarly in nerve-scarce and innervated portions of frog sartorius muscle. Twitches to electrical excitation of the nerve-scarce pelvic end of the muscle are also rapidly reduced. The results indicate that succinylcholine probably acts generally on the muscle membrance to diminish excitability.

Properties of electrolyte-filled glass microelectrodes: an experimental study

The electrochemical and electrical properties of geometrically defined electrolyte-filled microelectrodes were studied at various transelectrode current passages, using radiotracer (38Cl and 42K) and electrical techniques. Geometrically, the electrodes were defined by their tip properties that, for standard (single-barrelled, 3.0 M KCl-filled, approximately 10 M[ohm]) electrodes implied a tip opening radius of 0.135 microm and a tip taper of 0.0215 microm/microm in the most distal (0-150 microm), and of 0.0105 microm/microm in the next most distal (150-1000 microm) tip regions. From the radiotracer studies it followed that (a) in the absence of transelectrode current passage, K+ and Cl- are leaking from the electrode tip in amounts corresponding to currents of +/- 3.8 nA, and (b) in the presence of transelectrode current passage, the flow of K+ and Cl- through the electrode tip changes with the transelectrode current in a statistically linear fashion so that K+ carries about 80% and Cl- about 20% of any electrode-injected current. From the electrical measurements it appeared that the standard electrodes are characterized by (a) a tip potential of -2.6 mV, and (b) a resistance that changes from an instantaneous, non-rectifying type to a steady state, outwardly rectifying type, within tenths of a second of constant current flow. The outward current rectification was seen to be reduced by raising [KCl] in the immersing solution, or by lowering it in the filling solution. Together, the observed electrode properties are consistent with the electrode electrolyte's solute and solvent turnover being governed by electro-osmotic as well as by electrodiffusion laws.

Threshold variability in fibers with field stimulation of excitable membranes

The central focus of this report is the evolution of transmembrane potentials following initiation of a point-source field stimulus, particularly when the stimulus is short and the stimulating electrode is close to the fiber. The transmembrane voltage threshold in response to a point-source field stimulus was determined in a numerical model of a single unmyelinated fiber. Both nerve (Hodgkin-Huxley) and cardiac (Ebihara-Johnson [1]) models of the fiber membrane were evaluated. A central question is whether it is possible to know in advance whether a stimulus of specific magnitude, duration, and location will result in a subsequent action potential. Such determination can be based on the membrane's "voltage threshold." In contrast to the commonly held view, the voltage threshold was found to vary markedly depending on the duration and location of the field stimulus. Voltage thresholds ranged from about 8 mV above baseline to more than 100 mV above baseline, the higher thresholds occurring with shorter stimuli and electrode locations closer to the membrane. A related question is whether the passive membrane response can be used as a tool in determining whether a subsequent action potential is elicited. If the answer is affirmative, this finding can be very useful, since passive properties are linear and thereby much simpler to evaluate than active ones. The results show that the passive response tracks active responses long enough to be a good estimator of subsequent action potential development. Examples show that the evaluation of Vm at 0.2-0.5 msec after stimulus initiation, times chosen on the basis of membrane characteristics, was a better predictor of subsequent excitation than was either initial transmembrane current or Vm at the time when the stimulus ends. Most of the circumstances analyzed here with electric field stimulation also appear likely to be valid with magnetic field stimulation.

Depolarizing neuromuscular block--a presynaptic mechanism?

It has been known since 1951 that drugs such as decamethonium and suxamethonium produced an acetylcholine like (agonist) effect on the postsynaptic membrane of the neuromuscular junction causing depolarization. Inspite of evidence of action of these drugs on the motor nerve terminals, it has been widely assumed that the neuromuscular block they produced is the result of depolarization followed by desensitization of the postsynaptic membrane. Evidence questioning the view that these drugs produce their clinical effects as a consequence of depolarization of the postsynaptic membrane or desensitization is presented together with the results of recent experiments which are more readily explained by proposing a presynaptic action of these drugs, initially stimulating then depressing acetylcholine release.

E-type prostaglandins depolarize primary afferent neurons of the neonatal rat

The mechanism of action of prostaglandins (PGs) to sensitize sensory terminals to noxious stimuli was studied in the isolated spinal cord-tail preparation of the newborn rat. Application of a small amount of capsaicin to the tail induced a nociceptive reflex that was recorded extracellularly from the lumbar ventral root. Pretreatment of the tail with PGE1 or E2 (0.8-4 microM) markedly potentiated the capsaicin-induced nociceptive reflex. In the isolated spinal cord preparation of the newborn rat, application of PGE1 or E2 (10 nM-1 microM) induced a depolarization of the dorsal root. Based on these results we propose a hypothesis that PGs regulate the resting potential of the peripheral terminals of nociceptive primary afferent fibers and that the depolarization is associated with lowering of threshold for various noxious stimuli.

Transport of electrolytes in muscle

The muscle fiber stands alongside the red blood cell and the giant axon as one of the three classical cell types that have had major application in investigating ion transport processes in cell membranes. Of these three cell types, the muscle fiber was the first to provide definite evidence for a sodium pump. The ability of the sodium pump to produce an electrical potential difference across the cell membrane was also first demonstrated in muscle fibers. This important property of the sodium pump is now known to have physiological significance in many other types of cells. In this review, electrolyte transport investigations in skeletal muscle are traced from their inception to the current state of the field. Applications of major research techniques are discussed and key results are summarized. An overview of electrolyte transport in muscle, this article emphasizes relationships between the muscle fiber membrane potential and ionic transport processes.

Prolonged exposure to acetylcholine: noise analysis and channel inactivation in cat tenuissimus muscle

1. Micro-electrodes were used to record membrane potential and associated noise at the end-plate region of cat tenuissimus muscle (37 degrees C), during applications of acetylcholine (ACh) in continuously flowing Krebs solution containing eserine and tetrodotoxin. 2. Densensitization was assessed from the frequency of channel opening calculated from the noise variance. 3. At higher concentrations of ACh (10-50 microM), desensitization occurred with an exponential fall to a plateau. 4. At low concentrations of ACh (1-2 microM) only slight desensitization occurred and at a much lower rate. Frequency of channel opening decreased at the rate of 0.045 +/- 0.024 min-1. Maximum frequency was (33 +/- 9) X 10(7)/sec while maximum depolarization was 20.5 +/- 1.4 mV (n = 11 cats). Depolarization was well maintained. 5. This slow rate of desensitization at low concentrations of ACh was confirmed in experiments where voltage clamped current, its associated noise, and miniature end-plate current amplitude were measured. 6. At low concentrations of ACh (1-2 microM) in the presence of eserine there was sustained block in neuromuscular transmission when twitch tension was measured. 8. It is concluded that the mechanism of neuromuscular block by ACh at around 1 microM concentration is by depolarization itself, not desensitization.

Manganese amd electrogenic phenomena in canine Purkinje fibers

Studies were performed on canine cardiac Purkinje fibers to evaluate the effects of manganese on membrane electrogenesis. The results indicate that manganese has a calciumlike effect on the excitatory sodium current and inhibitory effects on potassium conductance and slow inward current. The calciumlike effect of manganese on sodium current was reflected through a leftward (toward less negative potentials) and downward shift in the curve relating maximum upstroke velocity to membrane potential. The inhibitory action of manganese on potassium conductance was suggested by the following observations. (1) Manganese caused an initial increase in action potential duration largely due to a lengthening of the plateau and decreases in the rates of phase 3 and terminal repolarization. (2) Manganese increased the rate of diastolic depolarization. (3) Manganes blocked the initial fall in maximum diastolic potential accompanying rapid stimulation. (4) Manganese in high concentrations caused generalized depolarization which was reversed by rapid stimulation and by increased extracellular potassium concentrations. The action of manganese to block slow inward current was indicated by the eventual shortening of the plateau and by the elimination of responses initiated from low levels of membrane potential (less than minus 55 mv). In addition to these effects, manganese also reduced membrane excitability, eliminated arrhythmic beats occurring during low-frequency electrical stimulation, and caused membrane hyperpolarization which was blocked by tetrodotoxin.

Pre- and postjunctional neuromuscular blockade by carbachol

Carbachol, when applied to the bathing Ringer solution of frog sartorius muscles, caused depolarization of the endplate and a blockade of endplate potentials (EPP's), miniature EPP's (mepp's) and the iontophoretic acetylcholine potential. In muscles treated with an analog of hemicholinium-3, alpha, alpha'bis(dimethylammonium acetaldehyde diethylacetal)-p-p'-diacetylbiphenyl dibromide (DMAE), depolarization of the endplate by carbachol was blocked and the blockade by carbachol of the iontophoretic acetylcholine potential was prevented. These responses to carbachol were attributed to a postjunctional action that was antagonized by DMAE. In contrast, the blockade by carbachol of EPP's and mepp's was enhanced in DMAE-treated muscles at a time when carbachol-induced depolarization was blocked. This response to carbachol was attributed to a pre-junctional action. Carbachol either blocked transmitter release by a mechanism that was insensitive to DMAE or enhanced the prejunctional blocking actions of DMAE. Succinylcholine had actions similar to carbachol. DMAE prevented depolarization by succinylcholine but enhanced neuromuscular blockade by succinylcholine. SKF 525-A (beta-diethylaminoethyl diphenylpropylacetate hydrochloride), like DMAE, prevented depolarization but not transmission blockade caused by carbachol.

Effects of manganese on the electrical and mechanical properties of frog skeletal muscle fibres

1. The effects of Mn on the electrical and mechanical properties of frog muscle fibres have been studied.2. In normal saline 10 or 20 mM-Mn hyperpolarized the fibres and had no effect on the membrane resistance. In isotonic K(2)SO(4) saline, Mn increased the membrane resistance indicating that this agent reduced the conductance to K.3. The action potential is prolonged by Mn while the overshoot amplitude is unaffected. The threshold of the action potential is shifted to more positive values of membrane potential.4. The isometric twitch is reduced by 45% in 10 mM-Mn; this effect is observed within 8 sec of the application.5. Mn (10 mM) reduced K contractures induced by 40 or 75 mM-K (constant [K].[Cl] product) and shifted to the right in a parallel manner the curve tension vs. log K concentration. The calculated mechanical threshold for K contractures was shifted from -48 to -33 mV.6. Caffeine contractures (3-4 mM) and supramaximal K contractures (190 mM-K) were unaffected by 10 mM indicating that contractile proteins and the ability of the sarcoplasmic reticulum to release Ca are not impaired.7. It is concluded that Mn is mainly affecting the excitation-contraction coupling by altering the mechanical threshold. Since Mn reduces the permeability to Ca in several excitable membranes, it is suggested that the mechanical threshold depends on the entry of Ca to the muscle.

Influence of extracellular K+ concentration on cable properties and excitability of sheep cardiac Purkinje fibers

Conduction velocity was increased from 3.5 m/sec to 4.1 m/sec in sheep cardiac Purkinje fibers when the superfusing Tyrode solution was changed from 2.7 mM K + to 4.0 mM K + . Further increase to 7.0 mM K + resulted in a fall in conduction velocity to 3.3 m/sec. To understand the nature of the cellular change responsible for this effect, cable analyses were made. With increases in extracellular K + concentration membrane resistance decreased and membrane capacitance did not change. Resistance of the myoplasm tended to fall in 7.0 mM [K] o . Since these effects did not explain the change in conduction velocity, excitability was studied by strength-duration curves using intracellular micropipettes for current passage and recording. Increase in K + from 2.7 to 4.0 mM resulted in a shift of the entire curve to the left, with a fall in rheobasic current from 121 to 104 nanoamperes and a fall in the time constant from 2.79 to 2.4 msec. Normalized plotting of stimulating current over rheobasic current (I/I Rh ) against duration of stimulating current over the time constant (t/τ) suggested that the curves were not basically different. The increase in K + from 2.7 to 4.0 mM was associated with a depolarization of the resting membrane, consistent with alteration in the potassium equilibrium potential, but no change in the absolute membrane potential for threshold. In this way, the changes in membrane voltage and charge necessary for excitation were reduced in 4.0 mM [K] o .

Local activation of frog muscle fibres with linearly rising currents

1. Single fast muscle fibres isolated from the semitendinosus muscles of the frog were locally activated by applying linearly rising current pulses to a pipette whose tip (diameter, 10-40 mu) was in contact with the fibre surface.2. In normal or choline-Ringer solution with 1.8 mM-Ca, the threshold depolarization for producing a just perceptible local contraction was nearly constant over a wide range of the duration of linearly rising currents (0.5-20 sec or more).3. If [Ca](o) was reduced to 0.25-0.1 mM, the threshold depolarization increased steeply with the increase in the duration of linearly rising currents, indicating a marked increase in the rate of an accommodation process in excitation-contraction coupling mechanism.4. In the low-Ca media, the threshold depolarization was observed to rise exponentially during the application of a linearly rising current of 3-10 sec duration, and to return exponentially to its initial value within 3-10 sec after the removal of the current.5. The marked accommodation in the low-Ca media was partially inhibited by the addition of Mg (5-10 mM), and completely eliminated in the presence of caffeine (1 mM), suggesting that the accommodation process may occur in the link between membrane depolarization and release of Ca from sarcoplasmic reticulum.6. Microscopic observation of local contractions in the low-Ca media indicated that the accommodative rise of threshold depolarization was most marked at the superficial layer of the fibre.7. From these results, it is suggested that the removal of bound Ca or some other kind of charged particle from the transverse tubular system or from its vicinity might be the cause of accommodation in excitation-contraction coupling.