Potassium and calcium conductance in slow muscle fibres of the toad

Slow muscle fibres in isotonic potassium sulphate saline could be easily repolarized to -90 mV. From this membrane potential a regenerative response could be elicited with short depolarizing pulses. 2. This response is blocked by TEA, suggesting that potassium is the main ion involved. 3. In the presence of TEA, a transient depolarization is recorded when the steady hyperpolarization is withdrawn. This anode break response is dependent upon the external calcium and is blocked by cobalt, suggesting that it is due to a calcium conductance. 4. The membrane conductance change was continuously recorded with short pulses at the end of the hyperpolarization. The membrane conductance decayed with at least two components with an average t1/2 of 1-2 and 6-8 sec. TEA blocked the slow component, and the fast one was dependent upon calcium and was blocked by cobalt.

Twitch potentiation by potassium contractures in single muscle fibres of the frog

1. The modifications of isometric twitches after K contractures have been studied in single muscle fibres from frog muscles.2. The twitch was greatly enhanced and somewhat prolonged after a conditioning K contracture. This potentiating effect was more evident in fibres having low values of the ratio control twitch tension/tetanus tension (P/P(0)).3. In most of the fibres, the time course of the decay of the potentiation phenomenon could be represented by the sum of three negative exponentials with average half-times of 9.1, 40 and 485 sec.4. Between 3 and 10 min after the onset of the twitch potentiation the resting potential and action potential were unchanged.5. Mn blocked the K contractures and prevented the onset of the twitch potentiation.6. 40-140 sec after inducing a K contracture with 30 or 40 mM-K, a second exposure to the same [K] evoked a tension equivalent only to 57% of the first one. This partial refractoriness of the fibres is not seen when both contractures were elicited with 190 mM-K, and contrasts with the full recovery and potentiation of the twitch after a K contracture.

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.

Induction of the action potential mechanism in slow muscle fibres of the frog

1. The electrical and structural characteristics of ;slow' muscle fibres of the frog were studied in normal and denervated muscles, and in muscles undergoing re-innervation by a mixed nerve containing large and small motor axons.2. In agreement with previous studies, slow fibres in normally innervated muscles were incapable of producing action potentials.3. Approximately 2 weeks after the sciatic nerve was transected or crushed, slow muscle fibres acquired the ability to generate action potentials. These fibres were positively identified as belonging to the slow type, because their passive-electrical and ultrastructural characteristics remained essentially unchanged after the operations.4. The action potential mechanism induced in slow fibres is sodium-dependent, and is blocked by tetrodotoxin.5. After long-term re-innervation by a mixed nerve, slow fibres lose their acquired ability to generate action potentials, presumably because small motor axons re-establish connexion with the fibres.6. It is concluded that the action potential mechanism of slow muscle fibres is under neural control, and is normally suppressed by small motor axons.

Electrophysiological studies of neuromuscular transmission in hereditary 'motor end-plate disease' of the mouse

1. Studies have been made on isolated nerve-muscle preparations from mice with hereditary ;motor end-plate disease'.2. Spontaneous fibrillation was observed in the isolated preparation.3. Muscles gave only a weak twitch or failed to contract in response to nerve stimulation. Direct stimulation of muscles caused a twitch response which had a slower time course than normal. Peripheral nerve conduction was normal.4. Intracellular recording from single muscle fibres showed that with longer survival of the animal an increasing proportion of fibres failed to show end-plate potentials or action potentials in response to nerve stimulation.5. Miniature end-plate potentials (m.e.p.p.s) were recorded in almost all muscle fibres including those in which neuromuscular transmission had failed. The frequency of m.e.p.p.s was greater than normal, was not increased by tetanic stimulation of the nerve but was increased by a raised external potassium concentration.6. Muscle fibres were supersensitive to acetylcholine.7. The results suggest that the muscular weakness in this disease is due to the failure of nerve action potentials to invade motor nerve terminals so that muscle fibres become ;functionally denervated'.

Miniature potentials in denervated slow muscle fibres of the frog

1. Pyriformis and iliofibularis muscles of the frog were studied with micro-electrodes. Both muscles contain a mixture of fast and slow muscle fibres, which can be distinguished by differences in their miniature end-plate potentials (min.e.p.p.s).2. In addition, the membrane time constant of fast fibres is < 20 msec, while that of slow fibres is > 200 msec.3. The characteristic difference in time constant persisted after denervation and allowed the identification of fibre type.4. Denervated slow fibres had min.e.p.p.s of variable time courses and skew amplitude distributions. Their frequency was lower than in normally innervated fibres, and could be increased by hypotonic solutions.5. In analogy with similar observations made previously in fast muscle fibres, it is suggested that after nerve degeneration the Schwann cells of small motor nerve fibres release packages of transmitter which give rise to min.e.p.p.s.

Resting potential and electrical properties of frog slow muscle fibres. Effect of different external solutions

1. The electrical properties of frog slow muscle fibres were investigated with intracellular micropipettes to determine their characteristic length (lambda), specific membrane resistance (R(m)) and specific membrane capacitance.2. The value of lambda was about 1 cm in fibres of 1.2 cm length. The ;short cable model' was used to calculate R(m). Its mean value was 1.12 x 10(5) ohm cm(2), about 10-20 times larger than the value for twitch fibres. The mean value for C(m) was 3.24 x 10(-6) F/cm(2).3. Resting potentials measured immediately after penetration with a single micropipette were about - 80 mV. Lower values can be attributed to the effects of damage or leakage produced by micropipette insertion.4. Changes in external K concentration produced changes in the initially recorded resting potentials which follow the constant field theory using a ratio of Na: K permeabilities P(Na)/P(K) = 0.02. Changes in external Cl concentration produced little or no change in the resting potential or membrane resistance, indicating a low Cl permeability.5. In agreement with previous work, slow fibres showed a time-dependent decrease in resistance (;delayed rectification') for membrane potentials more positive than - 60 mV. ;Anomalous rectification' observed in twitch fibres was not seen in slow fibres. In high external K concentrations the resistance of slow fibres is almost unaffected by changes in membrane potential.6. Increasing the concentration of external Ca (up to isotonic) has two distinct effects on slow fibres. It increases R(m) up to ten times, and it improves the stability of trans-membrane recordings, probably by reducing the leakage due to micropipette penetrations. Magnesium does not appear to have either of these effects.

Ionic mechanisms of cholinergic excitation in molluscan neurons

Acetylcholine appears to be an excitatory transmitter at synapses on two different types of molluscan nerve cells: the so-called D- and CILDA neurons. The action of this substance is different in the two cases. In D-neurons, this compound increases the permeability of the subsynaptic or somatic membrane to chloride ions, and through a net efflux of this anion, depolarizes the cell. In CILDA neurons, on the other hand, acetylcholine depolarzies the cell by increasing its permeability to sodium ions.

Two different ionic mechanisms generating the spike "positive" afterpotential in molluscan neurons

The ionic bases of the "positive" afterpotential (ap) have been examined in the so-called DInhi neurons of the central nervous system of Cryptomphallus aspersa. In these cells E(K) has been determined and its value compared with the equilibrium, potential of the ap (E(ap)). It has been found that in half of the studied cells the E(K) value is very close to E(ap) whereas in another half, the difference (E(K) - E(ap)) is large and amounts to circa -10 mv. The effects of changes in the concentration gradients of K(+), Cl(-), and Na(+) were assayed in both groups of cells. When the [K(i)/[K](o) ratio is reduced in both groups of neurons, the ap amplitude and the E(ap) diminished. In cells displaying a large (E(K) - E(ap)), Cl-free Ringer's solution diminished the ap amplitude and E(ap), but produced no effect in the neurons with a reduced (E(K) - E(ap)). A similar effect was observed if [Cl], was increased by intracellular injection of NaCl. Changes in both [Na](o) and [Na](i) were ineffective. It is concluded that K(+) is the only ion involved in the origin of the ap in the groups of cells with a low value for (E(K) - E(ap)). On the contrary, the ap of the neurons presenting large (E(K) - E(ap)) is produced by a simultaneous increase in the fluxes of both K(+) and Cl(-).