The relation of membrane changes to contraction in twitch muscle fibres

507

472

4. The reversal potential for the early current appears to be consistent with the sodium equilibrium potential expected in hypertonic solutions.5. The variation of the equilibrium potential for the delayed current (VE) with external potassium concentration suggests that the channel for delayed current has a ratio of potassium to sodium permeability of 30:1; this is less than the resting membrane where the ratio appears to be 100: 1. VE corresponds well with the membrane potential at the beginning of the negative after-potential observed under similar conditions. 6. The variation of VE with the amount of current which has passed through the delayed channel suggests that potassium ions accumulate in a space of between X and W of the fibre volume. If potassium accumulates in the transverse tubular system (T system) much greater variation in VE would be expected.

mentioning

confidence: 84%

“…Inactivation of potassium currents A striking difference between frog muscle and squid nerve is that the delayed current in frog muscle inactivates fairly rapidly and completely during a prolonged depolarization (Nakajima, Iwasaki & Obata, 1962;Heistracher & Hunt, 1969;Stanfield, 1970). …”

Section: Voltage Clamp Experiments In Muscle Early Ionic Currentmentioning

confidence: 99%

Voltage clamp experiments in striated muscle fibres

Adrian¹,

Chandler²,

Hodgkin³

1970

507

472

“…The micro-electrodes were made flexible following the procedure described by Heistracher & Hunt, 1969). The voltage microelectrode was filled with 3 M-KCl and connected via a Ag-AgCl connexion to an amplifier with variable input capacitance compensation (NF1, Bioelectric Instruments).…”

Section: Methodsmentioning

confidence: 99%

“…The present work was carried out to study the membrane potential dependency of relaxation, and to gain knowledge on the extent by which Ca release may be modified by membrane potential changes. In order to do so, we have employed a two micro-electrode voltage clamp technique and short muscle fibres (1 to 1-5 mm) to improve longitudinal homogeneity of potential control (Heistracher & Hunt, 1969;Bezanilla, Caputo & Horowicz, 1971). A preliminary account of some of the experiments reported here has been presented elsewhere (Caputo, 1977).…”

Section: Introductionmentioning

confidence: 99%

Membrane potential, contractile activation and relaxation rates in voltage clamped short muscle fibres of the frog.

Caputo¹,

Bolaños²

1979

SUMMARY1. Voltage clamped short (a 15 mm) muscle fibres of the frog can develop maximum tension of 4-3 kg/cm2.2. The time course of contractile responses to prolonged depolarization is markedly dependent on the fibre membrane potential. With sufficiently long pulses the responses present a plateau and a spontaneous relaxation phase.3. At room temperature (20-22 TC), and at membrane potentials of -10 mV the plateau duration is about 2 see and the spontaneous relaxation rate is 0 50 sec-'. At membrane potentials of -35 mV the plateau duration is 4x6 see and the spontaneous relaxation rate is 0x28 sec-'.4. When the fibres are depolarized at room temperature with relatively short pulses ( < 2 see), the contractile responses are cut short at the end of the pulse, and the fibres relax at a rate (11 sec-') which is independent on the pulse amplitude and duration.5. The relaxation rate after a short pulse can be affected by membrane potential only in the region near the contractile threshold where further release of contractile activator is expected to occur.6. The time course of contractile responses to prolonged depolarization may be shortened by conditioning depolarization.7. The system responsible for the release of contractile activator may be tentatively described by three states, resting, activated, and inactivated, in analogy with the model proposed by Chandler, Rakowski & Schneider (1976) to describe the possible configurations of the potential dependent charge movement in muscle.

“…In some cases the electrode tip was flexibly mounted on a piece of thin silver foil (Heistracher & Hunt, 1969). In all cases the muscle was stretched over a wedge of Sylgard, and penetrations made opposite the nerve entry point into the muscle (which corresponds to the end-plate band running across the muscle).…”

Section: Single Unit8mentioning

confidence: 99%

Motor units in a skeletal muscle of neonatal rat: mechanical properties and weak neuromuscular transmission.

Jones

Ridge

1987

SUMMARY1. Isometric twitch and tetanic tensions were recorded from whole muscles and single motor units in isolated fourth deep lumbrical muscles from neonatal rats (most at 3-5 days old) and from older rats of various ages.2. Whole-muscle time to peak contraction reduced from about 120 ms at birth to about 20-25 ms at 20 days and older.3. The number of motor units in the muscle was constant with age (eleven on average) and there was no significant branching of motor axons below the common peroneal nerve branching point in the thigh.4. In the 3-5 days age range, mean twitch: tetanus ratio for whole muscles was 0-299 and for single units was 0-177. As a consequence, mean motor unit size (as a percentage of whole-muscle tension) was greater for tetani (29-7 00) than for twitches (19-9 %). This was not the case in muscles from animals 22 days or older. Evidence is given that the cause of this is low junctional efficacy in some neuromuscular junctions in neonatal muscle. Intracellular recordings supported this view.5. The relationships of motor-unit size to the contraction time, to the ratio of contraction time: half-relaxation time, and to fatigue index are given. There was no indication of clear segregation of motor units into more than one population, but it is concluded that small motor units are more likely to contain a higher proportion of slowly contracting, fatigue-resistant fibres than large units.6. The level of overlap by axons in the lateral plantar nerve onto muscle fibres in a single sural nerve motor unit was greater in tetani than in twitches. The results indicate that the distribution of weak and strong inputs was not random, but that there was a tendency for one strong input to accompany a number of weak inputs (on average about two) on each muscle fibre.7. Intracellular recording indicates that about 120 of fibres at 3-5 days may be electrically coupled.