## Electrical Properties of Striated Muscle

### Abstract

1 Cable Theory for Striated Muscle
2 Voltage‐Clamp Methods for Striated Muscle
2.1 Gap Methods
2.2 Microelectrode‐Clamping Methods
3 Action Potential in Striated Muscle
3.1 Sodium Current
3.2 Potassium Current
3.3 Calcium Current
3.4 Action‐Potential Calculations
4 Currents in the Inactive Membrane
4.1 Potassium Conductance
4.2 Chloride Conductance
4.3 Voltage‐Dependent Membrane Capacitance
 Figure 1. Three‐dimensional representation of potential (Vr=a; Eq. ) across membrane at surface of a cylinder of specific resistivity Ri (200 Ω·cm) and membrane resistance Rm (1,000 Ω·cm2). Current is delivered from a point source represented as the tip of a microelectrode impaling the fiber. Microelectrode tip is at coordinates x′ = 0, θ′ = 0°, r′ = 0.9 a where a is the fiber radius (50 μm). Potential across membrane at any point on surface of cylinder is represented as radial distance between surface of cylinder and surface surrounding cylinder. Marks on x‐axis are at ± 250 μm and ± 500 μm.From Adrian, Costantin, and Peachey Figure 2. Disk model of the transverse‐tubular system. Figure 3. Circuits referred to in text to illustrate the definition of effective capacitance by means of Equation .From Adrian and Almers Figure 4. Arrangement of microelectrodes in three‐electrode method adapted to the center of a long cylindrical fiber. Fiber ends are to be understood to be several (more than, say, 5Δ) length constants away from x = 0.From Adrian and Marshall Figure 5. Recorded (by S. Nakajima) and calculated action potentials for frog striated muscle. A: recorded action potential at 3.3°C (above) and 21.8°C (below). B: action potentials calculated at 2°C (above) and 20°C (below) by Eqs. and . Dotted lines are VT, the potential across the capacitance representing the tubular wall (CT) in the membrane equivalent circuit shown in Fig. A (but shown there without any elements to represent potential‐dependent ionic currents).From Adrian, Chandler, and Hodgkin Figure 6. Calculated action potentials across surface of a fiber and across wall of T system at various radial distances. Coordinates: vertical, potential in 20‐mV divisions; horizontal to the left, the fiber diameter divided into 6 equal divisions; horizontal to the right, time in 1‐ms divisions. In both calculations no additional resistance is assumed at entrances of the tubular system (access resistance = 0 Ω·cm2). A: no activable sodium or potassium current in tubular system. B: activable sodium and potassium currents in tubular system are assumed.From Adrian and Peachey Figure 7. Calculated action potentials across surface of a fiber and across wall of T system at various radial distances. Coordinates as in Fig. , and all parameters of calculation in A and B are same as in Fig. A and B, respectively, with the exception of access resistance of 150 Ω·cm2.From Adrian and Peachey Figure 8. A: records of currents required to impose voltage steps of long duration on a muscle fiber. Three‐electrode method with fiber in an isotonic sulfate Ringer's solution with 5 mM K at 1.5°C. Current for hyperpolarization is initially large but decays with time constant 0.5–1 s. B: initial (open circles) and final (crosses) current plotted against membrane potential during imposed potential step. Outward current is positive.From Adrian, Chandler, and Hodgkin Figure 9. Above: records of membrane potential (V), membrane current (ΔV), and electrode current (I0) for control and test 10‐mV steps of V at 2.5°C. Control step is from −90 mV; test step from −52 mV. Membrane capacity at control and test potential (CC, CT) is determined from integral of transient part of membrane current at “on” and “off” of 10‐mV step. Below: point‐by‐point differences in membrane currents (ΔVT − ΔVC) for test and control steps. These records show, for various starting potentials, current that is not present in the control step from −90 mV. Note that the kinetics of this additional polarization current can be complex. In both sets of records the 10‐mV step lasts for 128 ms. Three‐electrode clamp; fiber in a hypertonic solution designed to minimize ionic currents.From Adrian and Peres Figure 10. Voltage dependence of nonlinear membrane capacity (CT:CC). For all curves, CC measured at −90 mV; CT at membrane potential indicated on abscissa. Membrane potential was held at −90 mV (half‐filled circles), −40 mV (open circles), and −20 mV (filled circles) except during the measurements of capacity (as in Fig. ). Note that the behavior of the nonlinear capacity depends on the holding potential.From Adrian . Reproduced, with permission, from Annu. Rev. Biophys. Bioeng., vol. 7, ©1978 by Annual Reviews, Inc.