||1. Membrane currents during step depolarizations were determined by
a method in which three electrodes were inserted near the end of a
fibre in the frog's sartorius muscle. The theoretical basis and
limitations of the method are discussed.
2. Measurements of the membrane capacity (CM) and resting resistance
(RM) derived from the current during a step change in membrane
potential are consistent with values found by other methods.
3. In fibres made mechanically inactive with hypertonic solutions
(Ringer solution plus 350 mM sucrose) step depolarizations produced
ionic currents which resembled those of nerve in showing (a) an early
transient inward current, abolished by tetrodotoxin, which reversed
when the depolarization was carried beyond an internal potential of
about +20 mV, (b) a delayed outward current, with a linear instantaneous
current¡Xvoltage relation, and a mean equilibrium potential with a normal
potassium concentration (2¡P5 mM) of -85 mV.
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
(V¡¬K) 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. V¡¬K corresponds well with the membrane potential at the
beginning of the negative after-potential observed under similar conditions.
6. The variation of V¡¬K with the amount of current which has passed
through the delayed channel suggests that potassium ions accumulate in a
space of between 1/3 and 1/6 of the fibre volume. If potassium accumulates in
the transverse tubular system (T system) much greater variation in V¡¬K
would be expected.
7. The delayed current is not maintained but is inactivated like the early
current. The inactivation is approximately exponential with a time constant
of 0¡P5 to 1 sec at 20¢X C. The steady-state inactivation of the potassium
current is similar to that for the sodium current, but its voltage
dependence is less steep and the potential for half inactivation is 20 mV
rate more positive.
8. Reconstructions of ionic currents were made in terms of the parameters
(m, n, h) of the Hodgkin¡XHuxley model for the squid axon, using constants
which showed a similar dependence on voltage.
9. Propagated action potentials and conduction velocities were computed for
various conditions on the assumption that the T system behaves as if it were
a series resistance and capacity in parallel with surface capacity and the
channels for sodium, potassium and leak current. There was reasonable
agreement with observed values, the main difference being that the
calculated velocities and rates of rise were somewhat less than those