Biology Reference
In-Depth Information
(Hodgkin, 1964; Huxley, 1964). They begin by first discussing their careful
experimental results which had employed the voltage clamp apparatus, developed
in 1949 by Kenneth Cole. This experimental device permits the establishment
of a set of different potential differences across the squid nerve cell membrane,
and recording of the effects that the different membrane potential have on the
state of the cell. (A detailed description of the apparatus and technique can be
found in the textbox on p. 152 of (Kandel et al. 2000).) Their earlier papers
had indicated that the movement of currents based on ions across nerve cell
membrane could be well represented by an 'equivalent circuit' involving a
capacitor and three resistors, all in parallel, and with each resistor in series with
a source of an electrical potential difference. This circuit captures the sodium
(Na) and potassium (K) currents, as well as a small leakage current (l). This
equivalent circuit adapted from their 1952 paper is shown in Fig. 1 (compare
with Huxley, 1964).
The 'laws of working' (a term originally used by John Mackie, but see my
discussion of the phrase in schaffner 1993, pp. 287, 306-307) that govern this
circuit are the standard physical laws including Ohm's law as noted in the legend
to Fig. 1. Additionally, the potential difference across the membrane established
by differences in the Na and K ions is as required by the Nernst equation:
zF
ln
X
o
RT
V
ion
=
X
i
Outside
I
I
I
Na
I
K
C
M
R
Na
R
K
R
I
E
+
+
-
-
-
+
E
Na
E
K
E
I
Inside
Figure 1
Equivalent circuit of a small area of membrane of the giant axon.
R
K
and R
Na
obey Ohm's law for rapid changes in the potential difference across the membrane,
but change their values in times of the order of a millisecond if the membrane potential is held at
a new value. R is constant.
