470 Chapter 12. Nerve impulses[[Student version, January 17, 2003]]
Na+
channelspatch
electrodea
cell
tightseal membraneNa+cytoplasmFigure 12.14:(Schematic; optical micrograph.) (a)Asmall patch of membrane containing only a single voltage-
gated sodium channel (or a few) is electrically isolated from the rest of the cell by a patch electrode. The current
entering the cell through these channels is recorded by a monitor connected to the patch electrode. (b)Patch-clamp
manipulation of a single, live photoreceptor cell from the retina of a salamander. The cell is secured by partially
sucking it into a glass micropipette (bottom), while the patch-clamp electrode (upper left) is sealed against a small
patch of the cell’s plasma membrane. [Digital image kindly supplied by T. D. Lamb; see Lamb et al., 1986.]
instrumentation capable of detecting the tiny currents they carry. E. Neher developed the necessary
electronic techniques in experiments with ion channel proteins embedded in artificial bilayers. The
real breakthrough, however, came in 1975, when Neher and B. Sakmann developed thepatch-
clamptechnique (Figure 12.14), enabling the measurement of ion currents across single channels
in intact, living cells. Neher and Sakmann’s work helped launch an era of dynamical measurements
on single-molecule devices.
One of the first results of patch-clamp recording was an accurate value for the conductance of
individual channels: A typical value isG≈ 25 · 10 −^12 Ω−^1 for the open sodium channel. Using the
relationsV=IRandR=1/Ggives that at a driving potential ofV−VNaNernst+ ≈ 100 mVthe current
through a single open channel is 2. 5 pA.
Your Turn 12c
Express this result in terms of sodium ions passing through the channel per second. Is it reason-
able to treat the membrane electric current as the flow of a continuous quantity, as we have been
doing?a. Mechanism of conduction The simplest imaginable model for ion channels has proved to be
essentially correct: Each one is a barrel-shaped array of protein subunits inserted in the axon’s bi-
layer membrane (Figure 2.30a,b on page 58), creating a hole through which ions can pass diffusively.
(Problem 12.9 tests this idea for reasonableness with a simple estimate.)
b. Specificity The channel concept suggests that the independent conductances of the axon
membrane arise through the presence of two (actually, several) subpopulations of channels, each
carrying only one type of ion and each with its own voltage-gating behavior. Indeed, the patch-
clamp technique revealed the existence of distinct, specific channels. Figure 12.15 illustrates the
great specificity of the sodium channel: The conductance of a single sodium channel to sodium is