Page 359 - Color Atlas of Biochemistry

350 Tissues and organs

Resting potential and action uli (or more rarely electrical stimuli). Binding
potential of a neurotransmitter to an ionotropic recep-
tor results in a brief local increase in the
membrane potential from –60 mV to about
A. Resting potential
+30 mV. Although the membrane potential
A characteristic property of living cells is the quickly returns to the initial value within a
uneven distribution of positively and nega- few milliseconds (ms) at its site of origin, the
tively charged ions on the inside and outside depolarization is propagated because neigh-
of the plasma membrane. This gives rise to a boring membrane areas are activated during
membrane potential (see p. 126)—i. e., there is this time period.
electrical voltage between the two sides of [1] The process starts with the opening of
+
the membrane, which can only balance out voltage-gated Na channels (see p. 222). Due
when ion channels allow the unevenly distrib- to their high equilibrium potential (see A), Na +
uted ions to move. ions flow into the cell and reverse the local
At rest, the membrane potential in most membrane potential (depolarization).
+
cells is –60 to –90 mV. It mainly arises from [2] The Na channels immediately close
+
the activity of Na /K + transporting ATPase again, so that the inflow of positive charges
+
+
(“Na /K ATPase”), which occurs on practically is only very brief.
all animal cells. Using up ATP, this P-type [3] Due to the increase in the membrane
+
enzyme (see p. 220) “pumps” three Na ions potential, voltage-dependent K + channels
+
+
+
out of the cell in exchange for two K ions. open and K ions flow out. In addition, Na /
+
+
Some of the K ions, following the concentra- K ATPase (see A) pumps the Na+ ions that
tion gradient, leave the cell again through have entered back out again. This leads to
potassium channels. As the protein anions repolarization of the membrane.
that predominate inside the cell cannot fol- [4] The two processes briefly lead to the
–
lowthem, and inflowofCl ions from the charge even falling below the resting poten-
+
outside is not possible, the result is an excess tial (hyperpolarization). The K channels also
of positive charges outside the cell, while close after a few milliseconds. The nerve cell
anions predominate inside it. is then ready for re-stimulation.
An equilibrium potential exists for each of Generally, it is always only a very small
the ions involved. This is the value of the part of the membrane that is depolarized dur-
membrane potential at which there is no net ing an action potential. The process can there-
inflow or outflow of the ions concerned. For fore be repeated again after a short refractory
+
K ions, the resting potential lies in the range period, when the nerve cell is stimulated
+
of the membrane potential, while for Na ions again. Conduction of the action potential on
it is much higher at +70 mV. At the first op- the surface of the nerve cell is based on the
+
portunity, Na ions will therefore spontane- fact that the local increase in the membrane
ously flow into the cell. The occurrence of potential causes neighboring voltage-gated
action potentials is based on this (see B). ion channels to open, so that the membrane
Nerve cell membranes contain ion chan- stimulation spreads over the whole cell in the
–
2+
+
+
nels for Na ,K ,Cl ,and Ca . These channels form of a depolarization wave.
are usually closed and open only briefly to let
ions pass through. They can be divided into
channels that are regulated by membrane po-
+
tentials (“voltage-gated”—e. g., fast Na chan-
nels; see p. 222) and those regulated by
ligands (“ligand-gated”—e. g., nicotinic acetyl-
choline receptors; see p. 222).

B. Action potential
Action potentials are special signals that are
used to transmit information in the nervous
system. They are triggered by chemical stim-

Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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