Page 57 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

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Resting Membrane Potential tive) diffusion potential drives K back into the
cell and rises until large enough to almost
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An electrical potential difference, or mem- completely compensate for the K concentra-
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brane potential (E m), can be recorded across tion gradient driving the K -ions out of the cell
the plasma membrane of living cells. The (! A4). As a result, the membrane potential,
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potential of unstimulated muscle and nerve E m, is approximately equal to the K equi-
cells, or resting potential, amounts to – 50 to librium potential E K (! p. 32).
– 100 mV (cell interior is negative). A resting –
potential is caused by a slightly unbalanced ! Cl distribution: Since the cell membrane is
Nerve and Muscle, Physical Work ing the membrane potential (see also p. 32ff.). from the cell (! A4) until the Cl concentration
–
also conductive to Cl (g Cl greater in muscle
distribution of ions between the intracellular
cells than in nerve cells), the membrane poten-
fluid (ICF) and extracellular fluid (ECF) (! B).
–
tial (electrical driving “force”) expels Cl ions
The following factors are involved in establish-
–
gradient (chemical driving “force”) drives
! Maintenance of an unequal distribution of
them back into the cell at the same rate. The in-
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ions: The Na -K -ATPase (! p. 26) continu-
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–
tracellular Cl concentration, [Cl ] i, then con-
–
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ously “pumps” Na out of the cell and K into it
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tinues to rise until the Cl equilibrium poten-
–
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(! A2). As a result, the intracellular K concen-
tial equals E m (! A5). [Cl ] i can be calculated
–
tration is around 35 times higher and the intra-
using the Nernst equation (! p. 32, Eq. 1.18).
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cellular Na concentration is roughly 20 times
–
the intra- and extracellular spaces exists only
(! B). As in any active transport, this process
–
as long as there is no active Cl uptake into the
requires energy, which is supplied by ATP. Lack
cell (! p. 34).
2 lower than the extracellular concentration Such a “passive” distribution of Cl between
of energy or inhibition of the Na -K -ATPase
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+
results in flattening of the ion gradient and ! Why is E m less negative than E K? Although
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breakdown of the membrane potential. the conductances of Na and Ca 2+ are very low
in resting cells, a few Na and Ca 2+ ions con-
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Because anionic proteins and phosphates present in
high concentrations in the cytosol are virtually un- stantly enter the cell (! A4, 5 ). This occurs be-
able to leave the cell, purely passive mechanisms cause the equilibrium potential for both types
(Gibbs–Donnan distribution) could, to a slight extent, of ions extends far into the positive range, re-
contribute to the unequal distribution of diffusable sulting in a high outside-to-inside electrical
ions (! A1). For reasons of electroneutrality, and chemical driving “force” for these ions
–
[K +Na ] ICF ! [K +Na ] ECF and [Cl ] ICF " [Cl ] ECF. (! B; ! p. 32f.). This cation influx depolarizes
+
+
+
+
–
However, this has practically no effect on the the cell, thereby driving K ions out of the cell
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development of resting potentials. +
(1 K for each positive charge that enters). If
+
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! Low resting Na and Ca 2+ conductance, g Na, Na -K -ATPase did not restore these gradients
+
+
2+
g Ca: The membrane of a resting cell is only very continuously (Ca indirectly via the 3 Na /Ca 2+
+
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slightly permeable to Na and Ca , and the exchanger; ! p. 36), the intracellular Na and
2+
resting g Na comprises only a small percentage Ca 2+ concentrations would increase continu-
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of the total conductance (! p. 32 ff.). Hence, ously, whereas [K ] i would decrease, and E K
the Na concentration difference (! A3–A5) and E m would become less negative.
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cannot be eliminated by immediate passive All living cells have a (resting) membrane
diffusion of Na back into the cell. potential, but only excitable cells such as nerve
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and muscle cells are able to greatly change the
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! High K conductance, g K: It is relatively easy ion conductance of their membrane in re-
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for K ions to diffuse across the cell membrane sponse to a stimulus, as in an action potential
(g K ! 90% of total conductance; ! p. 32ff.). Be- (! p. 46).
cause of the steep concentration gradient
(! point 1), K ions diffuse from the ICF to the
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ECF (! A3). Because of their positive charge,
the diffusion of even small amounts of K ions
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44 leads to an electrical potential (diffusion poten-
tial) across the membrane. This (inside nega-
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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