Page 59 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Action Potential resulting in a hyperpolarizing afterpotential
+
+
(! A1). Increased Na -K -ATPase pumping
An action potential is a signal passed on rates (electrogenic; ! p. 28) can contribute to
through an axon or along a muscle fiber that this afterpotential.
influences other neurons or induces muscle Very long trains of action potentials can be
contraction. Excitation of a neuron occurs if the generated (up to 1000/s in some nerves) since
membrane potential, E m, on the axon hillock of the quantity of ions penetrating the mem-
a motor neuron, for example (! p. 42), or on brane is very small (only ca. 1/100 000 th the
the motor end-plate of a muscle fiber changes number of intracellular ions). Moreover, the
Nerve and Muscle, Physical Work rotransmitter-induced opening of postsynap- + unresponsive to further stimuli; this is called
Na -K -ATPase (! p. 26) ensures the continu-
+
+
from its resting value (! p. 44) to a less nega-
tive value (slow depolarization, ! A1). This
ous restoration of original ion concentrations
(! p. 46).
depolarization may be caused by neu-
During an action potential, the cell remains
tic cation channels (! p. 50) or by the (elec-
the refractory period. In the absolute refractory
trotonic) transmission of stimuli from the sur-
period, no other action potential can be trig-
roundings (! p. 48). If the E m of a stimulated
gered, even by extremely strong stimuli, since
cell comes close to a critical voltage or thresh-
Na channels in depolarized membranes can-
old potential (! A1), “rapid” voltage-gated Na
+
channels are activated (! B4 and B1 ! B2).
not be activated (! B3). This is followed by a
+
This results in increased Na conductance, g Na
+
tion potentials of smaller amplitudes and rates
(! A2). If the threshold potential is not
of rise can be generated, even by strong
2 (! p. 32), and the entry of Na into the cell relative refractory period during which only ac-
reached, this process remains a local (sub-
stimuli. The refractory period ends once the
threshold) response. membrane potential returns to its resting
Once the threshold potential is reached, the value (! e.g. p. 59 A).
cell responds with a fast all-or-none depolari- The extent to which Na channels can be ac-
+
zation called an action potential, AP (! A1). tivated and, thus, the strength of the Na cur-
+
The AP follows a pattern typical of the specific rent, I Na, depends on the pre-excitatory resting
cell type, irregardless of the magnitude of the potential, not the duration of depolarization.
+
stimulus that generated it. Large numbers of The activation of the Na channels reaches a
Na channels are activated, and the influxing maximum at resting potentials of ca. – 100 mV
+
Na accelerates depolarization which, in turn, and is around 40% lower at – 60 mV. In mam-
+
+
increases g Na and so on (positive feedback). As mals, Na channels can no longer be activated
a result, the E m rapidly collapses (0.1 ms in at potentials of –50 mV and less negative
nerve cells: fast depolarization phase or up- values (! B3). This is the reason for the abso-
sweep) and temporarily reaches positive levels lute and relative refractory periods (see above)
(overshooting, + 20 to + 30 mV). The g Na drops and the non-excitability of cells after the ad-
before overshooting occurs (! A2) because ministration of continuously depolarizing sub-
the Na channels are inactivated within 0.1 ms stances such as suxamethonium (! p. 56). An
+
(! B2 ! B3). The potential therefore reverses, increased extracellular Ca 2+ concentration
and restoration of the resting potential, the re- makes it more difficult to stimulate the cell be-
polarization phase of the action potential, cause the threshold potential becomes less
begins. Depolarization increases (relatively negative. On the other hand, excitability in-
slowly) the open-probability of voltage-gated creases (lower threshold) in hypocalcemic
+
K channels. This increases the potassium con- states, as in muscle spasms in tetany
ductance, g K, thereby accelerating repolariza- (! p. 290).
tion. The special features of action potentials in
In many cases, potassium conductance, g K is cardiac and smooth muscle fibers are de-
still increased after the original resting poten- scribed on pages 192, 70 and 59 A.
tial has been restored (! A2), and E m tem-
46 porarily approaches E K (! pp. 44 and 32 ff.),
Despopoulos, Color Atlas of Physiology © 2003 Thieme
All rights reserved. Usage subject to terms and conditions of license.