Page 61 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Propagation of Action Potentials in Action potentials normally run forward (or-
Nerve Fiber thodromic) because each segment of nerve
fiber becomes refractory when an action
Electrical current flows through a cable when potential passes (! A1b and p. 46). If, however,
voltage is applied to it. The metal wire inside the impulses are conducted backwards (anti-
the cable is well insulated and has very low- dromic) due, for example, to electrical stimula-
level resistance, reducing current loss to a tion of nerve fibers from an external source
minimum. As a result, it can conduct electric- (! p. 50), they will terminate at the next syn-
ity over long distances. Nerve fibers, especially apse (valve-like function, ! p. 42).
Nerve and Muscle, Physical Work roundings. Therefore, the cable-like, elec- this process is rather time-consuming (! B1).
unmyelinated ones (! p. 42), have a much
Although the continuous generation of ac-
tion potentials in the immediately adjacent
greater internal longitudinal resistance (R i)
and are not well insulated from their sur-
fiber segment guarantees a refreshed signal,
The conduction velocity, θ, in unmyelinated
trotonic transmission of neural impulses
(type C) nerve fibers (! C) is only around
dwindles very rapidly, so the conducted im-
1 m/s. Myelinated (types A and B) nerve fibers
pulses must be continuously “refreshed” by
(! C) conduct much faster (up to 80 m/s = 180
generating new action potentials (! p. 46).
mph in humans). In the internode regions, a
Propagation of action potentials: The start
myelin sheath (! p. 42) insulates the nerve
of an action potential is accompanied by a brief
+
cell membrane that previously was inside
nal currents strong enough to generate action
potentials can travel further down the axon
negative now becomes positive ( + 20 to
2 influx of Na into the nerve fiber (! A1a). The fibers from the surroundings; thus, longitudi-
+ 30 mV), thus creating a longitudinal poten-
(ca. 1.5 mm) (! A2). This results in more rapid
tial difference with respect to the adjacent, conduction because the action potentials are
still unstimulated nerve segments (internal generated only at the unmyelinated nodes of
–70 to –90 mV; ! p. 44). This is followed by a Ranvier, where there is a high density of Na +
passive electrotonic withdrawal of charge from channels. This results in rapid, jump-like pas-
the adjacent segment of the nerve fiber, caus- sage of the action potential from node to node
ing its depolarization. If it exceeds threshold, (saltatory propagation). The saltatory length is
another action potential is created in the adja- limited since the longitudinal current (1 to
cent segment and the action potential in the 2 nA) grows weaker with increasing distance
previous segment dissipates (! A1b). (! B2). Before it drops below the threshold
Because the membrane acts as a capacitor, level, the signal must therefore be refreshed by
the withdrawal of charge represents a capaci- a new action potential, with a time loss of
tating (depolarizing) flow of charge that be- 0.1 ms.
comes smaller and rises less steeply as the spa- Since the internal resistance, R i, of the nerve
tial distance increases. Because of the rela- fiber limits the spread of depolarization, as de-
tively high R i of nerve fiber, the outward loops scribed above, the axon diameter (2r) also af-
of current cross the membrane relatively close fects the conduction velocity, θ (! C). R i is pro-
to the site of excitation, and the longitudinal portional to the cross-sectional area of the
2
2
current decreases as it proceeds towards the nerve fiber (πr ), i.e., R i ! 1/r . Thick fibers
periphery. At the same time, depolarization in- therefore require fewer new APs per unit of
creases the driving force (= E m – E K; ! p. 32) for length, which is beneficial for θ. Increases in
K outflow. K fluxing out of the cell therefore fiber diameter are accompanied by an increase
+
+
accelerates repolarization. Hence, distal action in both fiber circumference (2πr) and mem-
potentials are restricted to distances from brane capacity, K (K ! r). Although θ decreases,
which the capacitative current suffices to the beneficial effect of the smaller R i predomi-
depolarize the membrane quickly and strongly nates because of the quadratic relationship.
+
enough. Otherwise, the Na channels will be
deactivated before the threshold potential is
48 reached (! p. 46).
Despopoulos, Color Atlas of Physiology © 2003 Thieme
All rights reserved. Usage subject to terms and conditions of license.