400
UNIT 3
Regulation and Integration of the Body
11
depolarization spreads as the neighboring membrane “patch” is,
in turn, depolarized.
As just explained, the flow of current to adjacent membrane
areas changes the membrane potential there as well. However,
the plasma membrane is permeable like a leaky water hose, and
most of the charge is quickly lost through leakage channels.
Consequently, the current dies out within a few millimeters of
its origin and is said to be
decremental
(Figure 11.10c).
Because the current dissipates quickly and decays (declines)
with increasing distance from the site of initial depolarization,
graded potentials can act as signals only over very short dis-
tances. Nonetheless, they are essential in initiating action poten-
tials, the long-distance signals.
Action Potentials (AP)
Te principal way neurons send signals over long distances is
by generating and propagating (transmitting) action potentials.
Only cells with
excitable membranes
—neurons and muscle
cells—can generate action potentials.
An
action potential (AP)
is a brief reversal of membrane
potential with a total amplitude (change in voltage) of about 100
mV (from
2
70 mV to
1
30 mV). Depolarization is followed by
repolarization and o±en a short period of hyperpolarization.
Te whole event is over in a few milliseconds. Unlike graded
potentials, action potentials do not decay with distance.
Te events of action potential generation and transmission
are identical in skeletal muscle cells and neurons. As we have
noted, in a neuron, an AP is also called a
nerve impulse
, and
is typically generated
only in axons
. A neuron generates a nerve
impulse only when adequately stimulated. Te stimulus changes
the permeability of the neuron’s membrane by opening specific
voltage-gated channels on the axon.
Tese channels open and close in response to changes in
the membrane potential. Tey are activated by local currents
(graded potentials) that spread toward the axon along the den-
dritic and cell body membranes.
In many neurons, the transition from local graded potential
to long-distance action potential takes place at the axon hillock.
In sensory neurons, the action potential is generated by the pe-
ripheral (axonal) process just proximal to the receptor region.
However, for simplicity, we will just use the term axon in our
discussion. We’ll look first at the generation of an action poten-
tial and then at its propagation.
Generation of an Action Potential
Focus on an Action Poten-
tial
(Figure 11.11)
on pp. 402–403 describes how an action
potential is generated. Let’s start with a neuron in the resting
(polarized) state.
1
Resting state: All gated Na
1
and K
1
channels are closed.
Only the leakage channels are open, maintaining resting
membrane potential. Each Na
1
channel has two gates:
a voltage-sensitive
activation gate
that is closed at rest
and responds to depolarization by opening, and an
in-
activation gate
that blocks the channel once it is open.
Tus,
depolarization opens and then inactivates sodium
channels.
and negative ions simultaneously move toward more positive
areas (Figure 11.10b).
For our patch of plasma membrane, positive ions (mostly
K
1
) inside the cell move away from the depolarized area and
accumulate on the neighboring membrane areas, where they
neutralize negative ions. Meanwhile, positive ions on the outer
membrane face move toward the region of reversed mem-
brane polarity (the depolarized region), which is momentarily
less positive. As these positive ions move, their “places” on the
membrane become occupied by negative ions (such as Cl
2
and
HCO
3
2
), sort of like ionic musical chairs. In this way, at regions
abutting the depolarized region, the inside becomes less nega-
tive and the outside becomes less positive. In other words, the
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Depolarized region
Stimulus
Plasma
membrane
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Distance (a few mm)
Membrane potential (mV)
–70
Resting potential
Active area
(site of initial
depolarization)
(a) Depolarization:
A small patch of the membrane (red area)
depolarizes.
(b) Depolarization spreads:
Opposite charges attract each other.
This creates local currents (black arrows) that depolarize
adjacent membrane areas, spreading the wave of depolarization.
(c) Membrane potential decays with distance:
Because current is
lost through the “leaky” plasma membrane, the voltage declines with
distance from the stimulus (the voltage is
decremental
).
Consequently, graded potentials are short-distance signals.
Figure 11.10
The spread and decay of a graded potential.
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