242 Chapter 13nerve impulses = action potentials
n A nerve impulse fires when a signal causes a neuron’s
resting membrane potential to reverse.
n Link to Concentration and electric gradients 3.10When a signal that is strong enough reaches a resting
neuron’s input zone, a change occurs in the membrane.
Sodium gates in it open, and Na^1 rushes into the neuron.
Sodium ions have a positive charge, so as they flow in,
the cytoplasm next to the plasma membrane becomes less
negative (Figure 13.4, steps 1 and
2). Then, more gates open, more
sodium enters, and so on—an exam-
ple of positive feedback. When the
voltage difference across the neuron
plasma membrane shifts by a mini-
mum amount called the threshold,
the result is a nerve impulse or
action potential.
The threshold for an action potential can be reached
where a neuron’s plasma membrane has voltage-sensitive
gated channels for sodium ions. When the threshold level
is reached, the opening of more sodium gates doesn’t
depend any longer on the strength of the stimulus. The
gates open on their own.
Keep in mind that an action potential occurs only if the
stimulus to a neuron is strong enough. A weak stimulus—
say, pressure from a tiny insect walking on your skin—that
arrives at an input zone may not upset the ion balance
enough to cause an action potential. This is because input
zones don’t have gated sodium channels, so sodium can’t
flood in there. On the other hand, a neuron’s trigger zone is
packed with sodium channels. If a stimulus that reaches an
input zone is strong enough to spread to the trigger zone,
an action potential may “fire.”action potentials travel away
from their starting point
To transmit messages within the body, action potentials
must spread to other neurons or to cells in muscles or
glands. Each action potential propagates itself, moving
away from its starting point. This self-propagation occurs
in part because the changes in membrane potential leading
to an action potential don’t lose strength. When the change
spreads from one patch of a neuron’s plasma membrane to
another patch, about the same number of gated channels
open (Figure 13.4, steps 3 and 4).a neuron can’t “fire” again until ion pumps
restore its resting potential
When a signal causes an action potential in a neuron’s
trigger zone, that area of the cell’s plasma membrane can’t
receive another signal until its resting membrane potential
is restored.
To understand how the resting potential is restored,
remember that a neuron’s resting membrane potential is
due in part to the different concentrations of Na^1 and K^1
on either side of the plasma membrane. Remember also
that the inside of the cell is a bit more negative than the
outside. Negatively charged proteins in the cytoplasm help
create this electric gradient. Together these factors mean
that sodium is always leaking into the neuron (down an
electrochemical gradient), and potassium is always leaking
out (down its concentration gradient).
A neuron can’t respond to an incoming signal unless
the proper concentration and electric gradients across its
plasma membrane are in place. Yet the Na^1 and K^1 leaks
never stop, opening the possibility that an imbalance might
develop in the necessary gradients. This imbalance doesn’tFigure 13.4 Animated! an inward flood of sodium ions triggers an action potential. 1, 2 Steps leading to an action potential.
3, 4 How an action potential propagates, or travels, along a neuron.fluid outside
neurongated sodium
channelNa+ Na+Na+In a membrane at rest, the inside of the neuron is negative
relative to the outside. An electrical disturbance (yellow
arrow) spreads from an input zone to an adjacent trigger
zone of the membrane, which has a large number of
gated sodium channels.A strong disturbance initiates an action potential.
Sodium gates open. Sodium flows in, reducing the
negativity inside the neuron. The change causes more
gates to open, and so on until threshold is reached and
the voltage difference across the membrane reverses.voltage reversed1 213.2
action potential A nerve
impulse.
threshold The minimum
change in the voltage dif-
ference across a neuron’s
plasma membrane that will
trigger a nerve impulse.
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