HUMAN BIOLOGY

the nerVOus system 243

hoW does an action potential, or nerve

impulse, take place?

• An action potential, or nerve impulse, occurs when a neuron’s
  resting membrane potential briefly reverses.


• Action potentials self-propagate and always move away from the
  trigger zone.


• After an action potential, sodium–potassium pumps restore the
  neuron’s resting potential.


• An action potential is all-or-nothing. Once the spiking starts,
  nothing can stop it.

taKe-hoMe Message

develop, however, because a resting neuron uses energy to
power a pumping mechanism that maintains the gradients.
Carrier proteins called sodium–potassium pumps span
the neuron’s membrane (Figure 13.5). With energy from
ATP, they actively transport potassium into the neuron and
transport sodium out.

action potentials are “all-or-nothing”

There is no such thing as a “weak” or “strong” action
potential. Every action potential in a neuron spikes to the
same level above threshold as an all-or-nothing event. That
is, once the positive-feedback cycle of opening sodium
gates starts, nothing will stop the full spiking. If thresh-
old is not reached, the disturbance to the plasma mem-
brane will fade away as soon as the stimulus is removed.
Figure 13.6 shows a recording of the voltage difference
across a neuron’s plasma membrane before, during, and
after an action potential.
Each spike lasts for about a millisecond. At the place
on the membrane where the charge reversed, the gated
sodium channels close and the influx of sodium stops.
About halfway through the action potential, potassium

channels open, so potassium ions

flow out and restore the original

voltage difference across the mem-

brane. Sodium–potassium pumps

restore the ion gradients. After the

resting membrane potential has

been restored, most potassium gates

are closed and sodium gates are in their initial state, ready

to be opened again when a suitable stimulus arrives.

Figure 13.5 Sodium–potassium pumps maintain ion gradients
across a neuron’s plasma membrane. This pumping, and
additional leaking of ions, maintain the proper balance of ions
across a resting neuron’s plasma membrane.

K+

K+

K+ K+ K+K+

K+

Na+ Na+

Na+

Na+

Na+

Na+

Na+/K+

pump

propagating action

potential

At the next patch of membrane, another group of gated sodium

channels open. In the previous patch, some K+ moves out through

other gated channels. That region becomes negative again.

After each action potential, the sodium and potassium

concentration gradients in a patch of membrane are not yet

fully restored. Active transport at sodium–potassium pumps

restores them.

3 4

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Figure 13.6 The action potential

spikes when threshold is reached.

123456

Time (milliseconds)

action potential

resting

level

threshold

level

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sodium–potassium

pump A carrier protein

through which active trans-

port moves potassium ions

into a neuron and sodium

ions outward.

fluid outside

plasma

membrane

neuron’s

plasma

membrane

cytoplasm

next to the

Na+ membrane

leaks in

Na+

pumped out

K+

pumped in

K+

leaks out

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