Human Physiology, 14th edition (2016)

174 Chapter 7

the axon. The nature of the stimulus in vivo (in the body),

and the manner by which electrical events are conducted to

different points along the axon, will be described in later

sections.

When the axon membrane has been depolarized to a thresh-

old level—in the previous example, by stimulating electrodes—

the Na^1 gates open and the membrane becomes permeable to

Na^1. This permits Na^1 to enter the axon by diffusion, which

further depolarizes the membrane (makes the inside less nega-

tive, or more positive). The gates for the Na^1 channels of the

axon membrane are voltage regulated, and so this additional

depolarization opens more Na^1 channels and makes the mem-

brane even more permeable to Na^1. As a result, more Na^1 can

enter the cell and induce a depolarization that opens even more

voltage-regulated Na^1 gates. A positive feedback loop ( fig. 7.13 )

is thus created, causing the rate of Na^1 entry and depolarization

to accelerate in an explosive fashion.

The explosive increase in Na^1 permeability results in a

rapid reversal of the membrane potential in that region from

2 70 mV to 1 30 mV ( fig. 7.13 ). At that point the channels for

Na^1 close (they actually become inactivated, as illustrated in

fig. 7.12 ), causing a rapid decrease in Na^1 permeability. This is

why, at the top of the action potential, the voltage does not quite

reach the 1 66 mV equilibrium potential for Na^1 (chapter 6,

section 6.4). Also at this time, as a result of a time-delayed

effect of the depolarization, voltage-gated K^1 channels open

and K^1 diffuses rapidly out of the cell.

the extracellular Na^1 concentration is not measurably changed.
However, the increased Na^1 within that tiny region of axon
membrane greatly affects the membrane potential, as will be
described shortly.
A fraction of a second after the Na^1 channels open, they
close due to an inactivation process, as illustrated in figure 7.12.
Just before they do, the depolarization stimulus causes the gated
K 1 channels to open. This makes the membrane more permeable
to K^1 than it is at rest, and K^1 diffuses down its electrochemical
gradient out of the cell. This causes the membrane potential to
move toward the potassium equilibrium potential (see fig. 7.14 ).
The K^1 gates will then close and the permeability properties of
the membrane will return to what they were at rest.
Because opening of the gated Na^1 and K^1 channels is
stimulated by depolarization, these ion channels in the axon
membrane are said to be voltage-regulated, or voltage-gated,
channels. The channel gates are closed at the resting mem-
brane potential of 2 70 mV and open in response to depolariza-
tion of the membrane to a threshold value.

Action Potentials

We will now consider the events that occur at one point in an
axon, when a small region of axon membrane is stimulated
artificially and responds with changes in ion permeabilities.
The resulting changes in membrane potential at this point
are detected by recording electrodes placed in this region of

Figure 7.13 Depolarization of an axon affects Na^1 and K^1 diffusion in sequence. (1) Na^1 gates open and Na^1
diffuses into the cell. (2) After a brief period, K^1 gates open and K^1 diffuses out of the cell. An inward diffusion of Na^1 causes further
depolarization, which in turn causes further opening of Na^1 gates in a positive feedback ( 1 ) fashion. The opening of K^1 gates
and outward diffusion of K^1 makes the inside of the cell more negative, and thus has a negative feedback effect ( 2 ) on the initial
depolarization.

More depolarization

Membrane potential

depolarizes from

–70 mV to +30 mV

Less

depolarization

Depolarization stimulus

Voltage regulated

Na+ gates open

Na+ diffuses

into cell

Voltage regulated

K+ gates open

K+ diffuses

out of cell

Membrane potential

repolarizes from

+30 mV to –70 mV

1

2

+30

0

–50

–70

Membrane

potential

(millivolts)

01234567

Time (msec)

Action

potential

Threshold

Resting

membrane

potential

Stimulus

Na+ in

1

K+ out

2

+