FOCUS
Action Potential
Figure 11.11
The action potential (AP) is a brief
change in membrane potential in a “patch” of
membrane that is depolarized by local currents.
Available at www.masteringaandp.com
The big picture
This graph shows how voltage changes over time at a given point inside an
axon during the course of an action potential.
The AP is caused by permeability changes in the plasma membrane:
Relative membrane permeability tells you the relative number of ion channels
that are open for each ion. Remember that open ion channels make the plasma
membrane permeable to those ions.
Relative membrane permeability
0
1
2
3
4
–70
–55
0
+30
Membrane potential (mV)
Time (ms)
Action
potential
Action
potential
Threshold
0
1
2
3
4
–70
–55
0
+30
Membrane potential (mV)
Time (ms)
Na
+
permeability
K
+
permeability
1
1
1
2
2
3
3
4
4
1
1
2
3
4
Resting state.
No
ions move through
voltage-gated channels.
Depolarization
is
caused by Na
+
flowing
into the cell.
Repolarization
is
caused by K
+
flowing
out of the cell.
Hyperpolarization
is
caused by K
+
continuing
to leave the cell.
K
+
channels open later,
over a longer time interval,
causing repolarization and
hyperpolarization.
Na
+
channels open,
causing depolarization.
 
402
previous page 436 Human Anatomy and Physiology (9th ed ) 2012 read online next page 438 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off