Neuronal Signaling and the Structure of the Nervous System
157
or to a spontaneous change in the neuron’s membrane poten-
tial, known as a
pacemaker potential.
The next section will
address the production of synaptic potentials, and Chapter 7
will discuss the production of receptor potentials.
Triggering of action potentials by pacemaker potentials
is an inherent property of certain neurons (and other excitable
cells, including certain smooth-muscle and cardiac-muscle cells).
In these cells, the activity of different types of ion channels in
the plasma membrane causes a graded depolarization of the
membrane—the pacemaker potential. If threshold is reached,
an action potential occurs; the membrane then repolarizes and
again begins to depolarize. There is no stable, resting mem-
brane potential in such cells because of the continuous change in
membrane permeability. The rate at which the membrane depo-
larizes to threshold determines the action potential frequency.
Pacemaker potentials are implicated in many rhythmical behav-
iors, such as breathing, the heartbeat, and movements within
the walls of the stomach and intestines.
Because of the effects of graded changes in membrane
potential on action potential generation, a review of graded and
action potentials is recommended. The differences between
graded potentials and action potentials are listed in
Table 6–4
.
Figure 6–23
Myelinization and saltatory conduction of action potentials. Potassium channels are not depicted.
Myelin
Intracellular fluid
Active node
of Ranvier;
site of action
potential
Node to which
action potential
is spreading
(dashed lines)
Inactive node
at resting
membrane
potential
Na
+
Na
+
channel
Na
+
++
+
+
++
+
+
+
+
+
+
+
++
––––
––
+
––
––––
–
++
–
–
–
––
++
–
–
Direction of action potential propagation
Table 6–4
Differences between Graded Potentials and Action Potentials
Graded Potential
Action Potential
Amplitude varies with size of the initiating event.
All-or-none. Once membrane is depolarized to threshold, amplitude is
independent of the size of the initiating event.
Can be summed.
Cannot be summed.
Has no threshold.
Has a threshold that is usually about 15 mV depolarized relative to the
resting potential.
Has no refractory period.
Has a refractory period.
Is conducted decrementally; that is, amplitude decreases
with distance.
Is conducted without decrement; the depolarization is amplifi ed to a
constant value at each point along the membrane.
Duration varies with initiating conditions.
Duration is constant for a given cell type under constant conditions.
Can be a depolarization or a hyperpolarization.
Is only a depolarization.
Initiated by environmental stimulus (receptor), by
neurotransmitter (synapse), or spontaneously.
Initiated by a graded potential.
Mechanism depends on ligand-gated channels or other
chemical or physical changes.
Mechanism depends on voltage-gated channels.
