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.
previous page 185 Vander's Human Physiology The Mechanisms of Body Function read online next page 187 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off