Neuronal Signaling and the Structure of the Nervous System
151
Figure 6–16
Graded potentials can be recorded under experimental conditions in
which the stimulus strength can vary. Such experiments show that
graded potentials (a) can be depolarizing or hyperpolarizing, (b) can
vary in size, (c) are conducted decrementally. The resting membrane
potential is –70 mV.
(a)
(c)
(b)
Membrane potential (mV)
0 mV
–70 mV
0 mV
–70 mV
0 mV
–70 mV
Depolarization
Hyperpolarization
Stimulus
Stimulus
Weak stimulus
Strong stimulus
Measured at
stimulus site
Measured 1 mm
from stimulus site
Stimulus
Stimulus
Time (msec)
millimeters). However, if additional stimuli occur before
the graded potential has died away, these can be added to
the depolarization from the first stimulus. This process,
termed
summation,
is particularly important for sensa-
tion, as Chapter 7 will discuss. Graded potentials are the only
means of communication some neurons use, while in other
cells they play very important roles in the initiation of signal-
ing over longer distances, as described next.
Action Potentials
Action potentials
are very different from graded potentials.
They are large alterations in the membrane potential; the mem-
brane potential may change 100 mV, from –70 to +30 mV, and
then repolarize to its resting potential. Action potentials are
generally very rapid (as brief as 1–4 milliseconds) and may
repeat at frequencies of several hundred per second. Nerve
and muscle cells as well as some endocrine, immune, and
reproductive cells have plasma membranes capable of produc-
ing action potentials. These membranes are called
excitable
membranes,
and their ability to generate action potentials is
known as
excitability.
Whereas all cells are capable of con-
ducting graded potentials, only excitable membranes can con-
duct action potentials. The propagation of action potentials
down the axon is the mechanism the nervous system uses to
communicate over long distances.
What properties of ion channels allow them to generate
these large, rapid changes in membrane potential, and how are
action potentials propagated along an excitable membrane?
These questions are addressed in the following sections.
Voltage-Gated Ion Channels
As described in Chapter 4, there are many types of ion channels,
and several different mechanisms that regulate the opening of
the different types.
Ligand-gated channels
open in response
to the binding of signaling molecules (as shown in Figure 6–15),
and
mechanically gated channels
open in response to physical
deformation (stretching) of the plasma membranes. While these
types of channels often serve as the initial stimulus for an action
Figure 6–15
Depolarizing graded potentials can be produced when transient
application of a chemical stimulus opens ion channels at a specifi
c
location. These channels close relatively quickly when the signal
molecules dissociate and diffuse away. (a) Local current through ion
channels depolarizes adjacent regions. (b) Different stimulus intensities
result in different degrees of depolarization, and regions of the
membrane more distant from a given stimulus are depolarized less.
Site of initial depolarization
Higher intensity
Lower intensity
Resting membrane
potential
0
–70
Area of depolarization
Open Na
+
channel
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Distance along the membrane
(b)
(a)
Membrane potential (mV)
Extracellular fluid
Intracellular fluid
+
+
+
+
+
+
Direction of current
Site of initial depolarization
Charge
Extracellular
fluid
Axon
Figure 6–17
Leakage of charge (predominately potassium ions) across the
plasma membrane reduces the local current at sites farther along the
membrane from the site of initial depolarization.
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