Chapter 11
Fundamentals of the Nervous System and Nervous Tissue
399
11
Graded Potentials
Graded potentials
are short-lived, localized changes in mem-
brane potential that can be either depolarizations or hyperpo-
larizations. Tese changes cause current flows that decrease in
magnitude with distance. Graded potentials are called “graded”
because their magnitude varies directly with stimulus strength.
Te stronger the stimulus, the more the voltage changes and the
farther the current flows.
Graded potentials are triggered by some change (a stimu-
lus) in the neuron’s environment that opens gated ion chan-
nels. Graded potentials are given different names, depending on
where they occur and the functions they perform.
When the receptor of a sensory neuron is excited by some
form of energy (heat, light, or other), the resulting graded
potential is called a
receptor potential
or
generator poten-
tial
. We will consider these types of graded potentials in
Chapter 13.
When the stimulus is a neurotransmitter released by another
neuron, the graded potential is called a
postsynaptic poten-
tial
, because the neurotransmitter is released into a fluid-
filled gap called a synapse and influences the neuron beyond
(post) the synapse.
Fluids inside and outside cells are fairly good conductors,
and current, carried by ions, flows through these fluids when-
ever voltage changes. Suppose a stimulus depolarizes a small
area of a neuron’s plasma membrane
(Figure 11.10a)
. Current
(ions) flows on both sides of the membrane between the depo-
larized (active) membrane area and the adjacent polarized (rest-
ing) areas. Positive ions migrate toward more negative areas (the
direction of cation movement is the direction of current flow),
Neurons use changes in their membrane potential as com-
munication signals to receive, integrate, and send informa-
tion. A change in membrane potential can be produced by (1)
anything that alters ion concentrations on the two sides of the
membrane, or (2) anything that changes membrane perme-
ability to any ion. However, only permeability changes
(changes in the number of open channels) are important for
transferring information.
Changes in membrane potential can produce two types of
signals:
Graded potentials
, which are usually incoming signals oper-
ating over short distances
Action potentials
, which are long-distance signals of axons
Te terms
depolarization
and
hyperpolarization
describe
changes in membrane potential
relative to resting membrane po-
tential
. It is important to clearly understand these terms.
Depolarization
is a decrease in membrane potential: Te
inside of the membrane becomes
less negative
(moves closer
to zero) than the resting potential. For instance, a change in
resting potential from
2
70 mV to
2
65 mV is a depolarization
(Figure 11.9a)
. By convention, depolarization also includes
events in which the membrane potential reverses and moves
above zero to become positive.
Hyperpolarization
is an increase in membrane potential:
Te inside of the membrane becomes
more negative
(moves
further from zero) than the resting potential. For example, a
change from
2
70 mV to
2
75 mV is hyperpolarization (Fig-
ure 11.9b). As we will describe shortly, depolarization increases
the probability of producing nerve impulses, whereas hyper-
polarization reduces this probability.
+
+ + +
+
+
+ + +
+
Depolarizing stimulus
Membrane potential (voltage, mV)
Time (ms)
0
–100
–70
0
–50
–50
+50
1
2
3
4
5
6
7
Hyperpolarizing stimulus
Membrane potential (voltage, mV)
Time (ms)
0
1
2
3
4
5
6
7
–100
–70
0
+50
Inside
positive
Inside
negative
Resting
potential
Depolarization
Resting
potential
Hyper-
polarization
(a) Depolarization:
The membrane potential
moves toward 0 mV, the inside becoming less
negative (more positive).
(b) Hyperpolarization:
The membrane potential
increases, the inside becoming more negative.
Figure 11.9
Depolarization and hyperpolarization of the membrane.
The resting
membrane potential is approximately
2
70 mV (inside negative) in neurons.
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