150
Chapter 6
in the cell’s environment acts on a specialized region of the
membrane. They are called graded potentials simply because
the magnitude of the potential change can vary (is “graded”).
Graded potentials are given various names related to the loca-
tion of the potential or the function they perform; for instance,
receptor potential, synaptic potential, and pacemaker potential
(
Table 6–3
).
Whenever a graded potential occurs, charge fl ows
between the place of origin of this potential and adjacent
regions of the plasma membrane, which are still at the rest-
ing potential. In
Figure 6–15
, a small region of a membrane
has been depolarized by transient application of a chemical
signal, briefl y opening membrane channels and producing
a potential less negative than that of adjacent areas. Inside
the cell, positive charge (positive ions) will fl ow through
the intracellular fl uid away from the depolarized region
and toward the more negative, resting regions of the mem-
brane. Simultaneously, outside the cell, positive charge will
ow from the more positive region of the resting membrane
toward the less positive regions the depolarization just cre-
ated. Note that this local current moves positive charges
toward the depolarization site along the outside of the mem-
brane and away from the depolarization site along the inside
of the membrane. Thus, it produces a decrease in the amount
of charge separation (i.e., depolarization) in the membrane
sites adjacent to the originally depolarized region, and the
signal moves along the membrane.
Depending upon the initiating event, graded potentials
can occur in either a depolarizing or a hyperpolarizing direc-
tion (
Figure 6–16a
), and their magnitude is related to the
magnitude of the initiating event (
Figures 6–15b
,
6–16b
).
In addition to the movement of ions on the inside and the
outside of the cell, charge is lost across the membrane because
the membrane is permeable to ions through open membrane
channels. The result is that the change in membrane poten-
tial decreases as the distance increases from the initial site of
the potential change (Figure 6–15b,
Figure 6–16c
). Current
fl ows much like water fl ows through a leaky hose, decreasing
just as water fl ow decreases the farther along the leaky hose
you are from the faucet. In fact, plasma membranes are so
leaky to ions that these currents die out almost completely
within a few millimeters of their point of origin. Because of
this, local current is
decremental;
that is, the fl ow of charge
decreases as the distance from the site of origin of the graded
potential increases (
Figure 6–17
).
Because the electrical signal decreases with distance,
graded potentials (and the local current they generate)
can function as signals only over very short distances (a few
Table 6–3
A Miniglossary of Terms Describing the Membrane Potential
Potential or potential difference
The voltage difference between two points
Membrane potential or
transmembrane potential
The voltage difference between the inside and outside of a cell
Equilibrium potential
The voltage difference across a membrane that produces a fl ux of a given ion species that is equal
but opposite to the fl ux due to the concentration gradient of that same ion species
Resting membrane potential or
resting potential
The steady transmembrane potential of a cell that is not producing an electric signal
Graded potential
A potential change of variable amplitude and duration that is conducted decrementally; it has no
threshold or refractory period
Action potential
A brief all-or-none depolarization of the membrane, reversing polarity in neurons; it has a
threshold and refractory period and is conducted without decrement
Synaptic potential
A graded potential change produced in the postsynaptic neuron in response to the release of a
neurotransmitter by a presynaptic terminal; it may be depolarizing (an excitatory postsynaptic
potential or EPSP) or hyperpolarizing (an inhibitory postsynaptic potential or IPSP)
Receptor potential
A graded potential produced at the peripheral endings of afferent neurons (or in separate
receptor cells) in response to a stimulus
Pacemaker potential
A spontaneously occurring graded potential change that occurs in certain specialized cells
Threshold potential
The membrane potential at which an action potential is initiated
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