144
Chapter 6
Figure 6–7
The electrical force of attraction between positive and negative
charges increases with the quantity of charge and with decreasing
distance between charges.
SECTION B
Membrane Potentials
Basic Principles of Electricity
As discussed in Chapter 4, the predominant solutes in the
extracellular fl uid are sodium and chloride ions. The intracel-
lular fl
uid contains high concentrations of potassium ions and
ionized nondiffusible molecules, particularly proteins with
negatively charged side chains and phosphate compounds.
Electrical phenomena resulting from the distribution of these
charged particles occur at the cell’s plasma membrane and play
a signifi cant role in signal integration and cell-to-cell commu-
nication, the two major functions of the neuron.
Charges of the same type repel each other—positive
charge repels positive charge, and negative charge repels nega-
tive charge. In contrast, oppositely charged substances attract
each other and will move toward each other if not separated
by some barrier (
Figure 6–7
).
Separated electrical charges of opposite sign have the
potential to do work if they are allowed to come together.
This potential is called an
electrical potential
or, because it is
determined by the difference in the amount of charge between
two points, a
potential difference.
The electrical potential
difference is often referred to simply as the
potential.
The
units of electrical potential are volts. The total charge that can
be separated in most biological systems is very small, so the
potential differences are small and are measured in millivolts
(1 mV = 0.001 V).
The movement of electrical charge is called a
current.
The electrical potential between charges tends to make them
fl ow, producing a current. If the charges are opposite, the cur-
rent brings them toward each other; if the charges are alike,
the current increases the separation between them. The
amount of charge that moves—in other words, the current—
depends on the potential difference between the charges and
on the nature of the material or structure through which they
are moving. The hindrance to electrical charge movement is
known as
resistance.
If resistance is high, the current fl ow
will be low. The effect of voltage
V
and resistance
R
on cur-
rent
I
is expressed in
Ohm’s law:
I =
V
R
Materials that have a high electrical resistance reduce current
fl ow and are known as insulators. Materials that have a low
resistance allow rapid current fl ow and are called conductors.
Water that contains dissolved ions is a relatively good
conductor of electricity because the ions can carry the cur-
rent. As we have seen, the intracellular and extracellular fl u-
ids contain many ions and can therefore carry current. Lipids,
however, contain very few charged groups and cannot carry
current. Therefore, the lipid layers of the plasma membrane
are regions of high electrical resistance separating the intracel-
lular fl uid and the extracellular fl
uid, two low-resistance water
compartments.
The Resting Membrane Potential
All cells under resting conditions have a potential difference
across their plasma membranes, with the inside of the cell
negatively charged with respect to the outside (
Figure 6–8a
).
This potential is the
resting membrane potential.
By convention, extracellular fl uid is assigned a voltage of
zero, and the polarity (positive or negative) of the membrane
potential is stated in terms of the sign of the excess charge on
the inside of the cell. For example, if the intracellular fl
uid
has an excess of negative charge and the potential difference
across the membrane has a magnitude of 70 mV, we say that
the membrane potential is –70 mV (inside relative to outside).
The magnitude of the resting membrane potential var-
ies from about –5 to –100 mV, depending upon the type of
cell. In neurons, it is generally in the range of –40 to –90 mV
(
Figure 6–8b
). The resting membrane potential holds steady
unless changes in electrical current alter the potential.
The resting membrane potential exists because of a tiny
excess of negative ions inside the cell and an excess of posi-
tive ions outside. The excess negative charges inside are elec-
trically attracted to the excess positive charges outside the
cell, and vice versa. Thus, the excess charges (ions) collect
in a thin shell tight against the inner and outer surfaces of
the plasma membrane (
Figure 6–9
), whereas the bulk of the
intracellular and extracellular fl
uids remain neutral. Unlike
the diagrammatic representation in Figure 6–9, the number
+
+
+
+
+
+
+
+
Force increases with the
quantity of charge
Force increases as
distance of separation
between charges
decreases
Electrical force
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