Biomedical Engineering Reference
In-Depth Information
move through the axon propagate through each branch to the presynaptic terminal. The
presynaptic terminals are the transmitting units of the neuron, which, when stimulated,
release a neurotransmitter that flows across a gap of approximately 20 nanometers to an
adjacent cell, where it interacts with the postsynaptic membrane and changes its potential.
12.3.1 Membrane Potentials
The neuron, like other cells in the body, has a separation of charge across its external
membrane. The cell membrane is positively charged on the outside and negatively charged
on the inside, as illustrated in Figure 12.2. This separation of charge, due to the selective
permeability of the membrane to ions, is responsible for the membrane potential. In the
neuron, the potential difference across the cell membrane is approximately 60 mV to
90 mV, depending on the specific cell. By convention, the outside is defined as 0 mV
(ground), and the resting potential is
60 mV. This charge differential is of
particular interest, since most signaling involves changes in this potential across the mem-
brane. Signals such as action potentials are a result of electrical perturbations of the mem-
brane. By definition, if the membrane is more negative than resting potential (i.e.,
V
m
¼
v
i
v
o
¼
60 to
70 mV), it is called
hyperpolarization
, and an increase in membrane potential from resting
potential (i.e.,
60 to
50 mV) is called
depolarization
. As described later, ions travel across
the cell membrane through ion selective channels.
Creating a membrane potential of
60 mV does not require the separation of many posi-
tive and negative charges across the membrane. The actual number, however, can be found
from the relationship
Cdv
¼
dq
,or
C
D
v
¼ D
q
(
D
q
¼
the number of charges times the electron
10
19
C)
F/cm
2
and
10
3
, the number
charge of 1
:
6022
.
Therefore, with
C
¼
1
m
D
v
¼
60
10
8
per cm
2
. These charges are located within a dis-
of charges equals approximately 1
tance of 1
m
m from the membrane.
Extracellular
Cell Membrane
Outside
Lipid
bilayer
Extracellular
{
Inside
Intracellular
Channel
Intracellular
FIGURE 12.2
The separation of charges across a cell membrane. The figure on the left shows a cell membrane
with positive ions along the outer surface of the cell membrane and negative ions along the inner surface of the cell
membrane. The figure on the right further illustrates separation of charge by showing that only the ions along the
inside and outside of the cell membrane are responsible for membrane potential (negative ions along the inside and
positive ions along the outside of the cell membrane). Elsewhere the distribution of negative and positive ions are
approximately evenly distributed as indicated with the large
symbols for the illustration on the right. Overall,
there is a net excess of negative ions inside the cell and a net excess of positive ions in the immediate vicinity out-
side the cell. For simplicity, the membrane shown on the right is drawn as the solid circle and ignores the axon and
dendrites.
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