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U N I T 1 0
Nervous System
(approximately −60 mV in large nerve fibers) represents
the membrane potential at which neurons or other excit-
able tissues are stimulated to fire. When the threshold
potential is reached, the gatelike structures in the ion
channels open. Below the threshold potential, these gates
remain tightly closed. The gates function on an all-or-
none basis, meaning they are either fully open or fully
closed. Under ordinary circumstances, the threshold
stimulus is sufficient to open many ion channels, trigger-
ing massive depolarization of the membrane (the action
potential).
Depolarization
is characterized by a rapid change in
polarity of the resting membrane potential, which was
negative on the inside and positive on the outside, to
one that is positive on the inside and negative on the
outside. During the depolarization phase, the membrane
suddenly becomes permeable to sodium ions. The rapid
inflow of sodium ions produces local electric currents
that travel through the adjacent cell membrane, caus-
ing the sodium channels in this part of the membrane
to open. In neurons, sodium ion gates remain open for
approximately a quarter of a millisecond. During this
phase of the action potential, the inner side of the mem-
brane becomes positive (approximately +30 to +45 mV).
Repolarization
is the phase during which the polar-
ity of the resting membrane potential is reestablished.
This is accomplished with closure of the sodium chan-
nels and opening of the potassium channels. The out-
flow of positively charged potassium ions across the
cell membrane returns the resting membrane potential
to negativity. The sodium/potassium–adenosine triphos-
phatase (Na
+
/K
+
-ATPase) pump gradually reestablishes
the resting ionic concentrations on each side of the mem-
brane. Membranes of excitable cells must be sufficiently
repolarized before they can be re-excited. During repo-
larization, the membrane remains refractory (i.e., does
not fire) until repolarization is approximately one-third
complete. This period, which lasts approximately one
half of a millisecond, is called the
absolute refractory
period
. During one portion of the recovery period, the
membrane can be excited, although only by a stronger-
than-normal stimulus. This period is called the
relative
refractory period
.
The excitability of neurons can be affected by condi-
tions that alter the resting membrane potential, moving
it either closer to or farther from the threshold potential.
Hypopolarization
increases the excitability of the post-
synaptic neuron by bringing the membrane potential
closer to the threshold potential so that a smaller sub-
sequent stimulus is needed to cause the neuron to fire.
Hyperpolarization
brings the membrane potential fur-
ther from the threshold and has the opposite, inhibitory
effect, decreasing the likelihood that an action potential
will be generated.
SynapticTransmission
Neurons communicate with each other through struc-
tures known as
synapses
. There are two types of synapses
in the nervous system: electrical and chemical. Electrical
synapses permit the passage of current-carrying ions
through small openings called
gap junctions
that pen-
etrate the cell junction of adjoining cells. Although not
important in synaptic transmission in the nervous sys-
tem, gap junctions are important in cell-to-cell commu-
nication in smooth and cardiac muscle.
Chemical synapses, which are the more common type
of synapse, involve special presynaptic and postsynap-
tic membrane structures, separated by a synaptic cleft.
The presynaptic terminal secretes one and often several
chemical transmitter molecules (e.g., neurotransmitters,
neuromodulators). The secreted neurotransmitters dif-
fuse into the synaptic cleft and bind to receptors on the
postsynaptic membrane. In contrast to an electrical syn-
apse, a chemical synapse permits only one-way commu-
nication. Chemical synapses are divided into two types:
excitatory and inhibitory. In excitatory synapses, bind-
ing of the neurotransmitter to the receptor produces
depolarization of the postsynaptic membrane; and in
inhibitory synapses it reduces the postsynaptic neuron’s
ability to generate an action potential.
The process of neurotransmission involves the syn-
thesis, storage, and release of a neurotransmitter; the
reaction of the neurotransmitter with a receptor; and ter-
mination of the receptor action. Neurotransmitters are
synthesized in the cytoplasm of the axon terminal. The
synthesis of transmitters may require one or more enzyme-catalyzed steps (e.g., one for acetylcholine and three for
norepinephrine). Each neuron generally produces only
one type of neurotransmitter. After synthesis, the neu-
rotransmitter molecules are stored in the axon terminal
in tiny, membrane-bound sacs called
synaptic vesicles
.
– 80
– 60
– 40
– 20
0
+20
Overshoot
Threshold potential
Depolarization
Repolarization
Resting potential
Time
(msec)
Membrane potential (mv)
A
C
B
FIGURE 34-4.
Time course of the action potential recorded at
one point of an axon with one electrode inside and one outside
the plasma membrane.The rising part of the action potential
is called the spike.
(A)
The rising phase plus approximately the
first half of the repolarization phase is the absolute refractory
period.
(B)
The portion of the repolarization phase that
extends from the threshold to the resting membrane potential
represents the relative refractory period.
(C)
The remaining
portion of the repolarization phase to the resting membrane
potential is the negative afterpotential.
