Biomedical Engineering Reference
In-Depth Information
Stimulation of the postsynaptic membrane along the dendrite and cell body must occur
for
V
m
to rise to the threshold potential at the axon hillock. As previously described, the
greater the distance from the axon hillock, the smaller the contribution of postsynaptic
membrane stimulation to the change in
V
m
at the axon hillock. Also, because of the mem-
brane time constant, there is a time delay in stimulation at the postsynaptic membrane
and the resultant change in
at the axon hillock. Thus, time and distance are important
functions in describing the graded response of
V
m
V
m
at the axon hillock.
Na
þ
conductance gates are opened and an inward
Once
V
m
reaches threshold, active
Na
þ
ions results, causing further depolarization. This depolarization increases
Na
þ
conductance, consequently inducing more
flow of
Na
þ
current. This iterative cycle, shown in
Na
þ
gates.
Figure 12.20, continues driving
V
m
to
E
Na
and concludes with the closure of the
K
þ
conductance occurs that drives
A similar but slower change in
back to the resting
potential. Once an action potential is started, it continues until completion. This is called
the “all or none” phenomenon. The active gates for
V
m
Na
þ
and
K
þ
are both functions of
V
m
and time.
The action potential moves through the axon at high speeds and appears to jump from
one node of Ranvier to the next in myelinated neurons. This occurs because the membrane
capacitance of the myelin sheath is very small, making the membrane appear only resistive
with almost instantaneous changes in
possible.
To investigate the action potential, Hodgkin and Huxley used an unmyelinated squid
giant axon in their studies because of its large diameter (up to 1 mm) and long survival time
of several hours in seawater at 6.3
C. Their investigations examined the then existing theory
that described an action potential as due to enormous changes in membrane permeability
that allowed all ions to freely flow across the membrane, driving
V
m
V
m
to zero. As they dis-
covered, this was not the case. The success of the Hodgkin-Huxley studies was based on
two new experimental techniques, the space clamp and voltage clamp, and collaboration
with Cole and Curtis from Columbia University.
The space clamp allowed Hodgkin and Huxley to produce a constant
over a large
region of the membrane by inserting a silver wire inside the axon and thus eliminating
R
a
V
m
. The voltage clamp allowed the control of
V
m
by eliminating the effect of further depo-
larization due to the influx of
as membrane permeability changed.
Selection of the squid giant axon was fortunate for two reasons: it was large and survived
a very long time in seawater, and it had only two types of voltage-time-dependent per-
meable channels. Other types of neurons have more than two voltage-time-dependent
permeable channels that would have made the analysis extremely difficult or even
impossible.
I
Na
and efflux of
I
K
Depolarization
Drives V
m
to E
Na
Inward I
Na
g
Na
FIGURE 12.20
The conductance gate for sodium.
