Biomedical Engineering Reference
In-Depth Information
ions constant. The mechanism responsible for the transport of sodium out of
the cell and potassium in the reverse direction is termed the 'sodium-potassium
pump'. The pump is fuelled by enzymatic hydrolysis of ATP.
The impulse stimulus of the axon of a neuron results in a number of electrical
potential changes collectively referred to as the 'action potential'. After a latent
period a depolarization of about 15 mV occurs; this is followed by a further
rapid depolarization. The point at which the latter occurs is often tensed the
'firing level'. The potential then rises above zero in an overshoot effect before
eventually returning to the resting potential. The time sequence of the events
associated with the action potential is summarized in Figure 2.7. The signal
represents an 'all or none' process with respect to firing.
We can now summarize the effect of a stimulus on the overall behavior of the
(unmyelinated) axon. Polarization of the membrane involves an excess of
positive charge on the outside and prevalence of negative charge on the inside
of the membrane. This polarization is eliminated during an action potential,
with the result of increase of flow of positive charge into the negative sink,
which in turn depolarizes the membrane ahead of the action potential. This
process represents a self-propagated response along the axon and circular flow
of current.
Myelinated axons with the insulating sheath and nodes of Ranvier in place
exhibit a similar circular (saltatory) flow of current to that described above.
However, in this case, the depolarization event jumps for one node to the next,
resulting in a propagation of the action potential which is many times faster
than for uninsulated axons. Although in principle axons can conduct current in
both directions,
d
n
4
t
3
n
g
|
0
n
3
.
in living animals they are unidirectional only to their
50
40
30
20
10
0
Time
-10
-20
-30
Firing
level
After-depolarization
-40
Stimulus
artifact
-50
After-hyperpolarization
-60
-70
-80
Latent
period
-90
Figure 2.7
Idealized time course of cellular action potential.
