RF/Microwave Interaction Mechanisms in Biological Materials - RF/Microwave Interaction with Biological Tissues - page 61

Outside

membrane

I Na I K I L

I c

g k g L

g Na

C m

E L

E k

E Na

Inside

FIGURE 2.1 Hodgkin-Huxley membrane model.

Depolarization

+30 Peak potential

0

Repolarization

Resting membrane
potential

-70

Hyperpolarization

0 1 2 3 4 5

Time (ms)

FIGURE 2.2 Membrane potential change in response to stimulation.

The absolute refractory period refers to the period of time during which a
second action potential cannot be initiated, even with very strong stimulus; it
is of the order of 0.4 ms for large fibers up to possibly 4 ms for small fibers [6].
Figure 2.2 shows a typical evolution of the membrane potential responding
to stimulation. The relative refractory period refers to the period of time during
which a second action potential can be initiated, but only by a stronger than
normal stimulus. It corresponds roughly to the period of increased K + perme-
ability resulting in a brief hyperpolarization, which means that the cell is more
negative than the resting level.
In other words, when considering the membrane in the resting state, the
concentration of Na + is high outside and low inside the cell while, in contrast,
the concentration of K + is higher inside the cell. Therefore, the K + ions tend
to diffuse (leak) out of the cell while the Na + ions tend to diffuse into the cell,