Page 92 - Electrical Safety of Low Voltage Systems

Electric Currents Through the Human Body 75

FIGURE 5.4
Excitability curve of
a cell.

This variation of the resting potential is called an action potential,
and represents the way information is carried within, and between,
tissues in the human body. The amplitude of the membrane potential
of the cell is not proportional to the stimulus, but obeys the all-or-none
law, that is, the action potential either occurs or does not occur. 3
The combination of stimulus duration and stimulus strength must
lie above the excitability curve of a cell, as qualitatively shown in
Fig. 5.4, to elicit depolarization of the cell membrane and cause the
action potential.
I s represents the strength of the stimulus required to activate the
action potential if applied for infinite time (defined as Rheobase). Chron-
axie is the duration of a stimulus of intensity 2I s that needs to be ap-
plied to trigger the action potential.
The above excitability curve describes the behavior of the cell only
when following stimuli are sufficiently spaced in time. The cell does
not respond to close subsequent stimuli, showing periods of refrac-
toriness. The absolute refractory period T AR is the time interval fol-
lowing the inception of the cell excitation, during which no action
potential can be triggered, regardless of the strength of the stimulus
(Fig. 5.5).
The relative refractory period T RR immediately follows T AR and is
the time frame in which the membrane can be activated, but only by a
greater stimulus (Fig. 5.5). The cell will return to its standard response
after a time given by the summation of the two aforementioned peri-
ods, called refractory period T R .
If an effective stimulus persists for a time exceeding T R , a natural
phenomenon known as accommodation takes place. The cell “adapts”
by elevating its excitation threshold necessary to trigger subsequent
action potentials after the refractory period has expired.