Page 90 - Electrical Safety of Low Voltage Systems

Electric Currents Through the Human Body 73

in fact, diffuse from areas of high concentration to areas of low con-
centration. In Fig. 5.1, the dotted line symbolizes the direction of the
electric field, while the solid one shows the force of diffusion.
The concentration of potassium K is larger inside the cell; there-
+
fore, the diffusion force tries to force these ions out. On the other hand,
the extracellular fluid is positively charged, with respect to the intra-
cellular fluid, and the resulting electric field will oppose the diffusion
of the positive ions. The two actions, then, balance each other. The
−
above explanation can be similarly applied to the Cl ions.
The process just described would not seem to explain, though,
the behavior of the Na ion. Sodium ions, in fact, are subject to both
+
forces, electrical and diffusion, which act in the same direction toward
the inside of the cell. The forces do not cancel each other. In equilib-
rium, then, the largest concentration of Na ions should be inside the
+
cell and not in the interstitial fluid. Studies have shown that another
active process, called the sodium–potassium pump, explains the lower
concentration of sodium in the intracellular fluid. Due to this process,
protein molecules, at the expense of the body metabolism, continu-
ously transport Na ions out of the cell and replace them with K +
+
ions.
In stable conditions, that is, in the absence of applied stimuli, the
concentrations of ions inside and outside the cell allow the establish-
ment and the holding of the membrane potential. The cell, therefore,
can be thought as a capacitor C. The two fluids, intracellular and ex-
1
tracellular, are good electrolytic conductors and act as the armatures
of a capacitor, whose dielectric is the membrane itself. The membrane,
7
in fact, has a high resistivity of approximately 10 · m and dielectric
constant of 7ε 0 .
The above-mentioned capacitor, though, is not an ideal compo-
nent, as leak currents can flow through the dielectric. As a conse-
quence, the model of the cell must include a leakage resistor R L in
parallel to the capacitor (Fig. 5.2).
The resting potential is represented by the electromotive force V RP .

5.2.2 Action Potential
Excitable cells have the property to remarkably increase the perme-
ability of their membrane to sodium ions upon application of a de-
2
polarizing stimulus, whose intensity and duration exceed the cell’s
threshold of excitation (Fig. 5.3).
The stimulus causes the voltage-sensitive sodium ions channels in
+
the membrane to open, allowing the “inrush” of Na ions into the cell,
driven by both electric and diffusion forces. This dramatically changes
the cell potential, which becomes positive from its resting negative
value. When the depolarization is complete, that is, the inside of the
cell is positive with respect to the outside, the sodium ion channels