Page 52 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy and Physiology of the Cell / 37

Closed channel
VetBooks.ir + + + + + + +

–60 mV

Voltmeter
_ _ _ _ _ _ _

Ion

+ + + + + + +

–45 mV

Voltmeter
_ _ _ _ _ _ _

Open Channel
Figure 2-11. Electrically‐gated channel opens and closes with changes in the electrical potential across
the cell membrane.

changes in the permeability of the channel two factors that may act synergistically.
(Fig. 2‐11). That is, both may have the same effect
Ions (atoms or radicals having a positive on the rate and direction of movement.
or negative charge) cannot diffuse freely Or they may act antagonistically, each hav­
through the lipid bilayer of the plasma ing opposite effects on rate and direction of
membrane. Thus, a channel that is perme­ movement. One factor is the concentration
able to a given ion must be present for that gradient between the two sides of the
ion to diffuse through cell membranes. membrane for the particular ion. Because
Most channels are permeable only to a of diffusion, ions have net movement
single specific ion or a small number of through permeable channels from areas of
specific ions. This characteristic is higher concentration to areas of lower
important from a clinical standpoint, as concentration. The second factor is any
some drugs are relatively specific for a electrical gradient generated by concentra­
given type of channel. With the use of tion differences among other charged ions
these agents, the movement of a specific on the two sides of the membrane. In all
ion across cell membranes can be regu- animal cells, the concentrations of charged
lated. For example, the inward move- ions on the two sides of the cell membrane
ment of calcium into cells of the heart are normally such that the inside of the cell
can be regulated with such drugs, and is negative as compared to the outside
this is beneficial in certain types of car- (Fig. 2‐11). The net negative charge on the
diac arrhythmias (abnormalities in the inside of the cell inhibits the inward diffu­
electrical activity of the heart). sion of negatively charged ions (anions),
The rate and direction of passage of a while it promotes the inward diffusion of
charged ion through a channel depends on positively charged ions (cations). The term