Biomedical Engineering Reference
In-Depth Information
FIGURE 5-1
Process of triggering cell action potential. (a) Equilibrium state.
(b) Depolarization process as sodium ions flow across cell membrane. (c) Depolarized state.
allow the entry of the potassium and chloride ions but block the sodium ions. Because the
ions seek to balance both potential and concentration across the membrane, the restriction
on the diffusion of sodium ions results in an imbalance in the concentration of sodium
ions, with fewer within the cell and more in the intercellular fluid. In an attempt to balance
the charge, additional potassium ions enter the cell. At equilibrium, a potential difference
of
70 mV exists across the cell membrane, with the interior being negative with respect
to the exterior. This potential difference, which has been measured at between
−
60 and
−
−
100 mV, is called the resting potential of the cell, and cells at this potential are referred
to as polarized.
When a section of the cell membrane is excited by the flow of ionic current or by some
other form of excitation energy, the membrane becomes permeable to sodium ions and
begins to flow across the boundary, as illustrated in Figure 5-1. This ionic current increases
the permeability further with the result that the current flow increases exponentially (the
avalanche effect), and sodium ions rush into the cell to try to reach equilibrium. At the same
time, some potassium ions begin to move out of the cell for the same reasons. Potassium
ions are slower than sodium ions, and as a result the cell is left with a slight potassium
imbalance, which results in a positive potential difference of about
20 mV across the cell
membrane. This is known as the action potential of the cell and is considered depolarized
when in this state.
At this time, the cell reverts to its semipermeable state, and an active process called
a sodium pump quickly transports sodium ions back out of the cell during the process
known as repolarization.
The time periods involved in this process vary with different cell types. For example,
in nerve and muscle cells the repolarization process is so quick that the action potential
appears as a sharp spike as little as 1 ms wide. Heart muscle repolarizes much more slowly,
with the action potential lasting from 150 to 300 ms. However, as shown in Figure 5-2,
regardless of the duration the resting and action potentials are always the same.
When a cell is excited and generates an action potential, ionic currents flow in the
intercellular fluid or in adjacent areas of the same cell and can excite neighboring cells.
In the case of nerve cells with a long axon fiber, the action potential is generated in a
very small segment of the fiber but propagates rapidly in both directions from the trigger
point. Under normal conditions, nerve fibers are excited only near their input end and thus
propagate in one direction only.
As an action potential travels down the fiber, it cannot reexcite the portion of fiber
immediately prior because of a refractory period that follows the action potential. The
rate at which the potential travels down the fiber is called the propagation rate, nerve
+
