Biomedical Engineering Reference
In-Depth Information
3.3.4 Action Potential
The transport of ions causes cells to change their polarization state. Many times a
small depolarization may not create a stimulus. Typically, cells are characterized
by threshold behavior (i.e., if the signal is below the threshold level, no action is
observed). The intensity of the stimulus must be above a certain threshold to trigger
an action. The smallest current adequate to initiate activation of a cell is called the
rheobasic current
or
rheobase
. The threshold level can be reached either by a very
high strength signal for a short duration or a low strength signal applied for a long
duration. The membrane potential may reach the threshold level by a short, strong
stimulus or a longer, weaker stimulus. Theoretically, the rheobasic current needs
an infinite duration to trigger activation. The time needed to excite the cell with
twice rheobase current is called
chronaxy
. If the membrane potential is given a suf-
ficiently large constant stimulus, the cell is made to signal a response. This localized
change in polarization, which is later reset to the original polarization, is called an
action potential
[Figure 3.5(a)]. This large local change in polarization triggers the
same reaction in the neighborhood, which allows the reaction to propagate along
the cell.
The transfer of Na
+
and K
+
ions across the membrane are primarily responsi-
ble for the functioning of a neuron. When the neuron is in a resting state, the cell
membrane permeabilities are very small as many of the Na
+
and K
+
ion channels
are closed; the cell interior is negatively charged (at
70 mV) relative to the exte-
rior. When a stimulus is applied that depolarizes a cell, the cell surface becomes
−
Figure 3.5
Action potential: (a) characteristics features of an action potential curve; (b) changes
in charge distribution during depolarization; (c) Hodgkin-Huxley equivalent circuit for a squid axon;
and (d) voltage clamp measurement circuit.
