STIMULATION OF EXCITABLE TISSUES - Design and Development of Medical Electronic Instrumentation - page 308

EXTRACELLULAR STIMULATION

In clinical practice, the stimulation methods used to elucidate the electrical properties of
excitable cells are simply not suitable. To start with, clinically useful stimulation requires
stimulating much more than a single nerve or muscle cell, making it impractical to build
electrode arrays that can impale a large number of cells simultaneously. Next, even if a
su cient number of cells could be impaled with microelectrodes, it is next to impossible to

keep them in place in a living, moving being. Because of this, in vivo stimulation almost
always involves delivering the stimulating currents between a pair of electrodes placed near
(but not inside) the target cells. Consider the simple model of Figure 7.3. Here the vector
current

fl flux is indicated by arrows and transmembrane current is assumed to fl flow only at

the anode and cathode (in reality it flows at all parts of the cell) when the switch closes. As

shown in Figure 7.3 b , the current through the membrane will hyperpolarize the intracellu-
lar membrane region under the anode and depolarize the intracellular region under the cath-
ode. Stimulation will occur when the transmembrane potential at the cathode crosses the

fl

(a)

+ -

I stim

Anode

I out Cathode

Cell

I in

Intracellular
Fluid

Extracellular
F l u i d

-

(b)

+

I

stim

I

out

R

out

C

C - - - - - - - - - - - - -

++++++++++++++

M_Anode

M_ Cathode

Outside of Cel l
Inside of Cell

- - - - - - - - - - - - - ++++++++++++++

I in

R

Figure 7.3 Simplified model of electrical stimulation of a cell by a current applied through extracellular electrodes. ( a ) A transmembrane
current is assumed to flow only at the anode and cathode. The vector current flux is indicated by arrows. ( b ) The current through the mem-
brane hyperpolarizes the intracellular membrane region under the anode and depolarizes the intracellular region under the cathode.
Stimulation occurs when the transmembrane potential at the cathode crosses the membrane's threshold voltage.