The equation governing a two-dimensional sheet of tissue perfused by a three-dimensional bath is similar to the equation derived by Rattay for a one-dimensional nerve axon [13]. The tissue obeys the conservation of current,

where β is the ratio of membrane surface area to tissue volume, *J*_{m }is the membrane current density, and *J*_{ix }and *J*_{iy }represent x and y components of the intracellular current density. Ohm's Law gives

*J*_{m }= *G*_{m }*V*_{m}, (4)

where *V*_{i }is the intracellular potential, *V*_{m }is the transmembrane potential, *G*_{m }is membrane conductivity per unit area, and σ_{ix }and σ_{iy }are the intracellular conductivities parallel to (x) and perpendicular to (y) the fiber axis. Substituting Eqs. (2), (3), and (4) into Eq. (1), and letting *V*_{i }= *V*_{m }+ *V*_{e}, we find that

where λ_{x }and λ_{y }are defined as

In order to solve Eq. (5), we must first determine the extracellular potential, *V*_{e}. We assume that *V*_{e }is from a point electrode in an infinite, homogeneous bath

where , σ_{e }is the conductivity of the bath, *I *is the stimulus current, and *d *is the distance from the tissue sheet (*z *= 0) to the electrode.

We discretize Eq. (5) using a finite difference formula, and solve it using a relaxation method. The number of nodes in each direction is 100, the space step is 0.2 mm (implying a tissue size of 20 × 20 mm), σ_{e }is 1 S/m, *I *is 1 mA, and *d *is 1 mm.