In the classical theory of electricity, we use the abstraction that the propagation of electric signals is instant and some discrete elements change the electric features of the circuits, the wires are only a passive medium. One of the discrete elements is the condenser an one can observe that the voltage on that element is proportional to the charge it stores; we call it ”capacity”.
When considering the finite speed of the charge carriers, one can identify two components, a resident one and a transient one. The resident capacity means that the ions sit on the surface as we can calculate from the concentration of the ionic solution, see Eq.(2.71). This capacity is estimated by [2], page 12, to be and the surface area , . The number of uncompensated ions , that raises on the membrane.
The transient capacity arises from the finite speed and finite size of the neuron.