This paper considers the quantitative description of intracellular and extracellular fields of a single circular cylindrical fiber resulting from the propagation of an action potential (AP). Several formulations are noted, but one, which permits identification of free-space source-sink relationships, is examined in some detail; the physical models which it gives rise to are described and developed. Desirable approximations are considered and the conditions of their validity are discussed. A convolution integral formulation to field patterns (from their sources) is presented. Axially symmetric anisotropic media are also considered.