Extracellular potential field of unmyelinated active axons

Experimental and model investigations were carried out on the extracellular action potentials and potential fields of some unmyelinated axons: the medial and the lateral giant axons of Lumbriscus terrestris and small unmyelinated axons (nerve fibres of group C). Close to the membrane the extracellular action potentials are similar in shape to the second space derivative of the intracellular action potentials--they are triphasic in case of intracellular action potential with depolarization after-potential, having four phases in case of intracellular action potentials with hyperpolarization after-potential. Upon increasing the radial distance, the amplitudes of the different phases of the extracellular action potentials decrease at a different rate, while their maxima are shifted, which results in considerable changes in their shape.