Anatomically speaking, there are two types of nerve fibers in vertebrates: myelinated and unmyelinated fibers. In the former the axon is covered with a protein-lipid structure (myelin) which is interrupted at more or less regular intervals (Ranvier nodes). In unmyelinated fibers this myelin structure is absent. Myelinated fibers conduct impulses much faster than unmyelinated fibers. In order to account for the high conduction velocities of medullated nerves, Lillie has proposed that excitation takes place only at the nodes of Ranvier with the circuit between adjacent nodes being made through the axoplasm of the fiber and the medium external to the fiber. This is the theory of saltatory conduction.
The literature on saltatory conduction is critically reviewed and it is concluded that only two experiments that have been done seem to indicate that saltatory conduction occurs. These are the narcosis experiments of Taskai and the air gap experiments of Huxley and Frankenhaeuser. These experiments have been carefully repeated and improved. From the results of these experiments it is concluded that medullated nerve can conduct in a situation where the conditions necessary for saltation do not exist. The possibility that saltation is the normal physiological process of conduction cannot be discounted.
