Allocation of Function to Specific Fiber Types in Peripheral Nerves

Abstract

In a recent communication (Heinbecker, Bishop and O'Leary 1 ) the fibers in a skin nerve associated with the sensations of touch and of pricking pain were identified. Knowing that the fibers responsible for such pain are included in a well-defined group of small myelinated ones, and that those for touch fall in a different group of larger fibers, we have two points of reference for locating other functions. The differential susceptibility of nerve fibers to novocaine permits a further assignment of various functions mediated by nerves, to known fiber groups. Such fiber groups can be recognized by their potentials when stimulated, and in novocaine anesthesia the differential blocking of these potentials can be correlated with the differential disappearance of sensations.

Observations were made on 3 surgical patients to determine the order of disappearance and reappearance of various sensations and of skin temperature changes during low spinal anesthesia produced by dissolving novocaine crystals in spinal fluid. The changes noted were first a very early increase in skin temperature, after 2 to 3 minutes a loss of sensitivity to heat and cold, and then to cutaneous and pressure pains. The order of disappearance of these sensations was definitely distinct but the time interval between their disappearance was brief. After a distinctly wider gap there occurred a loss of motor function and after another short interval a loss of joint sense, sensation of pressure and touch. In recovery the order of sensibility return was reversed with the gaps between the return of different sensations much wider.

In vitro experiments to determine the effect of novocaine on the action potential of the cat's saphenous nerve definitely showed that the first potential to disappear was that derived from unmyelinated fibers.