AXON DIAMETERS IN RELATION TO THE SPIKE DIMENSIONS AND THE CONDUCTION VELOCITY IN MAMMALIAN A FIBERS

The problem of the relationship between the diameters of nerve fibers and the velocity at which the fibers conduct impulses cannot be considered solved until it is possible, within the range of a homogeneous group of fibers, to predict correctly the form of the action potential on the basis of the histological picture. Of all the methods as yet proposed, the re-\ construction method is the most sensitive. As the method was first employed (11) it appeared to work satisfactorily on the basis of direct pro-portionality between conduction velocity and the diameter of the fibers. But the full range of the fiber velocities was then unknown and later attempts to apply the procedure to the complete series failed to give an acceptable result, or involved an assumption the validity of which can no longer be maintained (10). Recent developments have been made with other methods. Blair and Erlanger observed that in frog nerves the size of single-fiber spikes varies directly as the velocity of conduction, and they argued that if the size of the spike is proportional to the cross-section of the fiber, the velocity must vary as the square of the diameter. And Zotterman came to the same conclusion after confirming their results on mammalian fibers. Reconstructions , however, in accord with the power relationship fail to match the recorded potentials (Erlanger, 1937,' fig. 14). A different conclusion was reached by Hursh (1939a), who compared the maximal velocities in a series of mammalian nerves with the respective sizes of the largest fibers. All the points relating the two properties fell about a straight line. A still different relationship was found by Pumphrey and Young for squid fibers. The diameters of the large axons in the fresh state were measured and compared with the individually determined velocities; those of the small axons were measured after fixa-tion and compared with the maximal velocity for the group. The result was a distribution of points best connected by a curve describing the velocity as varying with the 0.6 power of the diameter. The fact that the relationship of velocity and fiber diameter has been 393