External Spiral Innervation of the Cochlea

Abstract

The descriptions of the external spiral innervation of the cochlea by various anatomists are incomplete; important data such as length of fibers, number of side branches, and direction in which fibers run (to apex or base) are not accurately known. About the function of this type of innervation only speculation exists. Branching nerve fibers in which impulses converge to the main fiber have interesting logical properties. At the branching point the refractory time constant can cause impulses from either branch to be blocked. Thus branching points become summing points where impulses from branches add in such a manner that output rate is generally less than the sum of input rates, but greater than either of the input rates. Using simple nerve analogs, a model of the spiral innervation was built; it was driven from a cochlear analog. Different hypotheses of anatomists were tested, such as direction and length of fibers and spacing of branches. Results indicate that the spiral innervation may serve to extend the dynamic range of the ear. The results also suggest a method to establish neurophysiologically the length of the spiral nerves, direction of their course, location along the basilar membrane, and perhaps the number and location of side branches.