Sequence of Degeneration of Corti's Organ and its First-Order Neurons

In our temporal bone material, sensory and neural degeneration do not occur independently. Usually there is a good correlation between the extent and severity of hair cell loss and the nerve degeneration in the osseous spiral lamina. Sensorineural degeneration in ototoxicity and after sound exposure is mostly sensory in nature in the initial phase of the process. Presence of supporting elements effectively delays secondary nerve degeneration. Degeneration of Corti's organ can often be complete, but the corresponding nerve degeneration is usually only severe to subtotal, never complete. Sensorineural degeneration which is predominantly neural (without parallel hair cell loss) is rare and occurred in less than 5% of our material. Severe subtotal nerve degeneration with hair cells still remaining was found only in one single temporal bone. Furthermore, Spoendlin's experiments in the cat suggest that in man also a portion of the first-order cochlear neuron could have an unusual degeneration behavior and may not degenerate in the case of compression or transection. The fact that sensory and neural degeneration rarely occur in reasonably “pure” forms may explain why it is often difficult to distinguish between them by means of audiological tests alone.