Dynamic firing properties of type I spiral ganglion neurons

Spiral ganglion neurons, the first neural element in the auditory system, possess complex intrinsic properties, possibly required to process frequency-specific sensory input that is integrated with extensive efferent regulation. Together with their tonotopically-graded sizes, the somata of these neurons reveal a sophisticated electrophysiological profile. Type I neurons, which make up ~95 % of the ganglion, have myriad voltage-gated ion channels that not only vary along the frequency contour of the cochlea, but also can be modulated by regulators such as voltage, calcium, and second messengers. The resultant developmentally- and tonotopically-regulated neuronal firing patterns conform to three distinct response modes (unitary, rapid, and slow) based on threshold and accommodation. This phenotype, however, is not static for any individual type I neuron. Recent observations have shown that, as neurons become less excitable with age, they demonstrate enhanced plasticity enabling them to change from one response mode to another depending upon resting membrane potential and the presence of neurotrophin-3. Thus, the primary auditory afferents utilized to encode dynamic acoustic stimuli possess the intrinsic specializations that allow them dynamically to alter their firing pattern.