The Electrophysiological Signature of Spiral Ganglion Neurons

Abstract

Examination of the basic features of primary sensory afferents has revealed much about the fundamental principles of neural encoding. This approach has been particularly valuable in the auditory system, which is systematically organized according to sound frequency and has a multiplicity of tonotopic specializations. The first neural element of the auditory pathway, the type I spiral ganglion neurons, consists of unique primary afferents that, unlike other sensory afferents, have their somata positioned directly in the axonal conduction pathway and display both graded and heterogeneous morphological properties. Electrophysiological specializations are also evident, exemplified by multifaceted voltage-gated ionic currents carried by diverse ion channel subunits that likely fine-tune neuronal firing patterns. Ion channel subunit density and the resulting characteristic firing patterns are not uniform throughout the ganglion, but instead show specific distribution patterns, some of which are related to the frequency-specific contour of the cochlear endorgan. Moreover, these properties can be regulated by neurotrophins such that fast firing electrophysiological features predominate in primary afferents innervating the high-frequency regions, whereas slow firing features are prevalent within primary afferents innervating the low-frequency regions. Thus, the complex electrophysiological properties of the spiral ganglion neurons and their regulation suggest that the primary auditory afferents are capable of shaping the electrophysiological signals that they transmit into the brain.