Firing patterns of type II spiral ganglion neurons in vitro.

Type I and type II spiral ganglion neurons convey auditory information from the sensory receptors in the cochlea to the CNS. The numerous type I neurons have been extensively characterized, but the small population of type II neurons with their unmyelinated axons are undetectable with most recording methods. Despite the paucity of information about the type II neurons, it is clear that they must have a significant role in sound processing because they innervate the large number of outer hair cells that are critical for maintaining normal responses to stimuli. To elucidate the function of type II neurons, we have developed an approach for studying their electrophysiological features in vitro. Type II neurons obtained from postnatal day 6-7 mice displayed distinctly different firing properties than type I neurons. They showed slower accommodation, lower action potential thresholds, and more prolonged responses to depolarizing current injection than the type I neurons. These differences were most evident in neurons from the basal, high-frequency region of the cochlea. The basal type I neurons displayed uniformly fast firing features, whereas the basal type II neurons showed particularly slow accommodation and responses to depolarization. Interestingly, neurons from the apical, low-frequency region of the cochlea showed the opposite trend. These data suggest that the type I and type II neurons have specialized electrophysiological characteristics tailored to their different roles in auditory signal processing. In particular, the type II neuron properties are consistent with cells in other sensory systems that receive convergent synaptic input for high-sensitivity stimulus detection.