Properties of auditory afferents and efferents in the mammalian cochlea

The mammalian cochlea is an excellent sound analyzer. It functions through the intricate interplay of multiple components. Our studies encompass three neuronal components important to cochlear function, including two types of auditory nerve fibers (type I and type II afferent fibers) and olivocochlear lateral efferent fibers. Postsynaptic to the inner hair cells, type I afferent fibers provide the first step of neural coding of the sound signal in the auditory pathway. In a feedback loop, lateral olivocochlear efferents synapse onto the type I afferent endings and may modulate the neural coding of type I afferents. Finally, the type II afferent fibers contacting the outer hair cells constitute a much smaller group of auditory nerve fibers than type I afferents. To further our overall understanding of cochlear function, we studied different aspects regarding each of these components and found that: 1. Diversification of spontaneous firing rate in type I afferent fibers occurs mostly between the second and the third postnatal week in rat. Statistical properties of type I afferent fiber spike trains showed developmental changes that approach adult-like features in older preparations. Most properties of type I afferent fiber spike trains derive from the characteristics of presynaptic transmitter release. Endogenous firing presents in type I afferent fibers and diminishes gradually during maturation. Two ANFs contacting the same inner hair cell could have different spontaneous rates, with no correlation in their spike timing. 2. Dopaminergic lateral efferents form patches of dense terminals in the cochlea. These terminal patches are formed by cholinergic lateral olivocochlear efferent neurons that also express tyrosine hydroxylase. The expression of tyrosine hydroxylase in these neurons can be induced by sound exposure. 3. Tyrosine hydroxylase (Th) and calcitonin-related polypeptide alpha (Calca/Cgrpα) are specifically expressed in type II versus type I afferent neurons. Using a Th2A-CreER mouse line, we first studied the morphology of apical type II afferent fibers. Combined with a CGRPα-EGFP (GENSAT) mouse line, we found that the expression patterns of Cgrpα and Th in type II afferent neurons formed opposing gradients, with Th being preferentially expressed in apical and Cgrpα in basal type II afferent neurons