Functional development of hair cells

Embryonic hair cells in chicks and mammals have functional transduction channels and voltage-gated outwardly rectifying potassium (K+) channels, fast inwardly rectifying channels, and voltage-gated sodium (Na+) and calcium (Ca2+) channels. Together these channels may participate in spiking by the immature hair cells, which may drive rhythmic or bursting activity of neurons at higher levels of the auditory pathway. The electrical activity of immature hair cells may influence afferent synaptogenesis and differentiation indirectly by promoting neurotrophin release or more directly by glutamatergic transmission. With maturation, a number of changes tend to reduce hair cell spiking: Na+, Ca2+, and fast inwardly rectifying channels may become less numerous, whereas outwardly rectifying K+ channels become more numerous and diverse. These changes signal the transformation from a developing epithelium with active formation of synaptic contacts to a sensing epithelium where receptor potentials represent the mechanical input in a graded fashion. The composition of the late-arriving outwardly rectifying K+ channels is specific to the hair cell's type and location in the sensory epithelium and confers specialized properties on the receptor potentials. Fast, Ca2+-gated channels serve high-quality electrical tuning in certain tall hair cells of the chick cochlea. In rodent cochlear hair cells and type I hair cells of chick and rodent vestibular organs, large outwardly rectifying conductances lower the input resistance, enhancing the speed and linearity of voltage responses. © 2003 Elsevier Inc. All rights reserved.