Synaptic transfer from outer hair cells to type II afferent fibers in the rat cochlea

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Type II cochlear afferents receive glutamatergic synaptic excitation from outer hair cells (OHCs) in the rat cochlea. However, it remains uncertain whether this connection is capable of providing auditory information to the brain. The functional efficacy of this connection depends in part on the number of presynaptic OHCs, their probability of transmitter release, and the effective electrical distance for spatial summation in the type II fiber. The present work addresses these questions using whole-cell recordings from the spiral process of type II afferents that run below OHCs in the apical turn of young (5-9 d postnatal) rat cochlea. A "high potassium puffer" was used to elicit calcium action potentials from individual OHCs and thereby show that the average probability of transmitter release was 0.26 (range 0.02-0.73). Electron microscopy showed relatively few vesicles tethered to ribbons in equivalent OHCs. A "receptive field" map for individual type II fibers was constructed by successively puffing onto OHCs along the cochlear spiral, up to 180 m from the recording pipette. These revealed a conservative estimate of 7 presynaptic OHCs per type II fiber (range 1-11). EPSCs evoked from presynaptic OHCs separated by > 100μm did not differ in amplitude or waveform, implying that the type II fiber's length constant exceeded the length of the synaptic input zone. Together these data suggest that type II fibers could communicate centrally by maximal activation of their entire pool of presynaptic OHCs.

Original languageEnglish (US)
Pages (from-to)9528-9536
Number of pages9
JournalJournal of Neuroscience
Issue number28
StatePublished - Jul 11 2012

ASJC Scopus subject areas

  • Neuroscience(all)


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