Fig. 1.

Ca²⁺ signals in IHCs following efferent fiber electrical stimulation. (A) Schematic representation of an IHC in the postnatal developmental period previous to the onset of hearing. Both efferent cholinergic and afferent glutamatergic contacts are present at the basal pole of the IHC. (B) Sequence of wide-field microscopy images of an IHC filled with Fluo-4 and illuminated with 488-nm LED light during efferent electrical stimulation. “Stim” indicates when the electric stimulus was applied. The IHC base contour is outlined with a dashed line. The ROI is defined by the red shape depicted in the first image. (C) Representative whole-cell current traces during efferent electrical stimulation (V_h = −120 mV). Black traces represent those trials where an eIPSC was detected after the stimulus artifact (failures are shown in gray). (D) Fluorescence signals measured at the ROI indicated in B during the same trials shown in C. Red traces correspond to trials in C where an IPSC was detected, and failures are shown in pink. (E, i–iii) Representative responses activating fluorescence changes in three different portions of the IHC cytoplasm. Each panel represents a different single synaptic event. (Top) Black traces show the electrophysiological signal. (Bottom) Traces show the spatially diverse Ca²⁺ fluorescence signals measured in the radial ROIs, indicated by colors in the scheme. Images represent the peak of the Ca²⁺ signal measured for the corresponding synaptic event.