Figure 1
Electrical stimulation in the brainstem slowly excites vestibular afferents. (a) Right bony labyrinth w/dashed lines outlining positions of superior (sVIII) and inferior (iVIII) divisions of cranial nerve VIII (8n) dorsally obscured by the brainstem (i.e., cochlear nuclei). CC, common crus; SC, PC superior and posterior canal. Afferent recordings were confined to sVIII (red dashed oval), mediocaudal to the arcuate eminence (AE). (b) EVS electrode placement in floor of fourth ventricle: Circles—open, midline; filled, ipsilateral. FC, IC, SC: facial, inferior, and superior colliculus. (c) Coronal brainstem section showing EVS nuclei (i.e. group e) and projections to peripheral hair cells and afferents. In vestibular neuroepithelium, primary vestibular afferents (blue), innervating type I and/or type II hair cells, collect in 8n and project to the vestibular nuclear complex (VNC). In the brainstem, contralateral efferent neurons (orange) cross the midline to join ipsilateral efferents (green) with both exiting 8n. Efferent neurons pass through the vestibular ganglion (VG) without synapsing on cell bodies for primary vestibular afferents8,86. Upon reaching the neuroepithelium, EVS neurons produce many varicosities innervating type II hair cells and their bouton afferents (B), as well as calyx-bearing afferents (C) innervating type I hair cells. Key: 4V, 4th ventricle; 6n, abducens nucleus; 7n, facial nucleus; dcn, vcn, dorsal and ventral cochlear nuclei; gfn, genu of facial nerve. (d,e) Regularly (black; 2 mV, 100 ms) and irregularly-discharging (red; 0.5 mV, 100 ms) afferent showing onset of midline brainstem stimulation (333 shocks/s, green bar), respectively. (f) Corresponding continuous histograms showing changes in afferent firing rates (AFR) during three successive midline stimulation trials (333/s for 5 s delivered every 60 s, green boxes). (g) Corresponding average response histograms after background subtraction (n = 7 trials/unit). (h) Mean peak slow excitation, averaged from a 1-s block at t = 6–7 s, is plotted versus CV* for 144 units (n: 65 regular, 79 irregular). Vertical dashed line separates regular from irregular afferents. Filled color circles correspond to units shown in panel g. Red line; linear fit of the data. Scale bars: a = 1 mm; b = 0.5 mm. Binning in panels (f) and (g) is 500 ms.
