Vestibular receptors Divisions of ear Location of bony labyrinth
The Hair Cell and Hair Bundle Scanning EM of hair bundle Dissociated hair cell Transmission EM A.J. Hudspeth
Hair cell types Wersall, Acto Otol. 1956
There are different types of hair cells with different properties... Type I and Type II
Hair cell tuning Hudspeth and Corey, PNAS, 1977
Hair cells are mechanoreceptors that are spatially tuned.. • When the stereocilia are at rest, the vestibular afferents have a baseline firing rate of about 100 AP’s per second • Bending of the cilia towards the kinocilium depolarizes the cell (increase in AP’s) • Bending of the cilia away from the kinocilium causes hyperpolarization (decrease in AP’s)
Mechanotransduction Tip link Gated by tension on tip link Hudspeth, Nature 1989
How is motion transduced into neural firing? Several steps... (1) When the head moves, inertia bends the stereocilia in the opposite direction (2) Small actin filaments between adjacent stereocilia open ion channels allowing K+ to enter the hair cell (3) The hair cell depolarizes, releasing neurotransmitter (4) There is an increase in the frequency of AP’s in VIIIth nerve afferents • Perilympth (similar to extracellular fluid) is found between the bone and the membrane labyrinth • Endolympth (similar to intracellular fluid, e.g., high K+, low Na+) fills the inside of the membranous labyrinth
There are three semicircular ducts that are approximately perpendicular to each other; One is approximately horizontal, whereas the anterior and posterior are vertical (approximately 45° from the sagittal plane) The semicircular canals (SCC)... • Together, they can measure rotations about any direction
Each canal has a swelling at it’s base, called the ampulla The crista is a saddle shaped receptor epithelium that is covered with sensory hair cells The stereocilia of the hair cells are embedded in a gelatinous membrane (the cupula) which forms a fluid tight seal across the ampulla Each canal has a partner in the other labyrinth (i.e., SSC work in pairs). When one partner is maximally excited, the other is maximally inhibited (push-pull organization) The semicircular canals (SCC)...
Cupula motion during rotation Hillman & McLaren, Neurosci. 1979 Steinhausen, 1931
The SCC theoretical response Hs = Gs (Ts1 + 1)(Ts2 + 1)
How do the SCC • detect rotational • motion? • When there is a change in • head rotational speed, the • endolymph lags behind • due to inertia. This • pushes on the cupula which • displaces the stereocilia. • Excitatory response to • ipsilateral motion. Inhibitory • response to contralateral • motion. • Canals work in pairs as • push-pull system.
The otolith organs... • … are two membranous sacs called the utricle and the saccule • The receptor epithelium (the macula) contains hair cells innervated by afferents of the VIIIth nerve • The stereocilia project into a gelatinous substance (otolith membrane), with calcium carbonate crystals (otoconia or ‘ear stones’) that are embedded in this gel • With the head upright, the macula of the utricle is horizontal and the saccule is on the side • Within each macula, hair bundles are oriented in all possible directions (indicated by arrows) • The direction of linear acceleration or gravity is determined by which hair cells are most active
Utricle Saccule Otoconia
The otolith organs... Lindeman, Adv. Emb. Cell Biol., 1969
Planar cell polarity Lindeman, Adv. Emb. Cell Biol., 1969 Wersall, Acta Otol. 1956
Otolith receptors are inertial sensors … the inertia of the otoconia crystals in the otolith membrane
Different afferent innervations... Utricle Saccule Calyx Dimorph HC Bouton
Utricular afferent spatial tuning Otolith afferents, are cosine- tuned.
Otolith afferent spatial tuning Yaw plane - Utricle Fernandez & Goldber, JNP, 1976