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Vestibular receptors. Divisions of ear. Location of bony labyrinth. Membranous 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.

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slide1

Vestibular receptors

Divisions of ear

Location of bony labyrinth

slide3

The Hair Cell and Hair Bundle

Scanning EM of hair bundle

Dissociated

hair cell

Transmission EM

A.J. Hudspeth

slide4

Hair cell types

Wersall, Acto Otol. 1956

slide6

Hair cell tuning

Hudspeth and Corey, PNAS, 1977

slide7

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)
slide8

Mechanotransduction

Tip link

Gated by tension on tip link

Hudspeth, Nature 1989

how is motion transduced into neural firing
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
the semicircular canals scc
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
the semicircular canals scc1
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)...
slide13

Cupula motion during rotation

Hillman & McLaren, Neurosci. 1979

Steinhausen, 1931

slide15

The SCC theoretical response

Hs = Gs

(Ts1 + 1)(Ts2 + 1)

slide16

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
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
slide18

Utricle

Saccule

slide19

Utricle

Saccule

Otoconia

slide20

The otolith organs...

Lindeman, Adv. Emb. Cell Biol., 1969

slide22

Planar cell polarity

Lindeman, Adv. Emb. Cell Biol., 1969

Wersall, Acta Otol. 1956

otolith receptors are inertial sensors
Otolith receptors are inertial sensors

… the inertia of the otoconia crystals in the otolith membrane

slide24

Different afferent innervations...

Utricle

Saccule

Calyx

Dimorph

HC

Bouton

slide25

Utricular afferent spatial tuning

Otolith afferents, are cosine-

tuned.

slide26

Otolith afferent spatial tuning

Yaw plane - Utricle

Fernandez & Goldber, JNP, 1976