Anatomy of inner ear

Inner ear

1. Anatomy of inner ear ( vestibular apparatus )

2. Vestibular apparatus is the sensory organ for detection of sensation of equilibrium Within petrous (“ rock like”) part of temporal bone, which guards sensory organs of the inner ear. Arcuate eminence- important landmark seen on superior view( corresponds to superior semi circular canal), in middle cranial fossa approach

3. INNER EAR DEVELOPMENT 4 -8 wks

4. Otic placode -plaque like thickening of the surface ectoderm dorsal to 1 st branchial groove , at the end of 3 rd wk – forms bipolar neurons of cochlear and vestibular ganglion Auditory pit- invagination into underlying mesenchyme Otic cyst or auditory vesicle- utricular and saccular components of the membraous labyrinth Each vesicle divide into- Pars superior (dorsal component)-( phylogenetically older)- scc and utricle – more resistant to developmental malformations Pars inferio r (ventral component)-( phylofenetcally newer)- saccule and cochlear duct Otic capsule – mesenchyme tissue surrounding the otocyst forms otic capsule

5. The inner ear is composed of: a. Bony labyrinth:(Hard bony outer shell) Cochlea Vestibule( spherical recess lodges saccule , elliptical recess for utricle) Semicircular canals: Anterior (or superior), Lateral (or Horizontal) & Posterior semicircular canals b. Membranous labyrinth (which is found inside the bony Labyrinth ): Cochlear duct Utricle and saccule 3 Semicircular canals Endolymphatic duct and sac

6. The v estibular apparatus consists of 3 semicircular canals that are sensitive to angular accelerations (head rotations) O tolith organs "utricle & saccule " that are sensitive to linear accelerations.

7. Bony labyrinth Houses the sensory organs and soft tissue structures of the inner ear Consists of -cochlea -3 semicircular canals -vestibule Each bone has 3 layers inner endosteal layer middle enchondral and intrachondrial bone outer periosteal layer

8. Semi circular canals Lateral (horizontal) Superior (anterior vertical) Posterior( posterior vertical) Each canal has ampullated limb (2mm ) and non ampullated limb (1mm) Crus commune – non ampullated limbs of posterior and superior canals fuse Opens into the vestibule Solid angle – angle formed by 3 SCC

9. Dehiscent superior semicircular canal Thinning or frank dehiscence of bone of superior scc Sound or pressure induced vertigo ( tullio phenomenon )

10. Vestibule Central chamber of bony labyrinth 4mm diameter Medial wall – spherical recess for saccule -elliptical recess for utricle - cochlear recess for cochlear duct

11. Mike’s dot Macula cribrosa superior Passage of superior vestibular nerve fibres to the cristae ampullaris of lateral scc Corresponds to extreme lateral aspect of the IAC Important landmark in trans labyrinthine surgery

12. Fissures in bony labyrinth 3 in number Fissula antre fenestrum- evagination of perilymphatic space anterosuperior to oval window Fossula post fenestrum- evagination of perilymphatic space posterior to oval window Hyrtle’s fissure ( tympano meningeal hiatus) - remnant of embryonic development -route for csf leakage into middle ear 2 microfissures of temporl bone 1- extends between round window niche and ampulla of posterior scc 2- runs superoiferior to oval window

13. Membranous( endolymphatic ) labyrinth Houses within bony labyrinth Consists of Cochlear duct( scala media) 3 semicircular canal ducts and their cristae ampullaris Otolith organs (utricle and saccule ) Endolymphatic duct and sac

14. S emicircular canal ducts Lateral or horizontal, Anterior or superior and Posterior. The semicircular canals are connected to the utricle at their bases. Oriented at right angles to one another. Near one end of each canal is widening known as ampulla , which contains crista ampullaris and cupula

15. Crista ampullaris Saddle shaped sensory organ of semicircular duct, contains sensory hair cells of vestibular system Crista consists of gelatinous mass, the cupula Ciliae of hair cells are embedded in the cupula Cupula act as membranous diaphragm stretching from crista to opposite wall of canal Cristae detects rotational acceleration of head

16. Crista ampullaris : Each semicircular canal works in concert with a partner located on the other side of the head, which has its hair cells aligned oppositely. There are three such pairs: the two pairs of horizontal canals, and the anterior canal on each side working with the posterior canal on the other side. Head rotation deforms the cupula in opposing directions for the two partners, resulting in opposite changes in their firing rates.

17. The otolith organs: Utricle & Saccule The sensory organs of the utricle and saccule are the maculae . Each macula consists of hair cells and supporting cells. The ciliary bundles of the hair cells project into the overlying gelatinous matrix known as the otolith membrane , which is studded with otoconia Otoliths –crystalline particles composed of caco3 surrounding proteinaceous core.( composed of protein unique to inner ear - otogelin , alpha and beta tectorins & caecam 16)

18. Macula It detect and respond to the position of the head with respect to li near acceleration and pull of gravity . Each macula contains thousands of hair cells (type1 & type2) that synapse with sensory endings of vestibular nerve Each hair cell has 5-10 small cilia called stereocilia plus one large cilium called kinocilium . The kinocilium is always located in one side, and the stereocilia gradually become shorter. Minute filaments connect the Tip of each sterocilium to the n ext longer cilia and finally to kinocilium .

19. Macula When stereocilia bend to the direction of kinocilium , it results in opening of K+ channels at the tip of the stereocilia , allowing K+ ions to enter and depolarize the hair cell. In respone to depolarization voltage-gated Ca+2 channels are activated allowing for Ca+2 influx and the subsequent liberation of transmitters to produce an action potential.

20. Macula Conversely, bending of stereocilia in the opposite direction (backward to the kinocilium ) reduces the tension on attachments and this closes the ion channels causing receptor hyperpolarization and inhibition of the cell.

21. Macula When the head is upright, the hairs project upward into the gelatinous material. When the head bends forward, backward, or to one side, the hair cells are stimulated as the gelatinous material of the maculae sag in response to gravity causing the hair to bend. Stimulated hair cells signal nerve fibers resulting in impulses traveling to the CNS through the vestibular branch of the vestibulocochlear nerve and informing the brain of the head’s new position. Brain responds by sending motor impulses to skeletal muscles to contract/relax to maintain balance.

22. Macula In the utricle- macula lies in the horizontal plane of the inferior surface of Utricle- so it determines the orientation of the head in upright position, - senses motion in the horizontal plane In the saccule , macula lies in the vertical plane and senses motion in the sagittal plane ( eg , up- down movement)

23. BPPV Otoliths may dislodge from maculae of utricle and settle within endolymph of one or more of the 3 semi circular canals , most commonly posterior scc When the position of head changes these displaced otoconia moves within the canal and sends false signal to the brain , causing vertigo Particle repositioning maneuvers

24. Endolymphatic sac and duct Membranous structure in the inner ear arise from posterior wall of the saccule . Formed by union of 2 ducts, from saccule and utricle Passes through the vestibular aqueduct and terminal part is dilated to form endolymphatic sac Endolymphatic sac lies wihin epidural space of posterior cranial fossa Papillary cystadenomatous tumours can arise from the endolymphatic sac( can occur bilaterally in Von Hippel L indau disease ) appear as retrolabyrinthine destructive masses

25. Endolymphatic sac and duct

26. The vestibular fluids: Perilymph Perilymphatic space- btw bony and membranous labyrinth It is similar to the extracellular fluid. ( low potassium, high sodium ). Formed by blood filtrate from capillaries of spiral ligament and direct continuation of csf Absorbed in the loose connective tissue of the modiolum

27. ..Vestibular fluids Endolymph In the membranous labyrinth. It is similar to the intracellular fluid ( high potassium, low sodium ). Secreted by stria vascularis of cochlea, planum semilunatum and dark cells of vestibule Absorbed by endolymphatic sac 2 types of endolymphatic circulation- longitudial pattern -radial flow pattern

28. MENIERE’S DISEASE ( e ndolymphatic hydrops ) Alteration in absorption or secretion of endolymph

29. Endolymphatic sac – surgical anatomy

30. Trautmann’s triangle Important landmark to reach posterior cranial fossa from mastoid. Solid angle

31. Internal auditory canal 8mm long 3.4mm diameter within petrous part of temporal bone Between posterior cranial fossa and inner ear Medial end- porus acousticus internus in the posterior cranial fossa Lateral end – abuts the bony labyrinth at its fundus Provide passage for CN 7 , CN 8 & Labyrinthine artery Contains vestibular ganglion

32. Internal auditory canal At the fundus the vestibular, facial and cochlear nerves are in a constant relationship that is determined by Horizontal – falciform crust Vertical- bill’s bar Bill’s bar is useful landmark in translabyrinthine surgery of cerebello-pontine angle as it seperates superior vestibular nerve from anteriorly located facial nerve

33. Vestibular nerves S uperior - superior and lateral scc , utricular macula, superior portion of saccular macula Inferior – inferior saccular macula, ampulla of posterior scc (posterior ampullary branch / singular nerve )

34. Vestibular schwannoma 80% OF CPA tumours , 10% of brain tumours Originate from the schwann cells of the vestibular nerve at the transition zone of central and peripheral myelin ( obsteiner redlich zone)- present in medial part of IAC Radiotherapy – gamma knife Surgery- approaches Middle cranial fossa Retrosigmoid Translabyrinthine

35. Scarpas’s ganglion Ganglion of the vestibular nerve Located inside the IAC Contains cell bodies of bipolar neurons whose peripheral processes form synaptic contact with hair cells of vestibular sensory organs Vestibular nerve carries the impulses to the vestibular nuclei in the medulla and to vestibulo - cerebellum

36. Venous drainage V eins accompanying the labyrinthine artery ,drain the inner ear , pass out through the IAC and drain into the transverse or inferior petrosal sinus.