Internal Ear

  • Inner Ear Consists of an irregular system of canals and cavities:
  • Bony labyrinth(Hard, bony outer shell.)
  • Membranous labyrinth(Fully contained inside the bony labyrinth.)
Bony Labyrinth
  • Bony labyrinth is an example of cartilaginous bone (i.e. a bone which ossification cartilage).
  • Vestibule – central chamber of labyrinth. In its lateral wall lies the oval window.
  • Semicircular canals.
  • Cochlea.
  • Shortest part of VIIth cranial nerve
Membraneous Labyrinth
  • Cochlear duct
  • Utricle
  • Saccule
  • Three semicircular canals
  • Endolymphatic duct and sac:
  1. It is formed by union of saccule and utricle
  2. It connects scala media to subdural space
  3. Its terminal part is dilated to form the endolymphatic sac
  4. Endolymphatic sac lies between the two layers of dura on the posterior surface of petrous bone
  • Utricle
  1. Posterior part of vestibule.
  2. Horizontal acceleration with forward movement in the sagittal plane is detected by it.
  3. 5 openings of semicirular ducts.
  4. Macula
  5. Utricosaccular duct.
  • Saccule
  1. Lies anterior to utricle opposite the stapes footplate in the bony vestibule.
  2. Saccule is connected to the cochlea via a thin reunion duct.
  • Semicircular ducts
  1. 3 in number.
  2. Open into utricle
  3. Crista ampularis
  • Macula is a gelatinous matrix into which hair cells project and which is studded with tiny calcium carbonate granules called otoconia
  • Arrangement of stereocilia
  • Each row of stereocilia is taller than the next. The tip of each stereocilium is linked to the side of the stereocilium behind it by a tip link. Hair cells

Flask shaped cells with a rounded base, narrow neck. More columnar, kinocilium, sterocilia, cytoplasmic roganelles are similar to type 1.
Nucleus is basal, surrounded by mitochondria Golgi complex is larger, small vesicles found in great numbers in cytoplasm.
With supranuclear Golgi complex, occasional cisternae of RER & small vesicles. Synaptic ribbons are found in the peripheral cytoplasm opposite the plasmalemma of terminal boutons.
With 50-100 sterocilia on free surface.
Tallest hair is 10um near kinocilium & shortest is 1um on the opposite side.
Some endings contain clear synaptic vesicles (non granulated vesicles) carry afferent nerves info to brain.
  • Trautmann’s Triangle:The triangle bounded by the bony labyrinth, sigmoid sinus, and superior petrosal sinus is known as Trautmann’s triangle.
  • Cochlea:
  1. Snail shaped coiled tube.
  2. 2.5 to 2.75 turns round a central pyramid of bone called modiolus.
  3. 30 mm long
  4. 5 mm from base to apex & 9 mm around its base Anterio medial to vestibule
  5. Lumen of cochlear canal is partitioned into 3 spiral chambers:
  6. 1. scala vestibuli (above)Movement of stapes causes vibration in Scala vestibuli.
  7. 2. scala media (between):Filled with endolymph
  8. 3. scala tympani (below)
  • Modiolus
  1. Central(2, 3/4) pyramid of bone around which cochlea forms
  2. The base of modiolus directed towards internal acoustic meatus
  3. Transmits vessels and nerves to cochlea
  • Promontary:A bony bulge in the medial wall of middle ear , represents the basal coil of cochlea.
  1. Vestibule
  2. Central chamber
  3. Spherical recess-saccule
  4. Ellipticle recess-utricle
  • Osseous spiral lamina
  1. Bone winds spirally around modiolus .Gives attachment to the basilar membrane and divides the bony cochlea tube into three compartments.
  2. Basilar membrane,Vestibular membrane, Reissner’s membrane
  3. Rosenthal’s canal:Spiral ganglions are situated in this canal which runs along the osseous spiral lamina.
  • Aqueduct of cochlea
  1. Scala tympani of internal ear is connected with subarachnoid space via aqueduct of cochlea.
  2. It is thought to regulate perilymph & pressure in bony labrynth
  3. Basilar membrane:Has 2 zones:Zona arcuata, Zona pectinata
  4. Tectorial membrane:Secreted at the upper surface of interdental cells, forms a cuticular layer over these cells
  • Organ Of Corti
  1. The end organ of hearing situated in Scala media
  2. Inner hair cells - Broad base, resting on the basilar membrane, slender and conical, nucleus is basal.
  3. Outer hair cells -Longer and more oblique, leaning to the inner pillars.with broad thin base.Otoacoustic emissions arise from it. Sound produced by outer hair cells travels in a reverse direction:Outer hair cells→basilar membrane→ perilymph→ oval window→ ossicles→ tympanic membrane→ ear canal.
  4. Tunnel of corti- Wide triangular intercellular space continuous through cochlear length. Bounded above converging inner and outer pillar cells.
  5. Deiters cells -Supporting cells for the 3 -4 rows of outer hair cells. 
  6.  Base is columnar with cup shaped upper end.
  7.  The apex does not reach the free surface of the organ of Corti. Inner phalangeal cells - arranged in a row on the inner side of the inner pillar cells.
  8.  Contiguous with slender Border cells marking the inner boundary of the organ of corti. Lining epithelium is low cuboidal or squamous cells.
  9. Cells of Hensen- Delimit the outer border of the organ of Corti arrange in rows decreasing in height continuous with the cells of Claudius.Supports hair cells.
  • Internal auditory canal
  1. About 1 cm long
  2. Passes into petrous part of temporal bone in a lateral direction
  3. Lined by dura
  • Contents:
  • Vestibulocochlear Nerve
  • Facial nerve including nervus intermedius
  • Internal auditory artery and vein

Ant. Vestibular Common Cochlear
Main Cochlear Vestibulocochlear
cochlear Post. Vestibular
Venous drainage:
  • Internal auditory vein
  • Vein of cochlear aquaduct
  • Vein of vestibular aquaduct
  • Drain into inferior petrosal and sigmoid sinuses

Resembles ECF Resembles ICF
Rich in Na+ Rich in K+
Exam Question
  • Otoacoustic emissions arise from Outer hair cells
  • Shortest part of VIIth cranial nerve is labryinth canal
  • Scala media of the cochlea is filled with a fluid similar to intracellular fluid with high K+ and low Na+
  • Labyrinthine artery is a branch of Anterior inferior cerebellar artery
  • Vestible is the central chamber of internal ear
  • Promontory seen in the middle ear is Basal turn of cochlea
  • Inner ear bony labyrinth is Cartilaginous bone
  • Cochlear aqueduct Connects internal ear with subarachnoid space
  • Organ of corti is situated in Scala media
  • Endolymphatic duct connects scala media to subdural space
  • Bony labyrinth, sigmoid sinus, and superior petrosal sinus are the boundaries of Trautmann's triangle
  • Vestibule, Senicircular canal & cochlea are the structures included in bony labyrinth
  • Modiolus is the Central(2, 3/4) pyramid of bone around which cochlea forms
  • Sense organ for hearing is Organ of Corti
  • Hair cell of organ of Corti is supported by Deiters & Hensen Cells
  • Endolymph is rich in K+
  • Horizontal acceleration with forward movement in the sagittal plane is detected by utricle
  • Movement of stapes causes vibration in Scala vestibuli.
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