Internal ear

Otoacoustic Emissions (OAEs)

They are low intensity sounds produced by outer hair cells of a normal cochlea and can be elicited by a very sensitive microphone placed in the external ear canal and an analysis by a computer. Sound produced by outer hair cells travels in a reverse direction:

Outer hair cells→basilar membrane→ perilymph→ oval window→ ossicles→ tympanic membrane→ ear canal.

The cochlea of the inner ear is a coiled structure filled with endolymph and perilymph. Within it are three longitudinal compartments. The membranous labyrinth within the cochlea is called the scala media, or cochlear duct. This compartment contains the hair receptors and is filled with endolymph that is secreted by the stria vascularis. The composition of endolymph resembles intracellular fluid (K+> Na+).

The labyrinthine artery (auditory artery, internal auditory artery), a long slender branch of the anterior inferior cerebellar artery (85%-100% cases) or basilar artery.

C i.e. Vestible is the central chamber

Bony labyrinth is an example of cartilaginous bone (i.e. a bone which ossification cartilage).

Endolymphatic duct – It is a part of membranous labyrinth (Scala media)

• It is formed by union of saccule and utricle
• It connects scala media to subdural space
• Its terminal part is dilated to form the endolymphatic sac
• Endolymphatic sac lies between the two layers of dura on the posterior surface of petrous bone
• Surgical importance – Endolymphatic sac is exposed for drainage or shunt operation in Meniere’s disease

Trautmann’s triangle is bounded by the bony labyrinth anteriorly, sigmoid sinus posteriorly and the dura or superior petrosal sinus superiorly

Supporting cells in organ of corti are PHD i.e. Pillar cells, Hensen cells and Dieter cells.

Hellar cells are ethmoidal air cell that extend along the medial roof of the maxillary sinus.

They may exist as a discrete cells or the may open into maxillary sinus or infundibulum                                

Ans. is ‘d’ i.e., K⁺

Fluid in inner ear

• There are two main fluids in the inner ear : –

(i) Perilymph                  

(ii) Endolymph

Perilymph

• It resembles ECF and is rich in Nu⁺ ions. It fills the space between bony and the membranous labyrinth, i.e., Scala vestibuli and scala tympani. It communicates with CSF through the aqueduct of Cochlea which opens into the scala tympani near the round window. Therefore It closely resembles CSF. It is formed by : –

i) It is a filterate of blood serum and is formed by capillaries of the spiral ligament.

ii) It is a direct continuation of CSF and reaches the labyrinth via aqueduct of cochlea.

Endolymph

It fills the entire membranous labyrinth including scala media (cochlear duct). It resembles intracellular fluid, being rich in IC ions. It is secreted by the secretory cells of the stria vascularis of the cochlea and by the dark cells (present in the utricle and near the ampullated ends of semicircular ducts).

Ans. is ‘a’ i.e., Macula of utricle

The structure marked as A in the picture above represents the stapes.

Movement of stapes causes vibration in Scala vestibuli.