Internal Ear

Internal Ear consists of two labyrinthine compartments, one contained within the other (bony labyrinth and membranous labyrinth), and three fluid filled spaces:

• Endolymphatic space, contained within the membranous labyrinth. Endolymph circulate here (similar in composition to intracellular fluid, high K+, low Na+).
• Perilymphatic space, which lies between the wall of the bony labyrinth and the wall of the membranous labyrinth. Perilymph circulates here (similar in composition to extracellular fluid, low K+, high Na+).
• Cortilymphatic space, which lies within the tunnel of Corti. It is a true intercellular space. Cortilymph circulates here (similar in composition to extracellular fluid).

Bony labyrinth consists of three connected spaces within the temporal bone:

• Vestibule is a small oval chamber located in the center of the bony labyrinth. Semicircular canals extend posteriorly from the vestibule. The cochlea extends anteriorly from the vestibule, and it houses the saccule and utricle. It also contains oval (where stapes is related) and round window.
• Semicircular canals are 3 bony walled tubes that lie at right angles to each other: anterior, posterior, and lateral. They open into the vestibule. The end of each semicircular canal nearest to the vestibule is expanded to form the ampulla (3 ampullae).
• Cochlea is a cone-shaped helix connected to the vestibule. Between its base and apex, it makes 2.75 turns around a central core of spongy bone called the modiolus. A sensory ganglion called the spiral ganglion lies in the modiolus (cell bodies of the auditory nerve).

The structure of the membranous labyrinth consists of a series of communicating sacs and ducts containing endolymph. It is suspended within the bony labyrinth. The space between the membranous labyrinth and the bony labyrinth is filled with perilymph. There sensory cells are located in six regions:

• Three cristae ampullaris (ampullary crests) are located in the membranous ampullae of the semicircular ducts. They are sensitive to angular acceleration of the head (turning the head). They lie in the ampulla of the semicircular canal and have a thickened transverse epithelial ridge. They are oriented perpendicular to the long axis of the semicircular canal. They consist of hair cells with stereocilia and supporting cells. Each crista has a gelatinous protein-polysaccharide mass attached to the hair cells. The cupula projects into the lumen, is cone shaped with no otoliths, and surrounded by endolymph.
• During rotational movement of the head, semicircular canals and semicircular ducts move. Endolymph does not move. The cupula projecting into the endolymph is swayed by movement differential between the crista fixed to the wall of the duct and the endolymph. Deflection of the stereocilia of the hair cells generate nerve impulses.
• Two maculae are located in the utricle (macula of utricle) and saccule (macula of saccule). They sense the position of the head and its linear movement. It is innervated by the vestibular nerve. It consists of:
• Type I hair cells are flask shaped, rounded base, thin neck. They are surrounded by an afferent nerve chalice (cup-shaped structure) and a few efferent nerves.
• Type II hair cells are cylindrical. Afferent and efferent bouton nerve endings at the base. Similar to type I, they have rows of stereocilia called sensory hairs. Each row of stereocilia increases in height. Behind the row of the longest stereocilia a single true cilium is present (Kinocilium).
• Supporting cells
• Afferent and efferent nerve endings
It also have an otolithic membrane that overlies the stereocilia of the cells: A gelatinous membrane of glycosaminoglycans that contains crystals of calcium carbonate and proteins (otoliths or otoconia). Macula of utricle and saccule are oriented at right angles to each other.
• Spiral organ of Corti projects into the endolymph of the cochlear duct. It is a sound receptor. The Cochlear duct divides the cochlear canal into 3 parallel compartments or scalae:
• Scala media, which is the middle compartment in the cochlear canal. It is the cochlear duct itself and is attached to the osseous spiral lamina (bony extension of the modiolus). It contains endolymph and is continuous with lumen of the saccule. It contains the Organ of Corti (sensor of sound vibrations).
• Scala vestibuli, which begins at the oval window. It is the space above the scala media and contains perilymph.
• Scala tympani, which ends at the round window. It is the space below the scala media and contains perilymph.
Scala vestibuli and scala tympani communicate with each other at the apex of the cochlea through a small channel (helicotrema). It contains:
• Hair cells, which can be divided into inner hair cells (close to the spiral lamina) and outer hair cells (farther from the spiral lamina). Both types have stereocilia. Inner hair cells are sensory receptors, and outer hair cells can contract and amplify the movement of the basilar membrane.
• Phalangeal cells (supporting cells), which can be inner or outer. Inner phalangeal cells are associated with inner hair cells, and the outer phalangeal cells are associated with outer hair cells. The latter type sends apical processes toward the endolymphatic space.
• Pillar cells, which can be inner or outer. Inner pillar cells rest on the tympanic lip of spiral lamina, and the outer pillar cells rest on the basilar membrane. Between the two sets of pillar cells is a triangular-shaped tunnel, called inner tunnel of corti (filled with cortilymph).
The tectorial membrane of the Organ of Corti is attached medially to the modiolus. The lateral free edge extends over and attaches to Organ of Corti by stereocilia of inner and outer hair cells. It consists of type II, V and IX collagen embedded in amorphous ground substance. It also contains glycoproteins similar to those in the otolithic membrane and cupula.

Neural Transduction of sound production. Hair cells are attached through phalangeal cells to the basilar membrane, which vibrates during sound reception. Stereocilia of these hair cells are also attached to the tectorial membrane, which also vibrates. Stereocilia are therefore the only structures that connect the basilar membrane and its complex epithelial layer to the tectorial membrane. Tectorial and basilar membranes are hinged at different points, thus a shearing effect occurs between basilar and tectorial membranes when sound vibrations enter the inner ear. Shearing effect of both membranes distorts the stereocilia of hair cells. The apical membranes of hair cells ultimately depolarize.

• It results in neurotransmitter release ⇒ action potentials in bipolar neurons of spiral ganglion ⇒ cochlear nerve / cochlear division of cranial nerve VIII