The Internal Ear

Consists of cavities and canals filled with perilymp = bony labyrinth
It also contains sacs and tubes filled with endolymph = membranous labyrinth
The membranous labyrinth is located in the body labyrinth and is attached to it via connective tissue strands or thickenings of periosteum.

Fig. 12.1 Anatomy of the ear
Stevens and Lowe p.187

Bony Labyrinth

The oval window leads to the vestibule
Six openings are found in the vestibule:
1. posterior 5 for semicircular canals
2. anterior one for cochlea
The bony labyrinth is lined with a layer of squamous epithelium resting on a thin lamina propria that is fused with the periosteum

Membranous Labyrinth

Consists of two sacks:
1. the smaller anterior saccule
2. larger posterior utricle
These two sacks are joined via the two short legs of a Y-shaped tube
The long leg of the Y is formed by the endolymphatic duct
The cochlear duct emerges from the anterior side of the saccule and the semicircular canals emerge from the posterior side of the utricle

Cochlea and Cochlear Duct

The cochlear duct has 3 walls:

1. Lower wall
- Formed by the basilar membrane that extends from the edge of the osseous spiral lamina to the outer wall
- The organ of Corti is located on this membrane
2. Upper wall
- Formed by the vestibular/Reissner's membrane thar extands from the spiral limbus to the outer wall
3. Outer wall
- Formed by the spiral ligament and specialised epithelium

Microscopic Structures of the Cochlear Duct

Vestibular membrane

Extremely thin
Consists of two layers of epithelial cells with a small amount of connective tissue inbetween

Fig. 12.4 Cochlea
Stevens and Lowe p. 190

Spiral Ligament

Is a thickening in the connective tissue layer of the periosteum
Near the bottom of the cochlear duct a spiral groove, the external spiral sulcus, is located.
The ridge above this groove is called the prominentia spiralis and the lower ridge is called the crista basilaris

Stria Vascularis

Consists of specialised epithelium
Capillaries are present between the epithelium
Produces the endolymph

Spiral Organ of Corti

Located on the basilar membrane
The periosteum near the outer edge of the spiral lamina thickens to form the spiral limbus which is covered by a layer of epithelial cells
The upper edge of the limbus extends for a short distance, forming the vestibular lip
Between the lip and basilar membrane an internal spiral sulcus is formed and the tectorial membrane is attached to the upper surface of the limbus
The tectorial membrane consists of a gelatinous matrix in which thin keratinous fibers are embedded and it extends over the organ of Corti
The epithelium on the limbus continues as a single layer of squamous or cuboidal epithelial cells and then become taller to form the spiral ridge - spiral organ of Corti
The organ of Corti consists of two types of cells:
1. hair cells - receptors
2. supporting cells - supports the hair cells
The cells of the organ of Corti from inside outward are the following:
1. Border cells - cuboidal to columnar
2. Inner hair cells - single row of flask-shaped cells with microvilli arranged in a C or U shape on their free surface
3. Phalangeal/inner supporting cells - single row of cells that surrounds the inner hair cells.
4. Two rows of pillar cells - supporting cells with broad bases that forms the tunnel of Corti
5. 3-5 rows of outer hair cells - cylindrical shaped cells with microvilli arranged in a Y or W shape on the free surface
6. The outer hair cells rests on the Deiter's cells/outer phalangeal cells - Deiter cells are tall and their bases cylinder-shaped and their apical surfaces are forked
7. Seven rows of supporting cells that decrease in height
8. Cells of Claudius - cuboidal cells that do not form part of the organ of Corti
9. Boettcher's cells - beneath the cells of Cladius and they possibly absorbs endolymph

Fig. 12.5 Organ of Corti

Stevens and Lowe p.191

Fig. 12.5 Organ of Corti
Stevens and Lowe p.191

Fig. 24.17 Photomicrograph of the vestibular duct and spiral organ of Corti

Ross et al. p832

The Basilar membrane
- Two sections:

1. Pars arcuata - thin and contains blood vessels

2. Pars pectinata - contains no blood vessels

Utricle, Saccule and Semi-circular canals

- The walls of these structures are thin and consists of vascular connective tissue

- On the inside the connective tissue is lined with a layer of squamous or cuboidal epithelium

- On the lateral wall of the utricle and medial wall of the saccule, a thickening of the inner wall occurs - the maculae

- In the ampullae of the semicircular canals are transverse ridges - cristae ampullares

- The macule and cristae are called the vestibular organ because they register equilibrium.

Fig. 24.17 Photomicrograph of the vestibular duct and spiral organ of Corti
Ross et al. p832

Fig. 24.21 Structure of the maculae

Junqueira and Carneiro p. 484

Fig. 24.21 Structure of the maculae
Junqueira and Carneiro p. 484

Fig. 24.23 Crista ampullaris

Junqueira and Carneiro p. 485

Fig. 24.23 Crista ampullaris
Junqueira and Carneiro p. 485

Steps involved in hearing speech
Sound signals travel from the ear to the brain where they are recognized and decoded

Public domain: Zina Deretsky, National Science Foundation