EAR

Origin: ectoderm.

Function:

• In auditory system - perception of sound
• In vestibular system - maintenance of balance.

Anatomic features

• External ear is composed of an auricle and external auditory meatus
• Middle ear is an air-filled tympanic cavity in the temporal bone with three small bones inside (auditory ossicles) and two openings in the medial wall - vestibular (oval) window and cochlear (round) window. It contains an auditory (Eustachian) tube
• Inner ear consists of bony and membranous labyrinths, one contained within the other
  • Bony labyrinth consists of three connected spaces within the temporal bone:
    • semicircular canals with dilations called ampullae
    • vestibule
    • cochlea

• Membranous labyrinth is divided into three subcompartments:
  • membranous semicircular ducts
  • utricle and saccule
  • membranous cochlea (cochlear duct)

• There are three fluid-filled spaces:

• Endolymphatic spaces (scala media) with endolymph, contained within the membranous labyrinth
• Perilymphatic space (scala vestibular and scala tympani) with perilymph, lying between the wall of the bony labyrinth and the wall of membranous labyrinth
• Cortilymphatic space with cortilymph, lying within the organ of Corti

AURICLE (PINNA)

Location: oval-shaped appendage on the lateral surface of the head.

Function: sound localization and amplification.

Composition: thin skin with hair follicles, sweat glands, and sebaceous glands covers supporting structure of elastic cartilage

EXTERAL AUDITORY MEATUS (CANAL)

Location: between the auricle and tympanic membrane

Composition: skin that contains hair follicles, ceruminous - modified sweat glands, and sebaceous glands covers supporting structure of elastic cartilage (lateral one-third) or bone (medial two-thirds)

TYMPANIC MEMBRANE (EARDRUM)

Location: separates the external auditory canal from the middle ear

Function: sound in the form of airwaves causes the membrane to vibrate, and these vibrations are transmitted to the attached auditory ossicles

Composition: the layers from outside to inside

• The epidermis of skin
• Radially and circularly arranged collagen fibers
• Mucous membrane covered by simple squamous epithelium

AUDITORY OSSICLES

Location: cross the space of middle ear in series and connect the tympanic membrane to the oval window

Function: help to convert sound waves (vibrations in air) to mechanical (hydraulic) vibrations in tissues and fluid-filled chambers

Composition: movable joints connect the bones:

• Malleus (hammer), attached to the tympanic membrane
• Stapes (stirrup), whose footplate fits into the oval window
• Incus (anvil), linking the malleus to the stapes

AUDITORY (EUSTACHIAN) TUBE

Location: connects the middle ear to the nasopharynx

Function: allow pressure in the middle ear to equilibrate with atmospheric pressure.

Composition: lined with ciliated pseudostratified columnar epithelium with small mass of lymphatic tissue

CRISTAE AMPULLARIS

Location: three sensory regions of the semicircular ducts in the ampullae of the semicircular canals

Function: sensors of angular movement

Composition: the ridge of epithelium that is oriented perpendicular to the long axis of the semicircular canal

Cells:

• Hair cells - nonneuronal mechnoreceptors with 50-100 stereocilia (sensory hairs) - modified microvilli. They are associated with both afferent and efferent nerve endings
  • Type I hair cells - piriform in shape with a rounded base
  • Type II hair cells - cylindrical in shape
• Supporting epithelial cells
• Cupula - gelatinous structure projects into the lumen and is surrounded by endolymph.

MACULAE

Location: saccule and utricle of the vestibule

Function: sensors of gravity and liner movement

Composition: the innervated sensory thickenings of the epithelium facing the endolymph

Cells:

• {#hair} Hair cells - nonneuronal mechanoreceptors with 50-100 stereocilia (sensory hairs) - modified microvilli and single true cilium - kinocilium. They are associated with both afferent and efferent nerve endings
  • Type I hair cells - piriform in shape with a rounded base
  • Type II hair cells - cylindrical in shape
• Supporting epithelial cells
• Otolithic membrane - gelatinous material with otoliths - 3-5mkm crystalline particles of calcium carbonate and protein

SCALA MEDIA (COCHLEA DUCT)

Location: the middle compartment in the cochlear canal

Composition: appears in transverse section as a triangular space.

• The most acute angle is attached to a bony extension of the modiolus
• The upper wall is the vestibular (Reissner's) membrane -two layers of squamous epithelial cells
• The lateral or outer wall is the stria vascularis - thick pseudostratified epithelium rich in blood vessels, the site of synthesis of endolymph
• The lower wall or floor is the basilar membrane - a dense mat of collagenic and some elastic fibers on which organ of Corti rests

ORGAN OF CORTI

Location: on the floor of scala media

Function: sound perception

Composition: complex epithelial layer

Cells:

• Inner (close to the stria vascularis) hair cells with 50-100 stereocilia and outer hair cells with 100-300 stereocilia are arranged in the rows of cells
• Inner (close to the stria vascularis) and outer phalangeal (supporting) cells - completely surround the basal portions of sensory cells, preventing them from touching the basilar membrane
• Pillar cells - form a triangular-shaped tunnel of Corti with cortilymph
• Tectorial membrane extends over the hair cells and attaches to the stereocilia. It consists of ground substance and collagen fibers