Introduction:
The ear is the organ of hearing. The eighth cranial nerve carries sound sensations to the cerebral cortex of the brain, where interpretation occurs. Most parts of the ear are situated in the petrous portion of the temporal bone, except the pinna, which lies outside.
Anatomy of the Ear:
The ear is divided into three parts:
- External Ear
- Middle Ear
- Internal Ear
External Ear:
The external ear consists of the pinna and external auditory meatus.
Pinna or Auricle:
The pinna is the outer expanded part of the ear projecting from the side of the head. It is made of fibroelastic cartilage covered with skin. The expanded outer edge is known as the helix, and the lower part is called the lobule, composed of fibrous and adipose tissue with rich blood supply.
External Auditory Meatus:
This is an irregular tube about 2.5 cm long, extending from the pinna to the tympanic membrane (eardrum). It has two parts — a cartilaginous part (outer one-third) and an osseous part (inner two-thirds). Ceruminous glands in the cartilaginous portion secrete wax (cerumen) that traps dust or insects entering the ear.
Tympanic Membrane (Eardrum):
The eardrum separates the external ear from the middle ear. It is oval and composed of three layers:
- Outer layer – stratified epithelium
- Middle layer – fibrous tissue
- Inner layer – cuboidal epithelium
Middle Ear:
The middle ear or tympanic cavity is an air-filled cavity in the temporal bone. It communicates with the pharynx through the auditory (Eustachian) tube, which maintains equal air pressure on both sides of the eardrum, keeping it stretched for proper vibration.
The lateral wall is formed by the tympanic membrane, while the roof, floor, and posterior wall are made of the temporal bone. The posterior wall has a small opening leading to the mastoid air cells. The medial wall contains two openings:
- Oval window (fenestra vestibuli) – where the stapes fits.
- Round window (fenestra cochleae) – closed by a thin fibrous membrane.
Auditory Ossicles:
The middle ear contains three small bones called the auditory ossicles:
- Malleus (hammer) – attached to the eardrum.
- Incus (anvil) – connects malleus and stapes.
- Stapes (stirrup) – fits into the oval window.
These bones transmit and amplify sound vibrations from the eardrum to the inner ear.
Internal Ear:
The internal ear consists of a bony labyrinth and a membranous labyrinth. The space between them contains perilymph, and the membranous labyrinth contains endolymph.
The bony labyrinth has three main parts:
- Vestibule – central part connected with the oval window.
- Cochlea – a snail-shaped structure responsible for hearing.
- Semicircular Canals – responsible for balance.
The membranous labyrinth includes structures such as the utricle, saccule, and cochlear duct. Inside the cochlea, on the basilar membrane, lies the organ of Corti containing hair cells — the true organ of hearing. These cells connect to the auditory nerve (VIII cranial nerve), which carries impulses to the auditory cortex of the brain.
Physiology of Hearing:
Sound waves travel through the air at about 332 m/s. The auricle collects and directs them into the auditory canal, causing the tympanic membrane to vibrate. These vibrations are transmitted through the auditory ossicles, and the stapes moves the oval window, setting up pressure waves in the perilymph of the cochlea.
These waves stimulate the endolymph, which vibrates the basilar membrane, activating the hair cells of the organ of Corti. The resulting nerve impulses travel via the cochlear part of the vestibulocochlear nerve to the auditory area in the temporal lobe, where sound is perceived and interpreted.
Pitch and Volume:
Pitch depends on the frequency of sound waves (measured in Hertz, Hz), and different frequencies stimulate different parts of the basilar membrane. Volume (intensity) depends on the amplitude of the sound waves (measured in decibels, dB). Long-term exposure to loud sounds can damage the hair cells, leading to hearing loss.
Detailed Notes:
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