Anatomy and functions of the ear

Functions of the Ear

• Hearing relies on the transmission of vibration
• Equilibrium is maintained through balance

Structure of the Ear

• The ear is segmented into the outer, middle, and inner ear
• Auricle (Pinna)
• Temporal bone: Bone surrounding the ear, the Auricle is also apart of the temporal bone.

Outer Ear

• The auricle, or pinna, is made of cartilages and covered with skin
• Its function is to collect sound waves and funnel them into the ear canal.
• The ear canal or external auditory meatus is lined with skin and contains ceruminous glands.
• Ceruminous glands secrete cerumen, or ear wax.
• Cerumen protects the epithelial lining of the canal from infection
• Cerumen also repels water, discourages insects with its odor
• Further, cerumen helps cleanse and moisturize the epithelium of the ear canal
• Cerumen helps to trap dust, sand particles, and microorganisms

Middle Ear

• Inside an air filled cavity, the eardrum, or tympanic membrane, is stretched across the end of the ear canal.
• It vibrates when sound waves strike it
• The vibrations are transmitted to the three auditory bones: malleus, incus, and stapes
• The stapes transmits vibrations at the end of the oval window to the fluid-filled inner ear.
• Sound waves are amplified by vibration of ossicles during conduction from tympanic membrane to inner ear.
• The Eustachian tube (auditory tube) extends from the middle ear to the nasopharynx.
• This allows air to move in and out of the middle ear.
• Optimal hearing occurs when the tube permits eardrum to vibrate
• The air pressure in the middle ear should equal atmospheric pressure
• The Eustachian tube equalizes pressure during altitude changes and swallowing or yawning.
• The tube allows bacteria to pass and spread from the pharynx to the middle ear.
• Otitis media (in children) can come as complications of a strep throat

Inner Ear

• The inner ear consists of a fluid filled cavity inside called the bony labyrinth
• Lined with a membranous labyrinth.
• Perilymph: fluid between bone and membrane
• Endolymph: fluid within the membranous structures.
• The inner ear contains:
  • Vestibulocochlear nerve (8th cranial nerve)
  • Cochlear nerve
  • Scala tympani
  • Scala media
  • Scala vestibuli
  • Helicotrema
  • Oval and Round Window
  • Cochlea

Cochlea

• The cochlea is a snail shell shaped structure with 3 internal canals:
  • Scala vestibuli (vestibular duct)
  • Scala media (cochlear duct)
  • Scala tympani (tympanic duct)
• The cochlear duct contains receptors for hearing
• Hair cells (stereocilia) located in the organ of Corti, are cells containing the endings of the cochlear branch of the 8th cranial nerve
• These cells respond to sound waves and stimulate the cochlear nerve to transmit messages to the brain

Organ of Corti

• The Organ of Corti transmits waves and converts them to signals by bending

Physiology of Hearing

• A process:
• Sound waves enter the ear canal and stimulate vibration of the eardrum
• This is followed by malleus, incus, stapes, oval window of inner ear, perilymph and endolymph within the cochlea, and hair cells of the organ of Corti stimulate eardrum vibration
• When hair cells bend, impulses are generated and carried by the 8th cranial nerve to the auditory areas in the temporal lobes of the cerebral cortex. From there, sounds are heard and interpreted.
• The round window prevents pressure damage to the hair cells.

How Sounds Travel to the Inner Ear

• The movement of the eardrum causes bones to vibrate and sound makes the eardrum vibrate.
• The Stapes bone moves in and out like a piston into the inner ear.
• Fluid in the inner ear stimulates hair cells that communicate with the hearing nerve.
• The hearing nerve tells our brain we are hearing sound

Utricle and Saccule

• The Utricle and Saccule assist with the sense of balance.
• Membranous sacs in the vestibule, which is between the cochlea and semicircular canals
• Contains hair cells that are affected by gravity
• When position of the head changes, calcium carbonate bends the hair cells
• Impulses are generated along the vestibular branch of the 8th cranial nerve to the cerebellum, midbrain, and cerebrum.
• Impulses are then interpreted as position of the head at rest

Semicircular Canals

• 3 membranous ovals exist in three planes.
• An enlarged base is the ampulla, which contains hair cells (crista) that are affected by movement
• The body moves, hair cells bend in opposite direction
• Impulses are generated along the vestibular branch of 8th cranial nerve to the cerebellum, midbrain, and cerebrum.
• Those impulses are then interpreted as movement of the body, changing speed, stopping or starting.

Disorder of the Ear

• Otitis media: inflammation of the middle ear
• Otitis externa: inflammation of the external ear canal
• Deafness is the inability to hear properly
• Conduction deafness is the impairment of one of the structures that transmits vibrations.
• Nerve deafness is the impairment of the 8th cranial nerve or the receptors for hearing in the cochlea.

- Deafness

• Central deafness is damage to the auditory areas in the temporal lobes

Motion Sickness and other disorders

• Motion sickness is characterized by cold sweats, hyperventilation, nausea, vomiting sensation and discomfort during travelling.
• Presbycusis is the loss of the ability to distinguish between high sounds in elderly.