Physiology of hearing. Vestibular analyzer

Main Components of the Hearing Mechanism: Divided into 4 parts (by function): Outer Ear Middle Ear Inner Ear Central Auditory Nervous System

Structures of the Outer Ear Auricle (Pinna) Gathers sound waves Aids in localization Amplifies sound approx. 5-6 dB

External Auditory Canal: Approx. 1 inch long “S” shaped Outer 1/3 surrounded by cartilage; inner 2/3 by mastoid bone Allows air to warm before reaching TM Isolates TM from physical damage Cerumen glands moisten/soften skin Presence of some cerumen is normal

Tympanic Membrane Thin membrane Forms boundary between outer and middle ear Vibrates in response to sound waves Changes acoustical energy into mechanical energy (From Merck Manual)

Eustachian Tube (AKA: “The Equalizer”) Mucous-lined, connects middle ear cavity to nasopharynx “Equalizes” air pressure in middle ear Normally closed, opens under certain conditions May allow a pathway for infection Children “grow out of” most middle ear problems as this tube lengthens and becomes more vertical

Stapedius Muscle Attaches to stapes Contracts in response to loud sounds; (the Acoustic Reflex) Changes stapes mode of vibration; makes it less efficient and reduce loudness perceived Built-in earplugs! Absent acoustic reflex could signal conductive loss or marked sensorineural loss

Structures of the Inner Ear: The Cochlea Snail shaped cavity within mastoid bone 2 ½ turns, 3 fluid-filled chambers Scala Media contains Organ of Corti Converts mechanical energy to electrical energy

Central Auditory System VIIIth Cranial Nerve or “Auditory Nerve” Bundle of nerve fibers (25-30K) Travels from cochlea through internal auditory meatus to skull cavity and brain stem Carry signals from cochlea to primary auditory cortex, with continuous processing along the way Auditory Cortex Wernicke’s Area within Temporal Lobe of the brain Sounds interpreted based on experience/association

Summary: How Sound Travels Through The Ear Acoustic energy, in the form of sound waves, is channeled into the ear canal by the pinna. Sound waves hit the tympanic membrane and cause it to vibrate, like a drum, changing it into mechanical energy. The malleus, which is attached to the tympanic membrane, starts the ossicles into motion. The stapes moves in and out of the oval window of the cochlea creating a fluid motion, or hydraulic energy. The fluid movement causes membranes in the Organ of Corti to shear against the hair cells. This creates an electrical signal which is sent up the Auditory Nerve to the brain. The brain interprets it as sound!

Organ Of Corti The end organ of hearing Contains stereocilia & receptor hair cells 3 rows OHC, 1 row IHC Tectorial and Basilar Membranes Cochlear fluids (From Augustana College, “Virtual Tour of the Ear”)

Vestibular Apparatus Vestibular apparatus and cochlea form the inner ear Vestibular apparatus – provides sense of equilibrium consists of otolith organs (utricle and saccule) and semicircular canals

Vestibular Apparatus Sensory structures located within membranous labyrinth filled with endolymph and located within bony labyrinth

Vestibular Apparatus Utricle and saccule provide info about linear acceleration Semicircular canals, oriented in 3 planes, give sense of angular acceleration

Vestibular Apparatus Hair cells – receptors for equilibrium Each contains 20-50 stereocilia (hair-like extensions) 1 of these is a kinocilium---a true cilium

Vestibular Apparatus Stereocilia bend toward kinocilium – hair cell depolarizes releases NT that stimulates CN VIII When bent away from kinocilium – hair cell hyperpolarizes In this way, frequency of APs in hair cells carries information about movement

When stereocilia are bend away from kinocilium, hair cell is hyperpolarized, i.e. inhibited. It occurs because acceleratory force acts to flow of fluid in semicircular canals during circular motion of the head or whole the body. Hair cells are located along crista ampularis and protect their cilia in cupula. Hair cells are secondary sensor cells, which synapse with neurons. Axons of these nerve cells compose vestibular nerve.

Utricle and Saccule Have a macula that contains hair cells Hair cells embedded in gelatinous otolithic membrane contains calcium carbonate crystals (otoliths) that resist change in movement

Utricle and Saccule Utricle sensitive to horizontal acceleration Hairs pushed backward during forward acceleration Saccule sensitive to vertical acceleration Hairs pushed upward when person descends

Semicircular Canals Provide information about rotational acceleration Project in 3 different planes Each contains a semicircular duct Crista ampullaris – where sensory hair cells are located 10-42

Semicircular canal function Ampula is enlargement at one end of semicircular canal. It has a small crest on top of which is a gelatinous mass known as cupula. Hair cells have two kinds of cilia – kinocilium and stereocilia. Kinocilium is large cilium located at one end of hair cell. Stereocilia are small. When stereocilia are bent towards kinocilium, hair cell is depolarized, i.e. stimulated.

Semicircular Canals Hair cell processes embedded in cupula of crista ampullaris When endolymph moves cupula moves Sensory processes bend in opposite direction of angular acceleration

Neural Pathways for Equilibrium and Balance

Nystagmus and Vertigo Vestibular nystagmus – involuntary oscillations of eyes occur when spinning person stops Eyes continue to move in direction opposite to spin, then jerk rapidly back to midline Vertigo – loss of equilibrium Natural response of vestibular apparatus Pathologically, may be caused by anything that alters firing rate of CN VIII Often caused by viral infection

Vestibular System Consists of three semi-circular canals Monitors the position of the head in space Controls balance Shares fluid with the cochlea Cochlea & Vestibular system comprise the inner ear

Vestibular System Consists of three semi-circular canals Monitors the position of the head in space Controls balance Shares fluid with the cochlea Cochlea & Vestibular system comprise the inner ear