THE AUDITORY SYSTEM Flashcards
The auditory system
m is a part of the sensory system that account on vision, hearing, touch, olfaction and
taste. The Auditory System is responsible for sense of earing and balance.
The ear
The organ of the auditory system is the ear. It detects sound and aids in body equilibrium of balance and
body position. Anatomically we divide the ear into three parts:
1. Outer ear.
2. Middle ear.
3. Inner ear.
The Outer Ear
The outer ear is the external portion of the ear whose function is to collect sound waves and channel them
to the tympanic membrane (the eardrum).
Structure:
1. The pinna (auricle) – captures sound waves. It is the visible part of the ear that is composed mostly
of cartilage.
2. External auditory canal – a canal that concentrate and conduct sound waves.
3. Tympanic membrane (eardrum) – a thin membrane that separates the outer ear from the middle
ear. Its function is to transmit sound from the air to the ossicles inside the middle ear.
The Middle Ear
The middle ear is an air filled cavity. It is the portion internal to the eardrum and external to the oval
window of the inner ear. The middle ear contains three small bones known as osiccles. Osiccles transfer
the air vibration of the eardrum into fluid membrane wave in the inner ear.
The ossicles are:
* Malleus (hammer).
* Incus (anvil).
* Stapes (stirrup)
the Eustachian (auditory) tube and its continueasion
The middle ear opens into the Eustachian (auditory) tube. It connects the middle ear with the nasopharynx
(nasal cavity) and equalizes the pressure between the middle ear and the atmosphere.
The wall that separates the middle and the inner ears has two opening called fenestrae:
* Oval window – upper membrane covered opening beneath the stapes. The stapes transfer
vibration from the eardrum to the oval window.
* Round window – lower membrane covered opening. Where the soundwaves go out of the inner
ear.
The Inner Ear
The inner ear, also called the labyrinth, is the internal portion of the ear. Consisting of fluid filled chambers
whose function include sound detection and balance.
The inner ear contains:
* Vestibule.
* Semicircular canals.
* Cochlea.
Function units of the inner ear:
1. Vestibular System – accounts for balance equilibrium, consisting of vestibule and 3 semicircular
canals.
2. Cochlear system – account for sound detection, consisting of the snail like cochlea.
The Vestibular System
The vestibular system functions in the balance and equilibrium. It consists of two structures.
- Vestibule.
- Three semicircular canal.
Vestibule
Consists of two membranous sacs that respond to gravitational forces.
1. Utricle.
2. Saccule.
The utricle and the saccule have a macula. The macula is a patch of sensory cell called hair cells, which
when stimulated, they alter the rate of nerve impulses being sent to the brain.
Covering the macula is the otolith – a gelatinous layer covered by calcium carbonate crystals which tend to
get displaced during linear acceleration (when we move our head), causing stimulation of macula hair cells. Thus, producing a sensory signal.
Semicircular Canals
Three circular bony canal all lying within 90 degrees of each other. They contain membranous sacs filled
with fluid known as endolymph that detects changes in angular movements.
Each canal has an ampulla which contains mound of hair cells as well, supporting cells knowns as crista
ampullaris (the sensory organ od rotation.
Surrounding the crista ampullaris is gelatinous layer called cupula. Angular movement cause the cupula to
move and innervate the hair cells and crista ampullaris which can increase or decrease the rate of nerve
impulses carried to the brain.
The Cochlear System
The cochlea is a snail like structure which functions in sound detection. It has three scale/chambers.
- Scala vestibule (superior)
- Scala tympani (inferior)
- Scala media (cochlear duct)
Scala vestibule (superior)
Separated from the cochlear duct by vestibular membrane. It contains
a fluid known as perilymph.
Scala tympani (inferior)
separated from the cochlear duct by basilar membrane. Also contains
the same fluid.
Scala media (cochlear duct) –
The space between the vestibular membrane and basilar membrane.
Contain endolymph fluid and the organ of Corti.
* Organ of Corti – rest on the basilar membrane and contains ciliated hair cells that give rise to nerve signals in repose to sound vibration. Movement causes the tectorial membrane that sits above the hair cells to slide over the hair cells that in turn causes innervation of the auditory nerve.
Sound Transduction
Process done by few steps:
1. Sound waves are directed toward the ear canal by the auricle (pinna).
2. The waves that enter the auditory canal are concentrated and made to strike against the tympanic
membrane.
3. Vibration picked up by the malleus are transmitted to the oval window via the incus and stapes.
4. The vibration travels along the scala vestibule to the end of the cochlea (helicotrema – where scala
tympani and vestibule meet) then to the scala tympani.
5. The vibrations from the basilar membrane, causing the sensory hair cells of the organ of Corti to
move, and in turn, it transmit action potential to the auditory nerve fibers.
6. The action potential is then transmitted via the auditory nerve to the auditory cortex of the brain
that is located in the temporal lobe
Sound waves
Lower sound waves reach deeper in the cochlea, while higher waves reach only to the outer parts.
Therefore, the hair cells that sense the waves in the outer part of the cochlea are more often to get
damaged. Resulting in the loss of earing of high sounds.
