Session 8 - The Inner Ear Flashcards
Outline the nature of sound (type of wave, speed and how it is measured).
Sound is a Compressive Wave, which travels at 343m/sec in air and at over 1,500m/sec in water.
Sound has two properties:
o Frequency – Hertz (Hz)
o Volume – Decibels (dB)
What frequencies are detected by the ear?
Sound is a Compressive Wave,
Detected by cochlear at ranges between 20 and 20,000 Hz.
How can level of funciton of the tympanic membrane be assesed?
o Level of function can be measured via tympanometry, which involves insertion of probe into external acoustic meatus and generation of sound at 226Hz. If more reflected back from tympanic membrane than normal range suggests then there may be some hardening of tympanic membrane.
Give two parts of tympanic membrane
Pars flaccida
Pars tensa
What are three funcitions of the eustachian tube?
o Equalisation of air pressure in middle ear
o Mucus drainage from middle ear
o Closing insulates middle ear from loud sounds
What three muscles hold open the eustachian tube?
Salpingopharyngeus
Levator veli palantini
Tensor veli palantini
What happens if mastoiditis breaks through superior wall?
breaking into middle cranial fossa, affecting the meninges and temporal lobe of the brain, causing meningitis or abscess.
What happens if posterior cranial fossa is the site of mastoid breakthrough?
breaking into middle cranial fossa, affecting the meninges and temporal lobe of the brain, causing meningitis or abscess.
Name the five contents of the inner ear
Vestibule Semi-circular ducts and canals Cochlea Cochlear duct Organ of corti
What is the vestibule and what does it detect?
Small bony chamber, containing the Utricle and Saccule, which are sensitive to rotational acceleration and the static pull of gravity
What is the semi-circular duct and canal, and what does it detect?
Communicate with the vestibule
Contain receptors that respond to Rotational Acceleration in three different planes
What is the chochlea?
Shell shaped portion of the bony labyrinth containing the Cochlear Duct
What is the cochlear duct?
Accommodates the spiral Organ of Corti
What is the organ of corti?
Contains the receptors of the auditory apparatus
Outline the structure of the cochlea duct
- Scala vestibule
- ——-vestibular membrane
- ——-basilar membrane
- Scala tympani
What is the scala media filled with?
Endolymph
What separates scala media from the basilar membrane?
Vestibular membrane
What is found on top of the basilar membrane?
Spiral organ of corti
What is the principal sensory receptor of the spiral organ of corti?
Inner hair cells
How many nerves attached to each inner hair cell
20
What do outer hair cels odo
Serve as amplifiers
Outline the mechanotransduction of sound
o Vibrations of tympanic membrane are transmitted via ossicles to oval window
o Vibrations of the stapes are converted to pressure waves in the Scala Vestibuli (upper compartment)
o The pressure waves are transmitted through the Vestibular Membrane to reach the Basilar Membrane
o In response to local resonance causing the bending of their Stereocilia, Hair cell K+ channels open
o Influx of K+ ions from endolymph
Endolymph = 140mM[K+]O compared to 5mM normally
o Depolarisation of Hair Cells
o Opening of Voltage Gated Ca2+ channels
o Influx of Ca2+, triggering neurotransmitter release onto afferent nerves of the Spiral Ganglia
How is endolymph different from normal ECF?
140mM of K+ compared to 5
How does the inner ear distinguish between loud and quiet sounds?
o High frequency pressure waves, caused by high pitched sounds cause the short fibres of the basilar membrane at the base of the cochlea to resonate
o Low frequency pressure waves, caused by low pitched sounds cause the long fibres of the basilar membrane at the apex of the cochlea to resonate
How is volume moderated in hearing?
A louder sound produces more action potentials and in a larger number of axons.
Define the central auditory pathway to the brainstem and cerebral cortex
Cochlea -> Spiral ganglia -> Cochlear nucleus -> (decussation) -> Superior olivary complex -> Inferior colliculus -> Medial geniculate nucleus -> Primary auditory cortex
Name 7 causes of hearing loss
Age Loud noises Congenital defects Infections Otoxic drugs Trauma Meniere's disease
Give three congenital defects associated with hearing impairment (inheritance pattern & defect type)
DFN – X-linked – Hair cell defect
DFNA – Autosomal dominant – Tectorial membrane proteins
DFNB – Autosomal recessive – Gap junction proteins
What infection causes hearing impairmetn
Rubella
Name two drugs which damage hearing
Aminoglycosides
Furosemide
Give four ways in which hearing can be assesed
o Visual inspection Otoscope o Audiographs Plot sensitivity against frequency o Otoacoustic emission o Auditory Brainstem Response (ABR)
Give four causes of conductive hearing loss
Blockage
Ruptured tympanic membrane
Fluid Accumulation (Otitis Media)
Otosclerosis (progressive ossicles immobilisation)
Give three causes of sensory hearing loss
Hair cell destruction (Physical, noise related)
Hair cell death (Ototoxicity)
Menieres disease (too much endolymph)
Give four causes of neural hearing loss
Spiral Ganglion Damage (E.g. Acoustic neuroma)
Tinnitus (Phantom, ringing sound)
Auditory Neuropathy (associated with neonatal jaundice)
Monayrak deafness destroys ability to localise a sound
What is your diagnosis in a pateint wil loss of balance, facial palsy and the loss of corneal reflex
acoustic neuroma (benign tumour of schwanna cells) at the cerebellopontine angle, due its dual compression of CN VIII and CN VII.
