Lecture 18; Sound 2 Flashcards
How do we not hear all sound?
The outer and middle ear filter out sounds that dont need to be heard, essentially to focus on speech
Excludes a lot of other sounds and tries to focus on what needs to be heard
How does the pina work?
It reflects and deflects sound, filtering it, enabling sound to be heard in space
What is the role of the outer ear?
- Important for collecting sounds
- Protects middle ear
- Resonances increases sound pressure at the TM (esp sound Hz of speech)
- Sound localisation
- Skin lining contains serum glands (wax) for self cleaning
What is special about the ear canal epithelium?
It migrates
Describe ear canal epithelium migration;
“Epithelial Conveyor Belt”
- Skin growth/ production starts at the enter of the ear drum and moves along canal
- Similar speed to fingernail growth
What produces ear wax and what is its function?
- Produced my sebaceous and sweat glands
- Very important, has antimicrobial properties(antibodies) and helps clean the ear, as it carried out along with cells and debris
- Lubrication
- Humidifies
- Repels water
- With hairs it traps dust and debris
Describe the pressure gain of the outer ear;
- The outer ear is a resonator and there is a pressure gain b/w start and finish of ear canal
- 1/4 wave resonator with peak gain ~ 2-3khz
~20db gain
Describe the location of the eustachian tube;
Drains middle ear into nasopharynx
Describe how the eustachian tube functions;
- The middle ear contains mucosal layer and produces a lot of mucous which is drained down through the eustachian tube which has a ciliated lining to help with this.
- Maintains pressure of across the ear drum (as the TM moves)
Describe the TM strucutre:
Three layers;
- Epithelial layer, continuous with outer ear canal
- Fibrous layer, concentric or circular rings of collagen, or radial
- Mucosal membrane layer, continous with middle ear
Whats special about the TM?
It can self repair
- Designed not to lose much energy
What is the TM connected to?
The long arm of the malleous/ ossicular chain
What are the bones of the ossicular tree?
Malleous
Incus
Stapes
Suspended by ligaments
They’re articulating joints
Why is there an ossicular tree? Why not just a membrane?
- Bone conduction; (very good) vibration of the skull transfers energy to the cochlear sensory cells directly
- Less sensitive (40-50db) than air conduction, but very important for monitoring voices
Why do we have a middle ear and not just air conduction?
- The inner ear has a lot of impendance and therefore the middle ear needs to transfer a lot of energy through the oval window, thus if it was just air conduction (not ossicles) you would suffer a hearing loss of 30-40db as it requires little energy for sound to move through air. (also why the outer ear acts as a resonator)
Whats the function of the middle ear?
- Filters sound so that only hz (voices) you want to detect are heard
- Middle ear acts as a transformer to overcome resistance of inner ear fluid.
How does the middle ear act as a transformer?
- Area difference between TM and stapes (oval window) (F = P.A, so if A decreases P inc) (20 fold inc)
- Lever ratio of malleous(longer);incus(shorter) 1.5 increased
- Lever ratio of TM (the ear drum is connicle), large movement of TM, small more forceful oval window)
How much of the energy does the middle ear transfer?
90%, so 1-1.5db loss instead of 40db loss
Describe energy loss and sound filtering;
Transfer of energy across frequencies is not equivalent but greatest over speech frequencies
Gets rid of low and high hz
How does the middle ear filter sound
To do with the relationship between mass and stiffness;
Stiffness;
- Ossicular ligaments
- Eardrum
- Air in middle ear space
Mass
- Ossicular chain
What are the muscles of the inner ear?
Tensor tympani
Stapedius
What is the tensor tympani?
- Attached to membrane of malleous
- innervated by trigeminal nerve
- tenses ear drum (attenuation relfex)
What is the stapedius muscle?
- Attatched to stapes head
- Smallest straited muscle in bod
- Innervated by facial nerve
- Reflex initiated by loud sounds, limits excess movement by stapes (attenuation relfexes)
Also important for focusing on sounds in oud environment
Where is the inner ear located?
Deep within the temporal bone
What are the parts of the inner ear?
Vestibular system
Cochlear (coiled tube)
Where is the oval window/ stapes insertion?
The oval window is in the vestibular system
… so how can we hear?
What are the two fluids systems of the inner ear that are shared by the cochlear and vestibular system?
- Perilymphatic fluid
- Endolympatic fluid (has sac and duct)
Both connect with CSF to maintain fluid concentrations
Describe perilymph;
Found in the big ducts (scala tympani, scala vestibuli) of the cochlear, similar to that of normal ECF
Na = 150mm K = 5-10mm
Describe the tubes of the cochlear
Scala vestibule
- Scala media ( organ of corti in here)
Scala tympani
Describe endolymph;
Na 10mm
K 140mm
Scala media
Essential for endocochlear potential - generated by the stria vascularis(Potential ~80mV)
What maintains the fluid concentrations of the cochlear?
Stria vascularis generates this fluid concentrations
What seperates scala media from scala vestibule?
Reissners membrnae
Describe the organ of corti;
Tectorali membrane
IHC 1 row
-Piller cells (separates sensory cells)
OHC 3-5 rows
- Dieter cells
Basilar membrane (fibrous collagen)
Describe sensory cell innervation
IHC (lots of afferent innervation, some efferent) (predominant input)
OHC (lots of efferent innervation, some afferent) (frequency tuning)
Describe sensory cell projections;
plates of stereocilia,
Movement of these lead to transduction
Describes how the inner ear allows fluid movement?
The round window allows movement of the fluid, pressure relief valve, so the stapes and oval window can may and generate movement of the tectorali membrane.
