Lecture 19: Hearing 2 Flashcards
Function of the outer ear?
- Collects sounds
- protects the middle ear 2.5-3cm
- important for sound localisation
- Skin lining contains cerumen glands (wax)
- Is self-cleansing
Organisation of the middle ear?
Three ossicles that connect the tymanic membrane to the oval window, held in place by ligaments. Mucoasl lining that is ciliated pushing mucus down the eustachain tube to the back of the throat. Glue ear occurs when this part of the ear becomes blocked and the eustachian tube stops functioning.
Transfer of sound to the inner ear? Associated structures?
Malleus attcahed to the Tympanic membrane
Incus attaches in the middle
stapes attaches to the incus and the oval window
- All have synovial joints between the bones
- Tensor tympani attaches to Malleus and is innervated by Trigeminal nerve
- Stapedius muscle attaches to the head of the stapes and is innervated by the facial nerve.
- These two muscles stiffen the chain reducing the amount of energy passing through the middle ear.
Function of the Eustachian tube?
Drains the midle ear to the nasopharynx
Ciliated mucosa lines the middle ear to assist with drainage
Maintains air pressure across the eardrum
- In children this tube can be quite flat and so doesn’t drain nearly as well as in adults
Function of the middle ear? Why do we have it?
2 mechanisms of increasing the sound transfer?
Comparison of middle ear transfer to bone conduction.
Air conduction of sound by dispacement of eardrum and ossicular chain
Acts as transformer to match low impedance of air to high impedance of inner ear fluid. Increase transfer of sound to the inner ear allowing hearing of much quieter sounds than otherwise detectable.
- Greater area of eardrum transfers more pressure on small stapes footplate
- Malleus is 1.5x the incus in length generating a greater force at the stapes.
NB: bone conduction of sound through the skull. Is less sensitive than air conduction but still very important.
Structure of the inner ear?
A series of tubes within the temporal bone
Consists of the cochlea and vestibule process
The oval window is where the stapes bone of the middle ear sits
The cochlear Fluids?
Perilymph - high Na+ and low K+ this fluid bathes the nerves and allows good nerve conduction.
Endolymph - High K+ and Low Na+ and bathes the sensory organ. K+ is released into this fluid by the stria vascularis and is very important.
The auditory organ?
The Organ of Corti
ribbon like structure rotating around the central column. About 20 000 hair cells in each cchlear (5k inner and 15k outer hair cells)
Inner hair cells are the ones with the connection to the CNS and sit on the Basalar membrane.
Inner hair cells and out hair cells arrangemet?
Sit in rows of usually 3 with single molecule connections between the transduction chanel of one to the shaft of another.
How is pressure in the inner ear relieved? Function of this?
The oval window and the stapes are in the top chamber but below this is the round window that is a flexible membrane that allows the pressure from the stapes to actually be transfered to the inner ear.
This sets up a whip like pressure wave that travels along the organ of corti and will increase in amplitude at a point that is compatable to the frequency of the sound.
The sensory cells sit on the basilar membrane and so when this moves th organ of corti moves up and down forcing the hair fibres to push against the tectorial membrane
How to IHC create a depolarisation?
The taller stereocillium moves more from the same pressure than the smaller stereocilia and the connecting molecule attached to a mechanically gated ion channel opens this channel allowing the flow of K+ from the endolymph causing depolarisation.
The linkage is connected to a myosin actin motor that keeps the tension allowing instantaneous/direct link to the CNS. These linkages are susceptable to damage from loud noise and don’t really heal.
How do K+ ions get back to the Stria vascularis?
They flow back to the stria vascularis through cells (fibroblasts) connected together by gap junctions (Connexons 26). Majority of congenital deafness is due to genetic abnormalities in these gap junctions.
What do outer hair cells do?
They are involved in sensitivity and also discrimination frequencies. They reinject energy into the traveling wave by activating small motor proteins. They are said to be electromotile and have thousands of prestin molecules that oscilate at the same frequency of the sound driving this energy into the inner sensory cells
These are the cells that die off leading to loss of discrimination and poor hearing usually at higher frequency sounds.
