Functional Anatomy of the Cochlear Flashcards
What is the overview of the anatomy of the ear?
Outer ear, middle ear and inner ear
- outer ear: external auditory canal and pinna
- middle ear: tympanic membrane, ossicles, (hammer, anvil and stirrup)
- inner ear: oval window
How is the inner ear split into?
The perilymph and endolymph
Describe the inner ear
- all the gaps around blue and pink in diagram filled w bone
- perilymph - surrounded by bone, filled w perilymph, high in sodium, low in potassium
- there is a continuous chamber that is separated from the perilymph by a membrane
- within that chamber, there is another fluid called endolymph - high in potassium, low in sodium
◦ also has a potential, more +ve in comparison to rest of body
What consists of the inner ear?
auditory and vestibular system
How is the cochlear made up?
- each spiral has 3 chambers
◦ upper chamber - scala vestibuli
◦ lower chamber - scala tympani
◦ membrane separating cochlear duct (w endolymph) - cochlear branch of the vestibulocochlear (VIII) nerve
- 2 perilymph chambers (scala vestibuli and tympani) and endolymph chamber (cochlear duct)
- spiral ganglion where receptors located
endolymph part of ear - cochlear duct
Where are the inner and outer hair cells found?
- inner hair cells. - towards middle
- outer hair cells - towards outside
- sit within supporting cells but have apical membrane sitting within perilymph filled chamber
- stereocilia stick out in endolymph and some are embedded in tectorial membrane
◦ important for transduction of sound wave - stereocilia linked to each other by tip links - are strands of glycoproteins
- hair cells found in the spiral organs found in the basilar membrane which spirals all the way up the cochlear
How are soundwaves captured?
- sound waves captured by outer ear, travels to tympanic membrane, causing it to vibrate
- vibrations cause the small bones to vibrate causing head of bone to vibrate against oval window
- jointed bones have leverage to increase amplitude of vibrations but main job to get vibration between air and liquid in inner ear
- w’o inner bones, vibrations will reflect back on oval window so wouldn’t allow liquid to vibrate
- vulnerability to system: conduction hearing loss occurs when there is a blockage preventing transmission of sound waves to the oval window =. all problems to hearing in this section
- vibrations introduced into fluid of inner ear and they spread to the channels of the cochlear initially through the scala vestibuli
- sound waves pass through the membranes, causing them to vibrate * wave of vibration passes from scala vestibuli to scala tympani, causes tectorial membrane and basilar membrane to vibrate up and down
- stereocilia embedded in tectorial membrane so pushed towards the tallest stereocilia
- rush of fluid causes the hair to also move back and forth
◦ with each cycle of vibrations, stereocilia are being tilted - tip links cause all rows of stereocilia to move, also cause mechano-gated channels at the base of the link to open* there is an electrical gradient, causes K+ to enter hair cell
- causes the depolarisation of hair cell
◦ releases more glutamate into its afferents - when stereocilia tilt in other direction, channels close, and so the cell hyperpolarises
What happens if there is a vulnerability in the hearing system?
- vulnerability in the system: endolymph hydrops
◦ endolymph continuously produced and absorbed
‣ produced in the stria vascularis and removed to the CSF of the brain
◦ so the production and removal must be perfectly balanced
‣ if not, pressure will build up in cochlear duct - Meniere’s disease in humans characterised as disorder of the endolymph in which endolymph hydrops develops
- usual symptoms: low freq hearing loss, tinnitus, rotatory vertigo, sense of fullness or pressure in the ear
◦ endolymph volume decreases slowly, disturbing motion of the basilar membrane and giving rise to hearing loss
◦ episodic vertigo: hydrops develop to the pint where boundary membrane breaks results in endolymph into the perilymphatic system
‣ high K+ toxic to cells that aren’t specialised to deal w it
* cells depolarise and swell in a high K+ environment resulting in auditory and vestibular dysfunction - inner hair cells responsible for encoding sound characteristics
◦ only have around 3500 inner hair cells per ear to deal w the entire range of freq we can detect - ototoxic damage to hair cells
◦ hair cells highly specialised receptors that are vulnerable to their environment
◦ sensitive to toxic chemicals including some medications
‣ platinum based anti cancer drugs and aminoglycoside antibiotics
What do hair cells capture?
the loudness and frequency of a sound
* louder sounds produce : louder vibrations, bigger R potentials, more transmitter released, afferents fire more AP, more afferents activated
* the mechanics of the cochlea separate out sound frequencies to create a place code
* each freq produces vibration at a characteristic location on the basilar membrane
* activates acharacteristic set of hair cells, their afferents in the auditory pathway
* tonotopic location of the neural activity represents the a freq of the sound that triggered it
What determines the sound origin?
- auditory brainstem
◦ location of a sound source determined by comparing the sound detected by the two ears
◦ task carried out separately for high and low pitched sounds - cues to vertical localisation given via slight shifts int he quality of sound (spectral cues) at diff elevations, due to the interference set up by the pinnae
What do outer hair cells do?
sit within spiral ganglion on the basilar membrane, also have stereocilia on their apical surfaces
* every time stereocilia tilt in direction of tallest one, the depolarise
* act as an amplifier so that sound waves can reach inner hair cells
* sound damage: hair cell function relies on delicate stereocilia and tip links which are trapped between two large and rapidly vibrating membranes
◦ will leak ions and die
noise damage will kill outer hair cells first
What are the differences between type 1 and 2 afferents?
T1:
- Big cells, thick axons, well-myelinated, local connections
T2:
- Small cells, thin axons, unmyelinated, long-ranging connections
How type 1 afferents work?
- True afferents
- Connect to one hair cell, preserving frequency information = frequency selective
- Synapses are fast and powerful, preserving detailed timing info
- Large well-myelinated axons transmit info quickly
- Low threshold/high spontaneous firing fibres tend ot contact the pillar face of the IHCs whereas high threshold/low spontanseousfiring fibres tend to contact the modiolarfaces of the IHCs
How do type 2 afferents work?
- Rarely fire APs but dendrites are both post and presynaptic to OHCs and make synapses w each other
- Recieve input from up to 30 cells covering a broad range of frequencies
- Synapses are weak and slow so timing details are lost
- Very slow axons
- Studies of the function of type II afferents currently focus on nociception and prevention of harm bc of their high threshold for firing APs
How does the cochlear control volume?
The Medial Olivocochlear (MOC) efferents
- respond well to sound
- have narrow frequency tuning
- feedback in humans
- Block OHCs
