9. AUDITORY & VESTIBULAR SYSTEM Flashcards
Describe the structure of a hair cell
- Hair cells can act as water motion, gravity or sound detectors
- Stereocilia are found on top of the hair cell
- An extracellular matrix surrounds the hair cell & protects it known as the tectorial membrane
What do hair bundles consist of?
- Actin
- Stiff rods
What’s the extracellular matrix for hair cells?
- tectorial membrane
What’s the extracellular matrix for the macula?
- otoconial membrane is found in the macula of the vestibular system
How are sterocilia arranged?
- Stereocilia are arranged in bundles
What two structures link the sterocilia?
- Connectors - link the stereocilia laterally
2. Tip links - found at the top of the stereocilia
What are the three types of connectors?
- Top connectors
- Shaft connectors
- Ankle links
How does current flow through the stereocilia in response to motion?
- When the hair bundle moves in one direction, there’s pull or stretch at the tip links
- This distorts the tip links mechanically
- tip links are associated with ion channels, so distortion of the stereocilia causes ion channels to open or close
- The movement of the hair bundle determines if depolarisation or hyperpolarization occurs
- This allows current to flow through the stereocilia
In what direction does the hair bundle move for depolarisation?
- Towards the tallest stereocilia
In what direction does the hair bundle move for hyperpolarisation?
- Away from the tallest stereocilia
Describe the process by which depolarisation & neurotransmitter release occurs in the tip links
- Tip links are associated with ion channels, so distortion of the tip links causes ion channel opening
- The stereocilia are surrounded by endolymph which is high in K+
- The K+ influx causes depolarisation
- VGCC open due to depolarisation
- Ca2+ results in neurotransmitter release
- Current flows proportionately
What cells detect water motion in hair cells?
- Neuromasts or Mechanoreceptors can detect the flow of water including the velocity & direction
What is the cupula?
- A gelatinous extracellular matrix surrounding the hair cells to prevent direct contact with water
How does the cupula help to detect water motion?
- As water flows in the lateral line system (fish), the cupula will move in the opposite direction of water flow to protect the hair cells
What does the vestibular system detect?
- Motion
- Head position
- Spatial orientation
What are the three regions of the ear?
- OUTER EAR
- MIDDLE EAR
- INNER EAR
What two structures make the inner ear?
- Semi-circular canals - vestibular system
2. Cochlea - auditory system
What are the three semi-circular canals?
- Posterior semi-circular canal
- Anterior semi-circular canal
- Horizontal posterior canal
What type of motion does the posterior semi-circular canal detect?
- Roll - shoulder to shoulder movement
What type of motion does the anterior semi-circular canal detect?
- Pitch - forwards & backwards movement
What type of motion does the horizontal semi-circular canal detect?
- Yaw - side to side movement
What are the cilia of the semi-circular canals attached to?
- Gelatinous cupula
Describe motion detection in the semi-circular canals
- The semi-circular canals are filled with endolymph, the endolymph moves in the opposite direction to head rotation
- Head rotation causes fluid motion in the semi-circular canals
- However, the fluid motion in the semi-circular canals lags due to inertia. This pulls the cupula in the opposite direction to head movement
- The cilia are displaced causing depolarisation
What are the otolith organs?
- The otolith organs are part of the vestibular system
- The otolith organs detect linear acceleration (forwards & backwards and gravitational forces)
- Utricle & Saccule or Utricular macular & Saccular macular
- Detect different directions due to arrangement of hair cells
What does the utricular macular detect?
- Right & left
- Horizontal
What does the saccular macular detect?
- Forwards & backwards
Describe motion detection in the otolith organs
- The hair cells of the otolith organs are surrounded by a layer of otoconia crystals
- Upon linear acceleration (movement), the otoconia crystals are displaced, deflecting the cilia
- The movement of the otoconia crystals in response to motion, results in deflection of the stereocilia beneath
- The stereocilia are located in the otolithic membrane
What is rarefaction & compression?
- Compression = high pressure, molecules close together
- Rarefaction = low pressure, molecules spread out
What determines the wavelength & frequency of a sound wave?
- The rate at which rarefaction & compression occurs determines the wavelength
- The rate at which the pressure cycles between rarefaction & compression determines frequency
What is the pathway of sound waves to the auditory cortex?
- Changes in air pressure detected as sound eaves
- Causes motion in the cochlea
- Electrical activity conduction to cochelar nucleus
- Cochlear nucleus -> Olivary complex -> lateral lemniscus -> inferior colliculus -> medial geniculate body -> auditory complex
What is the pinna & what does it do?
- The pinna is found in the outer ear
- It gathers sound from the environment and passes it on to the eardrum
- Made entirely of cartilage & covered with skin
What is microtia?
- Malformation of the external part of the ear (pinna)
- Varies in severity (Grade I - 4)
What is Grade 1 microtia?
- Less than complete development of external ear
- identifiable structures and a small but present ear canal
What is Grade 2 microtia?
- Partially developed ear
- closed stenotic external ear canal producing conductive hearing loss
What is Grade 3 microtia?
- Absence of the external ear
- small peanut like vestige structure
- absence of external ear canal and drum
What is Grade 4 microtia?
- absence of total ear/ anotia
What is otitis media/glue ear?
- Collection of fluid in the middle ear which impedes the movement of the ossicles
- Loss of amplification as eardrum is pushing against the fluid
What structures make up the middle ear?
- Tympanic membrane
- Ossicles
- Oval window
What is the tympanic membrane?
- The tympanic membrane is the eardrum, it is connected to the ossicles
- Arriva of a sound wave causes vibration of the tympmanic membrane, separating the middle ear from the inner ear
What are the ossicles?
- Ossicles = trio of tiny bones on the other side of the tympanic membrane
- Vibration is passed on to ossicles form tympanic membrane which then conducts it to oval window of the cochlear
- Smallest bones of the body
What is the cochlea?
- Cochlea is found in the inner ear
- It consists of fluid filled canals that run parallel to each other.
- The cochlea is divided by a flexible basilar membrane
What are the three chambers of the cochlea?
- Scala vestibuli
- Scala media
- Scala tympani
Where is the basilar membrane & it’s function?
- It’s between the scala tympanis and the scala media of the cochlear
- Basilar membrane filters sound according to frequency
What does the stapes do?
- Stapes causes compression of fluid within cochlea
What is the organ of corti?
- Movement of the basilar membrane is converted into electrical activity in the organ of corti
- Organ of corti lies on top of cochlear membrane
Describe how the movement of the basilar membrane is converted into electrical activity?
- Inner & outer hair cells are attached to the organ of corti
- Motion of the basilar membrane causes displacement of the stereocilia (hair cells) on the Organ of Corti
- Displacement of the cilia leads to depolarisation which causes the outer hair cell to touch the tectorial membrane
What’s the function of an inner hair cell?
Inner hair cells have afferent synapses which take information away from the hair, towards the CNS
- Movement of the tectorial membrane causes inner cell to move which causes depolarisation
- There’s an influx of K+ from the endolymph
- A smaller influx results in a lower inner hair cell output
What’s the function of an outer hair cell?
- Outer hair cells have efferent synapses meaning they receive information
- They are involved in cochlear amplification
Describe the process of cochlear amplification?
- Outer hair cells are motile, upon displacement there’s depolarisation
- The influx of positive ions causes the outer hair cell to contract & shrink, contracting the tectorial membrane
- Basilar membrane is pulled inwards towards the basilar which causes inner hair cell to move
- K+ influx results in inner hair cell output to CNS
