Auditory Flashcards
(35 cards)
First amphibians
They already had some sensory cells that were able to sense the movement of water around the body and a vestibular system.
When they migrated into land they learnt how to detect sounds. This lead to the appearance of the tympanic ear in mammals and birds.
Sound is important for
Communication
Emotion
Navigation
Survival
What features of sound need to be encoded.
Frequency in hertz-
Number of waves per second. We can hear a wide range. 10^3
Sound intensity loud in decibels-
A massive range can be heard 10^12. This is achieved by the firing rate of afferent nerve fibres.
Onset/latency- a rapid onset is important for localising different sounds. And creating a topographic map of auditory space.
Duration- the ear has to remain sensitive to sound for long periods without fatigue. It never rests. The sensory cell synapses are specialised for sustained ntm release.
What is the function of the oscicles.
They amplify the signal so that the fluid in the cochlea will vibrate.
How does sound enter the ear and get to the cochlea
Sound is shaped by the auricle (outside of ear).
It travels down the ear canal and causes a vibration on the tympanic membrane.
This will vibrate the tiny bones called the oscicles. (Malleus incus and stapes). The stapes is the smallest in the body.
This will vibrate the round window and cause vibration if the fluid in the cochlea.
Labels
UTICLE and saccule
Cochlea nerve
What is in the organ of corti
At the base of the cochlea
It enters through the Modiolus (core of the cochlea)
Sensory cells
What are spiral ganglions and look at the picture of the cochlea.
Spiral ganglions come from the three chambers and carry info and join to the cochlea nerve.
What do the three chambers of the cochlea contain.
Scala vestibuli and tympani contain perilymph. This is like normal extra cellular fluid with low K (5nM) and normal ca (1.3mM).
-70mV
Scala media contains endolymph which is similar to intra cellular fluid. With high k (150nM) and low ca.
80mV
Stria vascularis
Pumps K into the scala media to give it the high K
Describe the hair cells in the organ of corti.
A single row of IHC
Three rows of OHC.
16,000 hair cells in each cochlea and 4000 are the IHCs which are the sensory ones.
When they are damaged they cannot be replaced.
Supporting cells in the cochlea
Dieters cells are below the outer hair cells.
Pillar cells are between the inner and outer hair cells.
Neurons in the cochlea
Type 1 spiral ganglion neurons innervate the IHCs. Carrying info to the brain.
Type 2 spiral ganglion neurons innervate the OHCs. Thought to be involved with loud noises and nociception.
Lateral efferent synapse with the type 1 fibres
Medial efferents synapse to the OHCs.
They give efferent feedback from the brain that allows some control over the sounds we concentrate on and this prevents the fibres being overstimulated.
Tonotopical organisation
The cochlea is tonotopically organised.
The cells at the base respond to high frequency sounds and the cells at the apex respond to low frequencies.
The cochlea relays the info to the cochlea nucleus in the brain stem.
The cochlea nucleus also has tonotopic organisation as all the apex cochlea fibres go to one side of the cochlea nucleus and the base cochlea fibres go to the other side.
Animals different hearing ranges.
Humans are 20-20,000
Mice, dolphins and bats can hear high frequencies.
Birds cannot heat high so they cannot heat mice. It makes them harder to hunt.
Blue whales use very low frequency sounds that travel well in water.
How is the tonotopicity established
By the basilar membrane
The movement of the fluid on the cochlea will cause a maximum stimulation of a particular region along the basilar membrane.
The membrane is narrow at the base for high frequencies and wide at the apex.
The base is stiff and the apex is floppy.
Low frequency sounds can travel further and vibrate the apex.
Maximum deflection
The biggest wave caused by a sound on the membrane.
The hair cells in the region if the basilar membrane that has the maximum deflection will be stimulated.
Tonotopicity preservation
It is preserved throughout the entire auditory pathway.
The cochlea and brain stem and cortex.
The general hair cell
Structure and stimulation
A hair bundle on top of the cell which is stimulated by the movement of fluid in the cochlea if mammals. Or by water in fish. The hair bundle is also called stereocillia.
On top of each stereocillia there is a transducer channel which is mechanically gated. The stereocillia are joined by tip links.
Pulling the tip link causes activation of the channels.
They also have a K channel for removing K so the cells can repolarise
The have ca channels for ca entry to allow excocytosis of synaptic vesicles.
When the hair cell is at rest
No sound.
There is resting tension on the tip links that opens some of the channels.
This allows some K into the hair cell and slight depolarisation to -55.
This activates some ca channels and produce the resting activity in the afferent fibres.
Inside the hair cell is
Outside the hair cell is
Perilymph
Endolymph
When the hair cell is stimulated
A sound occurs.
The stereocillia bend towards the taller ones.
This increases the tension in the tip links and open all the channels.
Lots of K enters and causes depolarisation.
It causes a maximum inward transducer current of -30.
Ca channels all open and lots of ntm released for high afferent firing.
then the sound pulls the stereocillia back in the opposite direction and this slackens the tip links and closes the transducer channels.
the cell will hyperpolarise and K will leave the cell
why is the hearing system so effective
no Na is required
how are the rows of stereocillia arranged on the IHC and where are the ion channels located
there is one tall row and two shorter rows.
the ion channels are located on the shorter cillia.
