What is sound important for?
- Communication
- Emotion
- To recognise different objects
- To create a 3D view of the auditory world (aware of objects without seeing them)
- Survival (detecting danger, catching prey)
What does sound begin with?
simple vibration of the eardrums
What features of sound need to be recorded and in what units?
- Sound frequency i.e. the pitch encoded in Hz
- Sound intensity i.e. loudness measured in dB (amplitude of soundwave from peak to peak)
- Onset (for quickly responding)
- Duration- ears remain sensitive to sounds for a long period of time
What is the simple way of explaining how we detect sounds?
Sound enters the outer ear
Travels through the middle ear through the ossicles
Enters the cochlea spiral
Activates sensory hair cells and nerve fibres causing a message to be sent to the brain
What are the three chambers that form a spiral in the cochlea?
- Scala vestibuli
- Scala media
- Scala tympani
Where is the organ of corti and where is it in relation to the cochlea?
- Contains the sensory hair cells
- Located in the scala media
- Organ of corti sits on the basilar membrane
What is perilymph and where is it found?
Solution with:
- Low K+ conc.
- High Na+ conc.
- Normal Ca2+ conc.
Found in SCALA TYMPANI and SCALA VESTIBULI
What is the endolymph and where is it found?
Solution with:
- High K+
- Low Na+
- Low Ca2+
Found in SCALA MEDIA
How is an electrical gradient created using the perilymph and endolymph solutions?
- High K+ comes about becomes of the cells in the Stria Vascularis - which use energy to actively pump K+ ions into the scala media
- This accumulation causes a positive potential in the scala media (+80mv) - - – Called the scala media potential
- Creates an electrical gradient of about 120 mv between the scala media and the hair cells
Describe the structure of the Organ of Corti
- Row of inner hair cells which are the main sensory receptors of the cochlea
- Transfer info to nerve fibres → brain
- There are 3 rows of outer hair cells which don’t have a sensory function , they act as a cochlea amplifier
- Sits on the basilar membrane
How does sound stimulate the sensory hair cells at specific locations? (3 steps)
- Sound waves enter ear and cause vibrations of the tympanic membrane - amplified by ossicles
- Sound wave is passed into the chambers of the cochlea which causes a travelling wave along the basilar membrane which moves from the base of the cochlea to the apex
- Sound of one frequency causes the maximal movement of the basilar membrane at one location called the characteristic frequency location for that sound frequency
** For a particular sound frequency this location is always the same
What is the difference between a low and high frequency wave?
Low frequency:
travels further along the basilar membrane and causes maximal movement towards the apex, long wavelength and high energy
High frequency:
travels less far along the basilar membrane and causes maximal movement towards the base, also has short wavelength and low energy
What is the difference between the characteristics of the basilar membrane at the apex and base?
Basilar membrane is wide and floppy at the apex and narrow and stiff at the base
What encodes sound frequency?
PLACE FREQUENCY CODE
- The inner hair cells which each encode a narrow frequency band, one frequency sound travels along cochlea and activates specific IHC at a specific location
- The brain is interpreting the position of this IHC as the sound frequency
What are the benefits of the place frequency code?
- Sound frequency itself does not need to be encoded in the firing pattern of the nerves
- The firing pattern can record sound intensity
Why is the cochlea spiralled?
To expand our hearing range as much as possible, longer organ of corti and more sensory cells
How is Tonotopic organisation preserved through the auditory pathway?
E.g. high frequency sounds are sent to specific parts of the brain
Origins in cochlea preserved in auditory areas of brainstem, midbrain and auditory cortex showing the same place code for sound frequency
Explain the features of Inner hair cells
Cochlea sensory receptors
- All hair cells are defined by the stereocilia hair bundle
-Mechanosensitive ion channels (METs) are at the tips of the shorter stereocilia
- METs are connected to the taller stereocilia by tip links
- Hair bundles also have voltage gated Ca2+ channels (for exocytosis) and voltage gated K+ channels (for repolarisation)
- IHCs are contacted by many afferent fibres because they are sensory cells which need to relay all the information to the brain
What is the stereocilia hair bundle?
3 rows that increase in height with one hair bundle on one inner hair cell
What do tip links do?
Pull on the channels and open them when the hair bundles are defected
How do inner hair cells work at rest?
- Slight tension on tip links with some channels open leading to resting current carried by K+ ions which enter down conc. gradient from the endocochlear potential
- There is also a large conc. gradient for K+ to exit as the intracellular solution has high K+ compared to that in the perilymph
- So the resting current causes slight depolarisation of the resting potential causing some Ca2+ entry and resting activity in the afferent nerve
How do Inner hair cells work in response to sound?
- Hair bundles pushed in the excitatory direction which is towards the taller stereocilia
- Displace hair bundle
- Increases tip link tension
- Opens MET channels leading to larger MET current
- Depolarises the hair cell
(Mature hair cells respond to stimulation with graded receptor potentials and not firing) - Depolarisation activates Ca2+ and K+ channels helping to repolarise the cell
What is the inhibitory phase of sound for Inner hair cells?
- Hair bundle defected in opposite direction towards the shorter stereocilia
- Tip links slacken which closes MET channels
- Turns off MET current
- Causes cell to hyperpolarise below the resting potential
- Little/no neuronal activity
How does a hair bundle respond to a sustained sound?
-Moves hair back and forth at the sound frequency
- Cycle of depolarisation and hyperpolarisation at the sound frequency
- Generates pulses of neurotransmitter release and afferent activity
