Principles of Neuroscience Lecture 9, Hearing Flashcards
What is meant by impedence?
Degree to which a material or substance reflects acoustical energy
Give a one sentence summary of auditory sensation
Sound waves transduced to nerve impulses
Describe the structure of the outer ear
Pinna
Concha
Meatus : ear canal
Tympanic membrane
Describe the structure of the middle ear
The ossicles :
Malleus
Incus
Stapes
Describe the structure of the inner ear
The cochlea
Describe the structure of the cochlea
Like a snail shell.
At the basal end, there are two openings : the round and oval window.
The base of stapes presses onto the oval window.
The cochlea has ‘tubes’ running all the way along it.
The scala vestibuli and scala tympani are separated by the cochlear partition.
These two chambers filled with perilymph.
The organ of corti and the basilar membrane lies at the partition.
The organ of corti is made up of hair cells (inner and outer - three rows) and the tectorial membrane.
The hair cells have stereocilia and one larger kinocilium that project out into the scala media.
The stereocilia are arranged on the apical end of the cell in height order.
These stereocilia are bathed in endolymph
Primary afferents link up with the inner hair cells
Describe how the perilymph in the scalas vestibuli and tympani is continuous
There is a gap at the top of the cochlea where the perilymph mixes. This is called the helicotrema
Describe the differences of constitution of endo and perilymph and where each is located
Perilymph : in the scala vestibuli and tympani. K+ poor, Na+ rich
Endo lymph : in the scala media. K+ rich, Na+ poor.
Describe the process of sound transduction in the ear
Sound waves travel down the ear canal after being focused by the outer ear.
The tympanic membrane amplifies the waves and pressure, causing the ossicles to move.
This motion presses on the oval window, displacing the perilymph.
Standing waves cause the basilar membrane to vibrate.
The wide and stiff basal part resonates at higher frequencies, and the narrow, more flexible, part resonates at lower frequencies.
The vertical component of these vibrations displace the organ of corti.
The different anchor points of the basilar and tectorial membranes result in a sheering force on the stereocilia of the hair cells.
When the stereocilia are pulled towards the tallest stereocilium, ion channels open in at the apex of the membrane. Potassium from the endolymph floods in, depolarising the hair cell. This depolarisation then leads to the opening of voltage gated calcium channels.
Calcium rushes into the hair cell.
Calcium inside the cell causes vesicles containing neurotransmitter to fuse with the basal end of the membrane.
These neurotransmitters target the primary afferents of the auditory nerve.
What is meant by labelled line coding?
This refers to the tonotopic resonance of the basilar membrane.
Different regions resonate at different frequencies
Describe the path that auditory information takes
Inner hair cell
Primary afferent
Auditory nerve
Brain stem, synapse, crossing over --> bilateral
MSO and LSO in the brain stem
Thalamus : Medial Geniculate Nucleus
Auditory cortex on superior temporal gyrus
Primary (A1) and secondaryDescribe the role of the MSO
The Medial Superior Olive
This is to do with the detection of the origin of sound
The time difference involved when sound originates closer to one ear is important.
The MSO has the greatest response when the information from both the right and left ear reaches the relevant area in the MSO at the same time.
The neurons from the ear that is closer to the source must travel a longer path to get to the relevant region in the MSO
Describe the role of the LSO
Interaural intensity differences locating the position of sound
When sounds of certain frequencies are localised to one side of the head, their wavelengths aren’t long enough to bend round the head.
The head acts as an auditory shadow.
Activation of the LSO on one side of the brain stem results in the inhibition of the LSO on the other side.
In this way, we can detect the localisation of sound.
Describe the location of the basilar, tympanic and tectorial membranes
Basilar: cochlea
Tympanic : outer ear
Tectorial: above the hair cells in the organ of corti
What is the name of the auditory centre in the thalamus?
Medial Geniculate complex
Which cells in the organ of corti are afferent, ie deliver information to the brain?
The inner hair cells
Describe the functional organisation in the primary auditory cortex
Tonotopic : the regions are devoted to sounds of different frequencies.
When the secondary auditory cortex receives information from A1, it is more diffuse and less tonotopically organised.
What three aspects of sound waves are encoded by the inner hair cells?
- Wavelength: pitch
- Amplitude: loudness
- Waveform: timbre
What is the role of the outer hair cells?
These are efferents from the brain. They control the tension of the basilar membrane of the organ of corti
Describe the arrangement of stereocilia on the surface of the organ of corti
By height
Shearing in which direction causes the K ion channels to open in the hair cells?
Shearing in the direction of the tallest stereocilium
What does ‘coincidence detectors’ relate to?
This relates to the localisation of sound by the medial superior olive
How does LSO output affect the detection of localisation of sound?
When the LSO on one side reacts more strongly, sound is coming from that end.
When they output the same amount, sound must be coming from directly in front of you
Which end of the Primary auditory cortex has information from the base of the cochlea?
The right hand side (flipped)
What is the difference between the two primary auditory cortices on either hemisphere?
For example?
They both receive information from both ears. However, each side does different stuff with the information
Speech on LHS
Music on RHS
Wavelength corresponds to…
Pitch
Amplitude corresponds to…
Volume
Waveform corresponds to…
Tone, timbre
Sound is coming from one’s left hand side
The left LSO is (more/less) excited
More
Describe inhibition between the left and right LSOs
When the left LSO is excited by sound coming from the left hand side, it inhibits the right hand side LSO
This gives us positional information about the source of the sound
