Sound conduction and transduction Flashcards
What scale is used to measure how loud a sound is?
Decibels (logarithmic scale)
What is the audible range for humans in terms of frequency?
20-20,000 Hz
What is the name given to the wing shaped flap skin and cartilage that makes up the outer ear?
Pinna
Describe the shape of the outer ear and its importance.
It is conical – starts off wide at the external auditory meatus and narrows to the tympanic membrane This focuses the noise and increases the pressure on the tympanic membrane
Is the tympanic cavity fluid-filled or air-filled?
Air-filled
State 2 ways in which the ossicles increase the pressure of vibration of the tympanic membrane.
Focussing the vibrations from the large surface area of the tympanic membrane to the small surface area of the oval window – this decrease in surface area means that the pressure is increased
The incus has a flexible joint with the stapes, such that the ossicles use leverage to increase the force on the oval window This amplifies the sound by 30 dB
What is the point of the middle ear? Why isn’t the tympanic membrane continuous with the cochlea?
The cochlea contains fluid, in which you are trying to induce a pressure wave If the tympanic membrane was continuous with the cochlea, you would go straight from air to fluid and 99% of the energy will bounce back due to impedance Sound waves require more energy to travel through fluid than air so the increase in pressure of vibration allowed by the ossicles is crucial for this conduction (Impedance matching)
What 2 muscles are involved in making sure that the ossicles aren’t damaged by excessive vibration due to loud noise?
Tensor Tympani
Stapedius
What is the name given to protective ear muscle reflex?
Auditory reflex
What is the latency period of the auditory reflex?
50-100 ms
What is hyperacusis?
Painful sensitivity to low intensity sounds – can occur in conditions that lead to flaccid paralysis of the auditory reflex muscles (e.g. Bell’s Palsy)
Which test is used to determine the site of damage to the auditory system, that is causing hearing loss?
Weber Test
What are the 2 specialised openings of the cochlea?
Oval Window
Round Window
What are the three compartments of the inner ear?
Scala Vestibuli
Scala Media
Scala Tympani
Which types of fluid do each compartment contain?
Scala Vestibuli + Scala Tympani = perilymph
Scala Media = endolymph
What structure connects the two perilymph compartments?
Helicotrema
Describe how the cochlea functions.
The vibration of the tympanic membrane is conducted and amplified to a vibration of the oval window by the footplate of the stapes. This vibration induces a pressure wave in the perilymph in the scala vestibuli. This vibrates the scala media leading to vibration of the basilar membrane. The round window vibrates as well to equalise the pressure in the cochlea.
Describe the difference in sensitivity of different parts of the basilar membrane.
Higher frequency sounds = base Lower frequency sounds = apex
What is the Organ of Corti?
The sense organ of the cochlea, which converts sound signals into nerve impulses that are transmitted to the brain via the cochlear nerve
Where is the Organ of Corti found?
It lies on top of the basilar membrane and beneath the tectorial membrane
What are the two types of cell in the organ of corti?
Inner and outer hair cells
Describe the features and function of inner hair cells.
Found on their own
Not in contact with the tectorial membrane
Send impulses to the brain
They have stereocilia that move in response to the movement of endolymph in the scala media
Roughly 3500 in the body
Describe the features and function of outer hair cells.
Found in groups of three
They are in contact with the tectorial membrane
They receive input from the brain
Electromotile so can expand and contract to amplify the amount of vibration (this is the basis of the cochlear amplifier)
Damage can result in sensorineural hearing loss Roughly 20,000 in the body
Which compartment of the cochlea does the stereocilia of the hair cells project into?
Endolymph (base is in the perilymph)
What internally generated sounds are the outer hair cells responsible for?
Otoacoustic emissions
What are stereocilia connected by?
Tip links (Line up with ion channels)
What bony conical structure is found at the middle of the cochlea?
Modiolus
Describe what happens when the basilar membrane is displaced upwards.
Depolarisation Stereocilia move away from the modiolus (towards tallest stereocilia)
K+ channels open K+ enters from the endolymph
Describe what happens when the basilar membrane is displaced downwards.
Hyperpolarisation Stereocilia move towards the modiolus
K+ channels close
Describe the difference in K+ and Na+ concentration in the different compartments of the cochlea.
Scala Media = High K+ and Low Na+
Scala Tympani = High Na+ and Low K+
NOTE: stria vascularis maintains this concentration
Describe the auditory pathway from the cochlea to the primary auditory cortex.
Spiral ganglion -> cochlear nuclei -> superior olive -> inferior colliculus -> medial geniculate nucleus -> primary auditory cortex
Up to what point is the auditory pathway from one ear ipsilateral?
Cochlear nuclei
Beyond this point there is bilateral representation
The inferior colliculus receives input from both cochlea. What is the inferior colliculus responsible for?
Reflex associations – turning your head towards loud noise
Frequency responses merge here
Formed of central nucleus (tonotopical) and dorsal and external cortices (not)
Responsive to complex sounds, sound locations and filters it as it bounces of surface – Precedence effect (must follow soon after original sound to work)
Describe a phenomenon that is involved in sharpening the signal coming from the cochlea.
Lateral inhibition
To which parts of the CNS do collaterals from the auditory pathway go?
Reticular formation
Cerebellum
In which lobe is the primary auditory cortex?
Temporal
What is the secondary auditory cortex responsible for?
Responding to sounds coming off/on Responding to the duration of sound
What is the name given to the axons that project from the medial geniculate nucleus to the primary auditory cortex?
Acoustic radiations (they travel via the internal capsule)
How do you localise short sound burst?
Interaural time delay - Bushy cells transmit information through delay lines forming a map of delay (found in birds), Occurs in medial superior olive of superior olivary complex
How do you localise continuous sound?
Interaural intensity difference (Sound is louder in ear it’s closer to) Occurs in lateral superior olive of superior olivary complex
What is conductive hearing loss?
When diseases of the middle ear damage the ossicles or stiffen their joints so that the amplification system is eliminated – results in conductive hearing loss
Causes
Meniere’s disease (fluid accumulation) common in children
Perforated tympanic membrane
Ostosclerosis (abnormal bone growth) obstructing ear canal
Barotrauma - Temporary form. Valsalva manoeuvre to accommodate and reopen Eustachian tube
What is sensorineural hearing loss and what can it be caused by?
When the cochlea or cochlear nerve get damaged, the signal transmitted to the primary auditory cortex is reduced or lost
Causes
Cochlear nerve
Acoustic schwannoma (tumour of the cochlear nerve) Cerebellar tumours expanding and putting pressure on thecochlear nerve
Cochlear
Loud noises - Can break tip link (temporary) or kill hair cell (Permenant)
Genetic mutations in Organ of Corti
Aminoglycosides toxic to hair cells
Congenintal hearing loss from rubella, toxoplasmosis etc
What is the term used to describe loss of hearing due to the death of hair cells in normal ageing?
Presbyacusis
How does a cochlear implant work?
Bypass hair cells and basilar membrane (mimic action with antennas but without active process) stimulate relevant parts of cochlear nerve, if cochlear nerve damaged implant will not work
Describe the cochlear nucleus
Tonotopically arrangement of nuclei
(Low frequencies ventral, high frequencies dorsal)
Dorsal cochlear nucleus detects sounds in vertical plane - does so as sounds with wavelegths similar size to the head shoulder and ear (high frequency) undergo interference both constructive and destructive.
The asymmetrical ear shape can pick up these spectral cues and process where the stimulus came from.
Achieved through fusiform cells
What do T-stellate and bushy cells do?
T stellate cells - encode sound frequency, intensity of narrowband stimuli their tonotopic array represnts sound spectra
Bushy cells - Produce sharper but less temporally precise version of cochlear nerve fibres. Provide resolution to encode relative arrival time of input to both ears
What does the superior olivary complex provide feedback to?
SOC neurones feedback to cochlear hair cells
MS - IHC (bilaterally)
LS - OHC (Ipsilaterally)
