Flashcards in Sound conduction Deck (48):
What is the formula for frequency?
Frequency= (Hz) Cycles/sec
What unit is sound measured in?
What is the human auditory threshold?
What is the sound pressure level that humans hear?
What is the frequency associated with human speech?
Describe the structures of the outer ear
Auricle/pinna: shape gives an idea about the elevation of sound
Auditory canal/external auditory meatus: Collects and conducts sound waves towards tympanic membrane to boost sound pressure
Describe the structures of the middle ear
Air-filled chamber between the tympanic membrane and oval and round windows
Acts to increase pressure of vibrations
Describe the structures of the inner ear
Consists of the chochlea (auditory) and vestibular organs
Functions to transduce vibrations into impulses; sensory hair cells turn mechanical energy into electrical -> CNVIII
What structures are contained within the middle ear and what is their purpose?
The ossicles; malleus, incus and stapes
Tympanic membrane vibrates due to air waves and the ossicles allow you to improve/amplify the signal
How does the middle ear increase vibrations and by what amount?
Change in surface area size from the tympanic membrane to the ossicles causes increase in pressure.
The incus has a flexible joint with the stapes; leverage is used to increase the force on the oval window.
Increased by up to 30dB.
What are the protective mechanisms of the middle ear to not damage sensitive hair cells?
Contraction of tensor tympani muscle; increases tension and reduces vibrations so less sound is conducted
Contraction of Stapedius muscles; reduces leverage of ossicle bridge so the amplitude is reduced
[Part of the auditory reflex- 50-100ms latency]
What is the purpose of the auditory/eustachian tube?
Usually closed; it opens when chewing or swallowing to equilibrate air pressure on either side of the tympanic membrane
What is the Weber test?
A tuning fork is placed on the scalp and can detect conductive or sensorineural hearing loss.
What is a Rinne test?
Performed by placing a tuning fork on the mastoid bone.
Evaluates hearing loss by comparing air conduction to bone conduction.
Name the structural components of the cochlear; scala, membranes etc.
Scala vestibuli, scala media, scala tympani
Vestibular membrane, Basilar membrane
Organ of corti (lies on the basilar membrane)
Describe the movement from the stapes to the round window
The stapes vibrate against the oval window of the cochlear. Perilymph is pushed around, travels to the scala tympani and vibrates the basilar membrane. The round window vibrates outwards to equalise the pressure as the fluid is incompressible.
How is the scala media and scala tympani connected?
Connected at the apex through a small channel (helicotrema), perilymph flows through here.
What fluid flows in the scala media?
Endolymph; it has a high K+ concentration
What is meant by tonotopic organisation?
Different frequencies are associated with different positions on the membrane.
High frequency; base of basilar membrane vibrates, narrow and tight membrane
Low frequency; apex of basilar membrane, wide and loose membrane
Where is the organ of corti located and what is its function?
Underneath the tectorial membrane and above the basilar membrane in the scala media.
Receptor sense organ of the cochlea which converted sound signals into nerve impulses via CNVIII.
It is sensory for when the basilar membrane is deflected die to pressure.
What is the tectorial membrane?
It is the membrane in the scala media upon which streocilia are attached at the bottom (motion sensitive hair cells of the inner ear). It is stiff compared to the basilar membrane so will generate shear force when sound force enters the scala media.
Where does the cochlear nerve emerge from?
The modiolus (axis of the spiral cochlea)
Where is endolymph generated?
What are the features of the inner hair cells?
Around 3, 500
Densely innervated by about 10 sensory axons (afferent to the brain)
Move due to endolymph
Do not make contact with the tectorial membrane
What are the features of the outer hair cells?
20, 000 cells arranged in 3 rows
In contact with the tectorial membrane
Cochlear amplifier; amplifies vibration signals from the inner hair cells as it is electromotive (expand and contract)
Damage to them causes sensorineural hearing loss
Responsible for otoacoustic emissions
What makes up the auditory nerve?
Fibres of the inner and outer hair cells.
their cell bodies are found in the spiral ganglion (perilymph).
What are tip links?
They connect the stereo cilia
Describe depolarisation of the hair cells
Upward movement of the basilar membrane causes hair cells to move away from the modiolus
K+ channels open
K+ enters the endolymph
Hair cell depolarises
Depolarisation causes Ca2+ channels in the body of hair cell
Glutamate is released from the base of the cell which depolarises the axon of spina ganglion cell -> AP
Describe repolarisation of the hair cells
Downward movement of the basilar membrane displaces stereo cilia towards the modulus
K+ channels close
Hair cell hyper polarises
What is the ion composition of the different scala and what maintains the conc. gradient?
Scala media; contains endolymph with High K, Low Na
Scala vestibuli and tympani; contains perilymph, High Na, Low K
Stria vascular maintains the conc. gradient. Gives rise to the endocochlear potential across the stereo cilia membrane.
How does amplitude affect the polarising go the stereocilia?
Higher amplitudes cause greater deflection of the stereo cilia and K+ channel opening
What is the central auditory pathway?
Spiral ganglions from each cochlea project via the auditory nerve (VIII) to the ipsilateral cochlea nucleus.
Axons sent to the contra- and ipsilateral superior olive.
Travels to the inferior colliculus and converge
Travels to the medial geniculate body of the thalamus.
Primary auditory cortex (temporal lobe)
[Collateral pathways to the reticular formation and cerebellum]
Which part of the auditory pathway is involved in auditory reflexes?
The superior olive projects pack to the cochlea as well as forward to the central pathways.
The inferior colliculus (caudal midbrain) is the centre for auditory reflexes.
What enhances the resolution of sound?
Lateral inhibition in the ascending pathway
What area of the brain is associated with the perception of sound?
Right posterior superior temporal gyrus
Where is the auditory association area and what does it do?
Next to Wernicke's area within the temporal lobe near the lateral cerebral sulcus
Processes auditory signals e.g. waveform shape, quality of sound
How is the medial geniculate nucleus and primary auditory cortex connected?
Connect via acoustic radiations via the internal capsule
Where is the secondary auditory cortex and what does it identify?
Located rostrally in the temporal lobe
Responds to sounds coming on and off
May respond to the duration of sound
What is a conductive hearing loss?
This is when sound does not reach the inner ear due to a problem with the outer or middle ear.
What are some causes of conductive hearing loss of the outer ear?
Congenital malformations (atresia)
Exostosis; benign bone growth, history of cold water exposure
What are some causes of conductive hearing loss of the middle ear?
Acute otitis media; starts with upper respiratory infection
Otitis media with effusion
Chronic otitis media
What is otorrhea?
Drainage exiting the ear
What is cholesteatomatous chronic otitis media?
It is destructive and expanding growth of keratinising squamous epithelium in the middle ear and/or mastoid process -> meningitis, death
What is otosclerosis?
Soft, spongy growth of new bone. Often appear near the oval window, the stapes are unable to move and in 10% of people reduced mobility causes conductive hearing loss. Treat via stapedectomy
What is sensorineural hearing loss?
Caused by dysfunction of the cochlear or cochlear nerve so the signal to the auditory cortex is reduced or lost
What are some causes of sensorineural hearing loss?
Presbyacusis; gradual loss of hearing due to aging
Sudden hearing loss; usually one ear, more than 30dB reduction
Ototoxic drugs e.g. antibiotics, beta blockers
Bacterial or viral infections
Diseases e.g Meniere's, meningitis
Noise induced hearing los; acoustic trauma or long-term noise exposure
Give an example of a neural cause of sensorineural hearing loss