Sound conduction and transduction Flashcards
Label the following diagram:
What is sound?
Change in air pressure.
At what speed do changes in air pressure occur?
343m/sec (767mph)- speed of sound.
What is frequency?
Sound causes a change in pressure that is periodic thus consists of compressed and rarefied air.
The frequency of sound is the number of compressed or rarefied patches of air that pass our ears each second.
What sound frequencies is the human ear sensitive to?
20-20,000Hz
What does sound intensity determine?
The loudness of the sound that we perceive.
Frequency remains unchanged- difference between the pressure compressed and rarefied air regions.
Intensity of the loudest sound (safe) is 1 trillion times greater than the faintest intensity perceived- dynamic range.
Amplification.
What structures make up the outer ear?
Pinna (auricle) + external acoustic meatus (external auditory meatus/ auditory canal).
What is the function of the outer ear?
Collects and conducts sound waves towards tympanic membrane.
What is the middle ear?
Air-filled chamber in bone, lying between tympanic membrane laterally, and oval and round windows medially.
Continuous with nasal cavity- Eustachian tube.
What structures make up the inner ear?
Cochlea + organs of balance.
Hair cells transduce mechanical energy of sound into electrical signal in cochlear nerve.
What are the mechanisms of sound amplification?
Conduction through middle ear amplifies sound by 30dB.
Achieved by lever system of articulated ossicles and ratio of area of tympanic membrane to oval window.
What are the protective mechanisms in the ear against loud noises?
Reflex contraction of tensor tympani and stapedius muscles reduces amplitude of vibrations passing through ossicles.
Protects against natural sounds but maybe not against man-made sounds.
Auditory tube allows equilibration of air pressure on either side of tympanic membrane.
What is conductive hearing loss?
Sound is prevented from reaching the cochlea.
What are the causes of conductive hearing loss?
Wax
Otitis media
Otosclerosis of ossicles
Perforated tympanic membrane
Congenital malformations
Describe the structure of the cochlea.
Hollow tube in bone, curled into spiral.
Divided longitudinally into 3 compartments, separated by 2 membranes.
Sound wave causes vestibular (Reissner’s) and basilar membranes to vibrate.
Cochlear hair cells are attached to basilar membrane.
Describe the structural and functional properties of the basilar membrane in the cochlea of the ear.
Structural properties determine how the membrane responds to sound: wider at apex than at base by 5x, stiffest at base and most flexible at apex.
Movement of stapes causes endolymph to flow causing a travelling wave in the membrane.
The distance the wave travels depends on its frequency.
Different locations of the basilar membrane are maximally deformed at different frequencies creating a placed code.
Describe the structural and functional features of the organ of Corti (spiral organ).
Hair cells are surrounded by supporting cells.
Tectorial membrane is gelatinous and does not vibrate with sound.
Spiral ganglion is embedded in modiolus and innervates hair cells.
Stria vascularis secretes endolymph (high in K+, low in Na+).
What are the 2 types of hair cells?
Inner hair cell- about 3500 cells arranged in single row, densely innervated by about 10 sensory axons/cell.
Outer hair cell- about 20,000 cells arranged in 3 rows sparsely innervated: one axon innervates several cells.
Both types of hair cell respond to sound but inner hair cells provide information for brain.
What is the sound transduction mechanism?
Basilar membrane vibrates in response to sound
Upward movement displaces stereocilia away from modiolus à K+ channels open à K+ enters from endolymph à hair cell depolarises
Downward movement displaces stereocilia towards modiolus.
K+ channels close.
Hair cell hyperpolarises.
Highly sensitive- response to threshold sound requires 0.3nm deflection.
Depends on maintenance of endolymph at +80mV by stria vascularis.
Depolarisation opens Ca2+ channels in body of hair cell.
Glutamate released from base depolarises axon of spiral ganglion cell.
Action potential.
What is the mechanisms for differentiation of pitch?
Basilar membrane acts as frequency analyser.
High frequencies vibrate basilar membrane nearer to base.
Low frequencies vibrate membrane nearer to apex.
Perceived pitch of a sound is determined by frequency.
What are the three compartments of the inner ear?
Scala vestibuli (contains perilymph fluid)
Scala tympani (contains perilymph fluid)
Scala media (contains endolymph fluid)
How is a place code or tonotopic map generated in the ear, and what is its purpose?
Different parts of the basilar membrane are sensitive to different frequencies.
A specific tone (frequency) is associated with a spatial position on the membrane.
This tonotopic arrangement is mirrored in higher levels of auditory processing.
Higher frequency sounds tend to make the basilar membrane towards the base of the cochlear vibrate more, whereas lower frequency sounds make the basilar membrane near the apex vibrate more.
What is the organ of Corti?
The sense organ of the cochlea of the inner ear which converts sound signals into nerve impulses that are transmitted to the brain via the cochlear nerve.
Lies on top of the basilar membrane and beneath the tectorial membrane.
Acts as a sensor for when the basilar membrane is deflected by a pressure wave.
Consists of inner and outer hair cells.
Describe the central auditory pathway.
Fibres from the organ of Corti project out of the cochlea via the spiral ganglion to form the vestibulocochlear nerve (VIII), which then projects to the ipsilateral cochlear nucleus.
After this point all connections are bilateral.
Fibres then project to the superior olive.
All ascending auditory pathways converge on the inferior colliculus.
Receives input from both cochlea.
Pathways are bidirectional.
The pathway continues to the medial geniculate nucleus and then to the auditory cortex.
There are also collateral pathways to the reticular formation and cerebellum.
