Neuroscience 9a - Hearing
Anil Chopra
1. Explain the relation between frequency and pitch of a sound
2. Explain the relation between the intensity and loudness of a sound and explain the usefulness of the decibel scale
3. Describe in outline the structure of the ear
4. Explain the mechanisms for amplification and safety in the middle ear
5. Label simple diagrams of the cochlea to show the 3 scala, basilar membrane, organ of Corti, and hair cells.
6. Explain the steps by which movements of stapes brings about depolarisation of hair cells.
7. Explain how the mechanical properties of the basilar membrane and the hair cells result in frequency analysis of sound waves by the cochlea.
8. Distinguish between outer and inner hair cells.
9. Draw a diagram of a tuning curve of a single, cochlea nerve fibre and explain its shape.
10. Label a diagram of the main structures and tracts forming the central projections of the auditory nerve.
11. Define tonotopic mapping.
12. Identify the part of the auditory pathway involved in auditory reflexes.
13. List the functions of the auditory cortex.
14. List the main causes of conductive and sensorineural deafness.
15. Describe the Rinne and Weber tests and explain how they can be used to differentiate between conductive and sensorineural hearing loss.
The Ear
The external ear: consists of
- pinna
- external acoustic meatus
The middle ear consists of
- tympanic membrane
- auditory ossicles
o malleus
o incus
o stapes
- oval window
- tensor tympani muscle on malleus
- stapedius muscle on stapes
The inner ear consists of
- cochlea
- semicircular canals
- utricle
- saccule
- vestibular and cochlear nerves
Sound Conduction
· Sound = pressure wave in air
· Frequency = cycles/sec and is measured in hertz (Hz)
· Pitch is how the ear perceives frequency
· Amplitude = intensity which is perceived as loudness
· Decibel range = log scale of loudness
The outer ear has a resonant frequency of 3 kHz. This therefore means that sounds between 2.5-3 kHz are easier to hear because the threshold volume for hearing is reduced by 15dB (decibels).
dB rating sound
0 threshold of hearing
30 whisper
50-60` normal conversation
90 shouting
120 gunshot
140 pneumatic drill
The middle ear amplifies sound by 30dB. It does this via the leverage of the ossicles as the tympanic membrane vibrates. The tympanic membrane is also 17 times the size of the oval window. This amplifies the sound 22-fold.
Protective Mechanisms
Vibrations of the ossicular chain are dampened down when they become too extreme by the tensor tympani muscle (malleus) and the stapedius muscle (stapes). There is however a slight delay between the vibration and the contraction of the muscles so it could not protect the cochlea from a sudden loud explosion. These are also activated before speech.
Equalisation
The tympanic membrane needs the pressure on either side of it to be equal for maximum efficiency. The ear mucosa constantly absorbs air, and therefore the pressure in the middle ear gradually drops below that of atmospheric pressure. The Eustachian tube allows the pressure to equilibrate when it is opened (by swallowing or yawning). Blockage of this tube leads to hearing defect.
Conductive hearing Loss
Sound is prevented from reaching the cochlea due to:
- Earwax
- Otitis media (inflammation of the middle ear)
- Otosclerosis of ossicles (hardening of he ossicles)
- Perforated tympanic membrane
- Congenital malformation
Sound Transduction
The cochlea is a hollow tube in bone, curled into spiral; divided longitudinally into 3 compartments separated by 2 membranes running the whole of its length:
- Scala vestibuli - contains perilymph (like CSF)
o Reissner’s membrane – attached to tectorial membrane.
- Scala media - contains endolymph (has high K+ conc. à 80mV)
o Basilar membrane – has attached cochlear hair cells
- Scala tympani - contains perilymph (Like CSF)
Vibration of the oval window causes the vibration of the perilymph and in turn causes the basilar and Reissner’s membrane’s to vibrate and in turn.
Organ of Corti & Hair cells
The organ of Corti is the functional unit of sound transduction in the cochlea. As the perilymph, endolymph, and basilar membrane vibrate, the hairs on the hair cells (attached to the basilar membrane) are displaced. This results in oscillating changes in the membrane potential of these cells, which causes depolarisation of the cell, a Ca2+ influx releasing glutamate when it reaches threshold (NB: very sensitive: threshold requires only 3nm deflection!). This causes the ganglion cell to fire action potentials to the brain via the cochlear nerve (VIII):
v Hair cells move up: hairs move away from modiolus
o K+ channels open
o Hair cells depolarise
v Hair cells move down: hairs move toward modiolus
o K+ channels close
o Hair cells hyperpolarise
Modulation of Transduced Signal
There are 2 types of hair cells:
Inner hair cells: about 3 500 cells arranged in a single row. These provide information for the brain i.e. they detect sound. The glutamate released by them can be inhibited by efferent axons (presynaptic inhibition)
Outer hair cells: about 20 000 cells arranged in 3 rows. These are affected by efferent nerves which cause them to change shape. This results in the amplification of the response of the inner hair cells and is used in “tuning” of the ear to important sounds.
The inner hair cells have projections to the spiral ganglion and then to the auditory nerve and on to the brainstem, while the outer hair cells receive input from the brainstem via the spiral ganglion cells and auditory nerve in order to modulate the inner hair cells
The tuning curve gives the frequency of maximum sensitivity for each cochlear nerve fibre
As can be seen in the diagram the maximum sensitivity for this nerve is at 2.0 with reducing sensitivity on either side. It has a wider reach of low frequency than high frequency sounds
Sharpening of the tuning curve by lateral inhibition is also shown below with a single tuning curve for a single nerve
Pitch Differentiation
Frequency determines the perceived pitch of sound.
The basilar membrane increases in width as it winds round the cochlea (base = 100μm, apex = 500μm). The longer fibres (at the apex) have lower resonant frequencies that then the shorter ones, hence they will vibrate more and send action potentials from those cells at lower frequencies. The shorter fibres (at the base) have higher resonant frequencies and hence will detect higher pitch sounds.
Furthermore the cells at the base are much stiffer than those at the apex and are therefore tuned mechanically.
This increase in width, coupled to a decrease in stiffness of the basilar membrane means that sound of high frequency maximall displaces BM at the stapedial end, while low-frequency sounds maximally active the apical end. i.e. high frequencies vibrate basilar membrane nearer to base, low frequencies vibrate membrane nearer to apex.
l The form of the sterocilia mirrors their position of the basilar membrane and so determines the hair cells response to sound frequency
l Hair cells are embedded in a gelatinous matrix containing calcium carbonate crystals called otoconia
l Two types of sensory hairs, 1) Stereocilia – arranged in rows of varying heights 2)single, long kinocilium
l Two types of nerve-endings present on hairs:
§ Types I; chalice like endings form ribbon synapses
§ Type II; simple nerve endings
Normal Human range = 20Hz – 20KHz
Most sensitive at 1-3 KHz (human speech)
Auditory Pathways
Ø Complex bilateral pathways through the brain
Ø Superior olivary nuclei project back to the cochlea as well as forward to the central pathways
Ø Inferior colliculi – reflexes e.g. startle, head turn
Ø Collateral pathways to reticular formation and cerebellum
Ø Lateral inhibition in ascending pathway sharpens tuning curve
Ø Descending pathways provide feedback at all levels
- From the cochlea, the vestibulocochlear nerve synapses first in the dorsal cochlear nucleus and ventral cochlear nucleus.
- From the dorsal cochlear nucleus, fibres pass through the dorsal acoustic stria and (contralaterally) then join the lateral leminiscus up to the inferior colliculus.
- From the ventral cochlear nucleus, the majority of the fibres pass contralaterally through the trapezoid body. There are some fibres which synapse again in the superior olivary nucleus on the same side. Both these then travel up the lateral leminiscus on their respective sides to the inferior colliculus.
- From the inferior colliculus in the midbrain, the nerves synapse once again in the medial genticulate body of the midbrain.
- From the medial genticulate body, they travel to the primary auditory complex.
- Throughout all of this, the spatial organisation of the fibres in response to frequency is preserved.
- In the primary auditory complex, the cells respond to specific features of sound and some complex patterns.
- In the secondary auditory complex, the neurones respond to more complicated sound patterns.
Localisation of Sound
Intensity and timing of the sound arrival at the two cochleas is very important. Most neurons in the auditory pathway respond to sound from either ear
» Localisation in the horizontal plane – Intensity and phase difference between the two ears is computed
» Localisation in the vertical plane – The pinna reflects sound waves from different directions in different patterns
» Localisation from distance – high frequencies are attenuated more than low frequencies
» Wernicke’s area is the cortical site of language comprehension and is found in the dominant hemisphere (usually left)
» Brocca’s area is connected to this and is responsible for the expression of speech and language.
Rinne and Weber Tests
Use a 512Hz tuning fork.
Rinne tests. The fork held at the meatus of a normal ear will sound slightly louder than when placed on the mastoid behind the pinna.
Conductive loss: the fork will sound louder on the mastoid.
Neurosensory loss: the fork is louder on the mastoid.
Weber test: the fork is placed on the mid-forehead above the brow level the tone is heard ‘in the middle of the head’. It tests sensor-neural hearing
Neurosensory loss: If there is a sensory loss to one side the spatial localisation of the tone is displaced to the good side. i.e. heard loudest in unaffected.
Conductive hearing Loss:
· Sound is prevented from reaching the cochlea:
· Caused by:
Ø Wax
Ø Otis media
Ø Otosclerosis of ossicles
Ø Perorated tympanic membrane
Ø Congenital malformations
Sensorineural Deafness:
· Sensory:
Ø Presbyacusis
Ø Exposure to loud noise
Ø Meniere’s disease
Ø Toxicity e.g. some antibiotics
Ø Hereditary disorders
· Neural:
Ø Acoustic neuroma
Ø Viral infection
· Central (rare):
Ø Demyelination in MS
Ø Injury to central auditory pathway (unlikely to cause serious deafness unless both auditory cortices are affected