S3: The Auditory System

Question 1

Describe anatomy of outer ear

Answer 1

• The outer cartilaginous bit of the ear forms the outermost part and this is the visible bit and forms a funnel shape called the pinna.
• This connects with the ear canal which projects back to where the tympanic membrane is (known as eardrum).

Question 2

Describe anatomy of middle ear

Answer 2

• Behind the tympanic membrane is an air filled cavity which is the middle ear. It is bridged by three articulated bones collectively referred to as the ossicles.
• The head of the first bone (hammer) sits against the tympanic membrane while the head of the far end bone (the stirrup) sits against the oval window (a membrane covering a hole in the bone of the skull).
• Behind is the inner ear.

Question 3

Describe anatomy of inner ear

Answer 3

• The inner ear is a set of channels and chambers carved out of the temporal bone and represents a space in the bone.
• The bony labyrinth is the rigid bony outer wall of the inner ear in the temporal bone, it consists of three parts the vestibule, semicircular canals and the cochlea. These cavities are hollowed out of the substance of the bone.
• The cochlea is the auditory part of the inner ear (spiral shaped). There is semicircular canals which loop around it and the vestibule is below it.

Question 4

What is the bony labyrinth filled with?

Answer 4

• The bony labyrinth is filled with a sodium-rich extracellular fluid called perilymph. Within the bony labyrinth there is also a smaller membrane bound compartment that is filled with endolymph, this is extracellular fluid low in sodium and high in potassium and high in other positive ions (+80mV compared to rest of body).
• This is the membranous labyrinth.

Question 5

Describe structure of cross section of chochlea

Answer 5

• There are upper and lower chamber, scala vestibuli and scala tympani and these contain perilymph.
• There is the third membrane which is separated by a membrane and this is the membranous labyrinth. At this location it is known as the cochlear duct. The cochlear duct is filled with endolymph (as part of the membranous labyrinth).
• The cochlear branch of the vestibulocochlear nerve runs up and its axons fan out to innervate each part of these structure.

Question 6

Describe structure of one of the turns on the cochlear

Answer 6

• There are two membranes, the basilar membrane on the bottom and vestibular membrane on the top which separates the endolymph compartment from the other two perilymph compartments.
• There is then a bony compartment in which the nerve fibres enter, the afferents go back to the spiral ganglion.
• Sitting on the basilar membrane is the spiral organ (previously called the organ of Corti).

Question 7

Describe the spiral organ

Answer 7

• Sitting here are the auditory hair cells which face upwards into the endolymph filled cochlea duct. They have projections called sterocilia (type of microvillae).
• The tectorial membrane is a flap and during life this covers over and attaches to the spiral organ.
• The inner hair cells lay closer to the origin of the tectorial membrane while the outer hair cells lay further back.
• The hair cells sit within the spiral organ which sits on the basilar membrane.
  The top of the hair cells contain the sterocilia, The sterocilia on the outer hair cells are embedded in the tectorial membrane and therefore these sterocilia are joining these two membranes (tectorial membrane to the spiral organ and therefore the basilar membrane).

Question 8

How is stereocilia from hair cells arranged spatially? How does it help their function?

Answer 8

• They are arranged in decreasing height in rows. If we look on even higher power we can see that there are a few strands of glycoprotein that link a steriocilia to its neighbour.
• It is these “tip links” that will essentially acts as the transduction mechanism and allow the sound waves to be converted into electrical signals (by pulling on tiplink that opens the channels that depolarise the hair cells).

Question 9

What are sound waves?

Answer 9

They are propagating waves of pressure.

Question 10

Describe the path of sound waves in outer,middle and inner ear

Answer 10

• The sound waves are captured by the pinna and channelled towards the tympanic membrane.
• This causes the tympanic membrane to vibrate and this vibration is carried through the ossicles and the final bone the stirrup transfers these vibrations into the fluid filled compartment of the inner ear via the oval window.
• The vibrations then travel from the base to the apex of the cochlea.
• The vibrations travel through the scala vestibuli (upper chamber) and pass easily into the scala tympani (lower chamber) though the membrane and through the hole in apex.
• The oval window is where the pressure waves start, these go through the spiral organ up to the apex! As the soundwaves pass through these membranes they cause vibration of those membranes. It is vibration of the basilar membrane that causes the hair cells to depolarise!
• So the basilar membrane moves up and down, vibrating due to the soundwaves,
• As the basilar membrane has the hair cells embedded in the spiral organ and the stereocilia of the outer hair cells join to the tectorial membrane it means that as the membrane moves up and down the stereocilia will tilt from side to side as the basilar membrane raises and lowers.
• For the lower hair cells, it is the currents in the fluid which cause their stereocilia to waft back and forth.
• As the stereocilia is pulled away from its shorter neighbours it tugs against the tip link!

Question 11

What is the function of ossicles?

Answer 11

The purpose of the ossicles is to capture the vibrations in the air and allow them to be converted to waves in the fluid. If they weren’t there, then the sound waves would just bounce off the membranes as fluid is much denser. They are also used to amplify the sound.

Question 12

What is conductive hearing loss?

Answer 12

When the conduction of the sound wave to hair cells is blocked leading to hearing loss. This can occur, when the sound waves have problems in the outer or inner ear e.g. ear wax. However, the hair cells are still intact.

Question 13

Describe how sound is transduced

Answer 13

• Hair cells are embedded in the spiral organ, on top of the basilar membrane. The sterocilia are arranged in height order with their tip links to their neighbour. At the end of each tip link is a mechanically gated ion channel.
• In neutral position, at rest the stereocilia are straight up and the cell is partially depolarised at about -40 mV. It will be releasing tonic glutamate onto the afferent nerve which will fire streams of spontaneous APs. RMP is partially depolarised as some of the channels on tip of stereocilia are open at rest.
• The stereocilia are projecting out into the endolymph which we know has very high K+, it also is positively charged in relation to the rest of the body and has a potential difference of +80mV.
• This means that when the basilar membrane vibrates, it moves up and the stereocilia move to the side, it tugs on the tip links and this pulls open some of the mechanically gated channels on the adjacent stereocilia membrane. This allows K+ to flow into the stereocilia, which they will do as they go down their electrical gradient. This causes depolarisation of the hair cell, resulting in an increased release of glutamate and a burst of action potentials fired.
• When the sterocilia tilts in the other direction, which it will do during the sound waves, the tip links relax and the mechanical channels close. This stops any K+ entering and the cell hyperpolarises which stops release of glutamate and the afferent stops firing briefly.

Question 14

Difference between how sound is transduced with low frequency sound waves and high frequency sound waves

Answer 14

• For a low frequency sound wave the afferent will fire bursts of action potentials at the frequency of the sound (i.e. every “wave” of pressure will cause the sterocilia to tilt and then hyperpolarise and over again).
• For high frequency sounds, there will just be continuous depolarisation and firing of the afferent because the stereocilia is unable to depolarise and hyperpolarise so quickly.

Question 15

Describe endolymph production and reabsorption

Answer 15

Endolymph is created by the stria vascularis, the endolymph needs to be continually replaced but also excess fluid must be removed. This is because too much fluid would lead to increased pressure which would damage the inner ear.
Hence production and removal must be perfectly balanced in order to keep pressure normal. Endolymph is continually produced and reabsorbed at a low rate.

Question 16

Whatis Meniere’s disease?

Answer 16

There is periodic increase in pressure in endolymph. It will lead damage the cochlea, there will be damage to the hair cells and they will gradually lose their hearing. They also get ringing in their ears and feel dizzy due to problems with the vestibular parts.

Question 17

Describe how loudness is encoded by the system

Answer 17

Louder sounds produce larger vibrations, which cause bigger receptor potentials (In the hair cells, due to greater tugging on tip links) and thus more action potentials
- However, noise-induced hearing loss used to be a serious work-related problem. In this current generation it will be due to personal music players.

Question 18

Describe how pitch is encoded by the system

Answer 18

Pitch is the frequency of the sound, a low pitch like a rumble or high pitch like a bad clarinet player. The whole point of the spiral shaped cochlea is that it is the method to separate out sound frequencies. The mechanics of the cochlea means some hair cells will only be stimulated by low frequency sounds while others only by high frequency sounds.

• At the apex of the cochlea, the basilar membrane is floppy and resonates only to low frequency sound. So only low frequency sounds will cause activation of afferents at this part of the cochlea.
• At the base, the membrane is narrow and stiff and resonates most strongly to high pitched sounds. When young we can hear up to 20kHz.
• There are then intermediates between the two limits.

Question 19

Describe the primary auditory pathway (discrimination)

Answer 19

• The inner hair cells send their afferents to the dorsal cochlea nuclei in the brainstem.
• Fibres from the dorsal cochlea nuclei project to the inferior colliculi contralaterally.
• Fibres from inferior colliculi project to the medial geniculate nucleus (in thalamus).
• Fibres terminate in the temporal lobe primary auditory cortex, located within the lateral sulcus.
• Other cortical areas also receive primary auditory input, not just A1 in the lateral sulcus.
• At the front of the primary auditory cortex are cortical cells that will respond to low frequency sounds, so the afferents that terminate here have come from the apex of the cochlea.
• At the far end of the primary auditory cortex are cortical cells that will respond to high frequency sounds, so the afferents that terminate here have come from the base of the cochlea.
• In between them are the frequencies in between!

Question 20

How is the auditory pathway tonotopically organised?

Answer 20

Importantly (and just like every other system) there is a tonotopic map of every step along the way. This is then seen in the primary auditory cortex, where the tonotopic map means there is spatial arrangement of different frequencies being processed by different parts of brain. Those frequencies that are next to each other will be processed next to each other and mapped onto the cortex.

Question 21

Give example of complex sound

Answer 21

Complex sound, such as speech require complex circuitry in order to decode human speech.

Question 22

Describe damage to Wernicke’s area

Answer 22

Speech comprehension is done at Wernicke’s area and damage here produces a form of deafness where the individual can hear what is being said but cannot comprehend language.
They can speak fluently back but their speech will be nonsensical as they have lost the ability to comprehend and self monitor their speech.
This is a fluent aphasia.

Question 23

Explain why bilateral lesions on both hemispheres damage discriminative hearing but do not cause total deafness

Answer 23

Other cortical areas also receive primary auditory input, not just A1 in the lateral sulcus. This is because there are still other areas in the brain processing sound.
So the individual will be aware a sound has occurred but unable to discriminate between the frequencies heard.
They would be unable to understand a voice as they would be unable to hear the different pitches.

Question 24

What is the vulnerability with high pitch sound?

Answer 24

The vulnerability here is that high-pitched sound is more energetic and hence more damaging! As we age we lose high frequency hearing (termed “presbyacusis”). This is seen in elderly people who lose the hearing of “ss” in speech (sibilance) and so they cannot understand some words. It is not necessarily a loudness issue where the younger person has to shout, as the elderly person will still not understand. To solve this the younger person has to speak slowly and clearly and sometimes at a lower pitch.

Question 25

How is sound origin located?

Answer 25

The location of a sound source is determined by comparing the sound detected by the two ears. This is done by the superior olivary nuclei, which compare the noises coming into the two ears and determine the laterality of that noise.
- High and low frequency sounds have to be done separately, because the sound waves themselves have different mechanical properties.

Question 26

Describe mechanism of lateral superior olivary nuclei in comparing the loudness of high frequency sound

Answer 26

• Here we have a high frequency sound coming from the left hand-side. Those sound-waves will hit the left ear before the right ear, but because they are so high frequency it is impossible to tell the timing of the sound in which ear it came in.
• However high frequency sounds have the characteristic that when they pass through solid objects they get absorbed and become quieter (get muted). Even passing through the head the high frequency sound will get quieter when it reaches the other ear.
• So the same noise hitting the left ear will be slightly louder than when it hits the right ear. Therefore the signal from the left ear will be stronger than that in the right ear.
• The lateral superior olives are very good at detecting these differences and converting it into the information about the origin of the sound.

Question 27

Describe mechanism of medial superior olivary nuclei in comparing the timing of low frequency sound in both ears

Answer 27

• Low frequency sounds pass straight through the skull without being affected (which is why you can hear the deep boom of neighbours dubstep but not the high pitched noises).
• However the sound waves are so far apart that the medial superior olivary nuclei can detect the arrival of the sound at one ear vs the other. (which ear it hits first!)
• They are very sensitive to differences in timing.

Question 28

Describe origin of sound in primary auditory pathway (localisation)

Answer 28

• Information about the origin of sound is sent via the ventral cochlear nuclei to the superior olives.
• These then project up bilaterally to inferior colliculus, medial geniculate nucleus and then up to the primary auditory cortex.
• In addition to a tonotopic map there is also a space map, generated by the superior olivary nuclei.
• This task is carried out separately for high- and low- pitched sounds.

Question 29

How are hair cells delicate?

Answer 29

The hair cells are very specialised and very delicate receptors, they are easily damaged and if they are lost they cannot be replaced. This is because there are no stem cells to replace them with.

Question 30

Describe how ototoxicity can cause damage to hair cells.

Answer 30

• Aminoglycosides (e.g. kanamycin, gentamicin).
• Anti-cancer drugs (e.g. cisplatin).
• Even NSAIDs in some cases.
  These can damage the stereocilia, meaning the transduction process will not work. If they are damaged enough the hair cell may just die.

Question 31

Describe how noise can cause damage to hair cells.

Answer 31

• Very high noises can damage the stereocilia and cause the hair cells to die.
• Worse, if the cells are traumatised (rupture), it may cause release of loads of glutamate which can be toxic and damage the afferents.
• 80dB (playground noise) is generally regarded as the level one can listen for indefinitely without damage.
• 95dB (hair dryer) 15-45 mins per day is safe, beyond that can damage hearing.
• 100dB (mp3 player limit) 5-15mins per day is safety limit.

Question 32

What is the difference in role of inner and outer hair cells?

Answer 32

• This is really important to appreciate because we only have about 3,500 inner hair cells per ear! It is the inner hair cells that are essential for discriminative hearing (to understand Penny Murphy’s voice for example!). The 3,500 are not all doing the same job, rather as a group they cover the whole of our hearing from 20Hz to 20kHz. So we only have a small number of inner hair cells for each pitch of sound that we can hear. Inner hair cells are also the source of afferent information that generate signals that our brain turn to sound.
• There are more outer hair cells than inner hair cells. The outer hair cells when they depolarise by the sterocilia being swayed due to basilar membrane vibrations, results in the outer hair cell shortening. This pulls together the tectorial and basilar membrane and amplifies the sensitivity of the system. They amplify at their own particular frequency and increase the vibration of the stimulus. The outer hair cells are essential for the inner hair cells to do their job, because otherwise the inner hair cells would get a very poor signal.

Question 33

Describe the central auditory pathway

Answer 33

• The two pathways for discrimination and localisation added together.
• Input to the ventral cochlea nuclei project to the superior olivary complex which give information to do with the localisation of sound.
• Input to the dorsal cochlea nuclei, gives discriminative hearing

Question 34

Where do afferents going to the cochlear nuclei have to run and what is their vulnerability?

Answer 34

Afferents going to the cochlear nuclei have to run through the auditory canal, this is a narrow space surrounded by thick bone.
The auditory nerve runs through the internal auditory canal, along with the vestibular and facial nerves.
- A narrow space gives vulnerability to space-taking lesions which can damage the auditory (cochlear) nerve.

Question 35

Describe acoustic neuromas

Answer 35

• The vestibular nerve, for most of its length is myelinated by oligodendrocytes as it is a central axon, but right at the end it turns into a peripheral nerve myelinated by neurolemmocytes (used to be called Schwann cells). These Schwann cells can start to proliferate to form a benign tumour, a tumour on a nerve is called a neuroma.
• As a neuroma forms in this space it will grow and press on the other nerves, the first thing the patient is likely to experience is ringing in the ears. This is why it is often called an acoustic neuroma (even though anatomically it is a vestibular schwannoma).
• As it gets bigger, patient will get dizziness (vestibular) and tingling in the face (the vestibular system is good at dealing with change so these symptoms appear later on when tumour is more severe).
• Acoustic neuromas are rare but account for approximately 6% of intracranial tumours (10 per million per year).