S3: The Auditory System Flashcards
Describe anatomy of outer ear
- 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).
Describe anatomy of middle ear
- 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.
Describe anatomy of inner ear
- 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.
What is the bony labyrinth filled with?
- 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.
Describe structure of cross section of chochlea
- 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.
Describe structure of one of the turns on the cochlear
- 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).
Describe the spiral organ
- 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).
How is stereocilia from hair cells arranged spatially? How does it help their function?
- 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).
What are sound waves?
They are propagating waves of pressure.
Describe the path of sound waves in outer,middle and inner ear
- 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!
What is the function of ossicles?
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.
What is conductive hearing loss?
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.
Describe how sound is transduced
- 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.
Difference between how sound is transduced with low frequency sound waves and high frequency sound waves
- 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.
Describe endolymph production and reabsorption
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.
Whatis Meniere’s disease?
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.
Describe how loudness is encoded by the system
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.
Describe how pitch is encoded by the system
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.
Describe the primary auditory pathway (discrimination)
- 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!
How is the auditory pathway tonotopically organised?
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.
Give example of complex sound
Complex sound, such as speech require complex circuitry in order to decode human speech.
Describe damage to Wernicke’s area
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.
Explain why bilateral lesions on both hemispheres damage discriminative hearing but do not cause total deafness
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.
What is the vulnerability with high pitch sound?
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.
