Lecture 18 and 19 : Hearing 1 & 2: Anatomy and Physiology Flashcards
Describe 4 reasons why hearing is so important
- Essential to communication; enriches life.
- Critical to learning and education: sensitive period during development when good hearing is essential (importance of early detection and treatment).
- Auditory system is a powerful processor; integrating complex information in time and space (spatial localisation of sound, e.g. can navigate with self-generated sound) (e.g. presence of aeroplane flying above you and the direction of movement)
- Importance of awareness of environmental sounds provides real world contact (3D precision): “never” turned off (constant exposure)
Why is hearing with 2 ears important?
- Hearing with 2 ears (binaural hearing) critical for listening in noisy background (extract signals in background noise, e.g. important in classroom for children development)
How does the ear interact with other systems?
- Multisensory interaction (e.g. vision and hearing)
- Importance of v_ision and auditor interactions_ e.g. facial expressions and lip movement for speech recognition
- Somatosensory (touch) and vestibular system interact with head turning (sound localisation)
What is Tinnitus?
Perception of noise or ringing in the ears. A common problem, tinnitus affects about 1 in 5 people. Tinnitus isn’t a condition itself — it’s a symptom of an underlying condition, such as age-related hearing loss, ear injury or a circulatory system disorder.
What are the statistics of hearing loss in NZ?
- 13-17% (up to 700,000); mod-severe about 200,000
- Prevalence of permanent loss increases with age above 55yrs, about 60% of those over 70
- Prevalence set to rise (in Aust to 26.7% by 2050) with ageing population
- Prevalence of temporary loss high in young children (ear infections); formative period (affect learning and development)
What are some causes of deafness in children?
- Congenital: 2-3/1000 children (120-180 per year) moderate-profound deafness (Newborn Hearing Screening Programme) (~50% genetic and~50% cytomegalovirus (CMV), e.g. in pregnancy)
- Otitis media with effusion (OME, or glue ear [mucus build-up, not much pain])
- Infections (meningitis)
- Trauma (pressure injury to head)
What are some causes of deafness in Adults?
- Age-related
- Noise exposure (complete modifiable risk factor)
- Genetic
- Trauma (e.g. temporal bone horizontal fracture)
- Tumours – commonly in the ear canal (acoustic neuroma)
- Ototoxic drugs (aminoglycoside antibiotics [cheap and effective, but acoustic side effects] and cytotoxic drugs)
- Other
8.
What are some implications of hearing impairment on the individual?
- Poor speech, language and cognitive skill development
- Reduced literacy, learning, education, employment
- Social isolation (especially elderly, reduced engagement to society)
- Social stigmatization; Depression
- Tinnitus (buzzing of ear)
What are some comorbidities with hearing loss?
- Dizziness and fall
- Cognitive decline, dementia (risk of developing dementia increases with level of hearing loss)
- Cardiovascular
- Diabetes
The ear is mostly located in the _________ bone
Temporal bone
What are the names of the 3 parts of the ear?
The ear can be split into three parts; external, middle and inner.
What are the ossicles?
ossicles (3 tiny bones that are attached) and found in the middle ear
1) malleus (or hammer) - long handle attached to the eardrum
2) incus (or anvil) - the bridge bone between the malleus and the stapes
3) stapes (or stirrup) - the footplate; the smallest bone in the body
Their role is to mechanically amplify the vibrations of the tympanic membrane and transmit them to the cochlea where they can be interpreted as sound.
Describe the structure of the inner ear
The inner ear includes 3 interconnected regions
1) 3 semicircular canals
2) vestibule of the vestibular system (contains the balance sensory organs, and the
3) spiral cochlea, containing the auditory sensory organ or organ of Corti).
The stapes is in contact with the oval window.
There is also a round window- which relieves pressure in the cochlea
The inner ear is the most distal part of the ear, housing the vestibulocochlear organs. It has two main functions:
- To convert mechanical signals from the middle ear into electrical signals, which can transfer information to the auditory pathway in the brain.
- To maintain balance by detecting position and motion.
Briefly describe the structure of the middle ear
The middle ear includes:
1) eardrum/tympanic membrane
2) cavity (also called the tympanic cavity)
3) ossicles (3 tiny bones that are attached)
- malleus (or hammer) - long handle attached to the eardrum
- incus (or anvil) - the bridge bone between the malleus and the stapes
- stapes (or stirrup) - the footplate; the smallest bone in the body
Their role is to mechanically amplify the vibrations of the tympanic membrane and transmit them to the cochlea where they can be interpreted as sound.
The ossicles are suspended by ligaments. Tensor tympani muscle (connected to the malleus) which is innervated by the Trigeminal nerve, and the Stapedius muscle (connected to the stapedius) which is innervated by the Facial nerve.
TTM pulls the drum inwards (stiffens ear drum up), whilst the stapedius muscle pulls the stapedius outwards. When both muscles contract. When both muscles contract, this stiffens the whole chain up, which limits sound from passing through (protective system)
separation between middle ear and brain is thin. Therefore infection can erode the bone to the brain, causing abscess from ear to brain, possibl deafness and brain abscess.
Briefly describe the outer ear
The outer ear includes:
1) auricle/pinna (cartilage covered by skin placed on opposite sides of the head)
2) auditory canal (also called the ear canal)
3) tympanic membrane/eardrum outer layer
- The outer part of the ear collects sound and protects the middle ear.
- Sound travels through the auricle and the auditory canal, a short tube that ends at the eardrum.
- There is a skin lining which contains _cerumen glands (_produce earwax). This auditory canal is self-cleaning.
- The canal skin sheds from the surface, and is pushed out of the canal.
What produces ear wax?
What is the purpose of ear wax?
cerumen glands inside the auditory canal/ear canal
Cleans, lubricates the auditory canal. It also has antibacteria properities, therefore provides some protection.
What do the cerumen glands do?
Produce ear wax
What is another name for the eardrum?
Tympanic membrane
Name the parts of the outer ear
1) auricle/pinna (cartilage covered by skin placed on opposite sides of the head)
2) auditory canal (also called the ear canal)
3) tympanic membrane/eardrum outer layer
Describe the Eustachian Tube
Drains middle ear to nasopharynx
- Ciliated mucosa lines middle ear and assists with muscosal damage
- Maintains air pressure across the eardrum (which prevents ear drum rupture, as changes in pressure can cause conductive hearing loss)
- In infants, the ET is flater, smaller and less functional. Therefore it doesn’t drain into the nasopharynx as well, which makes them prone to infections.
Why do we need a middle ear?
1) Because of higher density of inner ear fluids, inner ear provides high resitance to virbation for same force. Middle ear structures acts as a “transformer to overcome resistance of inner ear fluids”
Greater area of the eardrum transfers more pressure on s_maller stapes footplate_ (of the middle ear)
The malleus arm is longer than incus, which generates greater force at stapes (big movements of malleus transfers to small, powerful movements of incus)- so you can hear small sounds
- energy transferred to inner ear is ~97% compared with 0.3% without the middle ear.
2) Air conduction of sound by displacement of eardrum and ossicular chain
Describe the inner Anatomy of Cochlea
The cochlea is a fluid-filled tube spiralling around a central bony core, the modiolus containing the auditory nerve and vascular supply. The bony tube is filled with a perilymph (Scala Vestibuli and Scala Tympani) with a composition similar to extracellular fluid, [Na] = 140mM; [K] = 5nM (high Na, low K).
Within the perilymphatic cavity/cochlea lies a tube, the cochlear duct or membranous cochlea. The floor of this duct is the basilar membrane . The duct is bounded on its medial surface by the R_eissner’s membrane_.
The cochlear duct is filled with an extraordinary extracellular fluid called the endolymph (Scala Media) which has an ionic composition more similar to intracellular fluid [Na] = 4nM ; [K] = 140nM (really unique)
The Endolymph has high [K] which is critical for ear function, and bathes the top of sensory hair cells which carries transduction signals.
The duct divides the perilymph fluid space into the scala vestibuli, above and scala tympani below. The space in the middle is known as scala media.
What are the 2 types of perilymph?
There are two types of perilymph: the perilymph of the scala vestibuli, and that of the scala tympani.
Both have a composition similar to cerebro-spinal fluid (CSF): rich in sodium (140mM) and poor in potassium (5mM) and calcium (1.2mM). The perilymph in the scala vestibuli comes from blood plasma across a hemto-perilymphatic barrier, whereas that of the scala tympani originates from CSF.
There is a low [K] in the Scala tympani, so the potassium just diffuses out of the cell. (no need for high energy pumps to maintain the concentration gradient. This reduces the need for high level of blood flow.
What is the significance of the Endolymph?
It appears that the cochlea has found a way of regulating potassium flow without expending any energy (ATP). Generally speaking, if an ion enters a cell in a passive way, it requires an active mechanism to leave it, and vice versa.
Only the apical pole of the hair cells bathes in the potassium-rich endolymph, which has a positive potential of 80mV. K+ ions therefore enter these cells passively, as there is more potassium in the endolymph than in the hair cell and the latter have a resting potential of -60mV, which favours an influx of K+.
These ions also leave the hair cells in a passive manner, due to the higher concentration of K+ ions inside the hair cell, compared to outside of the cell body bathed in perilymph. This all results in a significant saving of ATP by the hair cell.
Describe the organ of Cordi
Attached to the basilar membrane is the auditory sense organ, the organ of Corti
The v_ertical motion_ at the organ of Corti is converted to r_adial motion_ at the sterocilia.
This is comprised of sensory cells called the hair cells (~20,000 because the snesory sterocilia which project from their apical surfae into endolymph.
There are 2 types of hair cells: inner hair cells (IHC) and outer hair cells).
IHC comprises 25% in a single row and OHC 75%, in 3-5 rows of the population of sensory cells.
Hair cells are surrounded by a matrix of supporting cells which provide the integral structural support for the organ of corti
Hair cells are innervated by both ascending (afferent) and descending (efferent) nerve fibres.
Afferent fibres have their cell bodies in the spiral ganglion in the modiolus. There are 2 types:
Type 1: myelinated nerves, 90% of all nerve fibres, innervates only IHC, projects to cochlear nucleus in the brainstem
Type 2: small unmeylinated, 10% of nerve fibre, innervates only OHC, projects to cochlear nucleus in the brainstem
Efferent fibres arise in the ipsilateral and contralteral superior olivary complex and innervate predominantly OHC
What are stereocilia?
In the inner ear, stereocilia are the mechanosensing organelles of hair cells, which respond to fluid motion in numerous types of animals for various functions, including hearing and balance.
They are about 10–50 micrometers in length and share some similar features of microvilli.
They are joined together by Tip Links
What is the Tectorial Membrane?
The tectorial membrane (TM) is one of two acellularmembranes in the cochlea of the inner ear, the other being the basilar membrane (BM).
The TM is located above the spiral limbus and the spiral organ of Corti and extends along the longitudinal length of the cochlea parallel to the BM.
What is the modiolus?
The modiolus is a conical shaped central axis in the cochlea. It consists of spongy bone and the cochlea turns approximately 2.5 times around it.[1] The spiral ganglion is situated inside it.
Describe the Transfer of Sound within the Inner Ear
- The Oval window vibrates to transduce sound signals (apply pressure to the inner ear)
- Organ of corti detect sound waves
- The Round window is a pressure reducing valvae and is not affected by acoustic signals coming through the middle ear. The round window faces 90 degrees to the eardrum, so is not influenced by it. (opens to relieve the pressure exerted by the stapes)
- It has a curved shape to withstand pressure from the middle ear.
- The movement of stapes in the oval window sets up a travelling wave (energy) on the organ of Corti and the basilar membrane
- The location of the peak is dependent on the frequency of the sound and the mechanical properties of the basilar membrane.
- Peak is corresponding to the resonance point on the membrane. All the energy will collapse into the sensory cell at that point to conduct signals. (e.g. like a organ pipe)
- The regions of cochlea is spatially tuned to different frequencies, this is called tonotopicity
- The cochlea base (close) is sensitive to high frequencies (consonance)
- The cochlea apex (away) is sensitive to low frequencies (vowels)
- Cochlear implants are electrodes that stimulate different regions of the cochlear.
The cochlea ____ is sensitive to ____________ frequencies (consonance)
The cochlea _____ is sensitive to __________ frequencies (vowels)
The cochlea base (close) is sensitive to high frequencies (consonance)
The cochlea apex (away) is sensitive to low frequencies (vowels)
What are the functions of the ‘windows’in the inner ear?
- The Oval window vibrates to transduce sound signals (apply pressure to the inner ear)
- Organ of corti detect sound waves
- The _Round window i_s a pressure reducing valvae and is not affected by acoustic signals coming through the middle ear. The round window faces 90 degrees to the eardrum, so is not influenced by it. (opens to relieve the pressure exerted by the stapes)
- It has a curved shape to withstand pressure from the middle ear.
Describe how the Hair cells work (Medchanoelectrical transduction mediated by hair cells)
In response to sound, the basilar membrane moves up and down, which causes the organ of corti to vibrate up and down, causing the tectoral membrane to cause in the window wiper action. This causes the sterocilia cells to bend causing a signal in the auditory nerve that goes to the brain.
The hair cells have sterocilia (occur in pairs- long and short ones). The short sterocilia have potassium channels. The long stereocilia are mechanicaly connected to the short sterocilia via tip links. So if the long hair stereocilia bends away, it causes the channels in the short ahri stereocilia to open.
The endolymph (high in K+) bathes the sterocilia- therefore K+ flows into the hair cells when the K_+_ gates open which increases the membrane potential. The increase in the MP causes voltage gated Ca2+ channels to open which causes the Ca2+ ions to also enter the hair cells. This triggers the neurotransmitters (Glutamate) to be released- causing the auditory nerve to be stimulated.
OHC is responsible for amplifying sound and tuning at low levels
The dominant innervation of IHC indicates main sensory cell input to CNS
Excitatory direction: When sound comes into the inner ear, it will cause the basilar membrane to move up, as contact is made between the s_tereocillia_ and _tectoral membran_e, deflection fo the stereocilia occurs away from the modiolus and towards the lateral wall.
Resting position: None of the transduction channels are opened.
Inhibitory direction: Basilar membrane moves down and stereocilia move towards the modiolus (away from the lateral wall)-> hyperpolarisation of cells.
Too much noise will damage the tip links
Why do we not hear blood rushing in our ears all the time?
There is a low [K] in the Scala tympani, so the potassium just diffuses out of the hair cells. (no need for high energy pumps to maintain the concentration gradient. This reduces the need for high level of blood flow/O2 to the sensory cells (so you don’t hear pulsing in the ears). (Potassium ions flow back to the stria vascularis through cells (fibroblasts) connected together by gap junctions)
So all of the energy dependent processes is pushed away from the sensory cells.
Stria vascularis (oustide wall of the endolymph) acts as a ““battery”driving sound detection by sensory cells. Energy is consumed at the stria vascularis to generate voltage by pumping K+ into the endolymph.
What are tip links?
When sound reaches the cochlear, the hair cells move against each other. This pulls on tip links, which opens the mechanically gated ion channels in the hair cells. This allows K+ influx into the cells which causes them to depolarise.
Too much noise will damage the tip links
Tip links join long and short hair of stereocilia on hair cells
What is the stria vascularis?
The upper portion of the spiral ligament (which forms the outer wall of the cochlear duct) contains numerous capillary loops and small blood vessels, and is termed the stria vascularis.
It produces endolymph for the scala media, one of the three fluid-filled compartments of the cochlea.
Stria vascularis (oustide wall of the endolymph) also acts as a ““battery” driving sound detection by sensory cells. Energy is consumed at the stria vascularis to generate voltage by pumping K+ into the endolymph.
How is the K+ recycled in the inner ear?
1) Vertical motion at the organ of Corti is converted into radial motion at the stereocilia (freq dependent). Stereocilia only activate in 1 direction due to tip links opening channels. Deflection of stereocilia away from the modiolus opens K+ channels via tip links.
2) K+ flows into the hair cells (leads to cell depolarisation).
3) Potassium ions flow back from hair cells to stria vascularis through cells (fibroblasts) connected by gap junctions. No energy is consumed by hair cells to undertake sound transduction (hair cells have no blood supply).
4) Stria vascularis acts as a “battery”driving sound detection by sensory cells. Energy is consumed at stria vascularis to generate voltage by pumping K+ into endolymph.
