23-10-23 - Anatomy and function of hearing, smell and taste (special senses) Flashcards
Learning outcomes
- a) Know what receptors detect taste and describe their structure
- b) Know the major locations of these receptors
- c) Know what the five taste modalities are
- d) Know how the receptor cells convert detection of molecules to an action potential in the afferent nerve(s)
- e) Know which cranial nerves supply each part of the tongue
- f) Know the major neural pathways involved in taste
- g) Know where the receptors detecting odour are located and describe their structure
- h) Know how the receptor cells convert detection of an odour molecule into an action potential
- i) Know the major neural pathways involved in olfaction
- j) Know how sound waves are translated to neural signals for hearing
- k) Be familiar with the principles of audiograms and common problems in hearing
- l) List the major neural pathways and structures involved in hearing
- m) Explain how the body tries to protect the inner ear from the effects of loud noise
What cranial nerves are involved in olfaction, taste, and hearing?
- Cranial nerves are involved in olfaction, taste, and hearing:
- Olfaction CN – CN1
- Taste CN – CN7, CN9, CN10 (intermediate nerve)
- Hearing CN – CN8
How does sound travel through air?
What properties does it have?
What frequencies and amplitudes can human ears hear?
At what amplitude can hearing damage occur?
What is the role of the hearing apparatus?
- Sound travels through air in waves
- Has a frequency (pitch) and an amplitude (volume)
- Humans hear 20 – 20,000 Hz (vibrations per second)
- 0 dB = limit of human detection
- 120-130 dB = physical pain threshold (every 10 dB increase is 10 times louder)
- Hearing damage can occur from exposure to 85 dB e.g. a lawnmower
- The hearing apparatus must convert these waves into a signal that is interpreted by the brain
What does the tympanic membrane (ear drum) separate?
What does the middle ear contain?
What does the middle ear communicate with?
- Tympanic membrane (eardrum) separates external ear from middle ear
- Middle ear contains the ossicles (bones) – malleus, incus, stapes
- Middle ear communicates with nasopharynx via pharyngotympanic tube
The external ear – auricle/pinna.
What is the role of the external ear?
What frequency is it most effective at?
What can humans not do despite having muscles here?
What is the external ear made from?
- The external ear – auricle/pinna
- The external ear collects sound waves and channels waves to tympanic membrane via external auditory meatus
- The external ear is most effective at 3kHz, which coincides with typical speech
- Humans typically cannot move ears voluntarily despite having auricular muscles
- The external ear is made from elastic cartilage
The external acoustic meatus.
How long is the external acoustic meatus?
What shape is it?
What does it consist of?
What glands does it contain?
What is cerumen?
What is its function?
What nerves innervate the external acoustic meatus?
- The external acoustic meatus
- ~2-3cm long
- Not perfectly straight
- Lateral 1/3, Cartilaginous, epithelialized as per auricular skin
- Medial 2/3 , bony, continuous epithelium with tympanic membrane
- Ceruminous and sebaceous glands
- Cerumen = ear wax – protective function but can become excessive
- Innervation of the external acoustic meatus is via Vagus (CN10 - auricular branch), and auriculotemporal branch of V3 (CN5), with some sparse facial nerve (CN7) innervation
What is the role of the tympanic membrane (eardrum)?
What is the umbo?
What does examining with an otoscope give?
What 4 nerves innervate the tympanic membrane?
- The tympanic membrane (eardrum) transmits incoming vibration to the malleus
- The Umbo is the tip of malleus in contact with membrane
- Examining with an otoscope gives a cone of light in anterior inferior quadrant
- 4 nerves innervate the tympanic membrane:
1) VII, V, X externally,
2) IX internal surface
Middle ear cavity aka tympanic cavity?
What is the middle ear cavity?
What 3 things is the middle ear cavity continuous with?
What 4 things does the middle ear cavity contain?
What nerve supplies the middle ear cavity?
- Middle ear cavity aka tympanic cavity
- The middle ear cavity is a Mucosa lined cavity
- 3 things the middle ear cavity is is continuous with:
1) Mastoid antrum
2) Mastoid air cells
3) Pharyngotympanic tube - 4 things the middle ear cavity contains:
1) Ossicles
2) Muscles (tensor tympani and stapedius)
3) Chorda tympani (not involved in hearing but relevant to taste)
4) Tympanic plexus - CN9 supplies the middle ear cavity
What are the 3 ossicles (bones) of the middle ear cavity?
When are they fully formed?
What are they held in place by?
What is their role?
What is located at the stapes base?
- 3 ossicles (bones) of the middle ear cavity:
1) Malleus – contact with tympanic membranes
2) Incus – communicates with malleus
3) Stapes – communicates with cochlea via oval window at the base of stapes - The ossicles are fully formed at birth
- They are held in place by ligaments
- The ossicles transmit sound from tympanic membrane to the oval window, and amplify force from tympanic membrane up to 22 times
- This is necessary as waves move from air to fluid
- Stapes base has annular ligament
What are the 2 muscles of the middle ear cavity?
Where do they each attach?
What nerve are they supplied by?
What is their role?
- 2 muscles of the middle ear cavity:
1) Tensor tympani
* Attaches from the pharyngotympanic tube cartilage, petrous temporal bone, greater wing of sphenoid to handle of malleus
* Supplies by CN5 V3 (mandibular division)
* Tenses tympanic membrane, reducing amplitude of vibration
* Protects from loud sounds
2) Stapedius
* Smallest muscle, 1mm long
* Attaches inside pyramidal eminence on posterior wall of tympanic cavity to neck of stapes
* Supplied by CN7
* Responsible for acoustic reflex – stiffens stapes, pulling away from oval window, reducing transmission
Middle ear cavity – nerves.
What is the chorda tympani a branch of?
What nerve branches in the tympanic cavity?
What does it supply?
What is the tympanic plexus formed from?
What does it supply?
What branches does it give off?
- Middle ear cavity - nerves
- The chorda tympani is a branch of the facial nerve
- The facial nerve branches within tympanic cavity and leaves via petrotympanic fissure to supply tongue
- The tympanic plexus is formed from the tympanic nerve (CN IX branch) on the promontory (prominence) of tympanic cavity
- The tympanic plexus supplies sensory to tympanic cavity and gives rise to lesser petrosal nerve, ultimately providing secretomotor supply to the parotid gland
Where is the cochlea located?
What does it consist of?
Where are ducts located?
What duct is involved in hearing?
- The cochlea is part of the vestibulocochlear organ located in the inner ear
- It is a bony and membranous labyrinth within otic capsule
- Ducts are within bony canals
- Hearing is concerned with the cochlear duct
Describe other functions of the vestibular components of the inner ear (in picture)
Inner ear – sound transduction. What are 4 components of sound transduction in the inner ear?
What are these components collectively referred to as?
- Inner ear – sound transduction
- 4 components of sound transduction in the inner ear:
1) Three co-axial spiral tubes (technically two, one is doubled up)
* Scali vestibuli and scala tympani continuous with each other and filled with perilymph (low K+)
* Scala media filled with endolymph (High K+)
2) Endolymph and perilymph
3) Basilar membrane with hair cells connected to spiral ganglion (cochlear part of CN8)
4) Adjacent tectorial membrane (forms a roof)
- These components are collectively referred to as the ‘Organ of corti’
Describe the 7 steps in sound transduction along the inner ear.
What is sound typically composed of?
- 7 Steps in sound transduction along the inner ear:
1) Sound propagates through the stapes and along the coiled cochlea
2) Basilar membrane has changing physical properties, resonating at different frequencies along its length
3) When sound reaches the portion of basilar membrane with the same resonance frequency as the wave then it is readily “absorbed” causing basilar membrane to be maximally displaced
* This is referred to as tonotopy
* Sound is typically composed of a range of frequencies
4) Mobile basilar membrane induces shear forces at the tectorial membrane
5) Cilia in the hair cells are displaced
6) Displacement towards tallest stereocilia causes depolarisation of hair cells
7) This depolarization will feed onto afferent nerve fibres
What are hair cells of the inner ear?
Where is there an influx of K+ from the endolymph?
What happens when there is an influx of K+ from the endolymph?
What happens when there is movement in the other direction?
- Hair cells of the inner ear are Mechanically gated K+ channels
- When there is movement in one direction, there is an influx of K + from endolymph
- This triggers voltage gated calcium channels, raising intracellular calcium, causing release of glutamate
- When there is movement in other direction causes closure of K + channels
Inner ear – frequency selectivity.
What % of auditory nerve fibres serve inner hair cells?
What is the role of outer hair cells?
What is prestin?
What is its role?
When do outer hair cells contract?
How does this affect the inner hair cells?
What does this allow discrimination between?
- Inner ear – frequency selectivity
- 90-95% of auditory nerve fibres serve inner hair cells
- Outer hair cells modify incoming signal
- Prestin is a motor protein in outer hair cells
- It allows the cells to extend and contract
- Outer hair cell contraction occurs when depolarised
- This retreat of outer hair cells causes greater fluid movement around inner hair cells and thus inner hair cells are more sensitive to that frequency
- This allows discrimination of similar frequencies (frequency selectivity)
Inner ear – frequency selectivity.
Where do outer hair cells receive motor input from?
How does this alter their response to incoming signal?
How is this process mediated?
Where do these efferent fibres come from?
- Inner ear – frequency selectivity
- Outer hair cells also receive efferent (motor) input from CNS that inhibits them
- This reduces their response to incoming signals and filters out sounds that may be less relevant – background noise etc
- This process is acetylcholine mediated
- The efferent fibres are from olivary nucleus, which travel in CNVIII
Inner ear – volume and position.
How does volume affect hair cells?
What is spatial summation?
How is sound in the horizontal and vertical plane perceived by the ears?
- Inner ear – volume and position
1) Volume
* Amplitude of vibrations (volume) causes more rapid excitation and firing of hair cells
* Also – Increased excitation of adjacent hair cells - spatial summation
2) Position
- Horizontal plane - Sound will reach and trigger impulses fractionally sooner from ear closer to source (better for low frequencies)
- Sound at closer ear will be fractionally louder due the head generating a “sound shadow” (better for high frequencies)
- Vertical plane – one ear can compare direct and reflected sound wave from pinna (outer ear)
Hearing - central pathways.
Describe the 6 steps in the central pathways for hearing
- Hearing - central pathways
- 6 steps in the central pathways for hearing:
1) Spiral ganglion
2) CN VIII
3) Dorsal and ventral cochlear nuclei (upper medulla)
4) Majority of second order neurones cross, going to superior olivary nucleus
5) Travel in lateral lemniscus then inferior colliculus, synapsing in midbrain
6) Then to medial geniculate nucleus of thalamus and via acoustic radiation to primary auditory cortex (superior gyrus of temporal lobe)
Hearing - central pathways.
Where is spacial orientation of fibres preserved between?
Where does the association cortex receive information from?
What are 2 areas collateral afferents travel to?
What does this allow us to do?
- Hearing - central pathways
- Spatial orientation of fibres is preserved from cochlea to cortex
- Association cortex (in adjacent temporal lobe) receives information from primary auditory cortex and thalamus
- 2 areas collateral afferents travel to:
1) Reticular activating system of brainstem
2) Vermis of cerebellum - These collaterals allow us to coordinate a response to a loud sudden noise
What are 3 different hearing pathologies?
Can hair cells regenerate?
- 3 different hearing pathologies:
1) Tinnitus
2) Conduction deafness
3) Sensorineural deafness - Hair cells can not regenerate
What is tinnitus?
What can it sound like?
Is it unilateral or bilateral?
What 7 things is tinnitus linked to?
- Tinnitus is Hearing noises that do not come from an external source
- May sound like buzzing/ringing/hissing/whooshing
- Can affect one or both ears
- 7 things Tinnitus is linked to:
1) Hearing loss
2) Ménière’s disease
3) Diabetes
4) Thyroid disorders
5) Multiple sclerosis
6) Anxiety/depression
7) Side effect of some chemotherapy medicines, antibiotics, NSAIDs and aspirin
What is conduction deafness caused by (4 causes)?
- Conduction deafness is caused by any of the following that affect conducting structures:
1) Blockage
2) Ossicle malformation
3) Perforated eardrum
4) Infection
What is sensorineural deafness caused by (5 causes)?
- Sensorineural deafness is caused by any of the following that affect peripheral or central nervous structures:
1) Presbycusis (age related hearing loss)
2) Trauma
3) Ototoxic drugs (inner ear damage from medications)
4) Stroke
5) Noise exposure
Describe the audiogram for Age related sensorineural deafness (in picture)
Describe the audiogram for conduction deafness.
Describe the audiogram for conduction deafness (in picture)
- In conduction deafness, the conduction process is impaired
- We can see from the audiogram, there is conduction through the mastoid process, not the ossicles themselves
- Audiogram for conduction deafness (in picture)
Olfaction - Peripheral anatomy of smell.
Where does human olfaction begin?
What lines the upper nasal cavity?
Where does the olfactory nerve pass through?
What are the 2 forms of olfaction?
- Olfaction - Peripheral anatomy of smell
- Human olfaction commences in the nasal cavity
- Specialised olfactory epithelium overlays the upper cavity
- Nerves pass through the bony cribriform plate, communicating with the olfactory bulb
- Olfaction may be orthonasal (through nose) or retronasal (through mouth)
Where does human olfactory epithelium extend between?
How does olfactory and respiratory epithelium differ?
What 4 structures does olfactory epithelium house?
What % of air reaches the olfactory area?
How can this be increased?
- ~5cm2 of human olfactory epithelium extends from below the cribriform plate to the middle turbinate, and septum
- Olfactory epithelium differs from normal respiratory epithelium – Different mucous layer and different cilia function
- 4 structures olfactory epithelium houses:
1) Olfactory receptor cells
2) Support cells
3) Basal cells
4) Specialised Bowman’s glands/ducts - 10% of air reaches olfactory area
- This can be increased by inhaling more forcefully
What type of cells are olfactory receptor cells?
What 3 substances do Bowman’s secretions contains?
- Olfactory receptor cells are bipolar, with a ciliated dendritic end, cell body and axonal end.
- 3 substances Bowman’s secretions contains:
1) Immune cells (IgG, IgM, lysozyme)
2) Odorant binding proteins
3) Enzymes involved in clearance of odorants
What do odorant molecules bind?
How many types of smell receptors are there in humans?
How many types of receptors per receptor cell?
- Odorant molecules bind specific receptors on the dendrites of receptor cells
- There are 350-400 types of receptors in humans
- There is one type of receptor per receptor cell
Describe the 3 steps in odorant binding to receptors on receptor cells.
- 3 steps in odorant binding to receptors on receptor cells:
1) GPCR binding of odorant to the receptor site initiates Adenylyl cyclase to produce cAMP
2) This molecule acts on nucleotide gated channels allowing Ca2+ and Na+ influx
3) Ca2+ further closes Cl- channels preventing Cl- efflux
How many types of receptors are there per olfactory receptor cell?
How are we able to detect more smells than this?
- Olfactory receptor cells express only one type of receptor
- We are able to detect more smells than this
- Although one receptor type can be activated by different ligands
- Olfaction is coded spatially both at the level of the epithelium as well as the olfactory bulb
Where does the processing of smell begin?
What goes through the cribriform plate?
What is the first layer of the olfactory bulb?
What are glomeruli formed from?
- Processing of smell begins at olfactory bulb
- Unmyelinated axons of odour receptor cells project through the cribriform plate and form glomeruli
- This first layer of the olfactory bulb is termed the glomerular layer
- Glomeruli are formed from a single type of receptor cell
What synapses with receptor cells at the glomeruli?
What do their dendrites form?
Where do second order olfactory neurones go?
What are signals modulated by?
- Mitral and tufted cells form second order neurons that synapse with receptor cells at the glomeruli
- Mitral and tufted cell dendrites form the external plexiform layer
- These second order olfactory neurones go on to relay signals to central brain structures
- Signals are modulated by inter-cell connections e.g. periglomerular cells and inhibitory granule cells
What structures form the olfactory tract?
Where are signals relayed from the olfactory bulb?
What is the role of Lateral olfactory striae?
What structures make up the primary olfactory cortex?
Why is olfaction a unique sense?
- Axons of mitral and tufted cells form the olfactory tract
- From the olfactory bulb signals are relayed ipsilaterally to regions collectively termed the “primary olfactory cortex” in the inferior temporal lobe
- Lateral olfactory striae conduct signal to primary olfactory cortex (conscious perception of smell)
- Primary olfactory cortex = piriform area (uncus + entorhinal area + limen insulae) + part of amygdala
- Olfaction is a unique sense in that fibres project to cortex prior to thalamus
Where does the primary olfactory cortex project to?
What 6 areas does the primary olfactory cortex project to?
- Primary olfactory cortex then projects to higher centres
- 6 areas the primary olfactory cortex project to:
1) Orbitofrontal cortex
* Odour perception and discrimination,
* Integration with taste signals
2) Insular cortex
3) Thalamus
4) Hypothalamus
5) Hippocampus
* Learning and episodic memory
6) Amydala
* Learning, reward system, emotion
What are 6 other areas the olfactory bulb also receives signals from?
What is the role of these signals?
- 6 other areas the olfactory bulb also receives signals from:
1) Horizontal limb of the diagonal band (HDB)
2) Amygdala (AM)
3) Primary olfactory cortex (POC)
4) Hippocampus (H)
5) Locus coeruleus (LC)
6) Raphe nuclei (RN) - These signals exert further modulation of the bulb and are believed to have a role in memory
Describe the route of the taste on the tongue.
Where are each of the 3 types of papillae located?
How many TRCs are there per taste bud?
What are the 4 types of TRCs?
Where are solitary chemosensory cells located?
How rapidly do they turn over?
- Route for tastes on the tongue – tongue to taste papillae to taste buds to taste receptor cells (TRC)
- Where each of the 3 types of papillae located:
1) Fungiform – anterior, and middle lateral
2) Foliate – lateral posterior
3) Circumvallate – posterior - Taste buds house ~50-100 TRCs each
- 4 types of TRCs:
1) Type 1 – support cells
2) Type 2 – sweet/bitter/umami
3) Type 3 – salt/sour
4) Type 4 – progenitor cells - There are also solitary chemosensory cells in oral mucosa, epiglottis and oesophagus
- They rapidly turn over ~10 days
Describe the innervation for each of the papillae
- Innervation for each of the papillae:
1) Fungiform papillae from Chorda tympani (CN VII branch)
* Supplies anterior 2/3rds of the tongue
2) Circumvallate papillae and foliate papillae from CN IX
* Supplies posterior 1/3rd of tongue
3) Epiglottis, pharynx and soft palate from CN X
* There are also solitary chemosensory cells in oral mucosa, epiglottis and oesophagus
What are examples of each of the 5 basic taste modalities?
- Examples of each of the 5 basic taste modalities:
1) Sweet
* Sucrose, glucose, fructose, also sweeteners
2) Bitter
* Typically, plant-derived or synthetics nitrogenous molecules (alkaloids), most diverse group of tastants
3) Umami
* MSG (monosodium glutamate or Inosine monophosphate, IMP)
4) Sour
* H+ ions
5) Salt
* Na+
What do TRCs express on their surface?
Describe the receptor/channel for each of the different tastants (in picture)
- TRCs express G-protein coupled receptors on their membrane surfaces as well as sodium and proton channels
- Receptor/channel for each of the different tastants (in picture)
Describe the 4 steps in the mechanism behind taste
- 4 steps in the mechanism behind taste:
1) To initiate taste the cells must be depolarised via binding of tastants to receptors or entry of acids/sodium
2) Depolarisation occurs, mediated by the G-proteins (phospholipase C/inositol triphosphate or adenylate cyclase/cyclic AMP) resulting in intracellular calcium release
3) This in turn causes release of ATP and/or serotonin neurotransmitters at the basal membrane synapse
4) These signals are then conveyed by the afferent nerves
Describe the 4 steps in taste sensation being carried from the mouth to the brain
- 4 steps in taste sensation being carried from the mouth to the brain:
1) CNVII/IX/X carry sensation to nucleus of the solitary tract in the brainstem
2) They synapse in gustatory nucleus in rostral part of solitary tract
3) Secondary neurones project to ipsilateral ventral posterior nucleus of thalamus
4) They then go to the primary taste cortex in the insula and postcentral gyrus
Describe the Two theories that persist on how the brain interprets signals from taste cells
- Two theories that persist on how the brain interprets signals from taste cells:
1) Labelled-line hypothesis
* Each taste receptor cell carries a specific signal to the brain
2) Across fibre hypothesis
* Patterns of broadly tuned TRCs are interpreted by the brain as a specific taste signal
What are 5 clinical considerations for gustation?
- 5 clinical considerations for gustation:
1) Taste disturbance linked to increased mortality, nutritional deficiency
2) Damage to taste (and smell) has QOL implications
3) Altered taste (medication)
4) Chemotherapy/radiotherapy – Nutritional impairment
5) Bitter taste genotype – SNP in TAS2R38 gene can predispose to avoidance of certain foods “supertaster”
