Anatomy and Function of Hearing, Smell and Taste Flashcards
What does the frequency of the sound waves equate to?
The pitch of the sound
What does the amplitude of the sound waves relate to?
The intensity of the sound (loudness)
What is the tympanic membrane commonly known as
Ear drum
What are the names of the 3 ossicles?
- Malleus
- Incus
- Stapes
What does the stapes hit against?
The oval window
What does the external ear comprise of?
- Pinna (auricle)
- External auditory (acoustic) meatus
What part of the ear is devoid of a single piece of cartilage?
Ear lobule
What is tragus?
Cartilagenous prominence on the inner side of the external ear
What is the clinical significance of tragus?
It is palpated to differentiate between pain from external ear and referred pain
WHat are Ceruminous glands?
- Within external auditory canal under the skin
- They secrete cerumen (ear wax)
What is the purpose of cerumen?
Keeps external acoustic canal soft and waterproof
- Prevents maceration of meatal skin when water trapped inside
What is the purpose of cerumen hairs?
Prevent small foriegn objects from reaching tympanic membrane
What are the two parts of the External acoustic canal?
Outer cartilage and inner bone
What is the nerve supply of the external acoustic canal?
- Auricular branch of Vagus
- Auriculotemporal branch of trigeminal
Why is the pinna pulled upwards and backwards in an autoscopy exam?
Because the cartilaginous and boney parts of the auditory meatus are not straight
What quadrant of the tympanic membrane is the chorda tympani located in?
Postero-superior quadrant (posterior to handle of malleus)
What is the safest quadrant of the tympanic membrane?
Antero-inferior quadrant (i.e. when entering middle meatus with a needle to drain pus)
What structure is used to differentiate between the different quadrants of the tympanic membrane?
Shadow of handle of malleus
What is the triangular shaped cone of light on the tympanic membrane?
Politzer’s triangle
What are the ligaments that attach the ossicles to the walls?
- Anterior ligament of malleus
- Superior ligament of malleus
- Posterior ligament of incus
What are the muscles of the middle ear?
- Tensor tympani
- Stapedius
Where does the air come from to fill the middle ear?
Auditory / pahryngotympanic / Eustachian tube
- Connects middle ear to nasopharynx
What is the nerve supply to the eustachian tube?
Glossopharyngeal - mucuos membrane is continous with nasopharynx
What is the chorda tympani?
Branch of the facial nerve that originates from the taste buds in the front of the tongue, runs through the middle ear, and carries taste messages to the brain
What is the lateral wall of the middle ear?
Tympanic membrane
What is the opening on the posterior wall of the middle ear cavity?
Aditus ad antrum (leads to mastoid antrum which leads to mastoid air cells)
What vein relates to the floor of the middle ear cavity?
Internal jugular vein
What lies of the medial surface of the middle ear cavity?
The promontory (rounded hollow prominence, formed by the projection outward of the first turn of the cochlea)
What makes up the roof of the middle ear cavity?
Thin plate of bone, the tegmen tympani, which separates the cranial and tympanic cavities
- Petrous part of temporal bone
How much larger is the tympanic membrane in comparison with the footplate of the stapes?
15 times larger (force is therefore also intensified 15 times)
What is the tendon which is found in the middle ear cavity and what is its function?
Tensor tympani tendon
- function is to tense tympanic membrane
- Pulls membrane medially
- Increase in tension in response to loud noises
- Reduces vibration of tympanic membrane
What is the muscle located in the middle ear cavity?
Stapedius muscle
What is the nerve supply to the tensor tympani?
MAndibular nerve
What is the function of the stapedius?
Pulls base of stapes away from oval window - protect inner ear from injury from loud noises
What is the nerve suppy to the stapedius?
Facial nerve (nerve to stapedius nerve)
What is the pharyngotympanic tube opened by?
- Walls normally collapsed
- Actively opened by the simulatneous contraction of tensor veli palatini and salpingopharyngeus muscles
What is the pharyngotympanic tube shaped like in children?
Short and straight (pharyngeal infections easily spread to middle ear)
What is the modiolus?
Conical shaped central axis in the cochlea
What are the boney spurs called that come off of the modiolus?
Osseus spiral lamina
What membranes are located either side of the cochlear duct / scala media?
Come off of osseus spiral lamina
- Vestibular membrane
- Basilar membrane
What is the name of the auditory receptor organ?
Organ of Corti
What is the fluid that fills the scala tympani and vestibuli?
Perilymph
What is the name of the structure that connects the scala vestibuli and tympani?
Helicotrema
How does the structure of the basilar membrane change along the length of the cochlea?
From short and stiff to long and floppy
What percentage of cochlear nerve endings terminate on the inner hair cells?
95%
What is the purpose of outer hair cells?
They increase the sensitivity of the inner hair cells
- Can tune the cochlea by amplifying select frequencies
What is the name of the structure which covers the hair cells?
Tectorial membrane
What causes K+ channels to open in the stereocillia?
Displacement of stereocillia towards the kinocillia
How is tuning controlled?
- Under active olivocochlear neuronal control
- Fibres along this path release Ach onto inner hair cells causing them to depolarise
- Effectively damps down hearing in areas of pitch which are no intrest to the listener (e.g background noise)
What drugs are highly ottotoxic?
Aminoglycosides (e.g kanamycin)
How can aminoglycosides cause deafness?
Kill outer hair cells at a specific point along cochlea
What is outoacoustic emission?
SOund that come out of ears
- 50% spontaneous
- Tinnitus
Describe the auditory pathway?
- Hair cells of the organ of corti generate an electrical signal
- Peripheral extensions of the bipolar neurons at the spiral ganglion synapse with hair cells of the organ of Corti
- Central extensions of bipolar neurons from the cochlear nerve (1st order)
- Cochlear nerve synapses at anterior and posterior cochlear nuclei
- Central extensions of 2nd order neurons splits up, with some travelling ipsilaterally but most contralaterally up to the respective superior olivary nucleus
- Lateral lemniscus (3rd order) ascend and synapse at inferior colliculus
- 4th order neurons project to the medial geniculate nucleus of the thalamus where they synapse
- 5th order neurons join thr auditory ratiation to the auditory cortex
What is the lateral lemniscus involved in?
Thereom
What is the inferior colliculus involved in?
Auditory reflex centre
Where do collaterals from the auditory pathway project to?
Reticular formation and the vermis of the cerebellum, causing arousal response to noise
How is sound relayed the cortex?
Tonographically, with lower frequencies to the anterior in most maps though there are variations
How is the brain able to detect the direction of sound?
- Volume and sound shadow - sound from one side hits the head, which then generates a sound shadow on the other side in which the volume is less. Comparison of signal intensities from both ears determines the ear closest to the sound
- Sound lag - sound from a particular direction enters one ear befpre the other and so there is a slight delay between the sound arriving ipsilaterally at the auditory cortex, and that arric=ving contralaterally. Enhanced version of this used by owls for prey location
Sound lag is better at determining horizontal direction at what frequencies?
Lower
Sound shadow is better at determining horizontal direction at what frequencies?
Higher
What allows us to tel where sound is coming from in terms of above and below?
Folds in the pinna
What can cause conduction deafness?
- Blockage of outer ear
- Infection in either the outer or middle ear
- Ossification of the small bones in the middle ear
- Rupture of the tympanic membrane
What can cause sensory-neural deafness?
- Breakdown of the cochlea and associated mechanisms
- Damage to the auditory nerve
- Damage to the auditory cortex
What noise levels (decibels) allow for what amount of exposure before damage occurs?
- 85 decibles (shouting at around 3 feet), roughly 8 hours
- 115 decibels, instantaneous damage
What is usually the first manifestation of noise damage / hearing loss?
Tinnitus
At what frequency can hearing loss be most marked?
4000Hz ?
What is Weber’s test?
Tuning fork in middle of the forehead, heard equally on both sodes
What is Rinne’s test?
Air conduction is better than bone conduction (AC>BC); positive test
What is taste?
Interaction of dissolved molecules with taste buds (saliva is important)
What are the 6 primary tastes that are recognised?
- Sweet: sugar, glycols, ketones
- Sour: H+
- Salty: NaCl
- Bitter: Quinine, alkaloids found in toxic plants (taste threshold 8x10-6
- Unami: Triggered by glutamate (meat, aged cheese and tomatoes)
- Oleogustus: taste of fatty acid (unpleasant)
Where are taste buds located?
Oral surface of the soft palate, posterior wall of oropharynx, and the epiglottis
Where are vallate papilla found?
Along sulcus terminalis
What are the most numerous papillae?
Fungiform (found along dorsum of tongue)
What nerve supplies the fungiform papillae?
Facial
What nerve supplies the vallate papillae?
Glossopharyngeal
What papillae are more sensitive to bitter?
Vallate papillae
What papillae are poorly developed?
Foliate
What are he largest taste buds?
Vallate papillae
What is taste from the anterior 2/3s of the tongue carried by?
Peripheral extensions of the sensory neurons in geniculate ganglion of facial nerve
- Chorda tympani
What nerve carries taste from the posterior 1/3 of the tongue and oropharynx?
Glossopharyngeal nerve (peripheral extensions of sensory neurons in inferior ganglion of glossopharyngeal)
Describe the passage of the chorda tympani?
- Travels with lingual nerve
- Infratermporal fossa -> petrotympanic fissure -> middle ear cavity -> Joins facial nerve `
What is taste from the epiglottis and soft palate conveyed by?
Vagus
What do central processes of neurons conveying taste converge to form?
Solitary tract
Where does the solitary tract synapse?
In nucleus of tractus solitarius (solitary nucleus, gustatory nucleus)
When do taste sensations cross the midline?
Axons of 2nd ordr neurons cross midline and join medial lemniscus
Where do 2nd order taste neurons synapse?
Thalamus
Where do 3rd order taste neurons synapse?
Cortex
How can taste trigger salivation or vomitting?
Gustation has limbic component via the thalamus this can activate brainstem nuclei for salivation, or vomitting
What are receptor cells?
Part of olfactory system (bipolar neurons)
Olfactory neuroepithelial cells have a life span of how many days?
40 - 60 days
What can basal cells be used for clinically?
Stem cells (can regenerate from basal cells)
What percentage of inhaled air comes in contact with olfactory receptors?
2%
What secretes mucus for olfaction?
Bowman glands (moistens olfactory cells, facilitates olfaction)
Is olfaction ipsilateral or contralateral?
Ipsilateral
What is the olfactory nerve covered with?
Connective tissue of meninges
What are the 2 olfactory tracts smell can take?
- Medial olfactory stria - limbic system
- Lateral olfactory stria - olfactory cortex in medial temporal lobe
What are the different causes of anosmia?
- Idiopathic (25%)
- Nasal / sinus disease (25%)
- Head trauma (15%)
- Alzheimer’s preceding
- Congenital anosmia (1%)
- Parosmia (20%)
What is parosmia?
Unpleasant sense of smell caused by damage to the lining at the top of the nose (URT viral infections)
