Anatomy and function of hearing, smell and taste Flashcards
What dictates the pitch of a sound?
The frequency: the higher the frequency, the higher the pitch
What dictates the intensity of sound?
The amplitude
Overview of hearing
- First transduction: sound wave strikes the tympanic membrane and becomes vibrations
- The sound wave’s energy is transferred to the 3 bones of the middle war which vibrate (malleus -> incus -> stapes)
- Second transduction: the stapes is attached to the membrane of the oval window, vibrations of the oval window create fluid waves within the cochlea
- Third transduction: fluid waves push on the flexible membranes of the cochlear duct. Hair cells bend and release the neurotransmitter
- Fourth transduction: neurotransmitter release onto sensory neurones creates action potentials that travel through the cochlear nerve to the brain
- Energy from the waves transfers across the cochlear duct into the tympanic duct and is dissipated back into the middle ear at the round window
What makes up the external ear?
- Pinna (auricle)
* External auditory (acoustic) meatus
What is the pinna?
- Also called auricle
- Single piece of cartilage
- Ear lobe, tragus, helix etc.
Describe the external auditory (acoustic) meatus
- Cartilaginous and bone parts: not in the same direction
- Ceruminous glands secrete earwax making it waterproof preventing the maceration of skin and preventing entry of foreign objects
How can you test to see if pain is referred or coming from the ear?
Palpate the tragus
What is the supply of the external auditory meatus?
- Auricular branch of the vagus
* Auriculotemporal branch of the trigeminal
Describe the tympanic membrane
- Concave
- 4 quadrants divided by the (shadow of the) handle of malleus
- Triangular reflection (Pulitzers triangle) in the anterior inferior quadrant
Which quadrant is chorda tympani found in?
Postero- superior quadrant
Which of the quadrants of the tympanic membrane is the safest?
• Antero-inferior quadrant
What is the middle ear?
- An air filled cavity with ossicles (malleus, incus and stapes- attached to walls by ligaments), muscles/tendons (tensor tympani and stapedius) and nerves (chorda tympani)
- it has 4 walls (lat. wall is the tympanic membrane), a roof (bone between the middle ear cavity and the middle cranial fossa) and a floor
What are the ossicles of the ear?
- Malleus
- stapes
- Incus
What muscles are found in the middle ear?
- Tensor tympani
* Stapedius
What is the auditory tube? What is the nerve supply?
- Also called pharyngotympanic/ Eustachian tube
- Mucous membrane continuous with the pharynx
- Supplied by glossopharyngeus
What happens if there is a fracture to the roof of the middle ear cavity?
• CSF will leak through, if the tympanic membrane is ruptured, CSF will leak out of the ear
What is the opening in the posterior aspect of the middle ear cavity?
- Adits as antrum
* Connects to the mastoid antrum and the mastoid air cells
What is chorda tympani a branch of?
The facial nerve
What is the role of tensor tympani?
- Pulls the tympanic membrane medially
* Increases the tension in response to loud noises which reduces the vibration of the tympanic membrane
What is the innervation of tensor tympani?
• Mandibular nerve (branch of the trigeminal)
What is the role of stapedius?
- Pulls the base of the stapes away from the oval window
* Protects the inner ear from injury from a loud noise
What is the innervation of stapedius?
• Facial nerve
What happens if there is injury to the muscles of the inner ear?
Hyperacusis
Describe the pharyngotympanic tube
- The walls are normally closed
- Actively opened by the simultaneous contraction of tensor veli palatini and salpingopharyngeus muscle
- This forces air into the cavity and equalises pressure
Describe the pharyngotympanic tube in children
- Straight and short
* pharyngeal infections can easily spread to the middle ear
What makes up the inner ear?
- bony labyrinth: vestibule (utricle and saccule), semicircular canals, cochlea
- Membranous labyrinth
- Perilymph (between the two labyrinths)
What contains endolymph?
The cochlear duct
What is the auditory organ and where is it located?
- Organ of corti
- On the basilar membrane
- It contains ciliated cells
Describe the basilar membrane
• Extends from the tip of the osseous spiral lamina to the wall of the spiral canal
How is frequency detected?
- The structure of the basilar membrane changes from short and stiff to long and floppy along the length of the cochlea
- Resonant frequencies vary along the cochlea with high frequency at the base and low at the apex
- When the basilar membrane vibrates at the resonant frequency, it absorbs all the kinetic energy of the wave and effectively stops it at that point
- This is tonotopic organisation
Describe signal detection at the organ of corti
- Upward deflection of the basilar membrane moves the inner and outer hairs laterally with respect to the tectorial membrane
- 95% of the cochlear nerve endings terminate on inner hair cells
- Outer hair cells increase the sensitivity of the inner hair cells (this can tune the cochlea by amplifying select frequencies)
What happens as a result of the displacement of stereo cilia?
• One direction opens K channels and closes them in the other
Describe how the auditory signal at the centre of a standing wave is enhanced?
- Inner hair cells become depolarised and send signals to the cochlear nerve then to the CNS - mechanical displacement does not provide the sharpness of pitch discrimination recorded
- Outer hair cells are stimulated by the basilar membrane to depolarise: this causes the cell to contract, and enhances the auditory signal at the centre of the standing wave and inhibits on either side
Describe how areas of pitch of no interest are dampened down?
- Olivocochlear neuronal control
- Fibres along this path release Ach onto the inner hair cells causing them to depolarise
- This damps down hearing in areas of pitch that are of no interest to the listener
Which antibiotic can cause an effect on hearing? What is this effect?
- Kanamycin
- Preferentially kills outer hair cells in a specific point along the cochlea resulting in specific frequency hearing loss at that point
Knockout of prestin
- Prestin in the cell membrane motility protein
- You lose 40-69 decibels of your hearing at that frequency i.e. the outer hair cell amplifier provides a 40-60 decibel gain in sensitivity
What is otoacoustic emission?
Sound comes out of the ears (tympanic membrane relieves vibration that has traveled from the cochlea to the middle ear)
Describe the auditory pathway
- Hair cells of the organ of corti generate an electrical signal
- Peripheral extensions of the bipolar neurones at the spinal ganglion synapse with hair cells of the organ of corti
- Central extensions of bipolar neurones form the cochlear nerve
- The cochlear nerve synapses at the anterior and posterior cochlear nuclei
- Central extensions of 2nd order neurones split up with some travelling ipsilaterally, but most contra laterally up to the respective superior olivary nucleus
- Lateral lemniscus (3rd order) ascend and synapse at inferior colliculus
- 4th order neurones project to the medial geniculate nucleus of the thalamus where they synapse
- 5th order neurones join the auditory radiation to the auditory cortex
What causes the arousal response to noise?
Collaterals from the pathway project into the reticular formation and the vermis of the cerebellum
where do secondary projections from the primary and thalamic association area go?
To the auditory association cortex - sound is relayed topographically with lower frequencies to the anterior in most maps
Describe volume and sound shadow
- Sound from one side hits the head which generates a sound shadow on the other side in which the volume is less
- The comparison of signal intensities from both ears determines the ear closest to the sound
- better for higher frequencies
Describe sound lag
- Sound from a particular direction enters one ear before the other and so there is a slight delay between the side arriving ipsilaterally at the auditory cortex and that arriving contra laterally
- Better at lower frequencies
What helps to determine horizontal direction of sound?
Sound lag
How is front/back or above to below detected?
Folds in the pinna
What is a conduction deafness?
• Something that stops vibration reaching the inner ear
For example:
• A blockage in the 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 causes a sensory- neural defect?
- Breakdown of the cochlea and associated mechanisms
- Damage to the auditory nerve
- Damage to the auditory cortex
What causes tinnitus?
- Damage of outer and inner hair cells
* Usually the first manifestation of auditory damage
Which tests can differentiate between a conduction and a sensory-neural deafness?
• Weber’s test: tuning fork in the middle of the forehead, heard equally on both sides
- Normal: sound perceived as coming from the midline
- Sensorineural: Sound perceived as coming from the neural ear
- Conductive: sound perceived as coming from the affected ear
• Rinne’s tesat: air conduction is better than bone conduction (AC>BC); if BC>AC then conductive loss
How do we taste?
Molecules dissolved in saliva interact with taste buds
What are the primary tastes?
- Sweet: sugar, glycols, ketones
- Sour: H+ ions
- Salty: NaCl
- bitter: Quinine, alkaloids found in toxic plants
- Umami: triggered by glutamate
- Oleogustus: taste of a fatty acid
Where are taste buds located?
On the oral surface of the soft palate, the posterior wall of the oropharynx and the epiglottis
What are the different taste buds?
- Foliate papilla
- Vallate papilla
- Filiform
- Fungiform papilla
Vallate papilla
- Along sulcus terminalis
- Supplied by glossopharyngeal nerve
- More sensitive to bitter
Foliate papilla
• Poorly developed
Fungiform papilla
- Most numerous
* Supplied by the facial nerve
What does chorda tympani innervate?
• Taste from anterior 2/3 of the tongue
Describe the course of chorda tympani
- Branch of the facial nerve
- Travels with the lingual nerve
- Infratemporal fossa -> petrotympanic fissure -> middle ear cavity -> joins facial nerve
What is the special sensory afferent nerve supply to the tongue?
- Anterior 2/3 chord tympani: the facial nerve
* Posterior 1/3: glossopharyngeal
What conveys taste from the epiglottis and the soft palate?
• Vagus
Describe the taste pathway
- Central processes of neurones conveying taste from tracts solitaires (solitary tract)
- Tractus solitarius synapse in the nucleus of tractus solitaires (solitary nucleus, gustatory nucleus)
- Axons of second order neurones cross the midline
- Join medial lemniscus
- 2nd neurones synapse in the thalamus
- 3rd neurones project to the cortex
- Gustation has a limbic component via the thalamus and can activate the brainstem nuclei for salivation, or vomiting
What do olfactory neuroepithelial cells regenerate from?
Basal cells
What percentage of air comes into contact with the olfactory receptors?
- 2%
* Sensitivity can be increased by forceful sniffing
What facilitates olfaction and moistened the olfactory canal?
Mucus produced by the bowman glands
What forms the olfactory nerve?
• Central processes of receptor cells
What causes anosmia?
- Idiopathic
- Nasal/ sinus disease
- Head trauma
- Alzheimer’s preceding
- Congenital anosmia
- Parosmia
