Hearing, Smell and Taste Flashcards
What determines the pitch and intensity of sound?
Pitch is determined by the frequency of the wavelengths whereas intensity is determined by the amplitude of sound
Briefly explain the process of hearing
1) First transduction is sound waves striking the tympanic membrane and becoming vibrations.
2) Sound wave energy is transferred to the three bones of the middle ear which vibrate.
3) 2nd transduction - stapes is attached to oval membrane so membrane vibrates when stapes does which creates fluid waves in cochlea.
4) 3rd transduction - Fluid waves push membranes of cochlear duct which causes hair cells to bend and release NTs.
5) 4th transduction - NT release onto sensory neurons creates action potentials which travel in cochlear nerve.
6) Energy waves dissipate back at round window
Describe features of the outer/external ear?
Consists of Pinna (auricle) and the external acoustic (auditory) meatus which is made from cartilagenous and bone, contains ceruminous glands and is supplied by the auricular branch of vagus and the auriculotemporal branch of trigeminal nerve
What is the function of ceruminous glands?
Secretes cerumen (ear wax) which keeps the external acoustic meatus moist and prevents foreign bodies reaching tympanic membrane
What is cauliflower ear?
Occurs if there is damage to pinna and blood accumulates between the skin and cartilage. If the haematoma is not drained adequately then it can cause damage and permanent disfigurement to pinna.
Describe features of the tympanic membrane
- Concave structure which is separated into 4 quadrants; chorda tympani is in the posterio-superior quadrant and the safest quadrant for surgical procedures is the antero-inferior quadrant. Also present here is the politzer’s triangle
Describe features of the middle ear
It is an air filled cavity with 3 occicles (malleus, incus and stapes), one muscle and one tendon (stapedius muscle and tendon of tensor tympani muscle) and one nerve (chorda tympani nerve).
- Connected to nasopharynx via eustachian tube whos mucosal membrane is continuous with pharynx.
What is the roof and floor of the middle ear cavity?
Roof - Thin bone from the petrous part of temporal bone. Lies next to mastoid air cells.
Floor - Jugular wall, thin layer of bone separating the cavity from the jugular bulb.
Label the following diagram
What are the attachments of tensor tympani and stapedius muscles?
TT - Originated from auditory tube and attaches to the handle of malleus. Supplied by mandibular branch of V.
S - Attaches to stapes. Supplies by VII
What are the functions of the middle ear muscles?
TT - Pulls tympanic membrane medially to increase tension in response to loud noises which reduces the vibration of the tympanic membrane.
S - Pulls base of stapes away from oval membrane so protecting the inner ear from injury from a loud noise.
What are the features of the pharyngotympanic tube (also called auditory tube or eustachian tube)
The walls are normally collapsed but are actively opened by the simultaneous contraction of tensor veli palatini and salpingopharyngeus muscles. Occurs when swallowing or yawning. In children it is shorted and straighter which increases their risk of middle ear infections due to spread from URT
What are the components of the inner ear?
Bony labyrinth consisting of vestibule, semicircular canal and cochlea. Then the membranous labyrinth and perilymph.
What is the bony structure of the cochlea?
It is made up of spiral canals around the modiolus which is the axis it consists of porous bone with spiral ganglia.
Osseus spiral lamina is a bony projection into the canal.
what divides the cochlear?
The vestibular membranous and Basilar membrane split the cochlear into 3 ducts; scala vestibuli, cochlear duct and scala tympani
What sits in the cochlear duct?
The tectorial membrane and organ of corti (auditory receptor organ)
Explain frequency detection of the cochlea
The structure of the basilar membrane changes from short and stiff to long and floppy along the length of the cochlea. Sound waves of low frequency cause pressure waves which vibrate the membrane near the apex whereas high frequency will vibrate the base of the basilar membrane. This is how the brain can determine frequency of a sound.
Describe the signal detection at the organ of corti
Upward deflection to the basilar membrane moves the inner and outer hairs laterally with respects to the tectorial membrane. Outer hair cells can increase the sensitivity of inner hair cells and this can tune the cochlea by amplifying select frequencies.
Where do the majority of cochlear nerve endings terminate?
Inner hair cells
Explain signal transduction through hair cells
Displacement of hair cells (stereocillia) in one direction opens K+ channels and if displaced in the opposite direction then it closes them
Describe cochlea tuning
It is under olivocochlear neuronal control. fibres along this path release Ach onto inner hair cells which causes them to depolarise. This effectively damps down hearing in areas of pitch which are of no interest to the listener (such as background noise). Outer hair cells act as an amplifier for vibrations at the organ of corti.
What is the negative side effect of Kanamycin?
It preferentially kills outer hair cells in a specific point along the cochlea and results in specific frequency hearing loss at that point.
What is otoacoustic emission?
They are sounds of cochlear origin.
Describe the neuronal auditory pathways
- 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. The central extensions of the bipolar neurons form the cochlear nerve which then synapses at anterior and posterior cochlear nuclei. 2nd order neurons split up with some traveling ipsilateral but majority traveling contralaterally up to superior olive nuclei.
- 3rd order neurons travel in lateral lemniscus and synapse at inferior colliculus. 4th order project to medial geniculate nucleus of thalamus and synapse. 5th order join auditory radiation to auditory cortex.
How is the brain able to tell the direction of sound?
1) Volume and sound shadow - sound hitting one side of head creates sound shadow on the other side. comparison of signal intensities from both ears determines ear closest to sound. Good for high frequencies
2) Sound lag - sound from one direction enters one ear before other so delay in sound arriving ipsilaterally at cortex and that arriving contralaterally. Good for low frequencies.
What causes conduction deafness?
Deafness caused by one of following;
- Blockage in outer ear,
- infection in either outer or middle ear,
- ossification of small bones in middle ear,
- rupture of tympanic membrane
What causes sensory-neural deafness?
Breakdown of cochlea and associated mechanisms, damage to auditory nerve or damage to auditory cortex.
What is taste and the 6 primary tastes recognised?
- Interaction of dissolved molecules with taste buds (therefore saliva is important),
- Sweet (sugars, glycols and ketones),
- Salty (NaCl),
- Sour (H+ ions),
- Bitter (Quinine - alkaloid found in toxic plants),
- Umami (Glutamate found in truffles, meat, cheese),
- Oleogustus (taste of fatty acid)
Name and locate the following taste buds
- Vallate papilla (supplies by glossopharyngeal nerve),
- Fungiform papilla (most numerus, supplied by facial nerve),
- Foliate (edge of tongue, poorly developed)
What is the function of corda tympani?
Taste from anterior 2/3rd of tongue.
- Travels with lingual nerve.
- Goes from infratemporal fossa, through pterygopalatine fissure through middle ear cavity and joins facial nerve
Describe features of the Glossopharyngeal nerve
It carries taste from posterior 1/3rd of tongue and oropharynx.
(vagus carries taste from epiglottis and soft palate)
Explain the neuronal taste pathways
- Central processes of neurons conveying taste form the tractus solitaris which will synapse in the solitary nucleus (nucleus of tractus solitaries/ gustatory nucleus).
- Axons of 2nd order neurons cross midline to join medial lemniscus.
- 2nd order synapse in thalamus, 3rd order project to cortex.
- Limbic component via thalamus can activate brainstem nuclei for salivation or vomiting
What is the olfactory system composed of?
Olfactory epithelium (can regenerate from basal cells), receptor cells (bipolar neurons), axons projecting through cribriform plate and neuronal tract to multiple destinations
Describe the process of activating olfactory cells
Odorants are dissolved in mucous secreted by bowmen glands which facilitates olfactory and moistens the olfactory cells. Certain chemicals can also activate other cranial nerves
Explain the neuronal olfactory pathways
Central processes of receptor cells from olfactory nerves pass through cribiform plate and synapse in the olfactory bulb, then info travels via olfactory tract and will divide into medial and lateral olfactory stria. Medial stria to limbic system and lateral stria to medial temporal lobe.
What are the causes of anosmia?
- Idiopathic,
- Nasal/sinus disease (colds or polyps),
- Head trauma,
- rare causes are alzheimers preceding and congenital anosmia.
What is parosmia?
Distorted, often unpleasant sense of smell caused by damage to the lining at top of nose. Can be caused by URT infections
