Anatomy and function of hearing, smell and taste (special senses) Flashcards
(42 cards)
What is Frequency ?
The pitch of sound, measured in Hertz
What is Amplitude ?
The intensity of the sound (loud/quiet)
How does hearing work in the ear?
1). First transduction: Sound waves strike the tympanic membrane and become vibrations
2). The sound wave energy is transferred to the three bones of the middle ear, which vibrate
3). Second transduction: The stapes is attached to the membrane ion the oval window. Vibrations of the oval window create fluid waves within the cochlea
4). Third transduction: The fluid waves push on the flexile membranes of the cochlear duct. Hair cells bend and release neurotransmitter.
5). Fourth transduction: Neurotransmitter release onto sensory neuron crates action potentials that travel through the cochlear nerve to the brain
6). Energy from the waves transfers across the cochlear duct / endolymph into the tympanic duct / perilymph and is dissipated back into the middle ear at the round window
What would normal hearing look like on a graph?
“The Speech Banana”
What are the features of the Outer (External) Ear?
Outer (External) Ear
Pinna (auricle);
- Tragus
External auditory (acoustic) meatus;
- Cartilaginous and bone parts; not in the same direction
- Ceruminous glands
- Suppled by auricular branch of Vagus and Auriculotemporal branch of trigeminal nerve
In external acoustic have ceruminous glands which are modified sweat cells that produce ear wax to keep canal and tympanic membrane healthy
What is the pathology with ear and how should would you treat it ?
Cauliflower ear - results in haematoma and impacts blood supply to cartilage. If left with nothing done cartilage can die and start to heal with fibrocartilage and give you a misshapen ear
Prevention is better than cure, head gear recommended. Can aspirate blood out in acute stage, or incision to get clot out. After that need plastics. Pinna plays role in hearing so may effect this.
What are the features of the Tympanic Membrane?
Tympanic Membrane;
- Concave
- Shadow of the handle of the malleus
- 4 quadrants
- Chorda tympani is in the postero-superior quadrant
- Safest quadrant is antero-inferior quadrant, has the Triangular reflection of light in this quadrant (Politzer’s triangle)
- Rich neural innervation
Middle ear infection causes pressure build up in middle ear and tympani membrane to bulge which is very painful
What are the features of Chorda Timpani nerve?
The chorda tympani is a branch of the facial nerve that originates from the taste buds in the anterior 2/3rd’s of the tongue, runs through the middle ear, and carries taste messages to the brain.
What are the features of the middle ear?
Middle ear;
- Air filled cavity with ossicles, muscles / tendon and nerves
Ossicles that transmit the vibration from the tympanic membrane to the inner ear;
- Malleus, incus, stapes
- Attached to the walls by ligaments
- Tensor tympani muscle
- Stapdius muscle
- Chorda timpani nerve
- Auditory / pharyngotympanic / Eustachian tube
- Mucous membrane continues with pharynx (supplied by glossopharyngeus)
What makes up the middle ear cavity and what is its clinical relevance ?
Aditus antrum -> Mastoid antrum
Within bone so ear infection can spread from middle ear to temporal bone (mastoid part)
Can cause osteomyelitis
What are the middle ear ossicles?
Middle ear ossicles;
- Malleus (including handle malleus)
- Incus
- Stapes (stirrup)
What runs through the middle ear cavity ?
- Chorda timpani comes from tongue ant 2/3rd taste, travelling through middle ear and joining facial nerve
- Tensor timpani goes through bony canal superior to Eustachian tube and attaches to handle malleus
- Stapedius is very small (over 1mm long) from wall to middle ear on stapes ossicle
What are the middle ear muscles and their function?
Tensor Tympani;
- Pulls the tympanic membrane medially which increases the tension in response to loud noise from the external ear canal, consequently reducing the vibration of the tympanic membrane, reducing the amount of sound heard.
- Does this when chewing contracts and reduces hearing when chewing
- Supplied by mandibular nerve
Stapedius;
- Stapedius muscle pulls base of stapes away from oval window
- Protects the inner ear from injury to a loud noise (moves stapes and dampens down vibration)
- Can still get damage before stapedius contracts as has to go from middle ear to get to there
- Supplied by facial nerve
- Runs from wall middle ear to stapes
What are the features of the Pharyngotympanic tube ?
- Walls are normally collapsed (open during yawning and swallowing)
- Actively opened by the simultaneous contraction of the tensor veil palatine and salpingopharyngeus muscles
- The tube is short and straight in children (Things can travel from nanocavity into pharyngotympanic tube and spread up into middle ear)
What are the features of the Inner Ear?
Inner Ear - a bony labyrinth
Comprised of;
Cochlea
Vestibule;
- Utricle
- Saccule
Semicircular canals;
- Ducts
- Membranous labyrinth (Made of soft tissue - different fluid, endolymph )
- Perilymph
What is the Cochlea and its functions?
- The Cochlea is a long tube spiralling 2.7 times, bony projection into tube all the way around called osseus spiral lamina
- Cochlear duct contains the Organ of Corti where sound waves translated into neural impulses, where hearing really happens, filled with endolymph and where hearing happens
- Basilar membrane helps determine pitch pushes organ Corti and hair cells up to the tectorial membrane and that’s how we generate action potentials
- Basilar membrane also vibrates In response to sound waves and helps to determine sound frequencies
- Scala tympani and vestibuli - both full of perilymph and vibrations travel through them before reaching cochlear duct
- Scala vestibuli and cochlear duct are separated by the vestibular membrane
- Cochlear duct and Scala tympani are separated by the basilar membrane
How does frequency detection work?
- Structure of basilar membrane changes from short and stiff to long and floppy along the length of thee cochlea
- Resonant frequencies vary along the cochlea with high frequency at the base (more stiff) and low at the apex (floppier)
- 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
- Tona topical organisation and is how you can discern high freq from low where they resonate along basilar membrane
How does Signal Detection work 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 cochlear nerve ending terminate on the inner hair cells
- Outer hair cells increase the sensitivity of inner hair cells
- This can tune the cochlea by amplifying select frequencies
How does Signal Transduction work?
Displacement of sterocillia in one direction opens K channels, and closes them in the other
What happens in Cochlea Tuning ?
- Inner hair cells become depolarised and send signals to the cochlear nerve thence to
the CNS - Mechanical displacement does not provide the sharpness of pitch discrimination
recorded so there must be a system to enhance this - Outer hair cells are stimulated by the basilar membrane to depolarise, but this causes
(as well as action potentials) the cell to contract. - Precise mechanism of the effect is still being researched but the effect is to enhance
the auditory signal at the center of the standing wave and inhibit on either side. - Tuning is also under active Olivocochlear neuronal control. Fibers along this path
release Ach onto the inner hair cells causing them to depolarise. This effectively
damps down hearing in areas of pitch which are of no interest to the listener
(background noise etc)
What are the different Auditory Pathways?
Auditory Pathways;
- Hair cells of the Organ of Corti generate an electrical signal
- Peripheral extensions of the bipolar neurons at Spiral ganglion synapse with hair cells of the Organ of Corti
- Central extensions of bipolar neurons form 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 ovary nucelus
- Lateral lemniscus (3rd order) ascend and synapse at inferior colliculus
- 4th order neurons project to the medial geniculate nucelus of the thalamus where they synapse
- 5th order neurons join the auditory radiation to too the auditory cortex
Note: Collateral from the pathway project into the reticular formation and the vermis of the cerebellum, causing arousal response to noise
Where do auditory secondary projections go to and how does this map onto the brain?
- Secondary projections from the primary, and some from the thalamic association areas then go to the association cortex
- Sound is relayed topographically to these cortical areas, with lower frequencies to the anterior in most maps though there are variations
How does Direction of Sound work and the 2 types its broken into?
1). Volume and Sound shadow (Good at High Freq);
- Sounds 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
2). Sound lag (Good at Low Freq);
- Sounds from a particular direction enters one ear before the other and so there is a slight delay between the sound arriving ipsilaterally at the auditory cortex and that arriving contralaterally. Enhanced version of this used by owls for prey location
- Sounds lag (which works at lower frequencies) is better at determining horizontal direction than sounds shadow (which is good high frequencies)
- Neither method detects front to back or above to below directionally
- This is achieved by the folds in the pinna which changes the characteristics of sounds coming from above compared to below etc
What are the 2 types of deafness / hearing pathologies ?
Conduction deafness;
- 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
- Weber’s test: Unilateral conductive loss, sound lateralizes toward affected ear
- Negative Rinne: Sound is heard louder from the mastoid (conductive hearing loss)
Sensory-neural deafness;
- Breakdown of the cochlea and associated mechanisms
- Damage to auditory nerve
- Damage to auditory cortex
- Weber’s test: Unilateral sensorineural loss, sound lateralizes to the normal or better-hearing side.
- Positive Rinne: air conduction is perceived louder than bone conduction (normal or sensorineural)
