What comprises the outer ear?
Auricle and external auditory meatus up to the tympanic membrane.
What bone do the middle and inner ear sit in?
Petrous part of the temporal bone.
What is the function of the auricle?
Collects sound, helping with localisation of sound in space.
What is the difference between the ear lobule and the rest of the auricle?
The ear lobule consists of fat and subcutaneous tissue, whereas the auricle is supported by cartilage.
What does the external auditory meatus do?
Collects sound and transmits it to the tympanic membrane.
What is nerve supply to the external ear done by?
Vagus (posteroinferior) and auriculotemporal nerve of CNV3 (anterosuperior and external surface of the tympanic membrane)
Where can damage to the anterosuperior external ear and tympanic membrane refer pain to?
Other structures innervated by CNV(III), e.g. the lower teeth.
What is the external auditory meatus comprised of?
Lateral 1/3: Cartilage
Medial 2/3: Bone (temporal)
What lines the external auditory meatus?
Hairy skin with cerumen glands
What are cerumen glands? What is their function?
Secrete wax, preventing the maceration of skin with water.
What can be seen on the lateral surface of the tympanic membrane?
Flaccid area, handle of malleus and a cone of light in the antero-inferior quadrant.
What shape is the tympanic membrane?
Concave externally and convex internally
What happens to the tympanic membrane in middle ear infections?
A build up of pressure in the middle ear changes the position of the cone of light and the shape of the tympanic membrane.
Where is the middle ear located?
From the tympanic membrane to the petrous part of the temporal bone.
Where does the auditory tube project?
Anteroinferiorly from the middle ear to the nasopharynx.
What is the function of the auditory tube?
Equalisation of pressure between the middle ear and external environment and drainage from the ear.
Where can infections of the epitympanic recess communicate to?
Air cells in the mastoid process, settling to the bottom due to gravity and causing recurring problems.
What is the difference between the auditory tube of infants and adults?
The auditory tube is more horizontal in infants, meaning bacteria can migrate to the middle ear easier and the area is also harder to drain.
How can sound transduction in the middle ear by bypassed in infants with chronic ear conditions?
By pacing a bone conductor onto the mastoid process.
What is a possible consequence of chronic middle ear infections, as seen in those with cleft palate due to palatine muscles not developing properly? What is the common treatment for this?
Problems with language.
A grommet can be inserted through the tympanic membrane to drain the middle ear.
What are the ossicles attached to?
The tympanic membrane laterally and the oval window in the petrous part of the temporal bone medially.
How do the ossicles transduce sound?
When the tympanic membrane vibrates, the ossicles vibrate, tapping on the oval window.
What types of joints do the ossicles have?
What are the 3 ossicles?
Malleus, incus and stapes.
What does the handle of malleus attach to?
What does the base of the stapes articulate with?
The oval window via the oval-shaped base of stapes.
What are the 2 muscles seen in the middle ear?
Stapedius and tensor tympani.
What is the role of tensor tympany?
Dampens the amplitude of vibration of sound passing through malleus when the sound is too loud - a protective mechanism.
What is tensor tympani innervated by?
What is stapedius innervated by?
Which nerve can be seen running through the superior part of the middle ear?
Chorda tympani (CNVII)
What comprises the medial wall of the middle ear?
The oval window opening into the inner ear and the round window.
What is the function of the round window?
To to release pressure from the ear back into the middle ear.
What is the large bulge seen on the medial wall of the middle ear?
Why can throbbing be felt when there is fluid in the middle ear?
Because the internal carotid artery sits close to the middle ear in the carotid canal. The throbbing is pulsations of the artery.
What comprises the inner ear?
Membranous and bony labyrinths.
What is the bony labyrinth?
A space in the petrous part of the temporal bone filled with perilymph.
What is suspended within the bony labyrinth?
The membranous labyrinth, filled with endolymph.
True or false: there is no communication of fluid or air in or out of the labyrinth.
True - because stapes is sitting on the membrane-covered oval window.
What are the different parts of the bony labyrinth?
Vestibule, oval and round windows, cochlea and semicircular canals.
What are suspended in the membranous labyrinth?
Sensory receptors for hearing and equilibrium
What are the different divisions of the membranous labyrinth?
Semicircular ducts (ant., horiz. and post.)
Which sensory receptors are in the utricle and saccule?
Sensory receptors predominantly for static equilibrium (head position)
Which sensory receptors are in the ampulla?
Sensory receptors for dynamic equilibrium
What are the 3 semicircular ducts?
Anteriorl, horizontal and posterior.
Where does the inner ear sit?
In the petrous aprt of the temporal bone.
Where do the semicircular ducts sit in relation to one another?
Anterior posterior sit at 90 degrees to one another and the horizontal sits below the anterior and posterior. All sit 45 degrees to head movements.
If the cochlea were to be rolled out, in which direction would it roll?
What are the percepts related to sound energy?
How is sound energy transduced?
Tympanic membrane moves ossicles
Ossicles move oval window
Cochlear transforms physical motion of the oval window into a neural response
How do the ossicles match the impedence of air to the impedance of the fluid within the inner ear?
Normally when sound passes from air to fluid, most is reflected.
This is overcome by pressure 200 fold greater at tympanic membrane compared to inner ear.
Size: tympanic membrane>>oval window (20:1)
Lever action of ossicles:1.3:1- sclerosis of the joints can lead to reduced hearing.
What comprises the inner ear?
The cochlea, composed of 3 chambers:
1. Scala vestibuli
2. Scala media
3. Scala tympani
The basilar membrane sits below the hair cells and the tectorial membrane sits above.
What are the characteristics of the basilar membrane?
Wider at apex than base
Stiffer at base
Base responds to high frequencies
Apex responds to low frequencies
How is the basilar membrane organised?
Tonotopically - cells next to each other down the basilar membrane will respond to slightly different frequencies.
What is the organ of Corti?
The auditory receptors are hair cells sandwiched between the basilar membrane and reticular lamina.
There are inner and outer hair cells.
3 rows of outer
1 row of inner
How do hair cells work?
Each has about 100 stereocilia
Bending of stereocilia causes neuronal signaling
Tectorial membrane sits on top of stereocilia.
The basilar membrane moves in response to sound, hair cells move up and down, shearing force comes from tectorial membrane, causing the stereocilia to move, transducing the signal.
How do hair cells transduce movement of their cilia into neuronal signals?
Potassium channels are partially open – some potassium leaking into cell permanently
Deflection of hair bundle towards tallest stereocilium causes opening of channels, and depolarization.
Hyperpolarization causes closure of channels.
Increased K+ causes depolarization and opening of VDCC, which leads to release of neurotransmitter glutamate.
How is the mechanoelectrical transduction by hair cells asymmetric?
Larger depolarisations than hyperpolarization
They tend to want to depolarize rather than hyperpolarize
What is the importance of potassium in mechanoelectrical transduction by hair cells?
Potassium serves to both depolarize and repolarize the cell
Potassium is recycled through potassium channels
Cell connections in the vasa vascularis are important.
Which hair cell is responsible for 95% of the nerves that project from the cochlea to the brainstem nuclei?
Inner hair cells
What do outer hair cells receive input from?
Superior olivary complex
What happens to outer hair cells during low intensity stimuli?
What does a change in length of the outer hair cells do?
Accentuates movements of the basilar membrane, i.e. amplification of signal that is received by the inner hair cells.
What does depolarisation of the outer hair cell cause?
What does hyperpolarisation of the outer hair cell cause?
What happens to basilar membrane movement without outer hair cells?
Movement is 100-fold less
Which antibiotics/pharmaceuticals can cause loss of outer hair cells and loss of hearing?
Gentamicin, aminoglycosides and aspirin (temporary)
Describe the auditory pathway.
Hair cells (within cochlear)
CNVIII (auditory-vestobuar nerve)
Cochlear nucleus (medulla)
Medial geniculate nuclear
Where does integration of information from both ears occur?
At the cochlear nuclei in the rostral medulla, although many pathways of auditory information flow from cochlear nucleus to auditory cortex with extensive crossing over of information.
Which relay nuclei within the brainstem mediate sound localisation?
Anteroventral cochlear nucleus to superior olivary complex.
What does the superior olivary complex consist of?
Lateral and medial superior olives & the trapezoid body.
MSO: Localization of sound by measuring time delay.
LSO: localization of sound by sensing intensity differences.
What is the duplex theory for sound localisation in the horizontal plane?
Time that sound arrives at each ear – based on the length of the path to the medial superior olive - especially for low frequencies (deep sounds)
Interaural intensity difference - for high frequencies
What role does the MNTP play in sound localisation in the horizontal plane?
Medial nucleus of trapezoid body.
Sends off inhibitory neurons to contralateral side.
The relative amount of excitation and inhibition determines where the brain thinks something is.
Where is the auditory cortex located?
At Herschl's gyrus on the dorsal temporal lobe.
How is the primary auditory cortex (Herschl's gyrus) mapped out?
Neurons are sharply tuned for sound frequency.
Columnar organization: cells in same column are tuned to same frequency.
Alternating regions of input from both ears - excitatory input from one ear and inhibitory input from the other ear.
How are different sounds represented in the cortex?
Asymmetrical representation of complex sounds occurs.
Speech sounds: left hemisphere
Environmental sounds: both hemispheres
Music: right hemisphere.
What are the different causes of sudden sensorineural hearing loss?
Peripheral causes: meningitis, Guillain-Barre, acoustic neuroma, metastasis.
Cochlear causes: Infection (esp HSV, autoimmune disease, traumatic, metabolic, vascular, ototoxicity)