Pathophysiology of sensory systems Flashcards
What parts of the ear can cause conductive deafness?
1) Auditory canal
- blocked by wax
2) Middle ear
- ruptured tympanic membrane
- otitis media (inflammation)
- otosclerosis (hardening of bones in ear)
What parts of the ear cause sensorineural deafness?
1) cochlea
- ageing
- acoustic trauma
- ménière’s disease (pressure imbalance)
2) cochlear nerve
- acoustic neuroma
3) central auditory pathways
What usually causes otitis media?
infection caused by fluid in the ear
Describe how sound travels through the ear?
1) sound conducted through external auditory canal to the tympanic membrane
2) ossicles transmit vibrations across middle ear to oval window
3) causes motion of fluid in scala vestibuli
4) vibration reach basilar membrane which cause Corti hair cells to generate signal
How can we tell the frequency and amplitude of sound?
Frequency - dependant on site of maximum resonance along basilar membrane
Amplitude - frequency of action potential
Should sound conduction be better in air or bone?
air so if better in bone than Rinne’s test will be positive
What are the two hearing tests?
Rinne’s
Weber’s
What is done in a audiometry test?
play different frequency sounds and test hearing level at each
What conditions will show disparities in hearing level at different frequencies?
acoustic trauma and ageing
hear better at higher frequencies
What are the visual tests
Snellens for visual acuity visual field test inspection fundus pupillary responses extra ocular movements
What pathway do auditory and cochlear signals travel through to reach auditory cortex?
- travel via CNVIII (vestibulocochlear)
- synapse in cochleae nuclei of brainstem
- ascend through inferior colliculus in midbrain to medial geniculate nucleus of thalamus
- ascend to auditory cortex in temporal lobes
What is the difference between sensorineural and conductive deafness?
sensorineural - defect in transduction mechanism converting sound to neuronal activity or processing of neural signals
conductive - defect in mechanical conduction of sound in inner ear
When will Rinnes test be positive?
pos = can hear better in air than bone
people with no hearing problem or people with sensorineural deafness will still be positive
When will Webers test show preference for one ear?
If patient has conductive deafness will be able to hear better in the other ear
How does the lens shape change to account for close objects?
- close objects parasympathetic innervation stimulate contraction of circular muscle in ciliary body
- reduces tension on zonule fibres and allows relaxation of lens
- has bulbous shape when relaxed
How does lens shape change to account for far objects?
- ciliary muscle relaxes
- lens pulled flat
What three aspects does the eye depend on for clear focused images?
1) cornea focussing
2) lens focussing
3) transparent light path (no cataracts or bleeding)
What cells are responsible for creation of neural signals in the eye?
Photoreceptors:
Rods - very sensitive
Cones - lower sensitivity but can assess colour
Where are cones and rods primary located?
Cones in macula specifically fovea
Rods across retina
What is the function of bipolar cells?
connect photoreceptors to ganglion cells in inner retina
What cranial nerve carries visual information from the eye?
Optic (CNII)
Where do the axons coming from the eye cross?
- optic chiasm
- only axons from nasal (left side) vision in each eye cross
- means signals from the right half of the visual field in each eye go to the left side of the brain (vice versa)
Where are the fibres carried after passing the optic chiasm?
- optic chiasm > lateral geniculate nucleus of thalamus
- then optic radiation passes from LGN to occipital cortex
What is the usual cause of increased intraocular pressure?
excess secretion of aqueous humour in the posterior chamber or impaired recirculation via trabecular network
