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
What pathologies can be seen by viewing of the retina?
Papilloedema (swelling of optic disc)
Hypertensive retinopathy
Diabetic retinopathy
What is presbyopia and what is the underlying cause?
- loss of accommodation for near objects
- loss of lens elasticity (takes up less convex shape)
What is myopia?
- short sighted
- image is formed in-front of the retina due to eyeball being too long for the refracting system
What is hypermetropia?
- long sighted
- image forms behind retina
- eyeball too short for refractive system
Where would you expect vision field defects with a retinal lesion?
Paracentral scotoma (opposite side of visual field to lesion)
Where would you expect vision field defects with a optic nerve damage?
monocular field loss
Where would you expect vision field defects with a optic chiasm lesion?
bitemporal hemianopia
Where would you expect vision field defects with a optic tract lesion?
homonymous hemianopia
Where would you expect vision field defects with lesion in temporal and parietal portion of optic radiation?
temporal - upper homonymous quadrantanopia
Parietal - lower homonymous quadrantanopia
Where would you expect vision field defects with a occipital cortex or occipital optic radiation lesion?
homonymous hemianopia
Where would you expect vision field defects with a occipital pole lesion?
homonymous hemiscotoma
What nerves control dilation and constriction of pupils?
both autonomic
Dilation - sympathetic
Constriction - parasympathetic
What muscles control dilation and constriction?
dilation - dilator pupillae
Constriction - sphincter pupillae
What is Horner’s syndrome?
sympathetic lesion causing ipsilateral defects
What are the symptoms of horner’s syndrome?
- miosis
- ptosis
- enophthalmous
- loss of sweating
What is ptosis?
dropping eyelid
What is miosis?
unilateral pupillary constriction
What is enophthalmous?
depression of eye in eyeball
What pathology would cause dilation of only one pupil when fixed to light
CNIII nerve compression interrupting parasympathetic supply
can indicate neurosurgical emergency
What pathology would cause bilateral fixed dilated pupils?
Deep coma or brain death
What pathology would cause fixed constricted pupils?
pontine lesions
opiates
What pathology would cause Argyll Robertson pupil?
near response but no light response
lesion in dorsal EW nucleus
What are some symptoms of sensory loss?
- numbness
- clumsiness
- specific areas of cutaneous sensory loss
What are some symptoms of abnormal sensations?
- tingling (paraesthesia)
- pain (neuralgia)
What is a dermatome?
the sensory distribution of a specific spinal root
What pathway to the pain, temperature and coarse touch senses travel to Brain via?
Contralateral spinothalamic tract (cross spinal cord close to the spinal root) to thalamus and from thalamus to sensory cortex
What pathway do proprioception and fine localised touch travel to brain via?
ipsilateral dorsal column reach dorsal column nuclei in the brainstem where they then cross to the contralateral thalamus and then to the sensory cortex
How will peripheral nerve damage present?
loss of sensation in distribution of nerve and weakness in muscle innervated by it
How will nerve root damage present?
loss of all sensation in affected dermatome, can be motor loss as well if ventral nerve root is affected
How will posterior column damage present?
ipsilateral sensory defects below level of defect involving position sense, vibration sense and light touch
How will spinothalamic tract damage present?
contralateral sensory defects below level of defect causing pain, temperature and coarse touch
How will bilateral spinal cord damage present?
loss of all sensation and bilateral weakness below lesion
What are the symptoms of Brown-Sequard syndrome?
1) ipsilateral loss of proprioception, vibration and light touch below lesion
2) contralateral loss of pain, temperature and coarse touch below lesion
3) ipsilateral weakness below the lesion and LMN sign at level of lesion
How will lesions at the brainstem present?
contralateral sensory defects in all modalities below sit of damage
how ill lesion above brainstem present?
contralateral sensory loss in all modalities of face and body
How will peripheral neuropathy present?
loss of sensation which can ascend as it progresses
can involve motor and autonomic activity and can appear as generalised defect in nerve excitation/conduction within affected region of body
