Visual Pathway Flashcards
what converts light into action potentials?
retinal rod, cone and ganglion cells
the lens is under what control and what does it do?
the ciliary muscle
- and it produced the image onto the retina
axons in the optic nerve decussate where?
at the optic chiasm
fibres from the nasal retina cross and join with the uncrossed fibres in the ……… retina to form the ……. …….
fibres from the nasal retina cross and join with the uncrossed fibres in the temporal retina to form the optic tract
-> thus each optic tract carries info from the contralateral visual hemifield
what is the LGN (lateral geniculate nucleus)?
connection of the optic nerve with the thalamus. it is a thalamic nucleus which provides a relay station for all axons of the retinal ganglion cells subserving vision.
fibres pass in the optic radiation from the LGN to reach visual cortex of the occipital lobe (6 and 7) through what lobes of the brain?
parietal and temporal lobes
visual information is further processed by neighbouring visual association areas which detect what?
they detect lines, orientation, shapes, movement, colour and depth: their is also a distinct area responsible for face recognition.
lesions posterior to the optic chiasm produce …… field defects
homonymous
-> indicating involvement of the same past of the visual field in both eyes. as info from 2 visual fields is seperated beyond this point
unilateral visual loss with a central or paracentral scotoma is hallmark of what kind of lesion?
an optic nerve lesion
- as most fibres in the optic nerve subserve macular vision.
describe some examination findings in optic neuropathy?
- reduced acuity in the affected eye
- a scotoma (central)
- impaired coloured vision
- afferent pupillary defect
- optic atrophy - pale disc
what is papilloedema?
swelling of the optic disc, shows as disc pinkness, blurring and heaping up of disc margins. nasal first.
what is optic neuritis?
one of the most common causes of subacute visual loss. symptoms may vary from a mild fogging of central vision w colour desaturation to dense central scotoma and complete blindness
- pain on eye movement
- confers a higher risk to developing MS
- recovering involves high dose IV steroids or oral
- can also be caused by infection or inflam disorders e.g. sarcoidosis or vasculitis
what is ant ischaemic optic neuropathy?
the ant part of optic nerve is supplied by post ciliary arteries, occlusion or hypoperfusion leads to infarction to optic nerve head
- results in sudden or stuttering visual loss (lower half of visual field)
bitemporal heminiopia or quadrantopia occurs with compression of the chiasm from above or below. name some common causes of compression?
- pituitary tumours
- meningioma
- craniopharyngioma
optic tract lesions cause a … ……
homonymous hemianopia (loss of contralateral visual field in both eyes)
optic radiation lesions cause ….. …… ….
homonymous contralateral quadrantopic defects
-> temporal lobe lesions (ie tumour, infarction) cause upper quadrantic defects (‘pie in the sky’)
-> parietal lobe lesions cause lower quadrantic defects (‘pie on the floor’)
for a left afferent pupillary defect what happens in the eye?
- the left pupil is unreactive to light (direct reflex)
- the consensual reflex is also absent
- however when left is shone in right intact eye, consensual reflex of the right eye remains intact
how does RAPD occur?
incomplete damage to one optic nerve relative to the other
what is Horners syndrome?
damage to the sympathetic nervous supply to the eye (hypothalamus -> down brainstem to cervical cord of T1 nerve root -> paravertebral sympathetic chain, on via carotid artery wall -> to the eye
what are the symptoms of Horner’s syndrome?
unilateral miosis (constricted pupil)
partial ptosis
loss of sweating on the same side
what causes Horners syndrome?
hypothalmic, brainstem lesions -> cerebral infarction, brainstem demyleniation
cervical cord -> syringomelia, and cord tumours
t1 root - alpical lung tumour, TB, cervical rib, brachial plexus trauma
sympatheyic chain and carotid artery in neck
miscellaneous -> congential, cluster headache, transient and idopathic
lesions affecting the calcarine sulcus of the occipital lobe tend to cause a?
homonymous hemianopia with sparing of the macula. macular vision is spared given the dual blood supply to ant portion of the visual centre (PCA, MCA)
optic nerve damage causes what kind of field defect?
ipsilateral monocular blindness
name 2 types of photoreceptors in retina?
rods and cones
- contain light-sensitive surface membrane proteins called rhodopsin
- rhodopsin in these cells are stimulated to propagate second messenger responses which convert light energy into electrical signals
- photoreceptors synapse with retinal bipolar cells which in turn transmit these signals to retinal ganglion cells.
what kind of cells converge at the optic disc forming the optic nerve?
the retinal ganglion cells
rods are used for?
seeing in dim light
cones are used for?
seeing in normal daylight
do rods or cones have higher convergence?
rods have more convergence, increasing sensitivity, while decreasing acuity. (more rods are required to stimulate ganglion cell)
where are cones found?
central retina (fovea)
where are rods found?
peripheral retina
what are the 3 basic types of retinal ganglion cells?
M - movement
P - form and colour
nonM-nonP cells - colour
long wavelength cones (L-cones) are sensitive to mostly light within what spectrum?
red spectrum
the medium wavelength cones (m-cones) are responsive to light in what spectrum?
green spectrum
the short wavelength cones (s-cones) primarily detects light in what spectrum?
blue spectrum
what photoreceptor cells are responsible for colour vision?
cones
-> they detect light within a particular spectrum
axons arising for the nasal retina eventually terminate on what layers of the contralateral geniculate nucleus?
layers 1, 4 and 6
axons of ganglion fibres from temporal retina will terminate on what layers of the ipsilateral geniculate nucleus?
2, 3 and 5
what broadman area is the primary visual cortex found?
broadmann area 17
photo transduction cascade of rod cells in response to light
- when light activates the rod cells. 11-cis is converted to trans-retinal releasing opsonin.
- opsonin activates transducin and activates PDE which breaks down C.GMP so Ca and Na influx cannot occur. thus cell becomes less pos = hyperpolarized. IPSP.
- less glutamate being released from rods
- as little glutamate is being released this stimulates the bipolar neurone
- bipolar cells become depolarized and produce EPSP. glutamate released stimulating ganglion cells
- ganglion cells produce A.P’s
opposite response in dim light
- all trans -> 11 cis
- more glutamate being released which inhibits bipolar = IPSP
- less glutamate moving down ganglion and optic nerve
the primary visual cortex is divided into …. functionally distinct layers. which layer recieves most visual input from the LGN?
6 layers
Layer 4 received most visual input. 4ca recieves mostly magnocellular input from LGN, while 4cb recieves input from parvocellular pathways.
