N20: Visual system Flashcards
what are the 3 layers of the eye?
- corneoscleral coat
- choroid coat
- retina
Describe the corneoscleral coat.
- tough connective tissue
- sclera white (hole for optic nerve)
- cornea transparent (refraction of light is zero)
Describe the choroid coat.
- highly vascular
- choroid: absorbs light – prevents reflection
- ciliary body: produces aqueous humor
- iris – regulates light entering the eye
Describe the retina.
contains photoreceptors - rods and cones
what permits accommodation for near vision?
biconvex lens which is elastic to allow it to change its shape
what are the anterior and posterior chambers filled with?
aqueous humor
what lies posterior to the lens?
larger vitreal cavity filled with vitrious humor (also called the vitreous body), which is avascular and gelatinous
what anchors the lens in place?
lens is suspended from ciliary body
what is the opening inn sclera through which the optic nerve passes?
lamina cribrosa
what causes a glaucoma?
distribution of balance of flow of aqueous humour from posterior to anterior chamber so pressure is exterted on the retina
what secretes aqueous humor?
cilliary body
what adjust the lens?
cilliary muscles
what stops stray light entering pupil?
melanocytes at the back of iris
what is the difference in melanin in blue and brown eyes?
blue eyes- low melanin
brown eyes- high melanin
what part of the eye allows us to read?
fovea centralis
what area of the eye is the blind spot?
optic disk (where optic nerve exits)
what happens to an image projected in retina?
turned upside down and back to front
what is higher in number in the retina?
much more rods than cones
what does each rod contain?
rhodopsin- a light sensitive pigment
what does each cone conatin?
one of 3 opsin pigments, sensitive to either red, green or blue light
what effect does light have on rods and cones?
hyperpolarizes them, decreasing their firing rate
where is information passed to in the retina?
Information passes from photoreceptor to bipolar cell to retinal ganglion cell
what modifies the information passing from photoreceptors to bipolar cell to retinal ganglion cell?
association neurons
what Is the point of fixation?
Detailed high resolution colour vision at the Fovea centralis
what are the reasons for the f fovea centralis’s high visual acuity?
(1) Thinning of retinal layers
(2) Lack of rods, high concentrations of cones
(3) Low convergence
when are axons of the otic nerve myelinated?
Axons are not myelinated within the eye, but are myelinated after passing the lamina cribrosa
what produces myelin in the optic nerve?
oligodendrocytes
what is the optic nerve surrounded by?
meninges
what causes the blind spot at the optic disc?
lack of rods and cones
where is visual information going to in the thalamus?
lateral geniculate nucleus
what are the basic elements of visual pathway?
- optic nerve
- optic chiasma
- optic tract
- Lateral geniculate nucleus (thalamus)
- optic radiation
- striate cortex (primary visual cortex)
what is the optic chiasma?
crossing point for fibres coming out of the eye
which of the fibres cross at the chiasma?
nasal retina fibres
which fibres stays on the ipsilateral side?
temporal retina fibres
what vision laterally crosses at the chiasma?
peripheral vision
what does each optic tract contain?
opposite visual world derived from both eyes
what causes objects to be projected onto the retina reversed and upside down?
lens
what does each optic tract, lateral geniculate nucleus (of the thalamus), optic radiation and visual cortex deal with?
visual information from the contralateral field
In addition to the projection to the primary visual cortex, where else do fibres from the retina project to?
- The pretectal area and nearby superior colliculus (probably functions in sustained pupillary contraction in bright light).
- A small number of fibres leave the dorsal surface of the optic chiasma and enter the suprachiasmatic nucleus of the hypothalamus (probably involved in circadian rhythm, particularly diurnal hormonal changes and sleep)
what does the superior colliculus function in?
saccadic eye movements
what does superior colliculi receive input from?
visual cortex, frontal eye fields and spinal cord
where does superior colliculi give output to?
Nuclei of CN III, IV and VI, motor nucleus of VII, and spinal cord
how does fibres from the lateral geniculate nucleus of the thalamus reach the primary visual cortex?
geniculocalcarine tracts (also called the optic radiation)
what do fibres from the lateral geniculate nucleus to the primary cortex initially form?
part of the internal capsule
what path do the fibres carrying visual information from the upper half of the visual field?
fibres first loop anteriorly around the temporal part of the lateral ventricle in meyer’s loop ending below the calcarine sulcus
where is the visual cortex found?
Surrounds the calcarine sulcus in the occipital cortex, it is thinner than most cerebral cortex
what is the visual cortex referred to as?
area 17 of Brodmann
why is the primary cortex also called the striate cortex?
prominent stripe of myelinated fibres found in layer IV which can be seen with the naked eye, called the line of Gennari.
where is the lower visual field projected to?
gyrus superior to calcarine sulcus
where does the macula project to?
posterior pole of the visual cortex and occupies a much greater proportion of the cortex relative to the size of the visual field it covers
where do objects in the centre visual field go on the visual cortex?
back of the visual cortex/occipital lobe
where do periphery vision go on the visual cortex?
further forward in the visual cortex/occipital lobe
where is colour, orientation and faces processed in the visual association cortex?
colour -area 18
orientation- area 19
faces- area 20
what is the main blood supply to primary visual cortex?
posterior cerebral
where do posterior and middle cerebral arteries overlap?
occipital pole-macular sparing
what happens in both pupils when light is shone into right eye?
- Right pupil constricts- direct light reflex
- Left pupil constricts - consensual reflex
what is required for the accommodation reflex?
requires input to oculomotor nucleus from the visual cortex?
what does the accomodation reflex produce?
- lens thickening (ciliary muscle)
- pupillary constriction (increase field depth)
- convergence of eyes (medial recti)
when is accommodation reflex used?
change in point of focus i.e from close to far away
what is the definition of hemianopia?
Blindness for half the field of vision in one or both eyes
what is it called when vision is lost in the same visual field in both eyes?
homonymous
what is it called when vision is lost in different visual field of each eye?
heteronymous
What would be the effect of a lesion of Meyer’s Loop on the right side?
lose one quadrant of vision
what provides the movement s in response to visual stimuli i.e tracking moving objects?
visual cortex
what provides movements of command (movements that are independent of moving visual stimuli) ?
frontal eye fields
what is tracking movements known as?
smooth pursuit eye movements
what is movements of command known as?
saccadic eye movements
Describe the effects of myopia.
- short-sighted
- able to focus near objects
- unable to focus distant objects
- eye ball relatively too long
what is the treatment of myopia?
treatment concave lens to diverge light
Describe the effects of hyperopia.
- long-sighted
- unable to focus near objects
- able to focus distant objects
- eye ball relatively too short
what is the treatment of hyperopia?
treatment convex lens to converge light
Describe the effects of presbyopia.
-normal age change
-gradual loss of accommodation
-lens less elastic
-ciliary muscles weaker
-Near point:
100 mm in young adults
800 mm in old age
what is the treatment of presbyopia?
treatment convex lens to converge light
