L29 30 Vision Flashcards
What are the nerve supplies for extraocular muscles? (move eyeball)
- superior oblique= trochlear nerve
- lateral rectus= abducens nerve
- all other extraocular muscles= oculomotor nerve
What is the range of light which we can see?
- 400 to 700 nm in wavelengths
- hte light we see is reflected and scattered from objects
Functions of pupil, iris and lens
- pupil= allows light to enter eyes
- iris= contains sphincter pupillae that increases/ decreases the size of pupil
- lens= curvature controlled by the ciliary muscles so changes where light falls (control by parasympathetic fibres of oculomotor nerve via ciliary ganglion)
Functions of cornea, choroid layer and aqueous humor
- cornea= curvature which light first enters so can bend the light
- choroid= vascular layer of the eye containing connective tissues and lie between retina and sclera. It contains dark melanin choroid pigments to help limit uncontrolled reflection of light in the eyes. Increase in vision acuity
- aqueous humour= remove waste and provide nutrients
What are the remarks you can see on retinal landmarks( looking into eyes using an olphathalmic scope)?
- optic disc= blind spot where axons of the ganglion cells and enter the optic nerve
- macula= central area of the retina in line with the visual axis (no vascular supply)
- fovea= depression in the centre of the macula where the vision acuity is the greatest (no vascular supply)
- foveloa= centre of the fovea contains only cone receptors
What is fovea?
- depression of centre of the macula where vision acuity is the greatest
- tissues are pushed away and allow light directly fall on photoreceptors( as they are on the bottom layer of structures)
What is the image like when formed on the retina?
- it’s upside-down and turned (left to right)
What is the general population which suffers from macula degeneration?
- 10% in patients age 66-74; 30% in patients age 75-85
- wet form= affects vasculature; dry form= builds up of waste products
What is retinitis pigmentosa?
-loss of peripheral vision due to damages to rod cells so have night vision problems too
What is aqueous humour?
- replaced every 60 mins
- produced in the posterior chamber by the ciliary process
- provide nutrients to the lens
- excess production leads to glaucoma (can be treated by using carbonic anhydrase inhibitor)
- normal pressure= 15-22mmHg
Pupils and pupillary light reflex
- a circular group calls sphincter pupillae contracts and constricts pupil
- a radial group calls dilator pupillae contracts and dilate pupil
- the pupillary light reflex should occur at both eyes even if the stimulus was applied only to one eye. Because there’s decussation of information feeding to the other eye.
- the reflex is completely damaged if both pupils can not constrict despite light shinning
- or only pupil constricts in the eye with stimulus applied means there’s a problem with information crossing over
Lens
- transparent bioconvex structure behind the iris
- held in position by suspensory ligaments
- thickens throughout life and cataracts can occur due to an accumulation of metabolic products
What is the accommodation reflex?
- when the eyes view an object close up, the ciliary muscle contract and allow the lens to become convex= increase in refractive power
- convergence of the two eyes occur so the image falls on the fovea of both eyes (otherwise double vision can occur)
- pupillary constriction improves the optical performance of the eye by increasing the depth of the focus
What is myopia?
- where light focused in front of retina. Can be corrected by using concave lens
What is hypermetropia?
- light focused behind the retina so needs to correct it with convex lens
How do the eyes view an object which is very close up? (involves the accommodation reflex)
- the ciliary muscles contract reflexively and allows the lens to become convex. This increases the refractive power
- convergence of the two eyes occurs so the image falls on the fovea of both eyes (otherwise diplopia, double vision occurs)
- pupillary constriction improves the optical performance of the eye by reducing aberration and increasing the depth of focus
What is the pathway for the accommodation reflex?
- info is trasferred from retina (via LGN) to visual cortex -> frontal eye field then to edinger-westphal nucleus -> ciliary muscle
OR
from frontal eye field then to occulomotor nuclei -> medial recti
What is emmetropia?
(basically means normal eye vision)
- this is when the eyes are relaxed and focused on an object more than 6 meters or 20 feet away. The light rays coming from that object are essentially parallel, and the rays are focused on the retina without effort
What is the structure of rods and cones?
- light sensitive part of the photoreceptor is the outer segment
- light has to pass through the retina before detection
- layers do not represent a barrier to light as the retina is transparent and is 0.4mm thick
What are the characteristics for rod cells?
- they detect low range of light (scotopic)
- 1 photon of light and can be easily saturated
- disks organised for efficient photon capture
- mostly found in periphery and allows convergence
