Physiology of Vision Flashcards
What are the three layers of the eyeball?
Outer fibrous layer (sclera)
Middle vascular layer (choroid)
Inner neural layer (retina)
In which layer does the biochemical activity and hyperpolarisation take place?
Inner neural layer
What happens at bright light enters the eye?
Pupil constricts
What happens when the light is dim?
Pupil dilates
What allows the pupil to change size?
Muscles of the iris
What produces a focused image on the retina?
Cornea & lens
How is focus changed in the eye?
By varying the shape of the lens
What do cataracts do?
Make the lens opaque - stops light entering the eye making it difficult to see
What changes the power of the image in the eye?
Iris (constricting and dilating) can vary diameter x4 - which varies retinal intensity by x16.
What absorbs unwanted light in the eye?
A pigmented layer being the retina
How many times does light bend in the eye?
Twice - once when passing through the cornea and once when passing through the lens.
Which part of the eye bends the light the most?
Cornea - bends 2/3rds
Lens - bends 1/3
What shape is the lens?
Biconvex
Does a convex lens converge or diverge light rays?
Converging - brings rays together
Does a concave lens converge or diverge light rays?
Diverges light rays - spreads them apart
What is the refractive power of a lens measured in?
Diopters
What is the focal length of a lens?
Distance between a convex lens and its focal point (where the rays converge)
What can affect the focal length of a lens?
Genetics
Pathology
Length of eyeball
Curvature of lens
What is long sightedness called? What is it caused by?
Where is the focal point of the lens?
Hypermetropia (Hypermetropic)
Eyeball too short or Lens too weak
Focal point - behind the retina
What is short sightedness called? What is caused by?
Where is the focal point of the lens?
Myopia
Eyeball is too long to lens is too strong.
Focal point - in front of the retina
What is the most common refractive disorder of the eyeball? What is it caused by? What does it do to vision?
Astigmatism
Caused by curved eyeball (more rugby than football)
Means that light is focussed in more than one place.
Which refractive disorder is more common in the elderly? What does it do?
Presbyopia
Caused by laxity of the lens - doesn’t return to its normal shape
What type of lens do you need as a corrective lens for myopic patients?
Concave (diverging)
What type of lens do you need as a corrective lens for hypermetropic patients?
Convex (converging)
Where does the optic nerve leave the eye?
In the optic disc
Which is most sensitive part of the retina? How does it appear under an ophthalmoscope?
Fovea
Small yellow spot - far eyeball
What does the fovea have?
Lots of special cones for detailed vision
Which cells are found in the neural layer?
Ganglion cells
Bipolar cells
Photoreceptor cells (rods & cones)
Amacrine & horizontal cells (supportive)
What do rods detect?
Black & white vision
Night vision
Dim light
What do cones detect?
Colour vision & bright light
What happens once photoreceptors are excited?
Signals are transmitted through the retina (photoreceptors to bipolar cells to ganglion cells) to the optic nerve fibres and then to the occipital cerebral cortex
What is the process by which photons of light are converted to electrical signals?
Phototransduction
What is the visual pathway in the brain?
From optic nerve to optic chaism to optic radiations and to the occipital cortex.
Do we have more rods or cones?
More rods (120m to 5m)
How many layers is the retina made of?
3 direct (receptors, bipolar cells and ganglion cells) and 2 transverse layers (horizontal and amacrine cells)
What are the function of the transverse layers of the retina?
Signal processing including lateral inhibition
What are the photosensitive pigments found in rods and cones?
Rods = Rhodopsin
Cones = Photopsin - each one has either red-sensitive, green-sensitive or blue-sensitive pigment
What type of channels are used in phototranduction?
cyclic Guanosine Monophosphate (cGMP)
What is the difference between different colours?
Their wavelength (Red 570, Green 535 and Blue 445)
What causes colour blindness?
Loss or modification of one of the three cone pigments.
Which gene carries the red and green pigments? What does this explain?
X
Why colour blindness is more common in males than females
Which gene carries the blue pigment? Why is this rarer?
Found on chromosome 7 (not sex-linked) = which means it is much rarer to find as usually are 2 copies of chromosome 7.
What does the visual system in the brain comprise of?
Eye
Optic nerve
Optic chiasm
Optic tract
Lateral geniculate nucleus
Optic radiation
Visual Cortex
Visual Association Cortex
Which nucleus is found after the optic chiasm?
Lateral geniculate nucleus
What are the two fields of vision?
Temporal
Nasal
What does the optic nerve from each retina do after leaving the eye?
Divides into L & R halves
What happens to the two halves of the optic nerves in the chiasm?
L halves and R halves from each eye combine to form optic tracts
Where do optic tracts relay? Where in the brain is this found?
Lateral geniculate nucleus
Found in the thalamus
Where does part of each optic tract travel to?
Superior colliculus in the mid-brain
Where does the output from the lateral geniculate nucleus go?
To the striate cortex in the occipital lobe.
Which Brodmann area is the primary visual cortex?
17
What do the following lesions cause?
What do the following lesions cause?
What is the term of loss of vision in the same half of both eyes?
Homonymous hemianopia
What is the term for loss of vision in opposite halves of both eyes?
Bitemporal hemianopia
What is the term of loss of vision centrally?
Scotoma
What can cause scotomas?
Retinal damage
Lesions in visual cortex
Tumours in optic nerve, chiasm, tract or radiation.
What is a common cause of bitemporal hemianopia?
Pituitary adenomas
Which stream of the cortex is concerned with locomotion, motion & action?
The dorsal stream
Where is the dorsal stream found in the cortex?
From the occipital to parietal cortex
Which stream of the cortex is concerned with object & face identity and conscious perception?
Ventral stream
Where is the ventral stream found in the cortex?
From the occipital to the temporal cortex
Which disorder means that a patient is unable to recognise and identify visually presented objects or persons despite having normally functioning senses?
Visual agnosia
What causes visual agnosia?
Problems between the occipital lobe and the temporal lobe - is a ventral stream problem.
What is the inability to recognise familiar faces called?
Prosopagnosia
