Eye Refraction and Accommodation Flashcards
What is refraction?
Light rays bend to from a sharp image on the retina
Can be biconcave and biconvex
What is accommodation?
Changing how much light rays are bending so can focus
What is the refractive media in the eyeball?
Cornea, AH, lens and VH are transparent to allow light to fall on retina
How does a sharp image fall on the retina?
Light waves from an object bend at cornea, bend more at lens to form a clear image on retina
Cornea is the most powerful bender but the lens has the power to change its bending power
What happens when objects are at a distance (>6m) compared to close up?
Only parallel rays from object reach the eye
When close up divergent rays from object reach the eye but need to be bent more to focus on retina
How does the eye focus on an object?
Lens becomes thicker so more powerful and clear image formed on retina again
What 3 things happen simultaneously and compromise accommodation?
Lens changes shape - thicker and more spherical
Pupil constricts
Eyes converge
How does the lens become thicker?
Ciliary muscle contracts making the body bulge which means space in middle decreases
So suspensory ligaments become lax - lens no longer under stretch
Explain pupil construction and increased focus
Pupil constricts to allow only a few rays to pass through
Pupillary constrictor - sphincter pupillae is around border of the pupil - parasympathetic innervation
Describe eye convergence
Eyes have to turn in to look at the object
Medial rectus muscle is in use - this muscle then thicker than lateral rectus
What is myopia?
Short-sightedness
What is hyperopia?
Long-sightedness
What is astigmatism?
Non-spherical curvature of cornea or lens
What is presbyopia?
Long sightedness of old age
What is emmetropia?
Perfect vision - 20/20
What is the most common cause of myopia?
Eyeball is too long
Describe myopia
When cornea and lens bend rays they make an image in front of retina so far off objects not seen clearly
Bending power is too much for the eyeball
Describe myopia when an object is brought close up
Divergent rays from object reach cornea and then use the bending power
Image is formed on retina without needing to increase curvature of the lens - no accommodative power
What are the symptoms of myopia?
Headaches, cant see blackboard or distant objects, can have divergent squint, toddlers can have more image in books and pictures and can lose interest in class
Describe myopia correction
Bending power needs to be decreased
Biconcave lenses - (-sign)
Spectacles, contact lenses and laser eye surgery
Describe hyperopia
Eyeball is too short or cornea and lens too flat
Image formed behind the retina
Person uses accommodation power and makes lens thicker causing image to form on retina - so using this power when usually not needed
Describe hyperopia and seeing up close
Uses more accommodation power until all used up as been used on focusing on long sight
Cant see nearby objects
What are the symptoms of hyperopia?
Eyestrain after reading, convergent squint which needs immediate correction to prevent lazy eye
What is the correction for hyperopia?
Biconvex glasses alleviates use of glasses for focussing on distant objects and rests accommodation power
Contact lenses
Laser eye surgery
How do images appear in astigmatism?
Close and distant images appear hazy as more than 1 image formed
Describe astigmatism
Surface has different curvatures in different meridians
Bending of light rays along one axis will never be the same as that of other axis so image hazy whatever distance
What is the correction for astigmatism?
Cylindrical glasses - curved in only one axis ( in degrees)
Laser eye surgery
Toric lenses - special contact lenses
Describe presbyopia
With age lens gets less mobile and elastic
When ciliary muscle contracts it is not as capable of changing shape so seeing near images becomes more difficult
What is the correction of presbyopia?
Biconvex reading glasses
What does the energy in light waves need to stimulate?
Photoreceptor cells on retina - photo transduction (conversion of light energy to electrochemical response by photoreceptors)
What do phototransduced rods and cons activate?
Optic nerve to generate an action potential
What are the main parts of the rods and cones?
Synaptic terminal, axon, nucleus, inner segment containing organelles and outer segment with lamellae (have photoreceptors)
Describe visual pigment in photoreceptor outer segments
Each lamellae is made up of cell membrane
In this cell membrane is visual pigment rhodopsin in rods and cone opsins S, M and L in cones
Molecules differ in their spectral sensitivity
What happens when light falls on 11-cis retinal?
Isomerises to all-trans retinal
Can not fit into opsin so rhodopsin splits resulting in bleaching of visual purple
What is opsin formed by?
Vit A
How does bleaching of visual pigment result in phototransduction?
Phototransduction cascade
Photoreceptor cells at rest kept at depolarised state - depolarised by open Na/Ca channels
What role does Vit A play on visual pigment?
Visual pigment regeneration
Cis-retinol transported back to rod/cone to reform opsin
Describe the cascade of reaction in rods and cones
Hyperpolarises - impulse is transmitted - becomes AP in optic nerve cells
Describe the effects of vitamin A deficiency
Vision affected - night blindness
Essential for healthy epithelium so conjunctiva and corneal epithelium are abnormal
