Vision 1 Flashcards
What two anatomical structures of the eye have the power to bend light?
Cornea (45D)
Lens (15D but can accommodate)
What is the scientific name for the bending of light?
Refraction
At what distance is an object too close to focus on?
20cm
What three things happen simultaneously during accommodation?
- Lens changes shape
- Pupil constriction
- Eyes converge
Describe the changes that occur to the shape the lens as an object gets closer?
Thicker and more spherical
Describe the process that allows the lens to thicken?
- Ciliary muscles contract
- Space in the middle decreases
- Suspensory ligaments become lax
- Lens is no longer under stretch
- Lens becomes thicker
What division of the autonomic nervous system governs the ciliary body?
Parasympathetic
Why do the pupils constrict when an object gets closer?
To only allow a few light rays from the object in question - producing a clearer image
What muscle is responsible for constricting the pupil and what division of the nervous system governs it?
Constrictor pupilae - parasympathetic
Where is the constrictor pupilae located?
In a concentric ring around the border of the pupil
What muscles allow for convergence of the eye?
Medial rectus
What nerve innervates the medial recti muscles?
CN III
What is the colloquial word for myopia?
Shortsightedness
What is emmetropia?
Perfect vision
What feature of the lens produces myopia?
Too powerful
What is the most common cause of myopia?
Eyeball too long
Where does the image focussing relation to the retina in myopic eyes?
Infront of the retina
What are some common symptoms of myopia?
Headaches, complaints of not being able to see
Infants/preverbal children - divergent squint
Toddlers - loss of interest in activities, more interest in books and pictures
Teachers may notice loss of interest at school
How can myopia be corrected?
Biconcave lenses (glasses/contacts)
Laser eye surgery
What is the colloquial word for hyperopia?
Longsightedness
What is the most common cause of hyperopia?
Eyeball is too short
In hyperopia, where does the image form in relation to the retina?
Behind the retina
What are the symptoms of hyperopia?
Eyestrain, convergent squint
Why does hyperopia require immediate correction?
To avoid lazy eye, ambulopia
How can hyperopia be corrected?
Biconvex lenses (contacts/glasses)
Laser eye surgery
What is presbyopia?
Age related long-sightedness
How does presbyopia occur?
Lens gets less elastic with age
When does presbyopia usually present in life?
5th decade
What is astigmatism?
Non-spherical shape of the cornea (or lens) therefore surface has different curvatures in different meridians
How is astigmatism corrected?
Laser eye surgery
Cylinder adjustments
Toric contact lenses
What is the process of phototransduction?
The converting of light into a neural impulse and ultimately a mental image
What are cones?
Photoreceptors which are responsible for colour distinction and which work in high light
What are rods?
Photoreceptors responsible for low light vision
What visual pigments are contained in rods?
Rhodopsin
What visual pigment is contained in cones?
Opsins S, M and L
Roughly outline the phototransduciton cascade?
Light stimulates visual pigments
Stimulates transducin
Stimulates
Phosphodiesterase
Na+ channels close (hyperpolarisation)
Stimulates retina
What is the special feature of the resting potential of photoreceptor cells?
Depolarised at resting state, due to open Na+ and Ca+ channels
How is visual pigment regenerated?
Dietary Vitamin A from the liver regenerates visual pigments
Other than visual pigments, what does vitamin A also do in the eye?
Maintains healthy epithelium (cornea and conjunctivae)
What pathologies can occur as a result of vitamin A deficiency?
Blindness (/night blindness) Ulceration of cornea Bitot's spots Corneal melting Corneal opacification
Why does colour blindness occur?
Different types of cones are responsible for perception of different colours
