Sudden Visual Disturbance - 130 Flashcards
In the dark what state are photoreceptors in? What is happening at a cellular level?
Rest. The cell is depolarised with Na diffusing IN and Ca being pumped OUT.
cGMP keeps the Na gate open. Glutamate is produced.
In the light what state are photoreceptors in? What is happening at the cellular level?
Cell hyperpolarises, becomes more -ve inside than out. Proton hits rhodopsin causing retinal dissociation. Retinal inactivates cGMP, which no longer keeps Na gate open. Na cannot get in the cell, but Ca can still get out.
Reduction in glutamate production.
What provides colour vision? What causes colour blindness?
Cones. Red, green and blue cones all have different rhodopsin molecules.
Colour blindness is the inability to produce opsin. Red-green most common. It is X-linked and mostly affects boys.
What cells carry out neural processing in the retina?
Horizontal cells, amacrine cells and bipolar cells
What happens with diffuse light?
Lateral inhibition (by horizontal and amacrine cells). This limits photoreceptor depolarisation and impulses if diffuse light is hitting an increased area of the retina.
What are on-centre and off-centre retinal ganglion cells? What is their purpose?
On-centre cells are stimulated when the centre of it’s receptive field is exposed to light. Inhibited when the periphery is exposed to light.
Vice versa with off-centre.
This allows for detail about contrast, edges and contours to be processed.
What are the 3 actions of the pupillary near response?
Convergence, accommodation, constriction
What is RAPD? What causes it?
Relative afferent pupillary defect - caused by a lesion anterior to optic chiasm
What is the most common cause of blindness and partial sightedness in the western world? What are symptoms and how would it be treated?
Age-related macular degeneration (AMD).
Symptoms = loss of central vision
Treatment = Anti Vegf (reverses vessel growth). Very expensive.
Name 2 drugs that cause mydriasis
- Phenylephine (a sympathomimetic). Lasts 5-7hrs, works in 60-90 mins. Least effective in elderly, diabetes or dark iris. Can cause acute glaucoma, hypertension and arrhythmias.
- Antimuscarinics, e.g. atropine, cyclopentolate. Work in 15 mins, last 4-6hrs. Can be used to treat infections - cyclopentolate immobilises iris.
What do the SNS and PNS do to pupils respectively?
SNS - dilates
PNS - constricts
What do parasympathomimetics do to the pupil? When might they be used?
They open up drainage channels in trabecular meshwork. Reverse mydriasis (constrict pupil) in 30 mins, last for 6 hours.
Example = pilocarpine, used to treat glaucoma.
What do beta blockers do regarding the eye?
Inhibit aqueous humour production
What can be used to examine the eye?
Fluorescein
Which part of the eye senses colour?
Fovea
