PBL3 Flashcards
what is the amount of light coming into the eye controlled by
The amount of light entering the eye is controlled by the size of the pupil- controlled by the autonomic nervous system
what happens to the eye in bright light
In bright light, the muscle of the iris contracts- pupil gets smaller (Parasympathetic)
what happens to the eye in dim light
In dim light, the radial muscle contracts- pupil dilates and gets bigger (Sympathetic)
what does the degree of focusing of the lens depend on
Degree of focusing of lens depends on the distance of the object we are looking at, i.e. the further an object is the less it needs to be focused
what is the light focused on when it enters the eye
Light entering the eye is focused by the cornea onto to lens- from here it gets focused onto the retina
what is the process of the eye adjusting called
accommodation
describes rod cells
Found throughout retina
One type
Active in dark
Rhodopsin found on outer surface reacts to light
Multiple rod cells converge with a single interneuron
describe cone cells
Only present in fovea Three types – Red, green and blue Active in light Cone opsins found on outer surface Each cone cell converges directly with an interneuron – Greater degree of visual acuity
describe how light is converted to electrical impulses
Light reaching the photoreceptors is converted into a biochemical signal which generates an action impulse.
Electrical impulses are processed in retina by bipolar, amacrine and horizontal cells.
In dark, constant inward leak of sodium keeps the cell depolarised and releasing glutamate.
Absorption of light changes the shape of rhodopsin which reduces the levels of c-GMP via a GPCR.
Reduction in cGMP closes Na+ channels and causes
the cells to hyperpolarise, stopping glutamate release
what are electrical impulses proceed by in the retina
- bipolar
- amarcine
- horizontal cells
how is the information relayed to the brain
Information is carried to brain on retinal ganglion cells which form to make the optic nerve.
Axons from optic nerve crossover at optic chiasm and travel to lateral geniculate nucleus.
Signals are then relayed to the primary and secondary visual cortex
describe how to measure visual acuity
Measured by standing 6m (20ft)from chart and reading smallest size print
The numberator is the distance in metres between subject and chart
The denominator is the distance at which a person with 6/6 vision could see the same optotype
what is visual acuity
Measure of the spatial resolution of the visual processing system
what is the main function of the aqueous humour
Main function: regulate intraocular pressure (helps maintain the shape of the eye)
describe how aqueous humour is produced and removed from the eye
Fluid secreted by ciliary epithelia- fills the anterior and posterior chambers of the eye
Aqueous humour is drained through the trabecular meshwork into Schlemm’s canal
Secondary route is via uveoscleral drainage which accounts for 10% and is independent of IOP
what is the difference between upper optic radiation and lower optic radiation
Upper optic radiation- information about superior quadrants
Lower optic radiation- information about inferior quadrants
describe the visual pathway
In the skull both optic nerves cross over to form the optic chiasm
- Here the left and right optic tracts are formed
Optic tracts synapse in the Lateral Geniculate Nucleus in the thalamus
Axons from the LGN carry visual information along the optic radiation.
describe pupillary light reflex
Axons in optic nerve (CNII) carry signals to prectectal nucleus in superior colliculus
Signals travel to Edinger-Westphal nucleus
Brought back to ciliary ganglion via oculomotor nerve (CNIII)
Axons sent to pupillary constrictor musle which constrict, reducing pupil size
describe how the accommodation reflex happens
Pupillary constriction- same as before
Lens thickening- parasympathetic output from CNIII causes suspensory ligaments to relax and lens thickens
Convergence- parasympathetic output from CNIII activates medial rectus muscles
what are the three parts of the accommodation reflex
- pupillary constriction
- lens thickening
- convergence
what is the red reflex
Refers to the reddish-orange reflection of light from the back of the eye (fundus) seen when using an ophthalmoscope
when is the red reflex considered abnormal
The red reflex is considered abnormal if there is any asymmetry between the eyes, dark sports, or white reflex
what does the red reflex rely on
Relies on the transparency of the lens, cornea and vitrous humour
what does the rombergs test test
Proprioception
Vision
Vestibular apparatus
how do you perform the rombergs test
Performed by asking patient to stand with feet together and eyes closed. Observe the patient for 1 minute
Positive Romberg’s test (i.e. patient sways) indicates problems with dorsal columns
Name some visual eye defects
Partial optic nerve lesion – Ipsilateral scotoma
Complete optic nerve lesion –
Blindness in that eye
Optic chiasm lesion – Bitemporal hemianopia
Optic tract lesion – Homonymous hemianopia
Damage to Meyer’s loop – Homonymous upper quadrant anopia
Optic radiaiton lesion – Homonymous hemianopia
Visual cortex lesion – Homonymous hemianopia (Macular sparing)
what is glaucoma
A group of diseases in which raised intraocular pressure causes damage to the optic nerve and visual field defects.
what is the normal intraocular pressure
10-21mmHg
what are the two main types of glaucoma
Primary open-angle glaucoma
Acute angle-closure glaucoma
what is the cause of primary open angle glaucoma
Reduction of outflow of aqueous humour through trabecular meshwork results in a gradual, painless loss of peripheral visual fields
how do you diagnose primary open angle glaucoma
Diagnosis is made through measurement of IOP
what are the risk factors for primary open angle
Age, race, family history and myopia
how do you treat primary open angle glaucoma
Aims to reduce IOP by increasing aqueous drainage or reducing aqueous production
Β-blockers and carbonic anhydrase inhibitors reduce aqueous production
Prostaglandin analogues increase aqueous outflow
what is acute angle closure glaucoma
Ophthalmic emergency
Sudden rise in IOP >50mmHg
Result of reduced aqueous drainage as a result of lens pushing the iris against the trabecular meshwork
Causes sudden onset red painful eye and blurred vision. Patient has N&V and headache; dilated, non-responsive pupil
what is the treatment of acute angle closure glaucoma
Acetazolamide (CA inhibitor) to reduce IOP
Pilocarpine - Muscarinic agonist which causes
- Iris sphincter muscle to contract causing pupil constriction
- Ciliary muscle contraction which opens the trabecular meshmork
Β-blockers and prostaglandin analogues can also be used
Surgery – Hole made in the periphery of iris
