PBL3

Question 1

what is the amount of light coming into the eye controlled by

Answer 1

The amount of light entering the eye is controlled by the size of the pupil- controlled by the autonomic nervous system

Question 2

what happens to the eye in bright light

Answer 2

In bright light, the muscle of the iris contracts- pupil gets smaller (Parasympathetic)

Question 3

what happens to the eye in dim light

Answer 3

In dim light, the radial muscle contracts- pupil dilates and gets bigger (Sympathetic)

Question 4

what does the degree of focusing of the lens depend on

Answer 4

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

Question 5

what is the light focused on when it enters the eye

Answer 5

Light entering the eye is focused by the cornea onto to lens- from here it gets focused onto the retina

Question 6

what is the process of the eye adjusting called

Answer 6

accommodation

Question 7

describes rod cells

Answer 7

Found throughout retina
One type
Active in dark
Rhodopsin found on outer surface reacts to light
Multiple rod cells converge with a single interneuron

Question 8

describe cone cells

Answer 8

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

Question 9

describe how light is converted to electrical impulses

Answer 9

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

Question 10

what are electrical impulses proceed by in the retina

Answer 10

• bipolar
• amarcine
• horizontal cells

Question 11

how is the information relayed to the brain

Answer 11

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

Question 12

describe how to measure visual acuity

Answer 12

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

Question 13

what is visual acuity

Answer 13

Measure of the spatial resolution of the visual processing system

Question 14

what is the main function of the aqueous humour

Answer 14

Main function: regulate intraocular pressure (helps maintain the shape of the eye)

Question 15

describe how aqueous humour is produced and removed from the eye

Answer 15

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

Question 16

what is the difference between upper optic radiation and lower optic radiation

Answer 16

Upper optic radiation- information about superior quadrants

Lower optic radiation- information about inferior quadrants

Question 17

describe the visual pathway

Answer 17

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.

Question 18

describe pupillary light reflex

Answer 18

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

Question 19

describe how the accommodation reflex happens

Answer 19

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

Question 20

what are the three parts of the accommodation reflex

Answer 20

• pupillary constriction
• lens thickening
• convergence

Question 21

what is the red reflex

Answer 21

Refers to the reddish-orange reflection of light from the back of the eye (fundus) seen when using an ophthalmoscope

Question 22

when is the red reflex considered abnormal

Answer 22

The red reflex is considered abnormal if there is any asymmetry between the eyes, dark sports, or white reflex

Question 23

what does the red reflex rely on

Answer 23

Relies on the transparency of the lens, cornea and vitrous humour

Question 24

what does the rombergs test test

Answer 24

Proprioception
Vision
Vestibular apparatus

Question 25

how do you perform the rombergs test

Answer 25

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

Question 26

Name some visual eye defects

Answer 26

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)

Question 27

what is glaucoma

Answer 27

A group of diseases in which raised intraocular pressure causes damage to the optic nerve and visual field defects.

Question 28

what is the normal intraocular pressure

Answer 28

10-21mmHg

Question 29

what are the two main types of glaucoma

Answer 29

Primary open-angle glaucoma

Acute angle-closure glaucoma

Question 30

what is the cause of primary open angle glaucoma

Answer 30

Reduction of outflow of aqueous humour through trabecular meshwork results in a gradual, painless loss of peripheral visual fields

Question 31

how do you diagnose primary open angle glaucoma

Answer 31

Diagnosis is made through measurement of IOP

Question 32

what are the risk factors for primary open angle

Answer 32

Age, race, family history and myopia

Question 33

how do you treat primary open angle glaucoma

Answer 33

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

Question 34

what is acute angle closure glaucoma

Answer 34

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

Question 35

what is the treatment of acute angle closure glaucoma

Answer 35

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