Vision 1 and 2

Question 1

What is refraction?

Answer 1

The bending of light when it passes from one optical medium to another. Light waves from an object bend at the cornea, then again at the lens so that a clear image can be formed on the retina.

Cornea is the most powerful bender of light, but the lens has the capacity to change its bending power via the ciliary muscles.

From distant objects, only parallel rays are hitting the lens. From close objects, divergent rays are also hitting and so these need to be bent a lot more.

Question 2

What is accommodation?

Answer 2

The ability to change focus from a far away object (over 6m away) to a close object (20cm). 3 things happen:

1) Lens changes shape via ciliary body contraction (CNVII): becomes thicker and more spherical as suspensory ligaments become lax
2) Pupil constricts: via parasympathetic innervation the constrictor pupillae muscle contracts so that only rays from the object you are trying to focus on get through
3) Eyes converge: eyes look inwards to focus on a close object via medial rectus muscles (CNIII).

Question 3

What is an emmetrope?

Answer 3

Someone with perfect vision.

Question 4

Define hyperopia.

Answer 4

Hyperopia: long-sightedness, long word. Eyeballs are too short and lens is too flat, distant images form behind the retina so the lens is made thicker to see objects that they should be able to see without contraction. When objects are closer all of the lens power is used up so hazy. Biconvex glasses. Convergent squint in toddlers.

Question 5

Define astigmatism.

Answer 5

Astigmatism is blurred vision that occurs when the cornea or lens aren’t perfectly curved - rugby ball shaped. Both close and far away objects are hazy - need cylindrical glasses or toric contact lenses.

Question 6

Define presbyopia.

Answer 6

Presbyopia is long-sightedness of old age. Lens becomes less mobile and so when ciliary muscle contracts it cannot change its shape. Need biconvex reading glasses.

Question 7

Define myopia.

Answer 7

Myopia: short-sightedness, short word. Eyeball is too big so distant objects are hazy - image is formed in front of the retina. For close images, rays are divergent so image is formed on the retina. Biconcave glasses. Divergent squint in toddlers.

Question 8

What is phototransduction?

Answer 8

The conversion of energy in light waves (photons) to electrical signals.

Light waves hit the photoreceptors of the retina (rods and cones) - these are now phototransduced and can activate optic nerve neurones by generating an AP, which can then be received by the brain.

Question 9

Which visual pigment is in rods?

Answer 9

Rhodopsin (opsin + 11-cis retinal) . When light hits this is isomerises all to trans retinal. 11-cis retinal is a derivative of vitamin A.

Question 10

Which visual pigments are in cones?

Answer 10

S, M and L. All are sensitive at different parts of the spectrum.

Question 11

How does bleaching of the visual pigment result in phototransduction?

Answer 11

The phototransduction cascade.

Question 12

What role does vitamin A play in the visual pigment?

Answer 12

Visual pigment regeneration. Trans and cis are in a cycle with vitamin A - some of the vitamin A becomes an ester and is lost which is why we need a dietary supply of vitamin A.

11 cis retinal –> trans retinal then vitamin A is needed to covert it to cis retinal again.

Question 13

What are Bitot’s spots?

Answer 13

Spots in the conjunctiva which are the first sign of vitamin A deficiency.

Question 14

Why can vitamin A deficiency cause night blindness?

Answer 14

There are more rods than cones in the eye and so these start malfunctioning first - rods work at low levels of light so night vision.

Question 15

What are some of the causes of vitamin A deficiency?

Answer 15

Vitamin A is acquired through the diet and so anything that affects absorption - coeliac disease, malabsorption etc.

Question 16

What are some of the presentations of vitamin A deficiency?

Answer 16

Bitot’s spots, corneal ulceration and corneal melting (which leads to future opacification of the cornea).

Question 17

What is the difference between intrinsic and extrinsic (extraocular) eye muscles?

Answer 17

Intrinsic: control diameter of the pupil and alter lens curvature to enable us to see objects that are close.

Extrinsic: move the eyeball.

Question 18

What are the 6 extraocular eye muscles that move the eye and their innervations?

Answer 18

Superior oblique, inferior oblique, lateral rectus, medial rectus, superior rectus and inferior rectus. Recti muscles are straight.

SO4 LR6 EE3

Question 19

Where do the extraocular eye muscles arise from?

Answer 19

Recti muscles arise from the annular fibrous ring. Superior oblique arises from the room of the orbit posteriorly. Inferior oblique arises from the roof of the orbit anteriorly.

Question 20

Which muscles open and close the eyelid?

Answer 20

Levator palpebrae superioris elevates the eyelid.

Orbicularis oculi closes the eyelid.

Question 21

Where do the recti and oblique muscles insert onto the eye?

Answer 21

Recti insert anteriorly into the sclera, obliques insert posteriorly into the sclera.

Question 22

What are the actions of the oblique muscles?

Answer 22

They are attached to the posterior part of the sclera, and so they pull the posterior part of the eye up/down and the anterior part moves in the opposite direction.

Question 23

Define intorsion and extorsion.

Answer 23

Intorsion is the inward rotation of the eye towards the nose.

Extorsion is the outward rotation of the eye away from the nose.

Question 24

What are the movements of the extraocular eye muscles?

Answer 24

Medial rectus: adduction

Lateral rectus: abduction

Superior rectus: elevation, adduction and intorsion

Inferior rectus: depression, adduction and extorsion

Superior oblique: depression, abduction and intorsion

Inferior oblique: elevation, abduction and extorsion

Question 25

What would be the consequence of a 6th nerve palsy?

Answer 25

Lateral rectus - eye not able to abduct.

Question 26

What would be the consequence of a 4th nerve palsy?

Answer 26

Superior oblique - eye not able to depress so moves upwards when adducted.

Question 27

What would be the consequence of a 3rd nerve palsy?

Answer 27

Drooping eyelid, eye can only move laterally and slightly downwards.

Question 28

What is strabismus?

Answer 28

A squint - misalignment of the eyes.

Question 29

Define esotropia and exotropia.

Answer 29

Esotropia is a convergent squint.

Exotropia is a divergent squint.

Question 30

What are the functional consequences of a squint?

Answer 30

Amblyopia: lazy eye due to the brain suppressing the image from one eye.

Diplopia: usually as a result of a CN palsy.

Question 31

Define visual field.

Answer 31

Everything seen with 1 eye, including the periphery. Images of objects in the field of vision are formed upside down and then inverted on the retina.

Question 32

Describe the visual pathway.

Answer 32

• All fibres pass from the optic nerve to the optic chiasma
• Medial (nasal) fibres cross over at the optic chiasma
• Optic tract contains fibres from the temporal half of the ipselateral eye and the nasal part of the contralateral eye ie all fibres from the opposite half of the visual field
• These fibres move down the optic tract and synapse in the lateral geniculate body (LGB) of the thalamus
• Optic radiation then passes behind the internal capsule (retro-lentiform fibres) to reach the primary visual cortex

–> Right visual cortex sees the left half of the visual field and vice versa

Question 33

Where is the primary visual cortex?

Answer 33

Occipital lobe which is area 17

Question 34

What would happen if the right optic nerve was damaged?

Answer 34

Blindness in the left eye

Question 35

What would happen if the optic chiasma was disrupted in the middle?

Answer 35

Bitemporal hemianopia

Question 36

What would happen if the right optic tract was damaged?

Answer 36

Blindness on the left temporal and right nasal fields - contralateral homonomous hemianopia

Question 37

What would happen if the right optic tract was damaged?

Answer 37

Blindness on the left temporal and right nasal fields - contralateral homonomous hemianopia

Question 38

What are the 3 intrinsic eye muscles?

Answer 38

1) Ciliaris muscle in ciliary body
2) Constrictor pupillae which in the iris at the pupillary border
3) Dilator pupillae which is a radially running muscle in the iris

Question 39

What are the innervations of the intrinsic eye muscles?

Answer 39

Ciliaris and constrictor pupillae - parasympathetic innervation by CNIII.

Constrictor pupillae - sympathetic innervation by from the plexus around blood vessels.

Question 40

What happens to the pupils in response to light?

Answer 40

Constrict - parasympathetic. Check direct and consensual reflexes.

Question 41

Describe the afferent limb of the light reflex.

Answer 41

On response to light, fibres that activate the light reflex do not go to the LGB. Instead they leave the optic tract and go to the midbrain where the nucleus of CNIII is located. Part of this nucleus is called the Edinger-Westphal nucleus (EWN) which is for parasympathetic fibres - pupillary reflex fibres go to the EWN of both sides.

Question 42

Describe the efferent limb of the light reflex.

Answer 42

1) Pre-ganglionic parasympathetic fibres go from EWS to the orbit
2) Parasympathetic fibres go to and synapse on the ciliary ganglion
3) Post-ganglionic fibres go through short ciliary nerves to constrictor pupillae
4) Pupillary constriction of both sides

Question 43

Define anisocoria.

Answer 43

When pupils are different sizes, eg in Horner’s syndrome. The pupils can look normal but there will be an abnormal light reflex.

Question 44

What can cause an absent/abnormal pupillary reflex?

Answer 44

Any abnormality of the afferent or efferent limb.

• diseases of the retina
• diseases of the optic nerve (optic neuritis eg MS)
• diseases of CNIII (efferent limb)

Question 45

What needs to be checked in CNIII palsy?

Answer 45

If this is due to eg DM there should be no damage to the parasympathetic fibres and so the pupil is unaffected. If pupillary reflex is absent this can indicate a cerebral artery aneurysm.

Question 46

Why does Horner’s syndrome cause aniscoria?

Answer 46

Damage to the sympathetic innervation of the pupil. Affected pupil is constricted (miosis), and also ptosis (drooping of the eyelid) and anhidrosis (inability to sweat on the affected side).

Question 47

What causes Horner’s syndrome?

Answer 47

Sympathetic ganglia have a thoracolumbar outflow. In the head and neck, post-ganglionic sympathetic fibres travel along with blood vessels as there is no sympathetic chain here - damage to these blood vessels can disrupt blood flow to the nerves and cause Horner’s syndrome. A pancoast tumour of the lungs can cause this.