Macular Degeneration PBL

Question 1

Name the 3 anatomical divisions of the eye?

Answer 1

Fibrous
Vascular
Inner layers.

Question 2

Describe the Fibrous layer of the eye?

Answer 2

Sclera: Provides attachment for extraoccular mucles.
Cornea: Refracts light entering the eye. IT is transparent.

Question 3

Describe the Vascular layer of the eye?

Answer 3

Choroid: Layer of CT and blood vessels. It provides nourishment to outer layers of the retina.
Ciliary Body: Composed of ciliary muscle and ciliary processes. The SM fibres are attached to the lens and control its shame and also makes aqueous humour.
Iris: Circular structure with an apperture (pupil) in the middle.

Question 4

What modulates pupil diameter?

Answer 4

Sphincter Pupillae and Dilator Pupillae muscles.

These are smooth muscles found in the iris.

Question 5

Describe the Inner layer of the eye?

Answer 5

The Retina - which consists of 2 layers.
Neural layer: Photoreceptors, located posterior and laterally..
Pigmented layer: Underneath the neural layer and attached to the choroid.

The centre of the retina is the macula which contains a depression called the fovea.
The optic nerve enters at the optic disc, which has no photoreceptors.

Question 6

What is the function of aqueous humour?

Answer 6

Nutrition for avascular tissues.
Maintains intraoccular pressure.
Contains immunoglobulins which have an immune role.

Question 7

Where is aqueous humour found?

Answer 7

Fills the anterior and posterior chambers of the eye.

Question 8

What does aqueous humour consist of?

Answer 8

Water, amino acids, electrolytes, immunoglobulins.

Question 9

What produces aqueous humour?

Answer 9

Ciliary epithelium.

Question 10

Describe the drainage of aqueous humour?

Answer 10

Ciliary body >
Posterior chamber >
Anterior chamber > 
Trabecular network >
Canal of Schlemm >
Episcleral veins.

Question 11

What is the arterial supply to the eye?

Answer 11

Mainly the Ophthalamic artery from the internal carotid.

It gives off many branches including the central artery of the retina.

Question 12

What is the venous drainage of the eye?

Answer 12

Superior and inferior Ophthalamic veins, which drain to the cavernous sinus.

Question 13

Describe Rod Cells?

Answer 13

Found near the periphery of the retina.
Can see in low levels of light.
See greyscale/black and white.
More rods than cones, but many rods synapse with one bipolar cell.
Contain Rhodopsin - formed by the protein scotopsin and Retinal.
Retinal is a Vitamin A derivative.

Question 14

Describe Cone Cells?

Answer 14

More central in the retina.
There are 3 different types - red, green and blue. Each absorbs a different wavelength of light.
There are less cones, but only one cone per bipolar cell.
They are harder to excite.
Contain the pigment Iodopsin

Question 15

What are bipolar cells?

Answer 15

Neurons with a central body and 2 synapses.
One synapses with rods/cones and the other to the ganglion cell.
They also interact with horizontal and amacrine cells.

Question 16

What are ganglion cells?

Answer 16

Receive information from bipolar/amacrine cells.

There axons then run into the optic disc and converge as the optic nerve to relay visual information to the brain.

Question 17

What are horizontal cells?

Answer 17

Have laterally interconnecting neurons which integrate and regulate info from photoreceptor cells.
Allow eyes to adjust to see in both dim and bright light.
Alter the sensitivity of the retina.

Question 18

What are amacrine cells?

Answer 18

Inhibitory interneurons.

Between bipolar and ganglion cells.

Question 19

Describe the physiology of vision?

Answer 19

When light is absorbed, retinal changes conformation and induces a change in the shape of opsin.
Opsin activates G-protein Transducin.
Transducin gains a GTP.
The alpha-subunit of transducin activates the enzyme cGMP Phosphodiesterase.
This converts cGMP to 5’GMP.
The decrease in cGMP results in Na+ channels of rods/cones closing.
This causes membrane hyperpolarisation.
This results in decreased glutamate (inhibitory) release at the synapse with bipolar cells.
Bipolar cells now depolarise.
This causes increased NT release at the synapse with ganglion cells.
The fewer the cone cells synapsing with a bipolar, the clearer the image.
Ganglion cell membranes depolarise, and there is an increase in AP’s being sent to the primary visual cortex of the occipital lobes.
Horizontal/amacrine cells facilitate/inhibit the passing of signals to ganglion cells. Prevents overwhelming of the brain when going from dark to light.

Question 20

Describe the pathway from ganglion cells to the brain?

Answer 20

The optic nerve leaves at the optic disc.
Both optic nerves arrive at the optic chiasm, and the nasal fibres cross. Temporal fibres stay on the same side.
The proceed as the optic tracts to the thalamus and synapse on the lateral geniculate nucleus.
Optic radiations then project back to the visual cortex at the calcarine sulcus.

Question 21

Describe the fate of fibres of the optic tract?

Answer 21

Mainly synapse on the LGN.
Some terminate in the superior colliculi, which controls external eye muscles.
Some terminate in the pre-tectal area to control pupillary and accommodation reflexes.

Question 22

What is the role of the pupillary light reflex?

Answer 22

Controls pupil diameter in response to the intensity of light detected by the retinal ganglion cells.

Question 23

Describe the process of the pupillary light reflex?

Answer 23

1. Light falls on retinal ganglion cells. This information is relayed via the optic nerve.
2. Some optic tract fibres travel to the pre-tectal area and send fibres to the Edinger-Westphal nucleus.
3. This sends parasympathetic signals via the occulomotor nerves, which synapses at the ciliary ganglion of both eyes.
4. Short, post-ganglionic ciliary nerves leave the ciliary ganglion to innervate the sphincter pupillae muscle of the iris.
5. The pupil of both eyes constricts.

Question 24

Why do both pupils normally constrict at the same time?

Answer 24

As efferent parasympathetic nerve fibres from the ED nucleus travel to both eyes.

Question 25

Where would you find a lesion if you had monocular vision loss? What does this mean?

Answer 25

In the optic nerve of one eye.

Cant see out one eye.

Question 26

Where would you find a lesion if you had bitemporal hemianopia?

Answer 26

At the optic chiasm.

Cant see anything out the temporal visual fields, as the nasal fibres have been affected as they cross at the chiasm.

Question 27

Where would you find a lesion if you had homonymous hemianopia?
What does this mean?

Answer 27

In the optic tract or optic radiations on one side.
No vision in the temporal field of one eye, and the nasal field of the other eye. Nasal fibres have crossed, so nasal and temporal fibres affected.

Question 28

Name the extraoccular muscles and their innervations?

Answer 28

Superior rectus: Occulomotor.
Inferior rectus: Occulomotor. 
Medial rectus: Occulomotor.
Lateral rectus: Abducens.
Superior oblique: Trochlear.
Inferior oblique: Occulomotor.

Question 29

What is the function of the superior rectus muscle?

Answer 29

Elevates eye ball.

Question 30

Function of the inferior rectus muscle?

Answer 30

Depresses the eye ball.

Question 31

Function of the medial rectus muscle?

Answer 31

Adducts the eye ball.

Question 32

Function of the lateral rectus muscle?

Answer 32

Abducts the eye ball.

Question 33

Function of Superior oblique muscle?

Answer 33

Medially rotates the eye ball.

Question 34

Function of Inferior oblique muscle?

Answer 34

Laterally rotates the eye ball.