Visual System

Question 1

Label eye anatomy

Answer 1

Slide 5 visual system, Slide 11

Question 2

What are the types of tears?

Answer 2

Basal, Reflex and Emotional

Question 3

What is basal tear?

Answer 3

Lacrimal gland is located in the orbit, latero-superior to the globe. It produces tears at a constant rate even in the absence of stimulation or irritation.

Question 4

What is reflex tear?

Answer 4

Production of tears in response to ocular irritation. The cornea is innervated by the sensory nerve fibres via the Ophthalmic Branch of the Trigeminal Nerve. As afferent pathway, the trigeminal nerve relays signal to the central nervous system. The efferent pathway is mediated by the parasympathetic nerve. It innervates the lacrimal gland

Question 5

Describe the pathway of tear drainage

Answer 5

Tear film drains through the two puncta, tiny openings on the upper and lower medial lid margins. The puncta form the opening of the superior and inferior canaliculi within the upper and lower eyelids. Both canaliculi converge as one single common canaliculus, and drain tear into the tear sac. Tear is finally drained out of the tear sac, into the nasal cavity through the tear duct.

Question 6

What are the functions of the tear film?

Answer 6

The tear film maintains a smooth cornea-to-air surface, important for maintaining clear vision and removing surface debris during blinking. Also a source of oxygen and nutrient supply to the anterior segment.

Question 7

What is the role of the lipid layer?

Answer 7

The lipid layer on the top responsible for protecting the tear film from rapid evaporation. The lipid layer is secreted by the Meibomian Glands, situated along the eyelid margins.

Question 8

What is the role of the aqueous layer?

Answer 8

Forms the main bulk of the tear film. It delivers oxygen and nutrient to the surrounding tissue. It contains factors against potentially harmful bacteria.

Question 9

What is the role of the mucinous layer?

Answer 9

Ensures that the tear film sticks to the eye surface. This renders the surface of the eye “wettable”. The mucin molecules act by binding water molecules, to the hydrophobic corneal epithelial cell surface.

Question 10

What is the conjunctiva?

Answer 10

Thin, transparent tissue that covers the outer surface of the eye. It begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids. It is nourished by tiny blood vessels that are nearly invisible to the naked eye.

Question 11

What are the 3 layers of the eye?

Answer 11

The outer fibrous opaque layer called the sclera, responsible for protecting the eye, and maintaining the shape of the eye. The middle pigmented vascular layer called the Choroid, responsible for providing circulation to the eye, and shielding out unwanted scattered light. The innermost Neurosensory Layer called the Retina, responsible for converting light into neurological impulses, to be transmitted to the brain via the Optic Nerve.

Question 12

What are the 5 layers of the cornea?

Answer 12

1 – Epithelium
2 – Bowman’s membrane
3 – Stroma – its regularity contributes towards transparency
4- Descemet’s membrane
5- Endothelium – pumps fluid out of corneal and prevents corneal oedema

Question 13

What is the cornea?

Answer 13

The transparent, dome-shaped window covering the front of the eye. Low water content unlike sclera. Powerful refracting surface, providing 2/3 of the eye’s focusing power. Like the crystal on a watch, it gives us a clear window to look through.

Question 14

What is the uvea?

Answer 14

Vascular coat of eyeball and lies between the sclera and retina. Composed of three parts – iris, ciliary body and choroid. Intimately connected and a disease of one part also affects the other portions though not necessarily to the same degree.

Question 15

What is the choroid?

Answer 15

Lies between the retina and sclera. It is composed of layers of blood vessels that nourish the back of the eye.

Question 16

What is the role of the iris?

Answer 16

Controls light levels inside the eye similar to the aperture on a camera.Round opening in the centre is the pupil. Embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size.

Question 17

Describe the structure of the lens

Answer 17

Outer acellular capsule encases regular inner elongated cell fibres (transparency). May loose transparency with age – cataract.

Question 18

Describe the function of the lens

Answer 18

Responsible for 1/3 of the eye’s focusing power - higher refractive index than aqueous fluid and vitreous. Plays a role in accomodation and elasticity. Is transparent allowing for passage of light.

Question 19

What is the retina and what is its function?

Answer 19

Very thin layer of tissue that lines the inner part of the eye. Responsible for capturing the light rays that enter the eye.These light impulses are then sent to the brain for processing, via the optic nerve.

Question 20

What is the role of the optic nerve?

Answer 20

Transmits electrical impulses from the retina to the brain. Connects to the back of the eye near the macula. Visible portion is called the optic disc.

Question 21

What is the blindspot?

Answer 21

Where the optic nerve meets the retina there are no light sensitive cells. It is a blind spot.

Question 22

What is the macula?

Answer 22

Located roughly in the centre of the retina, temporal to the optic nerve. A small and highly sensitive part of the retina responsible for detailed central vision. The fovea is the very centre of the macula. The macula allows us to appreciate detail and perform tasks that require central vision such reading.

Question 23

How does the fovea help perception of detail?

Answer 23

Fovea is the most sensitive part of the retina. It has the highest concentration of cones, but a low concentration of rods. This concentration of cones allows for perception of detail.

Question 24

What is central vision?

Answer 24

Detail day vision and colour vision – fovea has the highest concentration of cone photoreceptors. Includes reading and facial recognition. Assessed by visual acuity assessment. Loss of foveal vision – Poor visual acuity.

Question 25

What is peripheral vision?

Answer 25

Shape, movement, night vision. Navigation vision. Assessed by visual field assessment. Extensive loss of visual field – unable to navigate in environment, patient may need white stick even with perfect visual acuity.

Question 26

What do the 3 layers of the neuroretina consist of?

Answer 26

Outer layer contains photoreceptors (detection of light) which give rise to first order neurons. Bipolar cells give rise to second order neurons and are responsible for local signal processing to improve contrast sensitivity, regulate sensitivity. Retinal ganglion cells give rise to third order neurone and transmit signal from eye to brain.

Question 27

What is the role of retinal pigment?

Answer 27

The retina consists of an outer thin layer of Retinal Pigment Epithelium, situated right in front of the Choroid, and an inner thicker layer called the Neuroretina. The retinal pigment epithelium transports nutrient from the choroid to the photo-receptor cells, and removes metabolic waste from the retina.

Question 28

What are rods and their functions?

Answer 28

Longer outer segment with photo-sensitive pigment. Higher spatial and temporal (time) summation. 100 times more sensitive to light than cones. Slow response to light. Responsible for night vision (Scotopic Vision). 120 million rods and recognises motion.

Question 29

What are cones and their functions?

Answer 29

Less sensitive to light, but faster response. Responsible for day light fine vision and colour vision (Photopic Vision). 6 million cones.

Question 30

Describe the distribution of photoreceptors

Answer 30

Rod photo-receptors are widely distributed all over the retina, with the highest density just outside the macula. The density of rod photo-ceptors gently tails off towards the periphery. Rod photo-receptors are completely absent within the macula. Cone photoreceptors found only within the macula.

Question 31

What are the different cones and their functions?

Answer 31

The eye captures different colours through different photoreceptors:
S-Cones: Blue
M-Cones: Green
L- Cones: Red
Rods are used for night vision and spatial recognition and are not really sensitive to any particular colour. Rod vision has only one single peak light sensitivity,
at 498 nano-meters wavelength.

Question 32

What is deuteranomaly?

Answer 32

It is caused by the shifting of the M-cone sensitivity peak towards that of the L-cone curve, causing red-green confusion. Can’t perceive the colour red.

Question 33

What is anomalous trichromatism?

Answer 33

Colour vision deficits caused by a shift in the photo-pigment peak sensitivity. Colour Vision deficits can also be caused by the absence of one or more of the 3 cone photo-pigments. In dichromatism, only two cone photo-pigment sub-types are present. In monochromatism, there is complete absence of colour vision. This can be caused by Blue Cone monochromatism, with the presence of only blue S-cones.