The Visual Pathways - THE EYE L5 & L6

Question 1

This is a really good resource if youre unsure of anything sensory

Answer 1

https://nba.uth.tmc.edu/neuroscience/s2/index.htm

Question 2

Learning objectives for the eye

Answer 2

Describe the anatomy of the eye.

Explain how the retina is organised.

Explain photosensitive cells and their connections.

Question 3

What is the anterior chamber?

Answer 3

The anterior chamber is filled with a fluid called aqueous humour, a watery fluid.

Question 4

What is the cornea?

Answer 4

The cornea is a transparent membrane through which light can enter the eye. It lacks blood vessels and is therefore nourished by the fluid behind it, the aqueous humour. The cornea protects the eye, but it will also be responsible for some refraction together. Refraction is the bending of the light, and it ensures that visual images are not blurred.

Question 5

What is the pupil?

Answer 5

Behind the cornea, you can see the pupil which is an opening in the eye. The light can enter and reach the retina at the bottom of the eyeball.

Question 6

What is the iris?

Answer 6

The iris is a diaphragm. It’s a coloured muscle that regulates the light entry to the eye. The colour of the iris is the colour of our eyes.

Question 7

What is the lens?

Answer 7

Behind the iris, is the lens, responsible for most of the refraction.
We need to change the shape of the lens to adjust the focus on objects that are at different viewing distances.

Question 8

What are ciliary bodies?

Answer 8

The lens is held in place by the ciliary bodies, which help to change the shape of the lens.

Question 9

What is vitreous humour?

Answer 9

Vitreous humour is a jelly-like fluid, and it helps maintain the structure and shape of the eye and also allows the movement of vitamins throughout the eye.

Question 10

What is the retina?

Answer 10

The retina is the light-sensitive structure of the eye, where the photoreceptors are.

Question 11

What is the fovea?

Answer 11

At the centre of the retina, there’s the fovea, the most sensitive part of the retina. If you are looking directly at something, the light that comes from the object you are looking at (yellow arrow) will be, directly to the most precise area of the retina, which is the fovea. Precise means the area that can see fine details.

Question 12

What is the choroid layer?

Answer 12

Behind the retina. is the choroid layer. This layer has many functions, but one of these is to control the excessive light. When the light enters the eye is not all absorbed by photoreceptors, and this could cause reflection or glare. The choroid layer’s task is to absorb all those photons not used by the photoreceptors.

Question 13

What is the Sclera?

Answer 13

The Sclera is also what we would normally call the white of the eye. You might notice that the cornea continues into the sclera, and just like the cornea, it is a protective covering.

Question 14

What is the optic disc?

Answer 14

The optic disc is where the retinal blood vessels and fibres of the optic nerve exit the retina. also the blind spot

Question 15

What is the optic nerve?

Answer 15

The optic nerve is a bundle of axons that go from the retina to the brain.

Question 16

What is light?

Answer 16

The basis of visual perception is light. Light is a wave of electromagnetic energy.

Question 17

What wavelengths can we percieve?

Answer 17

We are only sensitive to a very narrow range of wavelengths.

Most wavelengths are invisible to us.

Too long and it goes straight through us, too short and it is damaging to us!

Question 18

What is a visual field?

Answer 18

Visual Field: How much of the outside world the retina can see.

Question 19

What are the degrees of visual angles?

Answer 19

Degrees of visual angle: In vision science, we like to refer to the size of an object in degrees of visual angle (deg).

We want to know the portion of the retina taken by an object. This is measured in degrees of visual angle.

Important= distance on the retina is measured in degrees of visual angle.

Question 20

What is the Macula?

Answer 20

The macula contains a high density of photoreceptors and is divided into different regions, and one of these regions is the fovea.

Question 21

What is the fovea in relation to the macula?

Answer 21

The fovea is the part of the macula that contains the highest density of photoreceptors, and therefore it is the area of the retina with the highest visual acuity.

Question 22

What are photosensitive cells called?

Answer 22

Photosensitive cells called photoreceptors are scattered across the retina: rods and cones.

Question 23

How are photoreceptors distributed in the retina?

Answer 23

The distribution is not uniform: there are more cones in the fovea and more rods in the periphery

Question 24

How is a blind spot made?

Answer 24

The optic nerve and retinal artery enter the eye above the retina, creating a physiological blind spot.

Question 25

What are the characteristics of rods?

Answer 25

120 million per eye.

Periphery.

Monochrome.

Low resolution.

Many to 1 with ganglion cells.

Question 26

What are the characteristics of Cones?

Answer 26

6 million per eye.

Fovea.

Colour.

High resolution.

1 to 1 with ganglion cells.

Question 27

What are the cells in the retina?

Answer 27

Horizontal cells (H): Inhibit adjacent cells (lateral inhibition)

Amacrine cells (A): form modified ‘3rd image’

Retinal ganglion cells (RGCs): produce APs that project to CNS via optic nerve

Bipolar cells (B): form modified ‘2nd image’

the light must pass several layers before it gets to the photoreceptor.

Question 28

What is the duplex retina?

Answer 28

we have two separate systems that deal with different light levels.

Question 29

What are the two systems in the duplex retina?

Answer 29

A cone-driven, photopic (light) system: high acuity, low sensitivity.

A rod-driven, scotopic (dark) system: low acuity, high sensitivity, colour-blind.

Question 30

How is our luminance range so high, if we have to operate in all different conditions we are likely to encounter?

Answer 30

The duplex retina: we have two
separate systems that deal with
different light levels.

Constriction/dilation of the pupil: 1-8mm, reduces/increases the light by 64x.

Adaptation: the photoreceptors increase the amount of photosensitive protein (OPSIN) they have, increasing sensitivity up to 1000x.

Question 31

What is our luminance range?

Answer 31

The luminance range is looking (almost) into bright sunlight, down to a dark, starless night.

This range covers nine orders of magnitude – a billion-fold difference!

Question 32

How do photoreceptors work?

Answer 32

Photoreceptors pass their signals through a network of other cells that modify the image before it projects to the brain through the optic nerve