The anatomy of the eye

Question 1

What is the conjunctiva?

Answer 1

A thin mucous membrane that covers the surface of the eye. The function of the conjunctiva is to lubricate the eye and protect it from dust, debris, and infection-causing microorganisms

Question 2

What is the cornea?

Answer 2

A circular transparent window that is involved in the refraction of light.

Question 3

What is the anterior chamber?

Answer 3

This is a chamber filled with a clear fluid called aqueous humour (controls secretion via SNS)- This provided nutrients and oxygen to the cornea and lens. These don’t have a blood supply and so need aqueous humour for nutrients. It also allows maintenance (AH) of intraocular pressure

Question 4

What is the ciliary body?

Answer 4

This allows the continuous secretion of aqueous humour- flows across the ens, out through the pupil and drains into the canal of scheme

Question 5

What is the iris?

Answer 5

The coloured part of the eye- it is made of smooth muscle to dial and constrict the pupil.

Question 6

What is the lens?

Answer 6

It is a transport organ which allows light through, the main role is the fine focus of light onto the retina.

Question 7

What do the suspensory ligaments do?

Answer 7

They hold the lens in place and attach it to the colliery body.

Question 8

What is the vitreous humour?

Answer 8

A viscous fluid that absorbed energy to protect the delicate structures of the eye.

Question 9

Comparison of vitreous and aqueous fluid.

Answer 9

The human eye is divided into two segments, the anterior (front) segment and the posterior (back) segment. The vitreous humor is located in the posterior segment and fills the vitreous chamber, which takes up about 80% of the eye. The vitreous humor is not to be confused with the aqueous humor, which is a clear watery fluid that fills the anterior segment.

s a transparent, colorless, gel-like substance that fills the space between the lens and the retina within the eye. The vitreous humor is composed of mostly water, along with a small percentage of collagen, glycosaminoglycans (sugars), electrolytes (salts), and proteins.The vitreous fluid is humor’s main role is to maintain the round shape of the eye. The size and shape of the vitreous humor also ensures that it remains attached to the retina, which is the layer at the back of the eye that is sensitive to light.

The vitreous humor is also a part of the eye that can help with vision clarity. Because the vitreous humor is a clear substance, light is able to pass through and reach the retina. Near the center of the retina is the macula, a pigmented region responsible for high-resolution color vision. When light travels through the vitreous humor to the retina and macula, it is then translated to visual information and transmitted by the optic nerve to the brain.

The vitreous humor can also be helpful in absorbing any unexpected disturbances to the eye, such as a thump to the side of head. Absorbing the shock associated with the thump or similar disturbance can help prevent eye damage.

The aqueous humor is a clear fluid located at the front part of the eye. Because the eye doesn’t contain blood vessels, the aqueous humor is responsible for providing nutrients to the eye. The aqueous humor also drains out any excess material and waste from the eye

Formation of the aqueous humor is correlated with and sensitive to your body’s circadian rhythm. It is a water-like fluid between the cornea and iris.

The aqueous humor’s main job is to:

Allow the cornea to expand, so it can protect the eye against dust, particles, and bacteria that can cause harm.
Preserve ocular pressure.
Transport nutrients, including vitamin C.
The aqueous humor is produced by a part of the eye called the ciliary body, located above the eye’s lens. The aqueous humor must enter and be drained from the eye at an equal rate, exiting the eye from a structure called the trabecular meshwork. This tissue lets fluid drain through it.

Question 10

What is the fovea?

Answer 10

Light is focused on to the fovea and is where highest visual activity occurs.

Question 11

What is the optic disc?

Answer 11

This is where axons from the retina leave the eye forming the optic nrve- takes visual info onto the brain. Also, blood vessels come in and out of the eye here
- The optic disc is referred to as the blind spot as nontight sensitive cells are found here.

Question 12

How is pupil size controlled>

Answer 12

Via the iris which is made of smooth muscle.
It has circular muscle- fibres that run circulaly and when these contract, the hole in the middle gets smaller,
Radial muscle: fibres that run radially = when contract, the fibres become shorter and the pupil becomes constricted

If, the PNS is stimulated, in the light:
- Contraction of circular muscle = pupil constrictes

In dark:
The radial muscle contracts, muscle fibres shorten = pupil is pulled wider and is dilated

Question 13

How is light focused?

Answer 13

The cornea is the first point of focusing and allows the greatest degree of refraction.

Accommodation; the curvature of the cornea can’t be changes but can at the lens, allowing for fine focus = accommodation.
Contraction and relaxation of the ciliary muscle is important as the lens is connected to he ciliary muscle by suspensory ligaments.
In far vision: The ciliary muscle is relaxed which pulls the lens taut (flat, thin) = less curvature and so focuses light from a distance
In near vision: Makes the lens more rounded. Cilliary muscle contracts = hole in middle gets smaller = suspensory ligaments slack as the cilliary muscle contracts = less becomes rounded with increased curvature = increased refraction, so can focus light from nearer.
This is under control of the PNS

As light comes in, the image is focused on the retina upside down and back to front = this is corrected by the brain.

Question 14

What happens if the cornea isnt smooth?

Answer 14

Will lead to uneven refraction = Astigmatism

Question 15

What does laser correction surgery (LASIK) do?

Answer 15

It changes the degree of curvature of the cornea- changes the plane of focus so light can be focused on the retina.

Question 16

What is presbyopia?

Answer 16

A condition where the lens becomes less flexible with age = no longer can become rounded and so it harder to focus on near objects.

Question 17

What is myopia?

Answer 17

Short-sightedness- Eyeball is too long and so the parallel light from a distance is focuses infant of the retina = becomes blurry.

Question 18

What is hyperopia?

Answer 18

Long-sightedness - eyeball is too short= can’t see near objects clearly as near objects are brought to a focus behind the retina = blurred.

Question 19

What do glasses do?

Answer 19

they change where light is focused.

Question 20

What are photoreceptors?

Answer 20

These are cells in the retina that transduce light energy into electrical signals- only at the visible spectrum 400-700nm can be transducer by photoreceptors.

Question 21

What are the 2 types of photoreceptors?

Answer 21

Rods and cones
rods = grey-scale
Cones = colour

Question 22

What is the structure of the retina?

Answer 22

The retina is made up of layers of neuronal cells with the photoreceptors at the back of the retina behind with the pigment and choroid layer behind them.
The photoreceptors synapse with bipolar cells. The bi-polar cells synapse with retinal ganglion cells which take signals to the brain via long axons.

Question 23

What are the 3 parts of the structure of a photoreceptor?

Answer 23

• The outer segment:
  Detects light stimulus- contains photopigment
  Is the most posterior part of the retina
  Flattened, stacked, membranous discs
  Turned over by cells of the retinal pigment epithelium (RPE)- phagocytose some, and then make new at the other end = constant turnover
• The inner segment:
  Is the metabolic centre- contains nucleus, ribosome, mitochondria
• The synaptic terminal:
  This is where the photoreceptors synapse with the bipolar cells and where NTs are released.

Question 24

How many rods and cones are found in the retina?

Answer 24

Approx 10 million rods and 6 million cones

Question 25

Which photoreceptors have a high sensitivity to light and which has a low and why?

Answer 25

Cones have a low sensitivity to light (therefore need bright light for colour vision). Day vision is dominated by cones
rods have a high sensitivity to light (required for night vision)

Question 26

Where in the eye are cones most abundant?

Answer 26

fovea

Question 27

Where in the eye are rods most abundant?

Answer 27

Peripheral retina

Question 28

Which photoreceptors have high convergence and which has little?

Answer 28

Cones in fovea have little to no convergence =1:1 ratio of cones to ganglion cell

Rods are highly convergent- many rods (up to 100) feed into one ganglion cell.

Question 29

prepare a compare and contrast cones and rods Q

Question 30

Which photoreceptors produce higher resolution images?

Answer 30

Cones have higher resolution images
Rods have a wider receptive field and increased sensitivity- in peripheral retina = resolution is low in comparison with fovea

Question 31

Compare the visual activity in the fovea and in the periphery?

Answer 31

Fovea is where the lens focuses the image and activity is highest- fine resolution if image.
- There are more cones- 1:1 coupling
- Lateral inhibition occurs- when 1 photoreceptor is stimulated, it can switch off the next one = defines edges in images

In periphery, it is dominated by rod vision.
The signals are largely black and white (greyscale)
Is very sensitive- can see movement, flashes of light etc- then once we see them in our periphery, we will turn to look at it.

Question 32

What is visual transdcuction and how is it performed by photoreceptors?

Answer 32

Photoreceptors convert light energy to electrochemical signals (Transduction). They contain photopigments in the outer segment membranes that can capture light energy.
Photopigment consists of a protein called opsin and a chromophore- retinal (retinene) which is derived from vitamin A (e.g. carrots).
Opsin is a GPCR and retinal is its ligand.

When retinal is activated by light, it changes shape from 11-cis-retinal to all-trans-retinal.
This causes a conformational change in opsonin and causes it to become activated. This leads to a cascade of events resulting in hyperpolarisation of the photoreceptor membrane

all-trans-retinal has a bend in its structure, that 11-cis-retinal doesn’t.

Question 33

What are photopigments and what are the 4 types?

Answer 33

Photopigment consist of a protein called opsin and a chromophore- retinal (retinene) which is derived from vitamin A (e.g. carrots).
Opsin is a GPCR and retinal is its ligand. They capture light energy

Types:
- 1 in rods: Rhodopsin
- 3 in cones: Sensitive to red, grey, and blue light

Question 34

How does phototransduction- photoreceptor activity in the dark work?

Answer 34

• the outer segment of the photoreceptor contain a cGMP-gated cation channel.
• cGMP opens the carton channel, allowing Na+ ions to move into the cell down its electrochemical gradient = depolarisation
• In the dark, cGMP levels are high due to action of granulate cyclase converting GTP to cGMP. The cGAMP- gated channels are therefore open.
• movement of Na+ into the cell = depolarisation to approx -40mV and causes opening of voltage-gated Ca2+ ions channels in the synaptic terminal
• This leads to a Ca2+ influx into the cell = stimulates neurotransmitter release. Ca2+ triggers synaptic vesicle exocytosis, thereby releasing the neurotransmitters contained in the vesicles and initiating synaptic transmission
• therefore in dark, photoreceptor is depilated and releasing NT

Note, the NT released is glutamate

Question 35

What is the activity of photoreceptors in response to light?

Answer 35

• Light causes 11-cis-retinal to change conformation to all-trans-retinal which causes a conformational changes in the opsin and stimulation of its GPCR activity
• Transducin is the G-protein associated with the receptor. Activation of this protein activates phosphodiesterase
• increased phosphodiesterase activity leads to increased degradation of cGMP.
• decreased cGMP causes closing of cGMP- gated cation channel and so no more Na+ ions can move into the cell. This leads to hyperpolarisation of the membrane potential.
• Hyperpolarisation leads to closing of voltage-gated calcium ion channels and therefore there is a decrease in neutrotransmitter release.

Note, the NT released is glutamate
- The response to light is graded = brighter light = less NT released

Processing by the retinal neurons means that in the light, action potentials are initiated in the ganglion cells, which take the information to the brain; in the dark, retinal ganglion cells are quiescent

Question 36

What happens to the photopigments after activation?

Answer 36

They are bleached and remain unresponsive until recycling of retinal has occurred. This terminates the signal and is mediated by enzymes that convert all-trans-retinal back to 11-cis-retinal

Question 37

What happens in colourblindness

Answer 37

the individual lacks one of the colour cones (red, blue or green).

Question 38

What is the visual pathway that takes information from the retina to the visual cortex?

Answer 38

Information is taken from the retina to the visual cortex via:
- Retinal ganglion cells leave the eye at the optic disc and make up the optic nerve
- These icons synapse in the thalamus (in the lateral geniculate nucleus)
- interneurones travel from the LGN to the visual cortex.

• RGC (retinal ganglion cells) axons from the different halves of the visual field go to opposite sides of the brain (information from the left visual field goes to the right side of the brain)
• Axons cross at the optic chiasm
• Processing of visual information starts in the retina and continues in the visual cortex and associated areas – a complex process