C27 - Monitoring Visual Function

Question 1

What are the (16) parts of the eye?

Answer 1

Pupil (aperture in iris diaphragm)

Iris (radial and circular muscles controlling light entering eye)

Lens (focuses light onto retina)

Cornea

Conjunctiva

Suspension ligaments (attach ciliary muscles to lens)

Ciliary muscle (radial and circular muscles that alter shape of lens)

Anterior chamber (containing aqueous humour)

Posterior chamber (contains jelly-like vitreous humour)

Retina (light-sensory layer composed of rods and cones)

Choroid (pigmented layer to prevent internal reflection)

Sclera (outer, tough protective layer)

Macula

Fovea centralis (higher concentration of cones)

Optic nerve (sensory neurones)

Blind spot (blood vessels and neurones pass through retina)

Question 2

Where is each eye found?

Answer 2

Each eye is a spherical structure located in the bony socket of the skull, called the orbit.

Question 3

How do the eyes move / what causes the eyes to rotate?

Answer 3

Each eye can be rotated in the socket by 2 pairs of rectus muscles and 1 pair of oblique muscles.

All of these muscles are attached to the tough, outer layer of the eye, called the sclera.

Question 4

What layer is found in the eye, beneath the outermost sclera layer?

Answer 4

The choroid layer.

It contains blood vessels and a layer of highly pigmented cells that prevent internal reflection.

Next is the layer of the retina which contains light sensitive rod and cone cells.

Question 5

What are the structures of the front of the eye?

Answer 5

The sclera is transparent and forms the cornea.

The conjunctiva is a thin, transparent membrane which covers the surface of the cornea and is continuous with the eyelids.
It is lubricated by tears produced from the lachrymal gland.

The choroid is modified to form the iris. The iris is a heavily pigmented diaphragm of involuntary smooth radial and circular muscle, which surround the pupil.
These antagonistic muscles contract and relax to change the size of the pupil and regulate the amount of light entering.

Question 6

What structures of the eye are found behind the pupil?

Answer 6

A biconvex lens, made out of transparent protein contained in a capsule held in place by suspensory ligaments attached to a ring of ciliary muscle.
The ciliary muscles and ciliary processes form the ciliary body.

The lens is flexible and elastic and has its shape altered by ciliary muscles (to focus light onto the macula).

Question 7

What is the space within the eye called?

Answer 7

The internal space within the eye is divided into 2 chambers.

• The one in front of the lens contains a clear, watery fluid called aqueous humour.
• The much larger chamber behind the lens contains a transparent, jelly-like fluid called vitreous humour. This maintains the eyeball’s shape.

Question 8

What are the 2 types of light sensitive cell?

Answer 8

Rod cells

Cone cells

Question 9

What are the two segments of rod cells?

Answer 9

Outer segment:
- contains up to 1000 vesicles of rhodopsin (photosensitive pigment) which are situated in the disc membranes.

Inner segment:
- contains many mitochondria and the nucleus.

The 2 segments are connected by a pair of cilia. Many rod cells form a synapse with a single bipolar (sensory) neurone, meaning rod cells have low visual acuity (clarity).

Question 10

What’s rhodopsin and what does it consist of?

Answer 10

A photosensitive pigment.

It consists of opsin (a protein) and retinal (vitamin A derivative).

Question 11

What’s bleaching?

Answer 11

Retinal (vitamin A derivative) normally exists as its cis-isomer.
When light hits it, it changes to its trans isomer form.

Rhodopsin splits in a process called bleaching and causes the rod cell (in the eye) to become impermeable to Na ions.

Question 12

How does the structure of cone cells differ to that of rod cells?

Answer 12

They’re very similar
- they too contain a photosensitive pigment however they have iodopsin (instead of rhodopsin).
There are also 3 different types of rod cell, each of which possesses a different type of iodopsin which absorbs either red, green or blue light.

• Cone cells aren’t as sensitive to light as rod cells so require more photons of light to produce an action potential.
• There are fewer cone cells than rod cells in the retina. Most cone cells are found in the macula.
• Most cone cells connect to one specific sensory neurone, producing greater visual acuity than rod cells.

Question 13

What are bipolar cells?

Answer 13

Specialised neurones that have 2 processes (extensions) coming out of their central cell body (axon and dendrite).

The processes closest to the photoreceptors cells are short and branch into many endings that form synapses with either multiple rod cells or a single cone cell.

Impulses from a number of rod cells summate before triggering an impulse in the bipolar neurone (which causes low visual acuity).
The other processes form synapses with a ganglion cell.

Question 14

What do bipolar cells do?

Answer 14

Stimulate a generator potential in the sensory neurone.
If it’s larger enough to overcome the threshold value t generates an action potential along the sensory neurone, from the rod cell to the optic nerve.
(Before the rod cell can be stimulated again the rhodopsin is actively reformed).

If rod or cone cells aren’t stimulated by light they depolarise and release an inhibitory neurotransmitter onto the bipolar cell.

This causes the bipolar cell to become hyper-polarised and prevents it from transmitting impulses to its related retinal ganglion cell.

Question 15

What’s the sclera?

Answer 15

The opaque outer layer of the eyeball.

It has many collagen fibres to protect and maintain the eyeballs shape under pressure.

Question 16

What does the choroid layer do?

Answer 16

It is highly pigmented and black, to absorb light and prevent total internal reflection.

Question 17

Where are lachrymal glands found and what do they do?

Answer 17

In the eye.

They produced is to lubricate the conjunctiva.

Question 18

What’s the ciliary body?

Answer 18

Part of the eye, made up of ciliary muscles, which attaches the iris to the choroid.

Question 19

What do ganglion cells do in the eye?

Answer 19

They have cell bodies in the retina and have many dendrites that form synapses with bipolar cells.
Action potentials are first generated here.

Question 20

What happens when an action potential is generated at ganglion cells in the eye?

Answer 20

The cells are activated by depolarised bipolar cells.
These cells have long axons to carry impulses to the brain, which are passed at a low frequency when the ganglion cell is not stimulated, and at an increased rate when stimulated.

The optic nerve then carries impulses from the eyes to the brain.

Question 21

What does the retina do?

Answer 21

Is has rod and cone (photoreceptor) cells which act as transducers - they convert light energy to chemical energy in the form of action potentials.

The action potentials are then transmitted along the optic nerve to the visual cortex of the brain.

Question 22

What happens when a rod cell is at rest/in dark/not stimulated by light?

Answer 22

No impulse is sent to the brain.

The rod cell has a resting potential of -40mV due to the Na/K pump and presence of Na and K channels in the cell surface membrane.
The pump moves 3 Na ions out for every 2 K ions it moves in, hydrolysing ATP as it does so.

A circulating electric current is generated by a constant flow of ions, as channels in the outer segment of the rod cell allow Na+ to diffuse in, and channels in the inner segment allow K+ to diffuse out.

The rod cell forms a synapse with a bipolar cell however, at rest, the synaptic bulb secretes glutamate (inhibitory neurotransmitter).
This diffuses across the cleft and prevents generator potentials in the bipolar cell (∴ no impulse sent).

Question 23

What happens when a rod cell is stimulated by light?

Answer 23

An action potential is carried from the optic nerve to the brain.

When rhodopsin in the rod cell disc membranes absorb light, the molecule changes shape, resulting in the closure of Na ion channels.
Na ions can no longer enter the rod cell and the outer segment membrane becomes hyper-polarised.

This change in the rod cell’s membrane potential causes voltage-gated Ca ion channels to close. Intracellular Ca ion concentration falls.
The fall in Ca concentration causes less glutamate to be released.

This removes inhibition of the bipolar cells and becomes depolarised, creating a generator potential, which is transferred to a ganglion cell.

Question 24

What occurs when rod cells are at rest? (When no light is present/too low level to stimulate rod cell)

Answer 24

Opsin and cis-retinal are joined to form rhodopsin.
Na+ ion channels open and Na+ ions enter the outer segment of the rod cell.
Rod cell membrane is depolarised.

The neurotransmitter, glutamate, is released by exocytosis from the rod cell into the synaptic cleft, and the bipolar neurone is hyperpolarised.

Inhibition of the release of neurotransmitter between the bipolar cell and ganglion occurs.
No generator potential is produced, so no action potential is generated.

Question 25

What occurs when rod cells are stimulated by light?

Answer 25

Light is absorbed by rhodopsin.
Rhodopsin is broken down to form opsin and trans-retinal.
Na+ ion channels close so no current is formed through the rod cell and the membrane is hyperpolarised.

No neurotransmitter (glutamate) is released into the synaptic cleft, and the bipolar neurone becomes depolarised.

Release of neurotransmitter occurs into the synapse between the bipolar cell and ganglion.
Generator potentials summate to form an action potential, and an action potential travels along the ganglion neurone.

Question 26

What eye tests are used to assess receptor activity?

Answer 26

Visual acuity test
- e.g. by use of a Snellen chart (with letters of decreasing size)

Colour vision test

• Ishihara colour test
• Farnsworth-Mussell 100 hue test

Optical coherence tomography (OCT scan)

Question 27

What’s a visual acuity test?

Answer 27

A quantitative measure of the eye’s ability to see an in-focus image at a certain distance (measuring clearness of vision).

This is usually with a Snellen chart (a series of letters decreasing in size).

Question 28

What are the 2 colour vision tests used and how do they work?

Answer 28

Ishihara test - uses pictures of coloured spots. A number is embedded in the picture as spots of a different colour.
People with colour-blindness are unable to see the number / the difference in colour.

Farnsworth Munsell 100 hue test - the patient must arrange a set of coloured caps in sequence to form a gradual transition of colour between 2 points.

Question 29

What happens in an OCT (optical coherence tomography) scan?

Answer 29

An optical beam is directed at tissue (normally the retina) and a small proportion of this light is reflected and collected.

 Most light isn’t reflected, and scatters off at large angles which causes background noise and obscures the image. 

Therefore, interferometry is used to build up a clear 3D image of thicker tissue samples, by rejecting the background signal.

Question 30

What can an OCT scan be used to diagnose?

Answer 30

Age-related macular degeneration

Diabetic retinopathy

Macular holes

Macular oedema