Neuroscience 11a - Structure and Function of the Eye
Anil Chopra
1. Draw the main components of the eye, including its focussing apparatus, and describe their
principal function(s)
2. Describe the production, circulation and drainage of aqueous humour and explain the
importance of its contribution to the maintenance of intraocular pressure
3. Explain how raised intraocular pressure can lead to glaucoma
4. Draw the main layers, cellular components and synaptic connections of the retina, and
describe the significance of the macula and fovea, especially in relation to visual acuity
5. Explain the basis of phototransduction, and distinguish the different contributions of rods
and cones to the visual process.
6. Explain briefly the most common forms of colour blindness
7. Describe the basis of physiological optics and of the common defects of refraction
8. Draw the binocular visual pathways, from retina to lateral geniculate nucleus to primary
visual (striate) cortex, with special regard to the partial decussation at the optic chiasm and its
consequences for the visual field representation at higher levels of the pathway.
9. Explain how specific visual field defects can arise from lesions at different sites, including
the optic nerve, chiasm and radiation, and at different locations within striate cortex. Give
examples of how some of these defects may arise.
10. Outline briefly the basic processes of visual integration occurring at different levels of the
visual pathway, and how these relate to interpretation of the retinal image.
11. Explain briefly the concept of functional specialization and the clinical effects of focal
extrastriate lesions, especially in relation to the visual perception of colour and motion.
12. Describe the afferent and efferent pathways underlying the pupillary light reflexes (direct and
consensual) and the near reflex.
13. Outline briefly the circadian visual system and indicate its significance
14. Explain briefly the simple assessment of visual field
15. List the most common uses of the ophthalmoscope
16. Outline the simple tests for visual acuity and colour blindness
17. Outline the tests for papillary responses
18. Understand the optics of the human eye
19. Know what errors of refraction are common in humans
20. Understand why spectacles do not correct all types of vision loss
21. Understand the optics underlying spectacle correction of vision
22. Know which eye problems are the commonest causes of vision loss
23. Know specifically which parts of the eye are affected by these conditions
24. Learn how to predict the specific nature of vision loss in individual conditions through
knowledge of eye anatomy and physiology
Purposes of Vision
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Scene selection (head and eye movement)
Light intensity (retina and pupil)
Colour (cones)
Night vision (rods)
Process images (post receptoral pathway)
The eye
Cornea: principal refraction of
light occurs here. It refracts
light by 60dioptres.
Lens: secondary refraction of
light occurs here. Its power
varies with age – average is 6
dioptres in adults =
accommodation.
Anterior Chamber: this is the region between the cornea and the
iris. It also provides the flow of the aqueous humour from the
ciliary body in the posterior chamber (where it is secreted) round
the iris and into the anterior chamber.. It is drained through the
trabecular meshwork. This provides the transparency for the lens,
nutrient for the cells and maintains intra-ocular pressure. (in
glaucoma a tumour in the ciliary body produces too much aqueous
humour and intra-ocular pressure increases causing damage to the
optic nerve).
NB: aqueous humour is plasma like and replaced every 100 mins.
Iris: contains circular and radial muscles to allow more or less light to reach the retina.
Retina
Retinal Function
The retina contains photosensitive cells
situated at the posterior of the eye with the neurones being
anterior. Both photosensitive cells contain outer segments
with a pigmented retinal layers, an inner segment containing
the nucleus and organelles and a synaptic terminal at their
most anterior.
 As can be seen the photoreceptor cells are attached to
the retina itself – therefore the light has to pass
through the ganglion cell layer, the bipolar cell layer
and through the photoreceptor layer itself before it
reaches the light sensitive tips
 Outgoing nerve impulses then travel back through
the cell layers to the ganglion cell layer where the optic nerve is formed at the optic disc
which also forms the blind spot (no photoreceptor cells present)
 Macula = a yellow spot/small depression formed by the outward displacement of vessels and
cells. At the centre of the macula is a fovea – a tiny rod free area with the highest
concentration of cones. The ratio of ganglion cells to cones is also higher in the fovea –
almost 1:1 relationship (improves acuity – the ability to resolve objects close together). The
further from the fovea, the reduced numbers of cones and increased numbers of rods. The
fovea is the principal focal point. The fovea is also avascular.
There are 2 main types of photosensitive cell:
Rods
 Scotopic – can work in low light
 Monochromatic – non-colour.
 High sensitivity
 Low Visual acuity
 Head is cylindrical in shape and hence arranged in many layers. If one disc does not pick up
a photon then the next one in the chain may.
 Not in fovea, generally in peripheral retina.
 More pigment than cones.
 20 times more rods than cones
Cones
 Photopic – bright light
 Colour Vision
 3 Types with different sensitivity ranges
o L – cone – 560nm (red)
o M – cone – 530nm (green)
o S – cone – 420nm (blue)
 Low Sensitivity
 High visual acuity
 Head is conical which makes it sensitive to light rays along its axis
 Concentrated in the fovea (macula) – centre of eye
 Less pigment than rods
 20 times fewer cones than rods.
 Diseases caused:
o Protanopia:(protanomally): missing (abnormal) L cone
o Deuteranopia :missing (abnormal) M cone
o Tritanopia : missing (abnormal) S cone
Visual Transduction
1. Light enters the eye through pupil and
travels through neurone layers.
2. It hits the visual pigment known as 11-cisretinal. 11-cis-retinal is bound to a
photopsin. There are different opsins in the
different cells:
 Opsin – rods (producing Rhodopsin)
 Erythrolabe – L cones
 Chlorolabe – M cones
 Cyanolable – S cones
3. The 11-cis-retinal absorbs a photon of light and is isomerised to all-trans retinal.
4. All-trans-retinal is unable to bind to the opsin and so a G-protein signalling pathway
causing a decrease in intracellular cGMP.
5. This results in cGMP gated channels closing and hence the cell becoming hyperpolarised.
Normally, in darkness, the cells are depolarised and the Na+ channels are open, maintaining
the membrane potential at -40mV.
6. The hyperpolarisation INHIBITS the release of glutamate (neurotransmitter) at the synapse.
7. The inhibition of glutamate can result in one of two reactions in bipolar cells depending on
the type:
a. In an ON bipolar cell, the reduction of glutamate release causes the cell to depolarise.
The depolarisation will then result in the release of neurotransmitter onto a ganglion
cell.
b. In an OFF bipolar cell, the reduction of glutamate release causes the cell to
hyperpolarise. The hyperpolarisation stops the release of neurotransmitter onto the
ganglion cell.
8. Signals are then projected onto the brainstem and thalamus.
Visual Processing
 It is initiated in the outer-plexiform layer of the retina
 Bipolar cells receive direct input at the receptive field centre together with surround input via
horizontal cells
 Centre-surround opponency provides the first spatial processing of visual information.
 Surround receptive fields result from horizontal and amacrine cells
Ganglion Cells: each ganglion cell has a
receptive field with a certain number of
bipolar and photoreceptor cells. There are 2
types of ganglion cell:
 “ON Centre” (off surround)
o If light is shined only on the centre, the
photoreceptor cells stimulate ON bipolar
cells  the ganglion cell fires more
frequently.
o If light is shined only on the outer part, the photoreceptor cells stimulate OFF bipolar cells
(or horizontal cells) the ganglion cell fires less frequently.
 “OFF Centre” (on surround)
o If light is shined only on the centre, the photoreceptor cells stimulate OFF bipolar cells 
the ganglion cell fires less frequently.
o If light is shined only on the outer part, the photoreceptor cells stimulate ON bipolar cells
(or horizontal cells)  the ganglion cell fires more frequently.
Furthermore, ganglion cells are further divided into:
 Parvocellular: these have small receptive fields and respond to colour
 Magocellular: these have large receptive fields and respond to movement and contrast.
Bipolar cells receive direct photoreceptor input at the receptive
field centre together with surround input fed via horizontal cells.
Horizontal Cells also feed into bipolar cells that may not be
directly connected to all the photoreceptor cells to create the
“centre-surround” organisation of the receptive field bipolar
cells. Horizontal cells are also responsible for shifting the
spectral sensitivity on the bipolar cell to match illumination.
Amarcrine Cells also feed into ganglion cells that may not be
directly connected to all the bipolar cells.
Electroretinography, is used to measure the electrical responses
of various cell types in the retina. Electrodes are placed on the
cornea and the skin near the eye. During a recording, the patient
is watching a standardized stimulus and the resulting signal is
interpreted in terms of its amplitude (voltage) and time course.
Stimuli include flashes (flash ERG) and reversing checkerboard
patterns (pattern ERG).
Colour opponent primate ganglion cells
Basic Currency of Parvocellular Pathway
Colour perception is created by colour
opponency of different cones which are
receptive to different wave lengths of light:
+
-
-
+
R on / G off
+
R off / G on
B on / Y off
+
-
-
+
G = M cone
G on / R off
G off / R on
B = S cone
R = L cone
Y=L+M
Visual Pathways
 Axons of the ganglion cell neurones travel posteriorly through the head to the optic chiasm
via the optic nerve (II)
 From here, the neurones from the temporal retina pass on to the ipsilateral lateral geniculate
nucleus, and those from the nasal retina pass on to the contralateral lateral geniculate
nucleus.
 From here they project onto the primary visual cortex.
In the lateral geniculate nucleus (in the cortex), clear sub-divisions can be seen between left and
right eye as well as with differing cell types.

the eye on the same side (the ipsilateral eye w.r.t the left or right LGN) sends information to
layers 2, 3 and 5
 the eye on the opposite side (the contralateral eye w.r.t the left or right LGN) sends
information to layers 1, 4 and 6.
A simple mnemonic for remembering this is "See I? I see, I see,". (CIICIC)
The cells from the lateral geniculate nucleus project onto simple cells. These cells are designed
to sense for edges of objects. Different aspects of visual information are carried in parallel
pathways and are processed simultaneously by the brain.
V1 is the primary visual cortex. It receives the input from the lateral geniculate nucleus. V1 has
a very well-defined map of the spatial information in vision. For example, in humans the upper
bank of the calcarine sulcus responds strongly to the lower half of visual field (below the center),
and the lower bank of the calcarine to the upper half of visual field. Conceptually, this
retinotopy mapping is a transformation of the visual image from retina to V1. The
correspondence between a given location in V1 and in the subjective visual field is very precise:
even the blind spots are mapped into V1. The visual information relayed to V1 is not coded in
terms of spatial (or optical) imagery, but rather as the local contrast.