Visual System I: The Eye And Retina
The visual system is responsible
for converting all incident light energy
into a visual image of the world. This information is coded for in the retina
which lies at the back of the eye, and transmits that information to the visual
cortical areas, the hypothalamus and upper brainstem (see Chapters 25 and 26).
The process of visual transduction is detailed in Chapter 23.
Optical properties of the eye
On reaching the eye, light has to
be precisely focused on to the retina, and this process of refraction is
dependent on the curvature of the cornea and the axial length of the eye.
Failure to do this accurately leads to an inability either to see clearly when
reading (long-sightedness or hypermetropia), or to see distant
objects clearly
(short-sightedness or myopia),
or both. In the latter case there is often an additional problem of astigmatism, in which the
refraction of the eye varies in different meridians.
In addition to the need to be
refracted precisely on to the retina, light must also be transmitted without
any loss of quality and this relies on the cornea, anterior and posterior
chambers and lens all being clear. Injuries or disease of any of these
components can lead to a reduced visual acuity (the ability to
discriminate detail). The most common conditions affecting these parts of the
eye are infec- tions and damage to the cornea (keratitis) or
opacification of the lens (cataracts).
Retinal anatomy and function
Photoreceptors
The light on striking the retina
is transduced into electrical signals by the photoreceptors that lie
on the innermost layer of the retina, furthest from the vitreous humour. There
are two main types of photoreceptors: rods and cones.
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Rods: The rods are found in all areas of the
retina, except the fovea; they are sensitive to low levels of light and
are thus responsible for our vision at night (scotopic vision). Many
rods relay their information to a single ganglion cell, and so this system is
sensitive to absolute levels of illumination while not being capable of dis-criminating
fine visual detail and colour. Thus, at night we can detect objects but not in
any detail or colour.
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Cones: The cones are found at highest density in the
fovea and contain one of three different photopigments. They are
responsible for our daytime or photopic vision. This, coupled to the
high density of these receptors at the fovea, where they have an almost
one-to-one relationship with ganglion cells, means that they are the receptors
responsible for visual acuity and colour vision. Alter- ations in the
photopigments contained within these receptors leads to colour blindness.
Diseases of the receptors leading to their death, such as retinitis
pigmentosa, lead to a progressive loss of vision that typically affects
the peripheral retina and rods in the early stages, resulting in night
blindness and constricted visual fields, although with time the disease process
can spread to affect the cones.
Horizontal cells
The photoreceptors make synapses
with both horizontal and bipolar cells. The horizontal cells have two
major roles: (i) they create the centre surround organization of the receptive
field of the bipolar cell; and (ii) they are responsible for shifting the
spectral sensitivity of the bipolar cell to match the level of back- ground
illumination (part of the light adaptation response; see Chapter 23).
The centre surround receptive
field means that a bipolar cell will respond to a small spot of light in
the middle of its receptive field in one way (depolarization or
hyperpolarization), while an annulus or ring of light around that central spot
of light will produce an opposite response. The horizontal cells, by receiving
inputs from many receptors and synapsing onto the photoreceptor bipolar cell,
can provide the necessary information for this receptive field to be generated. The mechanism by which they fulfil
their other role in light
adaptation is not fully understood.
Bipolar cells
The bipolar cells relay
information from the photoreceptors to the ganglion cells and receive synapses
from photoreceptors, horizontal and amacrine cells. They can be classified
according to the receptor they receive from (cone only, rod only, or both) or
their response to light. Bipolar cells that are hyperpolarized by a small spot
of light in the centre of their receptive fields are termed off centre (on-surround)
while the converse is true for those bipolar cells that are depolarized by a
small spot of light in the centre of their receptive field.
Ganglion cells
The ganglion cells are found
closest to the vitreous humour; they receive information from both bipolar and amacrine
cells and send their axons to the brain via the optic nerve. These nerve fibres
course over the inner surface of the retina before leaving at a site which
forms the optic disc and which is responsible for the blind spot as
no receptors are located at this site. This blind spot is not usually apparent
in normal vision. The ganglion cells can be classified in a number of different
ways: according to their morphology; their response to light as for bipolar
cells (‘on’ or ‘off’ centre); or a combination of these properties (the XYW
system in cats or the M and P channels in primates). The X ganglion
cells, which make up 80% of the retinal ganglion cell population, are involved
in the analysis of detail and colour while the Y ganglion cells are more involved
in motion detection. The W ganglion cells, which make up the remaining 10% of
the population, project to the brainstem, but as yet have no clearly defined
function. The X and Y ganglion cell system defined initially in cats is
equivalent to the P and M channel in primates, which is broadly responsible for
‘form’ and ‘movement’ coding, respectively. In addition, there is a small
population of ganglion cells that contain a protein called melanopsin, which
allows them to detect light independently of photoreceptors. These ganglion
cells project to multiple sites within the central nervous system, especially
the suprachiasmatic nucleus of the hypothalamus (see Chapters 11 and 25).
Amacrine cells
The amacrine cells of the
retina, which make up the final class of retinal cells, receive and relay
signals from and to bipolar, other amacrine and ganglion cells. There are many
different types of amacrine cells, some of which are exclusively related to
rods and others to cones, and they contain a number of different transmitters.
They tend to have complex responses to light stimuli and are important in
generating many of the response properties of ganglion cells, including the
detection and coding of moving objects and the onset and offset of illumination.
