The human retina
The retina (at the back of the eye) is composed of several types of cells, as well as blood vessels.
Surprisingly, light has to pass through at least 2 layers of neurones before reaching the photoreceptor
cells: 'rods' and 'cones'.
These are so called because of the shape of the outer segment. Here is a stack of membranes, containing a pigment which makes them light-sensitive.
Rod cells are responsible for monochrome ("black and white") vision, whilst cones give colour vision. They also vary in sensitivity, as explained below.
Impulses from the photoreceptors are passed via synapses to intermediate bipolar neurones
- so called because each has two processes on either side of the cell body - and these pass impulses on to ganglion cells
which feed into the optic nerve. The left and right optic nerves pass into the optic chiasma, effectively crossing to the other side of the brain and into the thalamus, which passes impulses on to the visual cortex, at the back of the cerebral hemispheres of the brain, where the nervous information is interpreted as vision.
Rods and cones are typically connected differently to their bipolar cells.
One bipolar cell normally receives impulses from several rod cells, and sends impulses on to more than one ganglion cell.
On the other hand, each cone cell normally sends its impulses on to a single bipolar cell and then on to a single ganglion cell.
This means that rod cells are particularly useful in low light levels, but they do not give such clear and high-definition vision as cones, which require higher levels of illumination.
The distribution of cones and the pattern of nerves in the fovea are responsible for vision with the highest definition - visual acuity
- as well as discriminating colours of light reflected from objects under view.
It also explains why it is necessary to (turn the head and) line up the visual axis of the eye with the object being studied, in class or reading a book, for example. However it is known that in the dark it is easier to see stars more clearly by looking slightly to one side.
The pigment in rods is called rhodopsin, and its molecule consists of a protein part - opsin - attached to retinal - a derivative of vitamin A. When this absorbs light energy, its molecule changes shape (from 11-cis to all-trans) and this alters membrane permeability, producing a generator poential.
The cone cells themselves are not differently coloured - they just respond to different colours of light
Similarly, cone cells have retinal attached to different opsin proteins.
There are actually three types of cone cell. Each one has a slightly different opsin, so that they respond to different wavelengths of light, corresponding to different colours of light: blue, green and red.
This graph shows the percentage of light of different wavelengths absorbed by the three
types of cone cell found in the retina of the human eye. Light of different colours is detected by stimulation of different combinations of these cells.
Layers of cells in the retina
Section through the retina, showing types of neurones
Photoreceptors labelled 1-6 are rods, 7 & 8 are cones
Cells 1 and 2 share the same ganglion cell and neurone, so light falling on both of these only registers as a single spot.
Cell 3 has a separate neurone so when light falls on 2 and 3, two separate spots are seen
When small amounts of light fall on a cell, e.g. 3 , they may not be seen because the receptor potential is sub-threshold.
But if a similar amount of light falls on several, e.g. 3, 4, and 5, this may be seen, because collectively they are above threshold, i.e. they cause sufficient depolarisation to
stimulate an action potential in the next cell. This is an example of spatial summation, with 3 receptors releasing enough neurotransmitter to generate an impulse.
Large numbers of cells similar to 7 and 8 are found in the fovea. These give higher visual acuity than rod cells 1-6, because each photoreceptor connects to a single bipolar cell and ganglion
They also give colour vision, because there are 3 differnt types of cones, each responding to different colours/wavelengths of light
All in the back of the eye
The retina consists of about 120 million rods, widely spread out, and about 5 million cones. About 2000 cones (and practically no rods) are concentrated in an area in the centre of the optical axis at the back of the eye. This region of maximal sensitivity is called the
, and it is served by about half the nerve fibres that make up the optic nerve (consequently feeding on to half the visual cortex). There is less obstruction by blood vessels and overlying connective neurones in the central region.
There are several zones, varying in the packing density of cones and bipolar cells. The term macula
may be used interchangeably with fovea, or it may represent a slightly larger area. Macular degeneration is an (age related) condition in which cellular changes in this area lead to reduced clarity of (central) vision.
There are no photoreceptors where the optic nerve leaves the eye, seen below as the optic disc. This is the 'blind spot'.
Can you see through this?
A photograph of the back of the eye ('fundus') showing the surface of the retina, and the dark orange colour is mainly due to visual pigments.
The fovea is the small blob in the indistinct dark area on the left.
The optic disc (composed of axons from ganglia) shows as a light circular zone on the right, with blood vessels emerging.
The distribution of rod cells and cone cells across the retina of a human eye
N.b. Even though there are high numbers of rods in some regions of the retina, they do not give such good resolution as cones do, because of the way their associated neurones are 'connected'.
What do you see?
The answer to the Ultimate Question of Life, the Universe, and Everything?
Colour blindness is a sex-linked condition, so it is more common in males.
If you can see two numbers, you have normal colour vision.
If you see only one number clearly, you probably have 'green colour blindness
' or 'red colour blindness
If you see one number clearly and another faintly, you probably have a mild version 'mild green colour blindness
' or ' mild red colour blindness
For more tests see web references below.