Index : Light , The eyeball , Optics , Accommodation , Focussing , Cornea , Lens , Errors of refraction , Aqueous humour , Light reflex , Retina , Photoreception , Visual coding , Receptive fields , Reading list , Sample exam questions , Links
Light is the visible part of the electromagnetic spectrum. We "see" because we have receptors which are excited by wavelengths between 400-700nm.
The light pass first through the cornea. The cornea and the lens represent the refractive system of the eye. The cornea is responsible for 70% and the lens for 30% of the refraction. The refractive system focusses the light upon the retina, where the photosensitive pigment lies. The image is inverted.
The lens is responsible for accommodation, that is, the adjustment of the focussing system for near objects. The shape of the lens is determined by the tone of the ciliary muscle. When it relaxes, as in far vision, the zonular fibres are pulled taut and the lens is under tension and flat. For near vision, the ciliary muscle contracts, releasing the zonular fibres from tension and the lens takes up its natural, rounder and more refractive state.
- moving the lens towards the back of the eye
- turning the eyes inward towards the nose (convergence)
- pupil constriction
- fattening of the lens
The cornea has an endothelium (facing the aqueous humour) and an epithelium (facing the tear film) sandwiching the stroma (fibroblast-like cells and collagen fibrils). The collagen fibrils exert a high colloid osmotic swelling pressure. The cornea will naturally swell and as it does so become opaque. This swelling must be counteracted by continual pumping of water out of the tissue to maintain the correct thickness and transparency.
The lens is transparent. It will naturally take up water and swell. This must be balanced by a continuous pumping of (ions and) water out of the lens. Anything that interferes with this process will cause the lens to swell and opacify - a condition known as cataract. There are many causes of cataract, ageing being the predominant of these - the so-called "sunlight" hypothesis for cataract is largely discredited.
ERRORS OF REFRACTION
- Presbyopia. With age lens thickens and becomes harder (won't accomodate)
- Myopia. The near sighted eye. Eyeball too long.
- Hypermetropia. Far-sighted eye. Eyeball too short.
- Astigmatism. Lens or cornea not smoothly spherical.
Secreted by the ciliary epithelium into the posterior chamber of the eye , it diffuses forward through the pupil and into the anterior chamber. From there it drains through the trabecular meshwork and into Schlemm's canal. Glaucoma is the raised intraocular pressure that arises from an imbalance of secretion and drainage, usually due to blocked outflow.
In bright light, the parasympathetic nervous system causes the circular sphincter muscle of the iris to contract and pupil constriction occurs. In dim light, the sympathetic nervous system causes the radial muscle to contract, dilating the pupil.
These are the main cells in the retina: ·
- Photoreceptors (rods & cones)
- Horizontal cells (lateral inhibition at the level of the photoreceptors)
- Bipolar cells ("on" and "off" cells) connect photoreceptors to retinal ganglion cells
- Amacrine cells (lateral inhibition at the level of the retinal ganglion cells)
- Retinal ganglion cells (the axons of which form the optic nerve)
Rods are responsible for "scotopic" or low intensity vision. Cones are responsible for "photopic" or high intensity vision. Both rods and cones contain a photopigment which absorbs the light. There are 4 photopigments, one in rods and one in each of 3 cones. The photopigment is called rhodopsin and consists of two parts - a filter called an opsin and a light sensitive chromophore called retinal. Retinal is common to all four photopigments. [See graph of distribution of cells across retina] In the peripheral retina there is a 1:1000 convergence of photoreceptors (mainly rods) onto retinal ganglion cells. In the fovea (mainly cones) there is a 1:1 correspondence.
In the dark the concentration of cGMP (cyclic guanosine monophosphate) in rods and cones is high. This opens cyclic nucleotide gated (non-selective cation) channels (CNG channels). Sodium (Na+) and calcium (Ca2+) enter the cell and this depolarizes, resulting in an increase in transmitter release.
In the light, a G-protein, transducin, is activated starting a cascade of biochemical events and resulting in a fall in cGMP. The receptor operated channels close and the cells hyperpolarise (receptor potential), resulting in a fall in transmitter release.
The release of transmitter either inhibits "depolarising" or (+) bipolar or stimulates "hyperpolarizing" or (-) bipolar cells which then relay the receptor potential to the retinal ganglion cells which increase/decrease their firing rate as appropriate. [Therefore, (+) bipolars lose their transmitter inhibition and depolarize in light, whereas (-) bipolars lose their stimulus and hyperpolarize in light. Nb. light causes photoreceptors to hyperpolarize and transmitter release, at synapses with bipolar cells, falls].
The retina analyses the visual image in terms of (a) colour, (b) form, (c) movement, (d) luminance and (e) depth.
a. Colour; green, blue and red cones. Cones contain a filter (opsin) and a light-sensitive bit (chromophore, called retinal). Green cones have green filters, red cones have red filters etc.
- red cones maximally sensitive to 559nm light
- green cones to 531nm
- blue cones to 419nm
Colour opponency. Red/green and blue/yellow "centre-surround" receptive fields of retinal ganglion cells (see below) mean that there are retinal ganglion cells sensitive to red/green contrasts and others sensitive to blue/yellow differences. (Yellow is made up of output from red/green cones combined). This aids in colour contrast definition. Colour-blind people usually lack either a red or green opsin and have trouble distinguishing red from green (both appear the same).
b. Form; edge detection, centre-surround receptive fields
Each retinal ganglion cell has a receptive field that corresponds to input from part of the visual field (a small region of the retina). This receptive field is made up of information from one to 1000s of photoreceptors arranged in a "centre-surround" fashion (see diagram).
"ON" and "OFF" cells
[Exercise: draw a line down a sheet of paper; (1) light on left, dark on right; assuming that a constant illumination gives a net output of the centre-surround receptive field of zero, determine the net output at the border, for an "ON" centre retinal ganglion cell - as on the left in the diagram above. You will see that this retinal ganglion cell detects edges. (2) Now do the same for red on the left, green on the right, for a (+)green centre/(-)red surround retinal ganglion cell. You will find that this cell detects red/green edges].
c. Movement; X (slow, high resolution) and Y (fast, low resolution) cells. These are different classes of retinal ganglion cells.
- X-cells; these are mainly connected to cones, slow conducting therefore slow response. Used for high resolution analysis. Found in fovea mainly.
- Y-cells; these are fast responding cells, mainly connected to rods, concerned with movement and found mainly in the peripheral retina.
d. Luminance; W-cells are another class of retinal ganglion cells that may detect the absolute levels of luminance and be involved with the light reflex, i.e. pupil constriction/dilation (and the Edinger-Westphal nucleus).
e. Depth; binocular vision. The visual field of each eye is slightly different. Fusion of the two images (in the cortex) confers the perception of depth. Cf. the stereoscope - an instrument presenting one, slightly different, image to each eye, give remarkable appearance of 3-D.
By analysing the visual image in terms of colour, form (edges), luminance, depth and movement, the retina has achieved a degree of pre-processing of the image before it leaves for the visual cortex via the optic nerve.
- VISION, an excellent site containing many links to a great range of topics relating to vision. Something for everyone.
- Howard Hughes Medical Institute, an excellent site for the Senses - especially vision
SAMPLE EXAM QUESTIONS (for year 2 Medical Students):
- List three types of cell in the cornea
- Name the different types of retinal ganglion cell
- What are the 5 types of visual coding in the retina?
"Human Physiology" by Vander, Sherman, Luciano
"The Senses" by Barlow and Mollon
"Neurophysiology" by R.H.S. Carpenter (editions 4 or 5)
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last update 6 Jan 2003