In the front of the eye the sclera gives way to the transparent cornea which admits light into the eye. Inside the sclerotic coat, is the pigmented, and highly vascularized, choroid coat. It makes the inside of the eye-ball black and non-reflecting. At the front, behind the cornea, the choroid becomes the iris. The iris is separated from the cornea by a pool of liquid called the aqueous humor. The aqueous humor (AH) is secreted into the posterior chamber, behind the iris and in front of the lens, by the ciliary process, a swollen, muscular, secretory area, at the base of the iris, where it merges with the choroid. The AH is reabsorbed into the venous system via a canal, the Canal of Schlemm, which runs around the border of the cornea, close to the base of the iris, at the corneo-scleral junction. If the entry ports to this canal are blocked then AH accumulates in the eye, the pressure increases, giving rise to glaucoma (increased intra-ocular pressure). The ciliary body is attached to a circular ligament, the suspensory ligament,or zonula, which inserts on to the lens.

With the eye at rest, such as when viewing distant objects, the suspensory ligament pulls out on the flexible lens, flattening it, and decreasing the focussing power. In the normal eye, the image of the object on the retina will then be in focus (emmetropia). When we look at near objects (nearer than 20 ft or 6 meters) the image will go out of focus. This triggers a reflex contraction of the ciliary muscle, reducing the outward pull of the suspensory ligament, and so causing the lens to become more rounded (increased focussing power), and restoring the focus of the retinal image. (Remember the lens rule: 1/v = [1/f - 1/u] where v is distance from lens to image; u is distance from object to lens; and f, focal length of the lens). Contraction of the ciliary muscles is induced by the parasympathetic nervous system. With old age, the lens stiffens, and cannot round up as much as in youth. The consequent inability to focus on very close objects, causes the near point (the closest distance at which an object can be seen clearly) to recede (presbyopia). If the length of the eye is too great or the focussing power of the lens too strong, then in the eye at rest, the image on the retina will be out of focus. The eye is then said to be myopic or nearsighted. The condition is described as myopia, and can be corrected with the use of diverging lenses of appropriate strength. If the eye is too short, or the lens too weak, the image is also out of focus in the resting eye, causing hyperopia or farsightedness, which can be corrected using converging lens of appropriate strength placed in front of the eye.The lens is responsible for changing the focussing power of the eye, but the majority of the focussing is actually done by the cornea. Hence, if the lens is removed, vision can still be quite good, with the aid of external lenses.

If the cornea (or lens) is not evenly curved in all directions, then the image on the retina will be distorted, causing astigmatism. In astigmatism, light rays (from a point source) oriented in the plane of greater curvature (BD) are brought to a focus closer to the lens, than light rays oriented in the less curved plane (AC). Astigmatism can be corrected using cylindrical lenses. Cylindrical lenses are flat in one plane, but curved in the plane at right angles (think of a solid cylinder of glass cut along the long axis). If properly oriented, they leave one plane unaffected (in the flat plane), but can enhance (if the curvature is convex) or decrease (if the curvature is concave) focussing power in the other.

Behind the lens, filling the main cavity in the eye, is a gelatinous, protein-rich substance, the vitreous humor. Posteriorly, this abuts on the retina. The cornea, aqueous humor, lens and vitreous body all have to be fully transparent and a-vascular for vision to be clear. Opaque areas or objects in these structures can obscure light passage to the retina, as in cataract. Cataracts frequently occur in uncontrolled diabetes mellitus, when, due to chronically high blood sugar levels, excess glucose entering the lens is metabolised to produce non-diffusible polyols such as sorbitol. These molecules become trapped in the lens, but are osmotically active, and pull water into the lens. The transparency of the lens depends upon the maintenance of a relatively dehydrated state.

The retina is the innermost layer of the eye. It is an outgrowth of the CNS, made up of nerve cells, covering most of the inner wall of the eyeball, behind the ciliary body. It has several layers, corresponding to different parts of just three main cell groups: the outermost layer -the photoreceptors, next to the choroid; the middle layer of bipolar cells, which connect the photoreceptors with the outer layer of ganglion cells, next to the vitreous body. Horizontal Cells connect the photoreceptors together laterally, at the level where they synapse with the bipolar cells. The nuclei of the horizontal cells lie among those of the bipolar cells. The synaptic layer between bipolar, horizontal cells and photoreceptors, forms the outer plexiform layer of the retina. The amacrine cells connect laterally between the ganglion cells at the level of their synapses on the bipolar cells (inner plexiform layer). Their nuclei also lie amongst those of the bipolar cells.

When we look directly at an object, its image falls on the retina at a fixed spot, along the visual axis. This spot on the retina lies in a slight depression, at the fovea centralis. The fovea lies in the centre of a yellowish area, the macula lutea.

Photoreceptors are of two types: rods and cones. Cones are found mainly in the central area of the retina, while rods are found in the peripheral retina (outside of the fovea centralis). Cones mediate color vision and operate best in bright light (have high thresholds) while rods mediate black-and-white vision at low light intensities.

The photoreceptors respond to light because the light causes the activation and consequent breakdown of a photo-sensitive pigment rhodopsin, bound in large quantities to the membranes of their distal (outer) segments.

The distal segment also has Na⁺ channels which are open at rest (in the dark) due to combination with cGMP, allowing a constant inflow of sodium ions. These ions are constantly pumped back out of the receptor at the proximal (inner) segment, which is rich in mitochondria and Na⁺/K⁺-ATPase (sodium pump) proteins.

 The photoreceptors release glutamate on to the bipolar and horizontal cells, causing either excitation via ionotropic receptors or inhibition via metabotropic receptors. If the photoreceptor inhibits the bipolar cell attached to it, then the bipolar cell would be excited (disinhibited) when the photoreceptor is hyperpolarized ("ON" response); if the photoreceptor excites the bipolar cell, then light would produce an "OFF" response.

The tips of the distal segments of rods are constantly sloughed off into the choroid at a high rate, and renewed by growth from the bases. Impairment of this process due to lack of Vitamin A, failure of the choroid to phagocytose the sloughed bits, or other causes, could lead to retinitis pigmentosa (night blindness). Cones also may undergo renewal of the distal segment, but only slowly, so they are not so susceptible to this problem.

There are three types of cone: red sensitive, green sensitive, and blue sensitive. This is what allows to distinguish between different colours. The opsin molecules in these three types are slightly different from each other, and from the opsin in the rods. The red and green opsins are very similar and are coded for by genes on the X-chromosome. Deficiency or loss of any one these genes may lead to different sorts of colour blindness. Dysfunction of the red cones: protanopia or protanomaly; of the green cones: deuteranopia or deuteranomaly. These are quite common among males (chromosomal make-up: XY), less so among females. Dysfunction of blue cones is less common, and equally distributed in males and females (not sex-linked). Many tests are available for colour vision, such as the Ishihara Charts.

Ganglion cells invariably have circular receptive fields on the retina. Photoreceptors in the centre of this field may either inhibit or excite the ganglion cell; those in the periphery of the field will have the opposite effect. This is called the "centre-surround" arrangement. Different ganglion cells may have overlapping receptive fields: receiving divergent output from the same receptor/bipolar cell combinations. Some ganglion cells are small with small receptive fields (X or A type cells); these are usually colour sensitive and respond well to detailed contrasts on the retina, contributing to form vision. Others have larger receptive fields (Y or B type cells); these usually are colour indifferent (although they receive input from cones) and poorly responsive to contrasts, serving to detect directional movement of objects across the retina.

The left part of the external visual field projects to the right occipital lobe, and the right half to the left occipital lobe. The superior external field projects below, and the inferior field projects above the calcarine fissure. In the cortex of each occipital pole, there is a topographic representation of the half retinae that receive input from the contralateral visual field. The area of cortex to which the fovea projects is, however, proportionately much larger than that receiving input from the peripheral retina.

Cortical cells in layer four have centre-surround fields like the geniculate cells. In other layers however, the receptive fields are larger, and rectangular or oval in shape, produced by convergence from geniculate cells with overlapping receptive fields. By a continued process of convergence and divergence from cells with particular characteristics, cells with progressively more complex response profiles are produced in cortical areas surrounding the primary visual cortex. We should note also that in addition to the optic tract-geniculate body-occipital cortex pathway, visual information also gets to the cerebral cortex via an optic tract-superior colliculus-pulvinar thalamic nucleus-posterior temporal cortical pathway. The processing of these inputs in the visual cortex, allows us to distinguish between, and recognize, objects of differing size, shape, colour, movement. Different aspects of this "processing" are carried out in different areas of the cortex. Our understanding of the way in which neurones in the visual pathway at different levels, respond to visual stimuli of different sorts, gives insight into the way in which the nervous system acts in order to decipher differing attributes of a stimulus.

Damage to the visual pathway at different points produces characteristic defects in the visual fields, ranging from simple scotomata, to loss of ability to recognise (visual agnosia), without impairment of ability to see or discriminate. It is even possible to affect ability to detect moving objects, without affecting perception of stationary forms.

September 16, 1999

RECOMMENDED READING:

Ganong: Review of Medical Physiology, Ch. 8 Vision; Berne & Levy: Physiology, Ch. 8 The visual system; Guyton: Textbook of Medical Physiology, Chs. 49 -51.