The Normal Eye - Retina
The retina forms the inner neuro-epithelium layer of the posterior segment and is concerned with the reception of light images and transmission of a nervous impulse along the optic nerve.
Structurally the retina has two main parts: the neural retina and the retinal pigment epithelium. While there is an intimate association between these two parts of the retina, there is not a strong attachment and this sub-retinal space has implications for retinal detachments. As a part of the brain, the neural retina has a very complex cellular structure which, in brief, consists of rod and cone photoreceptors, adjacent to the retinal pigment epithelium, which connect through a series of ganglion cells, finally ending as nerve fibres in the innermost layer of the retina. These fibres sweep from all areas of the retina to the optic nerve head. There is a very wide and varied population of nerve cells in the retina, each responsible for a particular aspect of the transmission or co-ordination of the visual impulse.
The retina has one of the highest metabolic demands of anywhere in the body and it requires an extensive and efficient blood supply, which comes from two sources. The dense choroidal capillary network (known as the choriocapillaris) supplies the retinal pigment epithelium and outer half of the retina. The central retinal artery enters the eye through the optic nerve head and sub-divides into an extremely intricate end-artery system, which supplies the other half of the retina adjacent to the vitreous. Capillary density is highest around the macular region of the central retina, the area which has the highest concentration of cone photoreceptors. In the centre of the macula is the fovea which is a vascular free zone and has responsibility for detailed vision. The intraretinal vasculature is worthy of special mention since normal functioning of the retinal blood vessels is extremely important for retinal function. This can be demonstrated by the pathological consequences of retinal vascular dysfunction, which leads to widespread retinal oedema and ischaemia as observed in several important diseases such as diabetic retinopathy and Age-Related Macular Degeneration.
Photoreceptors represent a remarkable adaptation of basic cellular structure that is optimized for presentation of photopigment on membranous discs and transmission of the converted light stimulus to accessory neurones (Figure 6). Photopigments are coloured substances with an ability to absorb light of various wavelengths. Chemically, the photopigments consist of a combination of water-insoluble opsin protein linked with an aldehyde of Vitamin A that can undergo a chemical realignment after light-stimulation. This realignment is responsible for initiating an electrical potential change leading to a nervous impulse. Rods and cones differ in their function. Cones are concerned with an appreciation of form and colour and under conditions of bright illumination give very high visual acuity. Rods on the other hand are responsible for perception under dim illumination and cannot discern colour or fine detail.
The highest density of cones is in the macular region where the ratio of cones to ganglion cells is greater than that of rods. Rods are present throughout the retina although they are dominant in the peripheral retina and absent from the fovea.