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.
• 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.
• 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.
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.
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.
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).
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.