Special Senses: Vision
Vision in humans involves the detection of a very narrow band of light ranging from about 400 to 750 nm in wavelength. The shortest wave-lengths are perceived as blue and the longest as red. The eye contains photoreceptors that detect light, but, before the light hits the receptors responsible for this detection, it has to be focused onto the retina (200 μm thick) by the cornea and the lens (Fig. 57a).
The photoreceptors can be divided into two distinct types called rods and cones. Rods respond to dim light and cones respond in brighter conditions and can distinguish red, green or blue light. The rods and cones are found in the deepest part of the retina, and light has to travel through a number of cellular layers to reach these photoreceptors. Each photoreceptor contains molecules of the visual pigments (rods: rhodopsin; cones: erythrolabe (red), chlorolabe (green) and cyanolabe (blue)); these absorb light and trigger receptor potentials which, unlike other receptor systems, lead to a hyperpolarization of the cell and not depolarization.
The layers between the retina surface and the receptor cells contain a number of excitable cells, called bipolar, horizontal, amacrine and ganglion cells. The ganglion cells are the neurones that transmit impulses to the rest of the central nervous system (CNS) via axons in the optic nerve. These cells are excited by the vertical bipolar interneurones which lie between the receptor cells and the ganglion cells. In addition, this complex structure also contains two groups of interneurones (horizontal and amacrine cells) that function by exerting their influence in a horizontal manner, by causing lateral inhibition on surrounding synaptic connections between receptor cells and bipolar cells, and bipolar cells and ganglion cells, respectively (Fig. 57a).
Each eye contains approximately 126 million photoreceptors (120 million rods and 6 million cones) but only 1.5 million ganglion cells. This means that there is a substantial amount of convergence of receptor and bipolar cells onto ganglion cells, but this is not uniform across the retina. At the periphery, there is a large amount of convergence, but, in the region of greatest visual clarity (the fovea centralis), there is a 1:1:1 connectivity between a single cone receptor cell, a single bipolar cell and a single ganglion cell. The fovea region has a very high density of cones and very few rods, whereas there is a more even distribution of rods and cones in the other regions of the retina.
Each ganglion cell responds to changes in light intensity over a limited area of the retina, rather than to a stationary light stimulus. This limited area is called the receptive field of the cell and corresponds to the group of photoreceptors that has synaptic connections with that particular ganglion cell. Ganglion cells are usually spontaneously active. Approximately half of the ganglion cells in the retina respond with a decrease in firing of their impulses when the periphery of their receptive field is stimulated by light, and increase their firing rate when the centre of the receptive field is lit up (the ON-centre cells); the other half increase their firing rate when the periphery is illuminated and decrease their firing rate when the central receptors are stimulated (the OFF-centre cells). This allows the output of the retina to signal the relative brightness and darkness of each area being stimulated within the visual field.
The ganglion cells are further subdivided into two main groups: P cells and M cells. P cells receive the central parts of their receptive fields from one or possibly two (but never all three) types of colour-specific cone, whereas M cells receive inputs from all types of cone. M cells are therefore not colour selective, but sensitive to contrast and movement of images on the retina. The division of P and M cells appears to be maintained throughout the visual pathway and they are involved in visual perception.
The optic nerves from the two eyes join at the base of the skull at a structure called the optic chiasma (Fig. 57b). Approximately half of each of the optic nerve fibres cross over to the contralateral side; the other half remain on the ipsilateral side and are joined by axons crossing from the other side. The axons of the ganglion cells from the temporal region of the retina of the left eye and the nasal region of the retina of the right eye proceed into the left optic tract, whereas the axons from the ganglion cells in the nasal part of the left eye and the temporal part of the right eye form the right optic tract. The neurones that make up the optic tract connect to the first relay stations in the pathway: the lateral geniculate bodies, the superior colliculus and the pretectal nucleus of the brain stem. Those fibres that synapse in the superior colliculus and the pretectal nucleus are involved in visual reflexes and orientating responses. A small number of fibres also branch off at this point to synapse in the suprachiasmatic nucleus, which is concerned with the body clock and circadian rhythms within the body. However, the bulk of the neurones reach the lateral genicu- late nucleus in the thalamus. Each nucleus contains six cellular layers and the information from the two eyes remains separate, each group of fibres synapsing in three of the layers. The M ganglion cells terminate in the lower two layers (called magnocellular because the cells are relatively large in these layers). Cells in the magnocellular layers are sensitive to contrast and motion, but not colour. The P ganglion cells synapse in the upper four layers of the lateral geniculate nucleus (two for each eye), called the parvocellular layers. These layers contain relatively small cells which transmit information about colour and fine detail. The fibres from the lateral geniculate nucleus fan backwards and upwards in a bundle (called the optic radiation) through the parietal and temporal lobes to an area of the cerebral cortex called the primary visual cortex. Each cortical cell receives inputs from a limited number of cells in the lateral geniculate nucleus, and therefore has its own receptive field or patch of retina to which it responds.