The central visual pathway 155
Vision is the most highly developed and versatile of all the sensory modalities and, arguably, the one on which humans are most dependent. The optic nerve and retina develop from the prosencephalic primary brain vesicle and, therefore, are regarded as an outgrowth of the brain itself. Vision commences with the formation of an image of the external world on the photoreceptive retina. The retina encodes visual information in the discharge of neurones that project to the brain through the optic nerve. Fibres of the optic nerve undergo hemidecussation in the optic chiasma and project to the lateral geniculate nucleus of the thalamus. Thalamocortical neurones in turn project to the primary visual cortex, in the occipital lobe of the cerebral hemisphere, where visual perception occurs.
The eyeball, or globe, is approximately spherical in shape (Fig. 15.1). Near its posterior pole emerges the optic nerve. The eyeball may be considered to consist of three concentric layers of tissue, the outermost of which is tough, fibrous and protective. Over most of the globe it forms an opaque white coat, the sclera, to which are attached the extraocular muscles that move the eyeball. Over the anterior pole of the globe it forms the transparent cornea, through which light enters the eye.
Figure 15.1 Schematic drawing of a horizontal section through the right eyeball.
Near to the anterior margin of the sclera, two rings of smooth muscle extend into the lumen of the eyeball (Fig. 15.2). The most anterior of these is the iris, which has a central aperture, the pupil, through which light is admitted to the posterior part of the eye. Some of the muscle fibres of the iris are arranged in a circular fashion, while others are oriented radially. They are under the control of the autonomic nervous system. Circular fibres are innervated by postganglionic parasympathetic neurones, which act to constrict the pupil and reduce the amount of light falling upon the retina (see p. 100). Radial fibres are innervated by postganglionic sympathetic neurones which dilate the pupil.
Figure 15.2 Schematic drawing of the sclerocorneal junction of the eyeball.
Behind the iris lies the ciliary body containing ciliary muscle, which receives innervation from the parasympathetic nervous system. The central aperture within the annulus of the ciliary body is occupied by the transparent, biconvex lens, which focuses light upon the retina. The lens is held in place by a suspensory ligament that is attached to the peripheral margin of the lens and to the ciliary body. Contraction of the ciliary muscle alters the shape and, therefore, the focusing power (focal length) of the lens, a process known as accommodation. The lens and suspensory ligament divide the lumen of the eyeball into an anterior and a posterior part. The anterior part, in front of the lens, contains a thin, watery fluid, aqueous humour, which is continuously secreted from the ciliary body. It is also reabsorbed into the ciliary body where it is drained by a small duct, the canal of Schlemm, through which it is returned to the venous system. The posterior part of the globe contains a gelatinous material known as vitreous humour. Behind the ciliary body, the inner surface of the sclera is lined by the choroid, the cells of which contain dark pigment that absorbs light and thus reduces reflection within the eye. Lining the inner surface of the choroid is the photoreceptive retina.
Light passes from objects in the field of vision (visual field), through the narrow aperture of the pupil to subtend an image upon the retina. An object in the visual field, upon which attention is focused, subtends an image that is centred near the posterior pole of the eye along the line of the visual axis (Fig. 15.1). At this point, which is known as the fovea centralis, and the surrounding 1 cm, which is known as the macula lutea, the retina is specially modified for maximal visual acuity (resolving power). The basic optical properties of the eye, which may be likened to those of a pinhole camera, dictate that the image so formed is inverted in both lateral and vertical dimensions (Fig. 15.3). Furthermore, objects that lie in the left half of the visual field form an image upon the nasal (right) half of the left retina and the temporal (right) half of the right retina, and vice versa. Medial to the macula is a region where retinal axons accumulate to leave the eye in the optic nerve. This is known as the optic disc. Photoreceptors are absent from this region, which is also, therefore, referred to as the blind spot.
Figure 15.3 The representation of the visual field upon the retinae.
The retina consists of a non-neural and a neural portion. The non-neural part is represented by the pigment epithelium, a single layer of light-absorbing, pigmented cells lying adjacent to the choroid (Fig. 15.4). The neural part of the retina contains photoreceptors and neurones as well as neuroglia and a rich capillary network. The photoreceptive cells lie deepest within the retina and interdigitate with the pigment epithelium. Light entering the eye, therefore, passes through, and is refracted and partially absorbed by, these additional elements before reaching the photoreceptors. By means of a series of photochemical reactions and physicochemical changes, retinal photoreceptors transduce light energy into electrical energy (changes in membrane potential). Retinal photoreceptors are of two types, rods and cones, of which the rods are about 20 times more numerous. These cells share many structural similarities but have important functional distinctions. Rods are exquisitely sensitive to light. They are particularly important for vision in dim lighting conditions. Cones are responsible for colour vision and, because of their arrangement and neuronal connections, they confer high visual acuity.
Figure 15.4 Schematic drawing showing the cellular organisation of the retina.
Rods and cones are heterogeneously distributed across the retina. Rods greatly predominate in the peripheral parts of the retina but their relative numbers decrease towards the macula, where cones are more abundant. At the fovea only cones are present. Furthermore, at the fovea the neurones and capillaries, through which light has to pass to reach the photoreceptors, are displaced so that the cones are directly exposed to light. This combination provides for maximal visual acuity.
In addition to photoreceptive cells, the retina contains the cell bodies of both the first- and second-order neurones of the central visual pathway (Fig. 15.4). The first-order neurone, or bipolar cell, lies entirely within the retina, while the axon of the second-order neurone, or ganglion cell, forms the optic nerve. Information is transferred from photoreceptors to bipolar cells and then to ganglion cells, with greater convergence for rods than for cones. The retina also contains interneurones known as horizontal cells and amacrine cells. These modulate transmission between photoreceptors and bipolar cells, and between bipolar cells and ganglion cells, respectively.
Objects in one lateral half of the visual field form images on the nasal half of the ipsilateral retina and the temporal half of the contralateral retina.
The retina contains photoreceptors (rods and cones), first-order sensory neurones (bipolar cells) and second-order neurones (ganglion cells).
The axons of retinal ganglion cells accumulate at the optic disc (blind spot) and pass into the optic nerve.
The central visual pathway
The axons of retinal ganglion cells assemble at the optic disc and pass into the optic nerve, which enters the cranial cavity through the optic canal. The two optic nerves converge to form the optic chiasma on the base of the brain (Fig. 15.5). The chiasma lies immediately rostral to the tuber cinereum of the hypothalamus and between the terminating internal carotid arteries. In the chiasma, axons derived from the nasal halves of the two retinae decussate and pass into the contralateral optic tract, while those from the temporal hemiretinae remain ipsilateral. The optic tracts diverge away from the chiasma and pass round the cerebral peduncle to terminate mainly in the lateral geniculate nucleus (within the lateral geniculate body) of the thalamus. A relatively small number of fibres leave the optic nerve, before reaching the lateral geniculate nucleus, to terminate in the pretectal area and the superior colliculus. These fibres are involved in mediation of the pupillary light reflex (Ch. 10). From the lateral geniculate nucleus, third-order thalamocortical neurones project through the retrolenticular part of the internal capsule and form the optic radiation, which terminates in the primary visual cortex of the occipital lobe. The primary visual cortex is located predominantly on the medial surface of the hemisphere in the region above and below the calcarine sulcus. Surrounding this area, the rest of the occipital lobe constitutes the visual association cortex. It is concerned with interpretation of visual images, recognition, depth perception and colour vision.
Figure 15.5 The central visual pathway.
There is a precise point-to-point relationship between the retina and the visual cortex. Because of the importance of the macula in vision, it is represented by disproportionately large volumes (relative to its size) of the lateral geniculate nucleus and the visual cortex. Within the visual cortex the macula is represented most posteriorly, in the region of the occipital pole.
As previously noted, objects in either half (left or right) of the visual field produce images upon the nasal hemiretina of the ipsilateral eye and the temporal hemiretina of the contralateral eye (Fig. 15.5). Each optic nerve, therefore, carries information concerning both halves of the visual field. Because of the decussation of fibres from the nasal hemiretinae at the optic chiasma, however, each optic tract, lateral geniculate nucleus and visual cortex receives information relating only to the contralateral half of the visual field. This combination of the images from both eyes is necessary for stereoscopic vision (depth perception). The upper half of the visual field forms images upon the lower halves of the retinae and the lower half of the visual field forms images upon the upper hemiretinae. As thalamocortical fibres leave the lateral geniculate nucleus they pass around the lateral ventricle, those representing the lower part of the visual field coursing superiorly to terminate in the visual cortex above the calcarine sulcus. Those which represent the upper part of the visual field sweep into the temporal lobe (Meyer’s loop, Fig. 15.6) before terminating below the calcarine sulcus.
Figure 15.6 The course taken by thalamocortical fibres projecting from the lateral geniculate nucleus to the primary visual cortex.
The central visual pathway
At the optic chiasma, axons from the nasal halves of the two retinae decussate and pass into the contralateral optic tract.
The optic tract contains axons that carry information relating to the contralateral half of the field of vision.
Optic tract fibres end in the lateral geniculate nucleus of the thalamus.
Third-order visual fibres from the lateral geniculate nucleus pass through the retrolenticular part of the internal capsule and the visual radiations to terminate in the primary visual cortex.
The primary visual cortex is located above and below the calcarine sulcus of the occipital lobe.
The rest of the occipital lobe constitutes the visual association area.
The visual field can be considered (Fig. 15.7) as comprising four quadrants (left/right, upper/lower), each projecting to its own quadrant of the primary visual cortex (left/right hemispheres, above/below the calcarine sulcus). There is both lateral and vertical inversion in the projection of the visual field upon the visual cortex such that, for example, the upper left quadrant of the visual field is represented in the lower right quadrant of the visual cortex.
Figure 15.7 Representation of the left half of the visual field at various levels in the visual pathway.
Visual field deficits
Disease of the eyeball (cataract, intraocular haemorrhage, retinal detachment) and disease of the optic nerve (multiple sclerosis and optic nerve tumours) lead to loss of vision in the affected eye (monocular blindness). Compression of the optic chiasma by an adjacent pituitary tumour leads to bitemporal hemianopia. Vascular and neoplastic lesions of the optic tract, optic radiation or occipital cortex produce a contralateral homonymous hemianopia (Fig. 15.8).
Figure 15.8 Visual field deficits produced by lesions of the visual pathway.
Retinitis pigmentosa is an inherited metabolic disorder of the photoreceptor and retinal pigment epithelial cells. There is progressive night blindness, peripheral visual field constriction and pigmentation of the retina visible on ophthalmoscopy.