Introduction

The visual pathways are of outstanding importance in clinical neurology. They extend from the retinas of the eyes to the occipital lobes of the brain. Their great length makes them especially vulnerable to demyelinating diseases such as multiple sclerosis; to tumors of the brain or pituitary gland; to vascular lesions in the territory of the middle or posterior cerebral artery; and to head injuries.

The visual system comprises the retinas, the visual pathways from the retinas to the brainstem and visual cortex, and the cortical areas devoted to higher visual functions. The retinas and visual pathways are described in this chapter. Higher visual functions are described in Chapter 29.

Retina

The retina and the optic nerves are part of the central nervous system. In the embryo, the retina is formed by an outgrowth from the diencephalon called the optic vesicle (Ch. 1). The optic vesicle is invaginated by the lens and becomes the two-layered optic cup.

The outer layer of the optic cup becomes the pigment layer of the mature retina. The inner, nervous layer of the cup gives rise to the retinal neurons.

Figure 28.1 shows the general relationships in the developing retina. The nervous layer contains three principal layers of neurons: photoreceptors, which become applied to the pigment layer when the intraretinal space is resorbed; bipolar neurons; and ganglion cells which give rise to the optic nerve and project to the thalamus and midbrain.

Figure 28.1 Embryonic retina. Green and red represent rods and cones, respectively.

Note that the retina is inverted: light must pass through the layers of optic nerve fibers, ganglion cells, and bipolar neurons to reach the photoreceptors. However, at the point of most acute vision, the fovea centralis, the bipolar and ganglion cell layers lean away all around a central pit (fovea), and light strikes the photoreceptors directly (see Foveal Specialization, later). In the mature eye, the fovea is about 1.5 mm in diameter and occupies the center of the 5 mm wide macula lutea (‘yellow spot’) where many of the photoreceptor cells contain yellow pigment. The fovea is the point of most acute vision and lies in the visual axis – a line passing from the center of the visual field of the eye, through the center of the lens, to the fovea (Figure 28.2). To fixate or foveate an object is to gaze directly at it so that light reflected from its center registers on the fovea.

Figure 28.2 Horizontal section of the right eye, showing the visual axis.

The axons of the ganglion cells enter the optic nerve at the optic papilla (optic nerve head), which is devoid of retinal neurons and constitutes the physiological ‘blind spot’.

The visual fields of the two eyes overlap across two-thirds of the total visual field. Outside this binocular field is a monocular crescent on each side (Figure 28.3). During passage through the lens, the image of the visual field is reversed, with the result that, e.g., objects in the left part of the binocular visual field register on the right half of each retina, and objects in the upper part of the visual field register on the lower half. This arrangement is preserved all the way to the visual cortex in the occipital lobe.

Figure 28.3 (A) Visual fields. Both eyes are targeted on the fixation point. The visual field of the right eye is shaded. (B) The right visual field. The white spot represents the blind spot of the right eye.

From a clinical standpoint, it is essential to appreciate that vision is a crossed sensation. The visual field on one side of the visual axis registers on the visual cortex of the opposite side. In effect, the right visual cortex ‘sees’ the left visual field. Only half of the visual information crosses in the optic chiasma, for the simple reason that the other half has already crossed the midline in space.

Visual defects caused by interruption of the visual pathway are always described from the patient’s point of view, i.e. in terms of the visual fields, and not in terms of retinal topography.

Structure of the retina

In addition to the serially arranged photoreceptors, bipolar cells, and ganglion cells shown in Figure 28.1, the retina contains two sets of neurons arranged transversely: horizontal cells and amacrine cells (Figure 28.4). A total of eight layers are described for the retina as a whole.

Figure 28.4 The layers of the retina. (1) Pigment layer; (2) photoreceptor layer; (3) outer nuclear layer; (4) outer plexiform layer; (5) inner nuclear layer; (6) inner plexiform layer; (7) ganglion cell layer; (8) nerve fiber layer.

Action potentials are generated by the ganglion cells, providing the requisite speed for conduction to the thalamus and midbrain. For the other cell types, distances are very short and passive electrical charge (electrotonus) is sufficient for intercellular communication, whether by gap-junctional contact or transmitter release.

Photoreceptors

The photoreceptor neurons comprise rods and cones.

Rods function only in dim light and are not sensitive to color. They are scarce in the outer part of the fovea and absent from its center. Cones respond to bright light, are sensitive to color (received in the form of electromagnetic wavelength energy) and to shape, and are most numerous in the fovea.

Each photoreceptor has an outer and an inner segment and a synaptic end-foot. In the outer segment, the plasma membrane is folded to form hundreds of membranous discs which incorporate visual pigment (rhodopsin) formed in the inner segment. The synaptic end-foot makes contact with bipolar neurons and horizontal cell processes in the outer plexiform layer.

A surprising feature of the photoreceptors is that they are hyperpolarized by light. During darkness Na⁺ channels are opened, creating sufficient positive electrotonus to cause leakage of transmitter (glutamate) from the end-feet. Illumination causes the Na⁺ channels to close.

Cone and rod bipolar neurons

Cone bipolar neurons

Cone bipolar neurons are of two types. ON bipolars are switched on (depolarized) by light, being inhibited by transmitter released in the dark. They converge onto ON ganglion cells. OFF bipolars have the reverse response and converge onto OFF ganglion cells (Figure 28.5).

Figure 28.5 Retinal circuit diagram. (Adapted from Massey and Redburn, 1987.) A, amacrine cell; C, cone; CB, cone bipolar neuron; GC, ganglion cell; H, horizontal cell; N, nexus (gap junction); R, rod; RB, rod bipolar.

Rod bipolar neurons

Rod bipolar neurons are all hyperpolarized by light. They activate ON and OFF ganglion cells indirectly, by way of amacrine cells (Figure 28.5).

Horizontal cells

The dendrites of horizontal cells are in contact with photoreceptors. The peripheral dendritic branches give rise to axon-like processes which make inhibitory contacts with bipolar neurons.

The function of horizontal cells is to inhibit bipolar neurons outside the immediate zone of excitation. The excited bipolars and ganglion cells are said to be on-line; the inhibited ones are off-line.

Amacrine cells

Amacrine cells have no axons. Their appearance is octopus-like, the dendrites all emerging from one side of the cell. Dendritic branches come into contact with bipolar neurons and ganglion cells.

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