The optic nerve meets the posterior part of the globe slightly nasal to the posterior pole and slightly above the horizontal meridian. Inside the eye this point is seen as the optic disc. There are no light-sensitive cells on the optic disc -and hence the blind spot that anyone can find in their field of vision. The optic nerve contains about one million nerve fibres, each of which has a cell body in the ganglion cell layer of the retina (Figure 2.6). Nerve fibres sweep across the innermost part of the retina to reach
the optic disc. They can be seen with the ophthalmoscope by carefully observing the way light is reflected off the inner surface of the retina (Figure 2.7). The retinal vessels are also embedded on the inner surface of the retina. There is therefore a gap, which is the thickness of the transparent retina, between the retinal vessels and the stippled pigment epithelium. Apart from the optic nerve, the posterior pole of the globe is also perforated by several long and short ciliary nerves. These contain parasympa-thetic, sympathetic and sensory fibres, which mainly supply muscles of the iris (dilator and sphincter) and ciliary body (ciliary muscles). Patients can experience pain when the iris is handled under inadequate local anaesthesia,
Figure 2.7. The normal fundus of a a Caucasian and b an African. The background is darker in the African owing to increased pigment in the retinal pigment epithelium (RPE).The nerve fibre layer is noticeable, especially along the superior and inferior temporal arcades. CO
and pain is also sometimes experienced during laser coagulation treatment of the chorioretina
- this would seem to prove the existence of sensory fibres in the iris and choroid. The cornea is extremely sensitive, but again, the only sensory endings are those for pain.
The visual pathways include the following:
1. The retina:
• bipolar cells
2. Axons of the ganglion cells visual and pupillary reflex pathways:
• nerve fibre layer of retina
3. Subcortical centres and relays:
• superior colliculus - reflex control of eye movements
• pretectal nuclei - pupillary reflexes
• lateral geniculate body - cortical relay.
4. Cortical connections:
• optic radiations
• visual cortex (area 17) - vision and reflex eye movements
• association areas (areas 18 and 19)
• frontal eye field - voluntary eye movements.
If the rods and cones are considered analogous to the sensory organs for touch, pressure, temperature, etc. then the bipolar cells may be compared to the first-order sensory neurons of the dorsal root ganglia. By the same token, the retinal ganglion cells can be compared to the second-order sensory neurons, whose cell bodies lie within the spinal cord or medulla.
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