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A. Anatomy of the Optic Nerve.
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As you can guess, the optic nerve connects the retina of the eye to the brain.
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As light excites the retina cells in the eye, action potentials are then transported through the optic nerves to the brain so that you become aware of what you see!
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But this connection between the eye and the brain is not as straightforward as you would expect.
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As you can see in the diagram, the connection depends upon whether the nerve originates from the right side or the left side of the retina…
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At first, all the nerves from the retina propagates as bundles, called the optic nerve, underneath the brain towards a central location, called the Optic Chiasma.
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In this chiasma (= intersection or cross-over) something strange happens! All the nerves from the inner side of the retina cross-over to the other side while the nerves from the outer side of the retina do not cross but stay at the same side of the brain!
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The inner side is called the ‘nasal’ side of the retina (closer to the nose) while the outer side is called the ‘temporal’ side of the retina.
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Why do these nerves cross-over like this? Nobody knows! But is, unfortunately for students, a fact.
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Therefore, from this chiasma, the optic tract (as it is now called), contains, at the left side, information collected from the ‘right’ side of both eyes, and the opposite happens in the right optic tract.
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After this tract, the nerves connect to a second set of nerves, one by one, in the lateral geniculate bodies, and form the nerves that travel to the optic (also called visual) cortex.
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Why is all this important? When something happens in this region of the brain, it depends on the location of the event (cancer, trauma etc) what happens to our visual perception!
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If the optic nerve is damaged (a), then the result is quite simple. That eye becomes blind; in the diagram, the right eye.
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If there is a damage to the optic chiasma, then only the two bundles that crossover to the other side are damaged. That makes that half the retina, both located at the nasal side, become blind, in both eyes!
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And, even more complex, is when a trauma occurs in the optic tract. Then again two halves of the retina become blind, but now either to the left or the right side of the retina.
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