5 THE VISUAL PATHWAY
The visual pathway consists of:
1. THE OPTIC NERVES
The fibres of the optic nerve originate in the retina. The retina is divided into the temporal and nasal halves at the level of the fovea centralis. The optic nerves join the optic chiasma at the anterolateral angle.
2. THE OPTIC CHIASMA
It is a flat band-like structure lying above the pituitary fossa. In the optic chiasma, where the semi-decussation of the nerve fibres lie.
- i. The nerve fibres from the nasal side (medial) of each retina cross-over to the opposite side.
- ii. The nerve fibres from the temporal side (Lateral) do not cross but pass into optic tracts of the same side.
3. THE OPTIC TRACT
The optic tracts originate from the postero-lateral angle of the optic chiasma. They are cylindrical bands running outwards and backwards to end in the lateral geniculate bodies. They consist of the temporal fibres of the same side and the nasal fibres of the opposite side.
4. THE LATERAL GENICULATE BODIES
These are oval structures situated at the posterior end of the optic tracts. The fibres of the optic tracts end in the lateral geniculate bodies and new fibres of the optic radiations originate from them.
5. THE OPTIC RADIATIONS
The nerve fibres proceed backwards and medially as the optic radiations to terminate in the visual centers situated in the occipital lobes.
6. THE OCCIPITAL CORTEX
It is situated above and below the calcarine fissure in the occipital lobes extending up to the occipital pole.
The visual nerve pathway can be divided into three parts:
- First order neuron: bipolar cell in the retina, with rods and cones being sensory end organs.
- Second order neuron: Ganglion cell in the retina, the process of which pass along the optic nerve, optic chiasma and optic tract to the lateral geniculate body.
- Third order neuron: Impulses via the optic radiations to the occipital lobe.