Light passes through the cornea – the avascular
part of the sclera – to enter the eye. Here, it is refracted by the biconvex lens to reverse
and invert the image, before the beams of light are received by the photoreceptors of
the retina. The retina is made up of six layers. Light
must pass through five layers before reaching the photoreceptor layer. Absorption of light
stimulates a chemical reaction in the discs of the photoreceptors, thus creating an electrical
impulse. There are two types of photoreceptors: rods and cones. Cones are necessary for perception
of colour and visual form, and are most dense in the fovea. Rods are necessary for the perception
of movement, depth of vision and small differences in brightness. Rods are absent from the fovea,
and are most dense in the perifoveal area. Once the electrical impulse has been initiated
within the rods and cones, it travels along the length of these cells, which synapse with
bipolar cells in the outer plexiform layer. The bipolar cells then synapse with the ganglion
cells in the inner plexiform layer. Ganglion cell axons are non-myelinated and carry the
impulse along the inner surface of the eye towards the optic disc, where the cell axons
pass between the fine collagen bundles of the lamina cribrosa as they leave the eye.
At this point, the axons become myelinated and collectively form the optic nerve. The
optic nerve is surrounded by the meninges and passes through the optic canal to exit
the orbit. The optic canal opens into the middle cranial fossa.
The temporal retinal fibres course laterally and do not decussate, while the nasal retinal
fibres decussate and join the temporal retinal fibres of the contralateral eye. At this point,
the optic tract is formed, with each tract composed of the ipsilateral temporal retinal
fibres and the contralateral nasal retinal fibres. The majority of the optic tract loops
around the cerebral peduncle to enter the lateral geniculate nucleus of the thalamus.
Here, the optic tract fibres synapse with the third order sensory neurons, whose cell
bodies are located within the lateral geniculate nucleus. The axons of these third order neurons
form the optic radiation, which relays the visual information to the primary visual cortex.
The majority of the primary visual cortex is located on the medial surface of the occipital
lobe along the calcarine fissure, with the remainder located on the posterior surface
of the lobe. The axons within the optic radiation follow the contour of the lateral wall of
the lateral ventricle. They retain retinoptic organisation forming a superior, intermediate
and inferior division. The superior division, also known as Baum’s loop, conveys information
from the superior retinal quadrants and therefore from the inferior visual field. The intermediate
division relays information from the macular region of the visual field. The inferior division,
also known as Meyer’s loop, courses posteriorly within the temporal lobe, looping around the
inferior horn of the lateral ventricle. Meyer’s loop relays information from the inferior
retinal quadrants and therefore from the superior visual field. The primary visual cortex projects
to the secondary visual cortex. Here, information is processed and projected to the association
visual cortex. Impulses then travel to the parietal lobe via the dorsal pathway or to
the temporal lobe via the ventral pathway. The dorsal pathway is concerned with spatial
vision; the ventral pathway is concerned with object recognition.