Although the retinal layer of the eyeball is often considered the beginning of visual perception, the anterior structures of the eye play an important role in visual perception too. The anteriormost structure is the cornea and is the first layer through which light, the visual stimulus, enters the eye (eFig. 9.72). This transparent layer overlies the aqueous humor of the anterior chamber and the pupil, a small central aperture that controls how much light is admitted into the eye. After passing through the pupil light is refracted through the lens, which along with the ciliary body, separates the anterior portion of the eye from the posterior portion (eFig. 9.72). The lens is a clear biconvex structure that rounds in shape as the ciliary muscle contracts and relaxes the suspensory ligaments attached to the borders of the lens, a parasympathetically controlled process referred to as accommodation.
After the lens, light passes through the vitreous humor of the posterior chamber and is projected onto the layers of the retina (eFig. 9.73). The retina is composed of a non-neural layer and several layers of neural cells with synaptic connections. To reduce the amount of light reflected in the eye, the choroid layer lining the inner surface of the sclera (eFig. 9.72), along with the pigment epithelium layer (non-neural) of the retina, absorb and refract some of the light stimulus (eFig. 9.73). Interdigitating between the pigment epithelial cells, the photoreceptors transduce the light stimulus into an electrical signal in a process called phototransduction. Images formed on the retina are inverted in both vertical and lateral dimensions (eFig. 9.74). Because of this, the visual field is defined as having four quadrants: left/right and upper/lower.
Photoreceptors include rods, which are very sensitive to light and essential for vision in dimly lit conditions, and cones, which are responsible for color vision and high visual acuity. Although rods and cones are both distributed across the retina, rods outnumber cones by twentyfold and are concentrated in the periphery of the retina. Cones predominate near the macula and are the only photoreceptors present at the fovea (eFig. 9.72). The fovea represents the primary visual axis of the eye and is the location of maximal visual acuity. Despite their functional differences, both the rods and cones have an outer light-sensing segment, an inner segment, and a synaptic terminal. These synaptic terminals contact bipolar cells, the first-order neurons in the visual pathway (eFig. 9.73). Bipolar cells then synapse with ganglion cells, the second-order neurons in the visual pathway. Two other cell types present in the retina are horizontal and amacrine cells. These transversely oriented interneurons moderate the excitation level of bipolar and ganglion cells.
Axons from the ganglion cells coalesce at the optic disc, which is absent of photoreceptors and thus creates a blind spot in the visual field. As they leave the optic disc they form the optic nerve, acquire a myelin sheath provided by oligodendrocytes, and are invested by the cranial meninges (eFig. 9.72). These morphological features derived during embryonic development, define the optic nerve as a component of the CNS. Anterior to the infundibular stalk, the optic nerves converge at the optic chiasm. Within the chiasm, axons from the nasal portion of the retina decussate and enter into the contralateral optic tract (eFig. 9.75). Conversely, axons from the temporal portion of the retina stay ipsilateral to enter the ipsilateral optic tract (eFig. 9.75).