20.3: Anatomy of the Eyeball
The eye is a spherical, hollow structure composed of three tissue layers. The outer layer — the fibrous tunic, comprises the sclera — a white structure — and the cornea, which is transparent. The sclera encompasses some of the ocular surface, most of which is not visible. However, the 'white of the eye' is distinctively visible in humans compared to other species. The cornea, a clear covering at the front of the eye, enables light penetration. The eye's middle layer, the vascular tunic, primarily consists of the choroid, the ciliary body, and the iris. The choroid is a highly vascularized connective tissue supplying blood to the eyeball, situated behind the ciliary body. The ciliary body, a muscular entity, is linked to the lens by zonule fibers or suspensory ligaments. These aid in lens curvature, facilitating the focus of light onto the rear of the eye. The iris, the eye's colored portion, overlays the ciliary body and is visible at the front of the eye. Iris, a circular muscle, dilates or constricts the pupil, the central eye aperture that permits light entry. The iris contracts the pupil in bright light, which widens the pupil in dim light. The innermost layer, the neural tunic or retina, houses the nervous tissue in light perception.
The eye can be segmented into two distinct sections: the front cavity and the back cavity. The front cavity between the cornea and the lens — encapsulating the iris and ciliary body — is filled with a light liquid known as aqueous humor. On the other hand, the back cavity expands from the area behind the lens to the back of the inner eyeball, where the retina is positioned. This cavity is filled with a thicker fluid referred to as vitreous humor.
The retina is a complex structure composed of numerous layers with distinct cells dedicated to the preliminary processing of visual signals. Photoreceptors, namely rods and cones, respond to light energy by altering their membrane potential. This change influences the quantity of neurotransmitters that the photoreceptors dispatch onto the bipolar cells in the outer synaptic stratum. In the retina, it is the bipolar cell that links a photoreceptor to a retinal ganglion cell (RGC) situated in the inner synaptic layer. Amacrine cells aid in processing within the retina before the RGC generates an action potential. Positioned at the retina's deepest layer, the RGCs' axons aggregate at the optic disc and exit the eye, forming the optic nerve. Since these axons traverse the retina, there is an absence of photoreceptors at the eye's rear, where the onset of the optic nerve lies. This results in a "blind spot" in the retina and an equivalent blind spot in our field of vision.
The intricate structure of the retina comprises multiple layers populated with different cells, all of which play a critical role in the initial interpretation of visual cues. The photoreceptors, specifically rods and cones, are sensitive to light energy, and this sensitivity prompts a shift in their membrane potential. This shift subsequently determines the amount of neurotransmitter released onto the bipolar cells in the outer synaptic layer. The bipolar cell is the intermediary between a photoreceptor and a retinal ganglion cell (RGC) in the inner synaptic layer within the retina. The processing within the retina is assisted by Amacrine cells before the RGC generates an action potential. The axons of RGCs, nestled in the innermost layer of the retina, converge at the optic disc, exiting the eye as the optic nerve. Due to the course these axons take through the retina, the rear of the eye, where the optic nerve originates, is devoid of photoreceptors. This results in a "blind spot" in the retina, reflecting an identical blind spot in our visual field.
It is important to note that the photoreceptors (rods and cones) within the retina are behind the axons, RGCs, bipolar cells, and retinal blood vessels. These structures absorb a considerable amount of light before it reaches the photoreceptor cells. Yet, the fovea is at the retina's center — a small area devoid of supporting cells and blood vessels, only housing photoreceptors. As such, visual acuity — the clarity of vision — is optimal at the fovea due to minimal absorption of incoming light by other retinal structures. As one moves away from the foveal center in any direction, there is a noticeable drop in visual acuity. Each of the fovea's photoreceptor cells is connected to a single RGC. It follows that the RGC does not need to amalgamate inputs from multiple photoreceptors, enhancing the precision of visual transduction.
Conversely, at the peripheries of the retina, several photoreceptors converge on RGCs (via the bipolar cells) in ratios as high as 50 to 1. The disparity in visual acuity between the fovea and the peripheral retina is starkly evident — focus on a word positioned in the middle of this paragraph without moving your eyes, and words at the beginning or end appear blurry and out of focus. The peripheral retina is responsible for creating the images in your peripheral field of view; however, these images often have indistinct, fuzzy edges, and the words must be more clearly discernible. So, a significant portion of the neural function of the eyes is concentrated on moving the eyes and head to ensure important visual stimuli are centered on the fovea.
Photoreceptors, the cells responsible for capturing light in the eye, are composed of two distinct components: the internal and external segments. The former harbors the nucleus and various other cell organelles, while the latter is a niche area enabling photoreception. Two distinct photoreceptor types exist — rods and cones — characterized by the morphology of their external segments. The rods — named for their rod-like segments — house membranous disks filled with the light-sensitive pigment rhodopsin. The cone photoreceptors, on the other hand, hold their light-sensitive pigments within the cell membrane's invaginations, and their external segments take on a conical shape. Cone photoreceptors possess three photopigments, namely opsins, each responsive to a specific light wavelength. The color of visible light is determined by its wavelength, and the photopigments in the human eye are adept at discerning three fundamental colors: red, green, and blue.
