Anatomy of the Eye
The eyes are located within the skull orbits, which provide protection for the eyes, as well as provide a place to anchor the soft tissues that support the functions of the eye. The eyelids, with lashes at their leading edges, help to protect the eye from abrasions by blocking particles that may get onto its surface. From the inner surface of each lid, a thin mucous membrane known as the conjunctiva folds in and covers the surface of the eye. Tears are produced by the lacrimal glands, which are superior and lateral to the orbit in each eye, and they flow over the conjunctiva to wash away particles that may have gotten past the lashes and the lids. Tears flow down through the nasolacrimal ducts, located on the medial side of each orbit, into the nasal cavity.
Anatomical features of the tissues surrounding the eye (a) and lacrimal system (b). This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).
Components of the Eye
The eye itself is a hollow sphere composed of three layers of tissue. The outermost layer is the fibrous tunic which is the white sclera and clear cornea. The two parts of the fibrous tunic are continuous, though they have different properties. The sclera accounts for 5/6 of the surface of the eye, most of which is not visible (though humans are unique in having so much of the “white of the eye” visible). The cornea covers the anterior region of the eye and allows light to pass into the eye where it will eventually stimulate photoreceptors. The next layer of the eye is the vascular tunic, which is mostly composed of the choroid, a highly vascularized connective tissue that provides a blood supply to the adjacent tissue. The choroid is posterior to the ciliary body, a muscular structure that is attached to the lens by the suspensory ligament. The ciliary body focuses light on the back of the eye. Overlaying the ciliary body, and visible in the anterior eye, is the iris, the colored part of the eye that opens in the center as the pupil. The innermost layer of the eye is the neural tunic, which is the retina or the nervous tissue that is responsible for photoreception.
Anatomical features of the eye. This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).
Chambers of the Eye
The eye is also divided into two cavities, the anterior and posterior. The anterior chamber, of anterior cavity, is the space between the cornea and iris. The posterior chamber sits between the iris and the lens. Both the anterior and posterior chambers are filled with a watery fluid called the aqueous humor. The posterior vitreous chamber (also posterior cavity) is posterior to the lens and is filled with a more viscous fluid called the vitreous humor (vitreous body).
Movement of the eye within the orbit is accomplished by the contraction of six extraocular muscles that originate from the bones of the orbit and insert into the surface of the eye.
Muscles that Control Eye Movement. This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).
Each of these muscles is innervated by one of the cranial nerves as summarized in the table below.
Focusing Light on the Retina
The retina, where the photoreceptors are found, is located at the posterior aspect of the eye. In order for the retina to transmit the most appropriate information to the brain, the light rays must land on the retinal cells in focus and with appropriate intensity. The cornea, pupil (the center of the iris) and the lens are responsible for meeting these requirements.
When light moves from one medium (such as air) into another medium (such as the cornea or lens), any rays not entering at a 90 degree angle will be refracted, or bent. Because both the cornea and lens have curved surfaces, they refract some of the light rays entering the eye. In doing so, they compress the image of what we see so that a large amount of visual information can be processed by a small amount of retinal tissue. The cornea refracts more light than the lens does because its surface is more curved, but the lens has the ability to change its shape, and therefore fine-tune the amount of refraction necessary to focus the light rays on the retina. This process is known as accommodation.
The refraction of light rays as they pass from one medium to another (a), such as through the cornea and lens (b). This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).
The lens changes its shape in response to changes in tension of the ciliary muscles on the suspensory ligaments (also called zonules) that hold the lens in place. When the ciliary muscles contract, the suspensory ligaments are less taught, causing the lens to become slightly more spherical and refract light more. This is what happens when objects that are being viewed are close, or moved closer. Light coming from objects that are far away do not require as much refraction and are viewed with the ciliary muscles relaxed and more tension on the lens, which makes it more oblong. The relationship between the ciliary muscles and the taughtness of the suspensory ligaments is a counterintuitive one for most individuals, but the eye has a unique anatomy the leads to this relationship. See the following video.
Accommodation of the lens with distant and near vision. This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).
Along with accommodation of the lens when objects are near, the pupil also tends to constrict to allow less peripheral light to enter the posterior chamber of the eye. In doing so, objects can be viewed more crisply. The pupil will also constrict when conditions are bright and dilate under low light conditions. This way the retina can receive an appropriate amount of light to activate its photoreceptors without bleaching them with too much light.
Changes in Vision
Sometimes the structures of the eye do not refract light appropriately, such that it focuses either in front of (myopia) or behind (hyperopia) the retina. This can happen, for instance, when the eye is not perfectly round. In order to correct for abnormalities in light refraction, glasses or contact lenses can be added to the system to better focus light on the retina and improve vision.
Correcting abnormalities in light refraction in the eye. This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).
Normal light refraction leads to the light rays converging on the retina (a). In the case of hyperopia, the light rays focus behind the retina. This is corrected using a convex lens to begin to bend the light before it reaches the cornea (b). In the case of myopia, the light rays focus in front of the retina. This is corrected using a concave lens to diverge the light rays before it reaches the cornea (c).
We have already discussed the structures of the eye that deliver and focus light on the retina. The retina is composed of a several layers and contains specialized cells for the initial processing of visual stimuli, with the rest of the visual processing occuring in the central nervous system.
The photoreceptors are found in the retinal layer closest to the back of the eye (outermost layer). When stimulated by light energy, they change their membrane potential and alter the amount of neurotransmitter released onto the bipolar cells. The bipolar cells connect to the retinal ganglion cells (RGC) where amacrine cells also contribute to retinal processing such as contrast enhancement and edge detection. The axons of RGCs, which are lying at the innermost aspect of the retina, collect at the optic disc and leave the eye as the optic nerve. Because of the axons passing through the wall of the eye at the optic disc, there are no photoreceptors resulting in a “blind spot” in the retina. The blind spot in either retina falls in the medial retina and does not process corresponding regions of the visual field.
Layers of the retina in stained tissue (a) and as a drawing (b). This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).
At the exact center of the retina is a point where light is focused by the lens and the greatest visual acuity is found. This is known as the fovea and it is a small dimple in the layers of the retina where there are no blood vessels, ganglion cells or bipolar cells to interrupt light reaching the receptor cells. Because more light passes to the receptor cells at the fovea, it is in this region that visual acuity is the greatest. From this central point of the retina, visual acuity drops off towards the peripheral retina. This difference is easily evidenced by looking directly at a word in the middle of this paragraph. The visual stimulus exactly in the middle of the field of view falls on the fovea and is in the sharpest focus. Without moving your eyes off that word, notice that words at the beginning or end of the paragraph are not in focus. Beyond the words on your computer screen, visual stimuli are less sharp to the point where the edges of vision have vague, blurry shapes that cannot be clearly identified. A large part of neural function to support the visual system is concerned with moving the eyes and head so that important visual stimuli are centered on the fovea of the retina.
Anatomy of the fovea. This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).
Light falling on the retina causes chemical changes to pigment molecules (called opsins) in photoreceptors, ultimately leading to a change in the activity of the retinal ganglion cells. Photoreceptor cells have two parts, the inner segment and the outer segment (Figure 9).Structure of the photoreceptor cells. This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).
The inner segment contains the nucleus and other common organelles of a cell while the outer segment is a specialized region of the cell where photoreception takes place. There are two types of photoreceptors, rods and cones, based on the shape of their outer segment. The rod-shaped outer segments of rod photoreceptors contain a stack of membrane-bound discs that contain a photosensitive opsin pigment called rhodopsin, which is sensitive to a wide bandwidth of light (white light). The cone-shaped outer segments of cone cells contain one of three photosensitive opsin pigments, called photopsins. Each of the three photopsins are sensitive to a particular bandwidth of light, corresponding to the colors of red, green or blue, allowing for the ability to distinguish color.Sensitivity of rod and cone photoreceptors to wavelengths of light. This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).
When a photoreceptor cell is activated by a photon near the wavelength it is sensitive to, the energy from the light creates a change in its opsin molecule called photoisomerization. Photoisomerization is the first step in a process that ultimately leads to a change in membrane potential of the photoreceptor. Until the opsin is changed back to its original shape, the photoreceptor cell cannot respond to light energy, which is called bleaching. When a large group of opsins are bleached, vision will be affected until enough opsins can return to the receptive state. You may have experienced this after the bright flash from a camera.
Light and Dark Adaptation
Because rhodopsin found in the rod cells is most sensitive to white light while the cone cells are color specific, rods are suited for vision in low-light conditions and cones are suited for brighter conditions. In normal sunlight, rhodopsin will be constantly bleached and the cones are active. In a darkened room, there is not enough light to activate cone opsins, and vision is entirely dependent on rods. Rods are so sensitive to light that a single photon can result in an action potential from the corresponding RGC. The three cone photopsins, being sensitive to different wavelengths of light, can aid in color vision. By comparing the activity of the three different cones, the brain can extract color information from visual stimuli. Since rods are bleached when cones are active and cones cannot react to low-intensity light, rods result in monochromatic vision. In a dark room, everything appears as a shade of gray shadow. If you think that you can see colors in the dark, it is most likely because your brain knows what color something is and relies on that memory. If you are walking through your dark living room and you are certain that the couch appears green, this is because you already know what color it is, not because you perceive it with rod photoreceptors.
Processing Visual Information
The photoreceptors, and other neuronal cells of the retina, send varied types of information to the brain. These include light intensity, colors and the spatial distribution of the information received. All of this information is then carried along the optic nerve and into the optic tract to be distributed to nuclei in the brain. At the point where the optic nerve becomes the optic tract, the optic chiasm is found. At this point, fibers carrying information from the nasal half of the retina on each side decussate (cross over), such that the information from the nasal half of the retina of the left eye crosses over to the right side of the brain and vice versa. In doing so, the left side of the brain receives information from the right visual field of each eye, and the right side of the brain receives information from the left visual field of each eye. This matches the sidedness of the brain to motor control. For example, visual information from the left side of the body, and motor control of the left limbs, are both processed by the right hemisphere of the brain.
Depiction of how visual information has sidedness in the brain. The diagram shows how information from the right visual field is delivered to the left brain and how information from the left visual field is delivered to the right side of the brain. This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/).
Visual information from the optic tract is sent to a variety of nuclei in the brain. These nuclei, along with the type of processing they are involved in are summarized in table below.
The majority of the visual information flows through the lateral geniculate nucleus of the thalamus into the occipital lobe for perception of vision. From here fibers will carry some information to regions of the parietal and temporal lobes, called the visual association areas. These areas contribute to object recognition (such as recognizing a face) and motion processing (such as catching a moving ball).
It is important to recognize when popular media and online sources oversimplify complex physiological processes so that misunderstandings are not generated. This video was created by a medical device manufacturer who might be trying to highlight other aspects of the visual system than retinal processing. The statement they make is not incorrect, it just bundles together several steps, which makes it sound like RGCs are the traducers, rather than photoreceptors.