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Vision is the special sense of sight that uses the eye to detect light waves and convert them into neural signals. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye. The eyelids, with lashes at their leading edges, help to protect the eye from abrasions by blocking particles that may land on the surface of the eye. Tears are produced by the lacrimal gland, located just inside the orbit, superior and lateral to the eyeball. Tears exit the lacrimal gland and flow over the surface of the eye, washing away foreign particles, exiting the eye through the nasolacrimal duct at the medial corner of the eye. This tube connects and drains tears into the nasal cavity.
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 eyeball. For more detailed information, please refer to the supplemental Axial Muscles.pdf. These muscles are innervated by three cranial nerves, the oculomotor (CN III), trochlear (CN IV), and abducens nerves (CN VI).
The eye itself is a hollow sphere composed of three layers of tissue called tunics (tunic, coat)—the fibrous tunic, vascular tunic, and neural tunic. The outermost layer is the fibrous tunic, which includes the white sclera and clear cornea. The sclera accounts for five-sixths of the surface of the eye, most of which is not visible, though humans are unique compared with many other species in having so much of the “white of the eye” visible. The transparent cornea acts like a curved window, covering the anterior tip of the eye and allowing light to enter the eye.
The middle layer of the eye is the vascular tunic, which is mostly composed of the choroid, ciliary body, and iris. The choroid is a layer of highly vascularized connective tissue that provides a blood supply to the eyeball. The choroid is posterior to the ciliary body, a muscular structure that is attached to the lens by suspensory ligaments, or zonule fibers. These two structures bend the lens, allowing it to focus 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. The iris is a smooth muscle that opens or closes the pupil, which is the hole at the center of the eye that allows light to enter. The iris constricts the pupil in response to bright light and dilates the pupil in response to dim light. The innermost layer of the eye is the retina, or neural tunic, which contains the nervous tissue responsible for photoreception.
The eye is also divided into two cavities:
Note that the photoreceptors in the retina are located behind the axons, RGCs, bipolar cells, and retinal blood vessels. A significant amount of light is absorbed by these structures before the light reaches the photoreceptor cells. However, at the exact center of the retina is a small area known as the fovea centralis, or fovea for short, that lacks the supporting cells and blood vessels and only contains photoreceptors. Because this is where the least amount of light is absorbed by other retinal structures, it is where visual acuity, or the sharpness of vision, is greatest. As one moves in either direction from this central point of the retina, visual acuity drops significantly. In addition, each photoreceptor cell of the fovea is connected to a single RGC. Therefore, this RGC does not have to integrate inputs from multiple photoreceptors, which reduces the accuracy of visual transduction. Toward the edges of the retina, several photoreceptors converge on RGCs (through the bipolar cells) up to a ratio of 50 to 1.
Light falling on the retina causes chemical changes to pigment molecules in the photoreceptors, ultimately leading to a change in the activity of the RGCs. Photoreceptor cells have two parts, the inner segment and the outer segment. The inner segment contains the nucleus and other common organelles of a cell, whereas the outer segment is a specialized region in which photoreception takes place. There are two types of photoreceptors—rods and cones—which differ in their shape and light sensitivity. Rods have a rod-shaped outer segment and contain the photosensitive pigment rhodopsin. These cells are sensitive to light intensity but do not detect colors. Rod photoreceptors only generate black-and-white images. Cones have a cone-shaped outer segment and contain one of three photosensitive opsin pigments, each sensitive to a particular wavelength (color) of light. The pigments in human eyes are specialized in perceiving three different primary colors: red, green, and blue.
At the molecular level, visual stimuli cause changes in the photopigment molecule that lead to changes in membrane potential of the photoreceptor cell. A single unit of light is called a photon. The energy of a photon is represented by its wavelength, with each wavelength of visible light corresponding to a particular color. Visible light has a wavelength between 380 and 720 nm. Light with a wavelength of 380 nm is blue whereas light with a wavelength of 720 nm is dark red. All other colors fall between red and blue at various points along the wavelength scale.
Opsin pigments are transmembrane proteins that contain a cofactor known as retinal. Retinal is a molecule related to vitamin A. When light hits retinal, it changes shape in a process known as photoisomerization. The shape change of retinal in the photoreceptor membrane initiates a change in its membrane potential. This causes the photoreceptor cell to decrease neurotransmitter release. This means that photoreceptor cells and the optic nerve decrease activity in the presence of light stimuli and increase in the absence of light stimuli.
Additionally, until the retinal molecule is changed back to its original shape by another enzymatic reaction, the opsin cannot respond to light energy, which is called bleaching. When a large group of photopigments is bleached, the retina will send information as if opposing (negative) visual information is being perceived.
EXAMPLE
If you look at a bright light and then look away, your vision will have an afterimage of a dark spot, also known as flash blindness.The same general process also works for colors. If you look at a relatively light red image and then look away, your vision will see the same afterimage but in green (the opposite color to red).
All visual pigments are sensitive to limited wavelengths of light. Rhodopsin, the photopigment in rods, is most sensitive to light at a wavelength of 498 nm. The three color opsins have peak sensitivities of 564 nm, 534 nm, and 420 nm corresponding roughly to the primary colors of red, green, and blue. The absorbance of rhodopsin in the rods is much more sensitive than in the cone opsins; specifically, rods are sensitive to vision in low light conditions, and cones are sensitive to brighter conditions. In normal sunlight, rhodopsin will be constantly bleached while 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 a rod’s corresponding RGC.
The three types of cone opsins, being sensitive to different wavelengths of light, provide us with color vision. By comparing the activity of the three different cones, the brain can extract color information from visual stimuli.
EXAMPLE
A bright blue light that has a wavelength of approximately 450 nm would activate the “red” cones minimally, the “green” cones marginally, and the “blue” cones predominantly.The relative activation of the three different cones is calculated by the brain, which perceives the color as blue. However, cones cannot react to low-intensity light, and rods do not sense the color of light. Therefore, our low-light vision is—in essence—in grayscale. In other words, in a dark room, everything appears as a shade of gray. If you think that you can see colors in the dark, it is most likely because your brain knows what color something is and is relying on that memory.
Source: THIS TUTORIAL HAS BEEN ADAPTED FROM OPENSTAX “ANATOMY AND PHYSIOLOGY 2E.” ACCESS FOR FREE AT HTTPS://OPENSTAX.ORG/DETAILS/BOOKS/ANATOMY-AND-PHYSIOLOGY-2E. LICENSE: CC ATTRIBUTION 4.0 INTERNATIONAL.
