SIW - Anatomy of the Eye Lesson
Anatomy of the Eye
The eye is the major sense organ for sight. Download "The Sense of Sight" note outline here Links to an external site. and complete the interactive learning object here Links to an external site. to learn about the structure of the eye.
How Do We See?
In order for us to see an object, light must first bounce off of that object and enter into our eyes, be refracted onto our retinas, be transduced into an action potential, and then processed by our brains.
Refracting Light
As light enters into the eye, the cornea and lens refract, or bend, light in order to focus an image on the retina. After passing through the cornea, light passes through the aqueous humor, which connects the cornea to the lens. This clear, gel-like liquid is rich in nutrients and helps maintain the convex shape of the cornea. The iris, the structure that gives our eyes color, is a ring made of smooth muscle lying between the lens and the aqueous humor that regulates the amount of light entering the eye. The center of the iris is called the pupil. Our pupils actively adjust their sizes to maintain a constant level of light entering the eye. This can easily be seen if a flashlight is shone into the eyes in a darkened room. In a darkened room, the irises will relax, enlarging (dilating) the pupils, to allow as much light into the eyes as possible. However, if a flashlight is shone into the eyes, the irises will immediately constrict reducing the pupil to a pinpoint size in order to protect the eyes.
The main function of the lens is to focus light on the retina. The lens is a transparent, convex structure located behind the cornea. On the other side of the lens is the vitreous humor, a clear, gelatinous mass that maintains the shape of the eye and suspends the lens. The lens will focus and re-focus light as the eye rests on objects near and far in the visual field. The lens is operated by muscles that change its shape. As the shape of the lens changes, its focal length also changes allowing it to focus images sharply on the retina.
Frequently, errors in focal length require people to wear corrective lenses. Learn about the most common of these disorders in the learning object below.
In the retina, there are two types of photoreceptors: rods and cones. Rods are strongly photosensitive (detecting dim light), located in the outer edges of the retina, and are used primarily for peripheral and nighttime vision. Cones are weakly photosensitive (detecting bright light), located near the center of the retina, and are used primarily for daytime and color vision. Human retinas contain about 120 million rod cells, but only 6 million cone cells.
The fovea centralis is a cone-dense region in the center back of the eye that is responsible for acute (central) vision. When you examine an object closely in bright light, the eyes orient so that the object's image falls on the fovea. However, when an object is in dim light, such as a star in the night sky, the object can be better viewed by the peripheral vision because of the rods at the edges of the retina.
Transducing Light
Although there is a large spectrum of electromagnetic radiation, humans are only able to perceive a small portion of it from, 400 nm to 700 nm, known as "visible light." Photoreceptors must be able to transform color and light into a neural signal in order for a human to be able to perceive it. Primates, such as humans, use a three-cone (trichromatic) system which results in full-color vision.
The color we perceive is a result of the ratio of the activity of our three types of cones. Humans have a very sensitive perception of color and can distinguish about 500 levels of brightness, 200 different hues, and 20 steps of saturation which results in about 2 million distinct colors.
The rods and cones are the sites of the transduction of light. Both rods and cones contain photopigments, which are pigments that undergo a chemical change when they absorb light. When light hits a photoreceptor, it causes it to change shape in the retina.
This starts a cascade of events that ends with the closing of Na+ channels in the membrane of the photoreceptor causing the neurons to hyperpolarize (rather than depolarize like other sensory neurons). This change in activity causes signals to be sent to the brain via the optic nerve.
Visual Processing
Once the signal has been sent, humans still rely on the brain to process the message in order to perceive an image. The optic nerve is responsible for transferring information from the retina to the brain. Not all information is processed in the same place. Some axons in the optic nerve carry information about form, movement, depth, and differences in brightness while others carry information on color and fine detail. Some visual information projects directly back into the brain, while other information crosses to the opposite side of the brain. This crossing of optical pathways produces the distinctive optic chiasma (crossing) found at the base of the brain and allows us to coordinate information from both eyes.
Once in the brain, visual information is processed in several places such as the thalamus, hypothalamus, primary visual cortex, parietal lobe, temporal lobe, and midbrain. The variety of locations where vision is processed reflects the complexity and importance of this sense in humans.
Remember, everything that we see must be processed by the brain. This means that injuries and disorders of the brain can affect how we perceive the world around us. Even healthy brains can be "tricked." Optical illusions are characterized by visually perceived images that differ from objective reality. They are often caused by our brains trying to make sense out of a variety of misleading stimuli.
To review the anatomy and physiology of the eye, watch the video below:
IMAGES CREATED BY GAVS OR OPENSOURCE