Figure \(\PageIndex<1>\) appears at first glance to be just a pattern of colored leaves, but hidden within it is the three-dimensional shape of an ant. Can you see the ant among the leaves? This figure is an example of a stereogram, which is a two-dimensional picture that reveals a three-dimensional object when viewed correctly. If you can’t see the hidden image, it doesn’t mean that there is anything wrong with your eyes. It’s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.
The human body has two basic types of senses, called special senses and general senses. Special senses have specialized sense organs that gather sensory information and change it into nerve impulses. Special senses include the vision for which the eyes are the specialized sense organs, hearing (ears), balance (ears), taste (tongue), and smell (nasal passages). General senses, in contrast, are all associated with the sense of touch and lack special sense organs. Instead, sensory information about touch is gathered by the skin and other body tissues, all of which have important functions besides gathering sensory information. Whether the senses are special or general, however, all of them depend on cells called sensory receptors.
Touch is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, but most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure \(\PageIndex\).
Vision, or sight, is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light and forms images. However, the eye is not sufficient for us to see. The brain also plays a necessary role in vision.
Figure \(\PageIndex\) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image and then changes the image to nerve impulses that travel to the brain. How the eye performs these functions is summarized in the following steps.
The optic nerves from both eyes meet and cross just below the bottom of the hypothalamus in the brain. The information from both eyes is sent to the visual cortex in the occipital lobe of the cerebrum, which is part of the cerebral cortex. The visual cortex is the largest system in the human brain and is responsible for processing visual images. It interprets messages from both eyes and “tells” us what we are seeing.
Vision problems are very common. Two of the most common are myopia and hyperopia, and they often start in childhood or adolescence. Another common problem, called presbyopia, occurs in most people beginning in middle adulthood. All three problems result in blurred vision due to the failure of the eyes to focus images correctly on the retina.
Myopia, or nearsightedness, occurs when the light that comes into the eye does not directly focus on the retina but in front of it, as shown in Figure \(\PageIndex\). This causes the image of distant objects to be out of focus but does not affect the focus of close objects. Myopia may occur because the eyeball is elongated from front to back or because the cornea is too curved. Myopia can be corrected through the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.
Hyperopia, or farsightedness, occurs when the light that comes into the eye does not directly focus on the retina but behind it, as shown in Figure \(\PageIndex\). This causes the image of close objects to be out of focus but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.
Presbyopia is a vision problem associated with aging in which the eye gradually loses its ability to focus on close objects. The precise cause of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, and the muscles that control the lens may lose power as people grow older. The first signs of presbyopia – eyestrain, difficulty seeing in dim light, problems focusing on small objects, and fine print – are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.
Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear in Figure \(\PageIndex\)). The sound waves strike the eardrum and make it vibrate. The vibrations then travel through the three tiny bones (hammer, anvil, and stirrup) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The cochlea is a coiled tube filled with liquid. The liquid moves in response to the vibrations, causing tiny hair cells (which are mechanoreceptors) lining the cochlea to bend. In response, the hair cells send nerve impulses to the auditory nerve, which carries the impulses to the brain. The brain interprets the impulses and “tells” us what we are hearing.
Taste and smell are both abilities to sense chemicals, so taste and olfactory (odor) receptors are chemoreceptors. Both types of chemoreceptors send nerve impulses to the brain along sensory nerves, and the brain “tells” us what we are tasting or smelling.
Taste receptors are found in tiny bumps on the tongue called taste buds. You can see a diagram of a taste receptor cell and related structures in Figure \(\PageIndex\). Taste receptor cells make contact with chemicals in food through tiny openings called taste pores. When certain chemicals bind with taste receptor cells, it generates nerve impulses that travel through afferent nerves to the CNS. There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty or savory taste is called umami.
Olfactory receptors line the passages inside the nasal passages (Figure \(\PageIndex\)). There are millions of olfactory receptors, which sense chemicals in the air. Unlike taste receptors, which can sense only five different tastes, olfactory receptors can sense hundreds of different odors and send signals to the olfactory bulb of the brain. Did you ever notice that food seems to have less taste when you have a stuffy nose? This occurs because the sense of smell contributes to the sense of taste, and a stuffy nose interferes with the ability to smell.
The most common cause of blindness in the Western hemisphere is age-related macular degeneration (AMD). About 15 million people in the United States have this type of blindness, and 30 million people are affected worldwide. At present, there is no cure for AMD. The disease occurs with the death of a layer of cells called retinal pigment epithelium, which normally provides nutrients and other support to the macula of the eye. The macula is an oval-shaped pigmented area near the center of the retina that is specialized for high visual acuity and has the retina’s greatest concentration of cones. When the epithelial cells die and the macula is no longer supported or nourished, the macula also starts to die. Patients experience a black spot in the center of their vision, and as the disease progresses, the black spot grows outward. Patients eventually lose the ability to read and even to recognize familiar faces before developing total blindness.
In 2016, a landmark surgery was performed as a trial on a patient with severe AMD. In the first-ever operation of its kind, Dr. Pete Coffey of the University of London implanted a tiny patch of cells behind the retina in each of the patient’s eyes. The cells were retinal pigmented epithelial cells that had been grown in a lab from stem cells, which are undifferentiated cells that have the ability to develop into other cell types. By six months out from the operation, the new cells were still surviving, and the doctor was hopeful that the patient’s vision loss would stop and even be reversed. At that point, several other operations had already been planned to test the new procedure. If these cases are a success, Dr. Coffey predicts that the surgery will become as routine as cataract surgery and prevent millions of patients from losing their vision.
Some people “see” sounds, “hear” colors, or “taste” words. This rare ability is called synesthesia, and it is thought to be caused by cross-wiring of the senses in the brain. To learn more about this intriguing phenomenon, watch this fascinating TED animation:
Many people experience the dizzying effects of vertigo at some point in their lives. Learn more here:
This page titled 11.7: Human Senses is shared under a CK-12 license and was authored, remixed, and/or curated by Suzanne Wakim & Mandeep Grewal via source content that was edited to the style and standards of the LibreTexts platform.
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