Our five senses; sight, hearing, touch, taste and smell, seem to operate independently, as five distinct modes of perceiving the world. In reality, however, they collaborate closely to enable the mind to better understand its surroundings.
1. Vision: Eyes detect color and light
The human eye functions somewhat like a camera; that is, it receives and focuses light upon a photosensitive receiver, the retina. The light rays are bent and brought to focus as they pass through the cornea and the lens. The shape of the lens can be changed by the action of the ciliary muscles so that clear images of objects at different distances and of moving objects are formed on the retina. This ability to focus objects at varying distances is known as accommodation.
The Role of the Retina
The retina—the embryonic outgrowth of the brain is a very complex tissue. Its most important elements are its many light-sensitive nerve cells, the rods and cones. The cones secrete the pigment iodopsin and are most effective in bright light; they alone provide color vision. The rods, which secrete a substance called visual purple, or rhodopsin, provide vision in dim light or semidarkness; since rods do not provide color vision, objects in such light appear in shades of gray.
Light rays brought to focus on the rods and cones produce a chemical reaction in those cells, in which the two pigments are broken down to form a protein and a vitamin A compound. This chemical process stimulates an electrical impulse that is sent to the brain. The structural change of pigment is normally balanced by the formation of new pigment through the recombination of the protein and vitamin A compound; thus vision is uninterrupted.
The division of function between rods and cones is a result of the different sensitivity of their pigments to light. The iodopsin of cone cells is less sensitive than rhodopsin, and therefore is not activated by weak light, while in bright light the highly sensitive rhodopsin of rod cells breaks down so rapidly that it soon becomes inactive. There is a depression near the center of the retina called the fovea that contains only cone cells. It provides the keenest possible vision when an object is viewed directly in bright light. In dim light objects must be viewed somewhat to one side so the light rays fall on the area of the retina that contains rod cells.
The Role of the Optic Nerve and Brain
The nerve impulses from the rods and cones are transmitted by nerve fibers across the retina to an area where the fibers converge and form the optic nerve. The area where the optic nerve passes through the retina is devoid of rods and cones and is known as the blind spot. The optic nerve from the left eye and that from the right eye meet at a point called the optic chiasma. There each nerve separates into two branches. The inner branch from each eye crosses over and joins the outer branch from the other eye. Two optic tracts exit thereby from the chiasma, transferring the impulses from the left side of each eye to the left visual center in the cerebral cortex, (see The Human Brain) and the impulses from the right half of each eye to the right cerebral cortex. The brain then fuses the two separate images to form a single image. The image formed on the retina is an inverted one, because the light rays entering the eye are refracted and cross each other.
However, the mental image as interpreted by the brain is right side up. How the brain corrects the inverted image to produce normal vision is unknown, but the ability is thought to be acquired early in life, with the aid of the other senses.
2. Hearing: Ears detect sound
In the course of hearing, sound waves enter the auditory canal and strike the eardrum, causing it to vibrate. The sound waves are concentrated by passing from a relatively large area (the eardrum) through the ossicles to a relatively small opening leading to the inner ear. Here the stirrup vibrates, setting in motion the fluid of the cochlea. The alternating changes of pressure agitate the basilar membrane on which the organ of Corti rests, moving the hair cells. This movement stimulates the sensory hair cells to send impulses along the auditory nerve to the brain.
It is not known how the brain distinguishes high-pitched from low-pitched sounds. One theory proposes that the sensation of pitch is dependent on which area of the basilar membrane is made to vibrate. How the brain distinguishes between loud and soft sounds is also not understood, though some scientists believe that loudness is determined by the intensity of vibration of the basilar membrane.
In a small portion of normal hearing, sound waves are transmitted directly to the inner ear by causing the bones of the skull to vibrate, i.e., the auditory canal and the middle ear are bypassed. This kind of hearing, called bone conduction, is utilized in compensating for certain kinds of deafness, and plays a role in the hearing of extremely loud sounds.
Balance and Orientation
In addition to the structures used for hearing, the inner ear contains the semicircular canals and the utriculus and sacculus, the chief organs of balance and orientation. There are three fluid-filled semicircular canals: two determine vertical body movement such as falling or jumping, while the third determines horizontal movements like rotation.
Each canal contains an area at its base, called the ampulla that houses sensory hair cells. The hair cells project into a thick, gelatinous mass. When the head is moved, the canals move also, but the thick fluid lags behind, and the hair cells are bent by being driven through the relatively stationary fluid. As in the cochlea, the sensory hair cells stimulate nerve impulses to the brain. The sensory hair cells of the saclike utriculus and sacculus project into a gelatinous material that contains lime crystals.
When the head is tilted in various positions, the gelatin and crystals exert varying pressure on the sensory cells, which in turn send varying patterns of stimulation to the brain. The utriculus sends indications of the position of the head to the brain and detects stopping and starting. The utriculus and sacculus also help control blood flow to the brain.
3. Smell: Nose detects scents
Smell, is a sense that enables an organism to perceive and distinguish the odors of various substances, also known as olfaction. In humans, the organ of smell is situated in the mucous membrane of the upper portion of the nasal cavity near the septum. It is made up of the olfactory cells, which are actually nerve cells that function as receptors for the -sense of smell. The free ends of the cells project outward from the epithelial tissue in the form of numerous hair-like processes. These fibers are buried in the mucus that coats the inner surface of the nasal cavity and are stimulated by various odors.
Nerve fibers extend from the olfactory cells to an area of the brain called the olfactory bulb. Any disturbance of the nasal cavity such as the common cold, in which the olfactory hairs are covered with excess mucus or other material, interferes with the sense of smell.
Most physiologists agree that although a substance must be volatile to be sniffed by the nose, it must subsequently be dissolved in the mucous lining of the nasal cavity to be smelled. It is also believed that there are only a few basic odors (perhaps about seven), and that all other odors are a combination of these. Attempts at classifying the so-called primary sensations of smell have not yet been successful.
The sense of smell is not as strongly developed in humans as in many other vertebrates, particularly carnivores which employ olfactory organs to locate food and detect dangerous predators. Too many invertebrates (especially insects) as well, smell is a highly developed sensory mechanism, necessary in obtaining food, in finding mating partners, and in recognizing other animals.
4. Taste: Tung detects sweet, salty, sour and bitter
Taste, response to chemical stimulation that enables an organism to detect flavors; In humans and most vertebrate animals, taste is produced by the stimulation by various substances of the taste buds on the mucous membrane of the tongue. A taste bud consists of about 20 long, slender cells a tiny hair projects from each cell to the surface of the tongue through a tiny pore.
The taste cells contain the endings of nerve filaments that convey impulses to the taste center in the brain. Five fundamental tastes, or a combination of these, can be detected by the buds: sweet, sour, salt, bitter, and umami. Umami, a meaty taste associated with glutamate and protein-rich foods, was identified by Kikunae Ikeda in Japan in the early 20th century and umami receptors were only discovered in 1996.
Only the buds most sensitive to salty flavor are scattered evenly over the tongue. Sweet-sensitive taste buds are concentrated on the tip of the tongue; sour flavors are detected at the sides of the tongue and bitter and umami flavors at the back. The close relationship of taste to smell gives the impression that a greater variety of tastes exists. This is also why an impairment of smell, as during a cold, may impart the feeling that the sense of taste is diminished.
5. Touch: Skin detects pain, pressure, heat and cold
Touch, tactile sensation received by the skin, enabling the organism to detect objects or substances in contact with the body. End organs (nerve endings) in the skin convey the impression to the brain. Touch sensitivity varies in different parts of the body, depending on the number of end organs present in any one area.
The tip of the tongue, lips, and fingertips are three of the most sensitive areas, the back and parts of the limbs the least so. The sense of touch is very closely related to the other four sensations received by the skin: pain, pressure, heat, and cold. There is a specific kind of sensory receptor for each of the five so-called cutaneous senses. For example, light-touch receptors convey only the sensation that an object is in contact with the body, while pressure receptors convey the force, or degree, of contact. The blind learn to read by the Braille system by making use of the sensitivity to touch of the fingertips.