Senses Info. (cont.)
Our sense of smell warns us of dangers, such as smoke or poisonous gases. It also helps us enjoy the flavors of our favorite food and drink. With our olfactory (another word for sense of smell) receptors, we are able to detect thousands of distinct smells. These receptors cover an area the size of a postage stamp at the roof of our nasal cavity, the hollow space inside your nose. Tiny hairs called nerve fibers dangle from these receptors, which are covered with a layer of mucus. When a smell, formed by chemicals in the air, dissolves in the mucus, the hairs absorb it and excite the proper olfactory receptors. Only a few molecules are needed to excite these sensitive receptors. Once they are stimulated, they transmit impulses to the brain. This pathway is directly connected to the limbic system, where the brain deals with emotions. Reactions to smells are rarely neutral - one usually either likes or dislikes a smell. Smells leave long lasting impressions and are strongly linked to our memories - think about fresh mown grass or grandmother's molasses cookies.
Our sense of taste protects us from eating poisonous and unsafe foods. We are likely to spit out something that tastes bad, before it has a chance to enter our stomachs. Taste also helps us to maintain a consistent chemical balance in our bodies. For example, our liking sugar and salt helps to satisfy our body's need for both carbohydrates and minerals. Eating sour foods, such as lemons, supplies our bodies with essential vitamins.
Our mouths contain around 10,000 taste buds, most of which are located on or around the tiny bumps on our tongues or in small trenches in the surface of our tongue. Each taste bud detects five primary tastes: sour, sweet, bitter, salty, and a more recent addition to the list, umami (salts of certain acids that are associated with savory flavors). Each taste bud has 50-100 specialized receptor cells. Sticking out of each receptor cell is a tiny taste fiber that checks out the food chemicals in your saliva. When these taste fibers are stimulated they send impulses to your brain. Each taste fiber responds best to one of the five basic tastes. The nerves of the tongue will send information to the taste center in the medulla, where signals will go out to the amygdale and the thalamus.
The ear is the organ for hearing. It is divided into three parts: the outer ear consists of the ear flap and the outer ear canal, which ends at the eardrum; the middle ear, which is the cavity between the eardrum and the inner ear and house the occicles, our body's three smallest bones; and the inner ear, which is a maze of bony chambers.
Hearing begins as sound waves generated by various sources travel into the ear canal and bounce off the eardrum causing it to vibrate. These vibrations pass through three very small bones, the ossicles, which are connected like a chain. At the end of the chain is the cochlea, a bony canal that is shaped like a garden snail and filled with liquid. Traveling through the three compartments of the cochlea, the sound arrives at the organ of Corti, which is the sensory transduction organ. The organ of Corti has hair cells that, when stimulated, begin moving and it is this mechanical movement that causes the cells to transmit the signal. The auditory nerve carries to information from the hair cells to the cochlear nucleus in the brain stem, and then on to the thalamus, which relays the information to the appropriate part of the cerebral cortex.
Eyes are the organs of sight. They are hollow fluid-filled spheres made up of three layers: the outside white sclera or cornea, the middle layer containing the iris, and the back of the eye, the retina. The retina contains millions of photoreceptor cells of two different types: rods and cones. Rods detect shades of gray and movement, are densest at the edge of the retina, and do not need a lot of light to work. We have about 120 million rods. Cones detect color and are fewer in number, only about 7 million. They are densest at the center of the retina and need bright light to function. This explains why, when there is not much light, we see only shades of gray. These photoreceptors are the neurons that send the information from the retina to the optic nerve to the thalamus and, finally, to the cerebral cortex.
Our skin has more than 4 million sensory receptors that are especially concentrated in the hands, lips, face, tongue, neck, fingertips, and feet. The receptors are sensitive to touch, pressure, temperature and pain. While the other senses are located in specific parts of your body, the sense of touch is distributed all over the body. The sense of touch originates in the dermis, which is the bottom layer of the skin.
These sensory receptors gather information and relay electrical messages through the central nervous system to the cerebral cortex for processing and possible reaction. The areas with the densest concentration of receptors, such as fingertips or lips, are the most sensitive. Sensitivity to pain also varies for different parts of the body: very little pressure to the eye causes pain but it takes a great deal more on the palm of your hand to cause similar discomfort. When any of these receptors is stimulated, it sends messages to the brain. Touch serves as a defense mechanism for the body. The feeling of pain can warn of danger or damage - if we feel something hot, we pull away!