How We Feel The Outside World
Our sense of touch is based primarily in the outer layer of skin called the epidermis. Nerve endings that lie in or just below the epidermis cells respond to various outside stimuli, which are categorized into four basic stimuli: pressure, pain, hot, and cold. Animals experience one or a combination of these sensations through a complex neural network that sends electrical impulses through the spinal cord to the cerebral cortex in the brain. The cerebral cortex, in turn, contains brain cells (neurons) arranged in columns that specialize in interpreting specific types of stimuli on certain parts of the body.
The sensation of touch begins with various receptors in the skin. Although these receptors appear to specialize in reacting to certain sensations, there is some debate concerning this specificity since most touch stimuli are a combination of some or all of the four major categories.
Scientists have identified several types of touch receptors. Free nerve ending receptors, located throughout the body at the bases of hair, are associated primarily with light pressure (such as wind) and pain. Meissner corpuscles are nerve endings contained in tiny capsules and are found primarily in the fingertips and areas especially sensitive to touch (in the form of low-frequency vibrations), like the soles of the feet and the tongue. The Pacinian corpuscles look like the cross section of an onion and are found in deep tissues in the joints, the genitals, and the mammary glands. They are extremely sensitive to pressure and are also stimulated by rapid movement of the tissues and vibrating sensations. Ruffini endings, which are also located in the deeper layers of the skin, respond to continuous stimulation, like steady pressure or tension within the skin. Merkel disks, are found near the base of the epidermis and also respond to continuous stimulation or pressure. The skin also contains specific thermoreceptors for sensing hot and cold and nociceptors that identify high intensity stimulation in the form of pain.
Most, if not all of these receptors, are designed to adapt or become accustomed to the specific stimulation they interpret. In other words, the receptor does not continue to register a constant "feeling" with the same intensity as when it first begins and may even shut off the tactile experience. Imagine, for example, putting on a wool sweater over bare skin. The initial prickly sensation eventually abates, allowing the wearer to become accustomed to the feeling. Other examples include wearing jewelry such as rings, necklaces, and watches.
These receptors are also found in greater numbers on different parts of the body. For example, peoples' backs are the least sensitive to touch, while their lips, tongue, and fingertips are most sensitive to tactile activity. Most receptors for cold are found on the surface of the face while thermoreceptors for warmth usually lie deeper in the skin and are fewer in number. A light breeze on the arm or head is felt because there tend to be more sense receptors at the base of the hairs than anywhere else.