Smell, taste and touch

Smell and taste are closely associated senses. The fact that a stuffy nose makes food seem tasteless demonstrates the close relationship between these senses. Smell and taste seem to oper­ate more simply than sight and hearing, but biologists do not yet know precisely how the receptors for smell and taste discrimi­nate among various chemicals.

The skin, the largest organ of the body, contains several types of receptors. These receptors register touch, pressure, pain, heat, and cold. The receptors for these sensations vary in number and location over the body.

The Nose

The nose, the chief sense organ of smell, contains receptors embedded in mucous membrane. About 50 million of these spe­cial cells, called olfactory receptors, are located in each nasal passage. Airborne substances dissolve in the mucus that covers the olfactory receptors. The receptors produce nerve impulses that travel through olfactory nerves to the olfactory lobe in the cerebral cortex.

Some biologists think that the perception of smell occurs when a specialized molecule on the receptor surface reacts with a specific chemical in inhaled air. The reaction generates an impulse that results in a particular smell. Other scientists believe that the outline, or shape, of a molecule is the cause of its partic­ular odor. They think that a molecule of a specific shape fits into an olfactory receptor that will accept only that shape, just as a lock works with one key. These researchers believe that the thousands of odors humans can distinguish are simply combina­tions of seven basic odors.

The Tongue

The tongue is the major sense organ of taste. The chemical receptors for taste are clusters of sensory hair cells located in the taste buds. Each taste bud consists of about 40 receptor and supporting cells and an opening called the taste pore. The taste buds lie in bunches called papillae, which are visible as the bumps on your tongue. Although most of a person's 10,000 taste buds are on the tongue, a few also exist on the roof of the mouth and in the throat.

Taste buds produce one or a combination of four main taste sensations: sweet, sour, bitter, and salty. A receptor cell may be stimulated by only one taste, but most cells are stimulated by two or more tastes. This combination of different tastes may be what produces the wide variety of flavors you enjoy.

Like smell, taste depends upon chemical reactions that take place only in solution. Saliva constantly bathes taste buds, reaching receptor cells through the taste pores. Food molecules also enter the taste pores. The chemical reactions that take place somehow cause the receptor cells to generate nerve impulses. The impulses travel through three different nerves to the taste center in the cerebral cortex. No one knows precisely how taste receptors function. Some researchers think that sensory cells have sites that accept specific chemical molecules. Other re­searchers think that, as with smell, the shape of a molecule determines its taste. Molecules of a certain shape, they think, activate specific sites on a taste bud to produce one taste.

The Skin

The skin is considered the organ of touch. It actually contains five distinct senses, most with their own type of receptor. These five senses are touch, pressure, pain, heat, and cold. Impulses travel from the various sense receptors to different areas of the sensory cortex.

Touch receptors are the ends of certain nerve fibers .Many touch receptors are located at the base of hairs and generate impulses when the hairs move even slightly. However, touch is most sensitive in the fingertips, palms, lips, and other places where hair is not present. Other receptors react to pressure. Some are sensitive to deep pressure and vibration, while others are sensitive to lighter pressure.

Unlike the other senses of touch, the sense of pain has no specialized receptors. Pain receptors are free ends of unmyeli-nated nerve fibers. Pain appears to stem from a variety of stim­uli. Some parts of the body are almost pain-free. Other parts may sense only one type of pain. Sensitivity to pain may be related to other body conditions, such as mental attitude.

Temperature receptors may be either bare nerve endings or specially shaped cells. Different types of receptors detect heat and cold.

Endocrine system

Introduction

Activities within the human body are regulated by two systems, the nervous system and endocrine system. Although both systems control body functions, their methods differ.

The nervous system sends its messengers, called impulses, to specific cells, generally muscle or gland cells. The nervous system acts quickly. Its messages travel rapidly and can change instantly. The response is immediate.

The endocrine system uses chemical messengers. They are widely dispersed to every cell throughout the body. However, only specific target cell, equipped with receptors, respond to the messages. The endocrine system generally does not act as quickly as the nervous system. Its messages travel more slowly, but the effect generated by those messages last longer than those from the nervous system.