About Senses Activity: Sensing the World Around Us

We are made aware of the world around us by means of our senses. For many centuries, man believed that human beings had only five senses : sight, hearing, touch, smell, and taste. Modern scientists have added to the list the senses of pressure, heat, cold, and pain.

1. A stimulus acts on the nerves in one of the sense organs.
2. Nerve impulses from the sense organ travel to the brain.
3. While in the brain, the impulses are interpreted as a feeling or sensation.
4. A message is then sent back to the sense organ. Even through the brain interprets the impulses, the feeling or sensation actually happens in the sense organ.

The sense of seeing is made up of messages your brain receives from your eyes. Light enters the eyes through an opening called the pupil. This hole looks like a black dot in the center of the eye. The pupil has a clear covering called the cornea. The amount of light that enters the eye is controlled by the size of the pupil. The size of the pupil is changed by muscles. These muscles are in the colored part of the eye called the iris. After the light goes through the pupil it enters the lens. The lens bends or focuses the light. Then the light passes through the inside of the eye, which is filled with a clear jelly. Finally the light reaches the back of the eye. Here a picture, or image is formed on the inside lining of the eye that is called the retina. The retina is coated with special nerve endings that are sensitive to light. These nerve endings then connect with the optic nerve that leads to the brain. When the light reaches the retina it causes impulses to begin to travel to the brain. These impulses are interpreted by the brain as an image of the object. This interpretation reverses the directions of the image so that we do not see things upside down and backward.

The sense of hearing is made up of messages your brain receives from your ears. First the sound must enter the ear. The outside of the ear is shaped so that it directs the sound waves inside your ear and to your eardrum. The eardrum then moves back and forth, or vibrates. On the other side of the ear are three tiny bones that are connected together and connected to the eardrum. Touching the inner surface of the eardrum is a tiny bone called the malleus or hammer. The malleus connects by a joint to another little bone, the incus or anvil. And the incus is jointed to a third bone, the stapes or stirrup. These tiny bones are connected to a structure called the cochlea (KAHK-lee-uh). The cochlea is filled with liquid. When the eardrum and tiny bones vibrate, they cause the liquid to vibrate. The nerve endings inside the cochlea start impulses on their way to the brain. These impulses form a message about the sound that is being heard.

The sense of feeling is made up of messages your brain receives from your skin. Your skin is sensitive to feelings of touch, pressure, pain , heat, and cold. The chief organs of feeling are free nerve endings in the epithelial cells of the body. On the outside of the body, the skins is the organ of feeling, inside the body it is the epithelial cells that line all cavities, such as the mouth, throat, stomach, intestines, ears, chest and sinuses. In the skin there are 16,00 nerve endings that detect heat and cold and more than four million that detect touch.

The sense of smell is made up of messages your brain receives from your nose. The nose has many little nerve endings in it. Each nerve ending is sensitive to certain odors in the air. The nerve endings cause impulses to travel along a pair of nerves to your cerebrum in the brain, where the impulses are interpreted as odors. Scientist are not sure exactly how this process takes place. Some think that the gases in the air dissolve in the dampness of the nose and cause a chemical reaction that stimulates the nerve endings. Your nose is very sensitive. It can detect up to 10,000 different scents.

The sense of taste is made up of messages your brain receives from your tongue. Small organs called taste buds are located just below the surface of the tongue and in three places in the throat. Taste, like smell, is probably the result of a mild chemical reaction. Your tongue is sensitive to four different tastes: bitter, sour, salty, and sweet. Not all tastes are detected by the same taste buds. The taste buds at the sides and tip of the tongue transmit saltiness. The buds at the tip of the tongue detect sweetness, those near the base detect bitterness and those on the sides detect sourness. When you put something in your mouth, impulses from the sensitive areas of the tongue are sent to the brain.