Senses are the means by which many-celled animals tell what is happening in their environment. Many people think that human beings have only five senses—hearing, sight, smell, taste, and touch. However, there are other kinds of senses that also give information about the position and movement of the body and about the body’s needs. These senses include balance, hunger, pain, and thirst.
Each sensory system responds to a particular kind of stimulus. These stimuli can affect the senses of touch, hearing, smell, taste, heat, and sight. Cells called sensory receptors alter electrical impulses along their membranes. The brain handles the information provided by the senses. Pathways inside and outside the brain move sensory information throughout the body.
Scientists divide the senses into two groups. External senses receive information about the outside environment—about things outside the body. The external senses include hearing, heat, sight, smell, taste, and touch. Internal senses receive information about the internal environment—about the changes that occur in the body’s organs and tissues.
External senses.
Some external senses detect things that occur far from the body, and others detect things that come in contact with the body. We detect things in the distant outside environment with our senses of sight, hearing, and heat. These senses require only a small stimulus to respond. This high level of sensitivity is necessary because the stimulus may occur far away from the sensory receptor. Therefore, the amount of energy that reaches the receptor is often very small.
The senses of taste, touch, and smell involve contact with the body. These senses require a fairly large stimulus to respond. Relatively heavy pressure must be applied to the skin for an object to be felt. Because these stimuli occur at the sensory receptors, the senses of taste, touch, and smell do not need high sensitivity.
Internal senses
detect changes that take place inside the body and send messages about these changes to the brain. The internal senses respond to chemical and physical stimuli in the circulatory, digestive, excretory, respiratory, and central nervous systems. They contribute to such feelings or sensations as hunger, fatigue, pain, and thirst. In addition, they respond to the position and movement of the head and the joints, to tension in the muscles, and even to changes in blood pressure. By responding to these chemical and physical changes, internal senses help maintain a proper environment inside the body.
The internal senses can be highly sensitive to small changes in the amounts of chemicals usually present in the body. These chemicals include oxygen, sodium, hormones, and blood sugar. The internal senses can also recognize changes in the shape of different bodily structures, such as the heart and blood vessels. This high sensitivity is necessary because the internal receptors must maintain a rather constant internal environment for life to continue.
Some animals respond to environmental stimuli that cannot be detected by human senses. These stimuli include ultraviolet light, Earth’s magnetic field, sounds of very high and very low frequencies, and extremely small electric currents in water.
By studying the senses, scientists determine what things in the environment are important to each kind of animal. Scientists also learn about the environment in which the evolution (gradual development) of particular groups of animals occurred.
