6.7. Sense organs

Introduction

You know what is going on around you and inside you because of special receptors in your body. Through them, you are able to see beautiful sights, hear wonderful sounds, taste delicious flavors, and smell appealing aromas. You can touch things and feel a vari­ety of sensations. More importantly, how­ever, these receptors help you survive. You can avoid injuries if you can feel, taste, smell, hear, or see potential danger. Without some of the special sense receptors, you could not move well, balance yourself, or judge your position in space.

Each kind of receptor reacts to a specific type of stimulus. The stimuli are transmitted to the central nervous system, which in turn determines the body's response to conditions in the external environment.

Receptors and Sense Organs

Many receptors that enable the body to obtain information from the environment are located in highly specialized organs called sense organs. The most familiar sense organs are the eyes, ears, nose, mouth, and skin. In addition to these, you have other sense organs that you may not be aware of. For example, recep­tors in your ears enable you to keep your balance. All sense organs have specialized receptors for stimuli. Most sense organs have receptors that pick up stimuli from the body's external environment. Other kinds of receptors pick up stimuli from the body's internal environment.

Types of Receptors

Sense receptors are highly selective. The receptors for taste will not respond to light, no matter how intense it is. The receptors for sight cannot be activated by sound vibrations.

Sense receptors can be classified according to the stimuli that activate them. Photoreceptors detect stimuli generated by light. The receptors for taste and smell are triggered by chemi­cals and are called chemoreceptors. Thermoreceptors respond to heat or cold, either inside or outside the body. Pain receptors generate impulses interpreted as pain. Mechanoreceptors re­spond to mechanical pressures. Such pressures may come from sound vibrations, touch, muscle contractions, or movements of joints. The pressure bends or distorts the part of the sense organ in which the mechanoreceptors are located. Hair cells, which have extremely fine projections like cilia, are the most common type of mechanoreceptors.

Sense Organs

Sense organs act as transducers—that is, they transform one form of energy into another form. For example, when light rays strike the inner lining of the eye, they are changed into im­pulses. These impulses move along a nerve to the brain's visual center where they are interpreted as sight.

Impulses from all sense organs are basically alike. The way the brain interprets impulses from various sense organs dif­fers. Impulses from each sense organ travel to a different part of the brain. The impulses from a particular sense organ are inter­preted in only one way, according to where they are received in the brain. For example, when the eye receives light signals, it produces impulses that the brain interprets as an image. When the ear receives pressure waves, or sound vibrations, it produces impulses that the brain interprets as sound. The brain never interprets impulses from the eye as sound or impulses from the ear as an image. Even if some other type of energy generates an impulse in a receptor cell, the brain will interpret the impulse exactly as it does all other impulses from that receptor. For example, a blow to the eye may cause you to see an image, even though the impulse was generated not by light but by mechani­cal pressure.

Thinking About Biology: The Other Senses

The human body has many special receptors other than those in the familiar sense organs. While some scien­tists claim that all these receptors are "senses," others feel they are more accurately described as "controls." Regardless of the term used, the fact remains that these special receptors react to inter­nal stimuli rather than external.

The internal controls primarily maintain homeo-stasis. For example, thirst is triggered by the hypo-thalamus, which responds to salt concentration in the blood. When the water level in the blood is low, salt be­comes more concentrated. When salt concentration is high, the hypothalamus reacts by generating im­pulses that trigger a thirst sensation. When the water level is high, salt concen­tration is low, and the body eliminates more water. Similarly, chemicals in the cerebrospinal fluid and a low level of glucose in the blood seem to trigger hunger.

Another type of internal control monitors your skeletal muscles. Muscle spindles, a special type of muscle fiber, are part of skeletal muscle. The spin­dles contain two types ofsensory neurons. One type alerts the central nervous system to a change in the stretch or contraction of a muscle. The other type registers how much stretch is involved. This constant monitoring of muscular contraction helps you main­tain posture and keeps your body steady. Joint and tendon receptors work with the muscle spindles. Joint receptors register the angle of ligament move­ment. Tendon receptors indicate the amount of stretch in the tendons.