SENSORY RECEPTORS
Sensations, or nerve impulses, begin with the excitation of sensory receptors, structures that detect changes in an animal’s external and internal environment. Some sensory receptors are stimulated by physical bending or stretching caused by forms of mechanical energy such as pressure, touch, and motion. A common type of sensory receptor that detects motion is the hair cell. When hairs bend in one direction, they stretch the hair cell’s membrane, sending a nerve impulse to the sensory areas of the brain. Hair cells can respond to the direction of motion as well as to its strength and speed.
听力材料：
The cat has several specialized nerve endings, or sensory receptors, throughout its skin that respond to various sensations. The hair follicles of the cat are richly supplied with several types of touch receptors. These are responsive to the slightest movement, and so they provide a close-range detection system. As with most meat-eaters, this system is highly developed in the stiff, coarse whiskers on the cat’s face.
The cat’s whiskers are long, stiff, highly touch-sensitive hairs. Cats generally have about twelve whiskers in rows on each upper lip, a few on each check, and a few bristles on the chin. The long sensory whiskers are more than twice as thick as the rest of the hairs in the cat’s coat, and extend three times deeper into the skin. The whiskers have a rich supply of blood vessels and a dense network of nerves. The slightest whisker movement stimulates the nerve endings and provides the brain with information on the cat’s immediate surroundings. Whiskers can even detect very slight motion such as air currents.
The whiskers are important in sensing and investigating close objects. Cats rely heavily on their whiskers in dim light, when their eyes cannot focus well on very close objects. The whiskers are sensitive to air currents deflected by objects. This is why the cat can feel its way in the dark without actually making contact with objects. Loss of the whiskers will hinder a cat’s ability to get around in the dark.
Question：
Describe the whiskers of a cat, and explain how the whiskers function as sensory receptors.
