Sensory receptors are specialized neurons that respond to specific types of stimuli. Transduction is the conversion of sensory stimulus energy into an action potential. Sensation is a physical process, whereas perception is psychological. Perception is the way sensory information is organized, interpreted and consciously experienced. The just noticeable difference, or difference threshold, is how much difference between two stimuli a person needs in order to notice that they differ.

Light goes into your eye through an opening called the pupil. Muscles attached to the iris change its shape to control how much light gets in, and when light levels are high, the pupil constricts to reduce the amount of light that enters the eye. The lens and the cornea work together to focus light on the retina. Light entering the eye forms an upside-down image on the retina, and the brain turns it right-side up. The retina uses special cells called rods and cones to process light, and special cells in the retina turn light into electrical signals.

Sound waves go into your ears and make your eardrums vibrate. Three tiny bones in the middle ear, called the malleus, incus and stapes, pass on the eardrum's vibrations. The cochlea is a snail-shaped part of the inner ear that turns vibrations into nerve signals. Hair cells in the cochlea sit on the basilar membrane and ride the wave made by sound vibrations. Low-frequency sounds are lower pitched, and high-frequency sounds are higher pitched. Hair cells near the wide end of the cochlea detect higher-pitched sounds.

Special smell receptors high in the nose detect odor molecules that travel through the air. The nose has about 10 million smell receptors, and the brain can recognize about 10,000 different smells from the combination of smell receptors that fire. Food has to dissolve in saliva before you can taste it, and once food chemicals have dissolved, receptors on the taste buds can detect them. You may have heard that there is a map on your tongue, with sweet at the tip and bitter at the back. In fact, in humans the different types of taste cells are scattered throughout the tongue. Flavor comes from taste and smell working together.

Touch starts with touch receptors in your skin, which send nerve impulses to the brain. Fingers and lips have more touch receptors than your back, so they feel more. Braille is a way of reading with the fingers, using raised dots. A full braille cell has six raised dots arranged in two rows of three. A single braille cell can stand for a letter, a number, a punctuation mark, or even a whole word. Sixty-four combinations are possible using one or more of the six dots in a braille cell.

American Sign Language is the primary language of many North Americans who are deaf or hard of hearing, and it is expressed by movements of the hands and face. You may have heard that it is English on the hands. In fact, ASL is a language completely separate and distinct from English. Fingerspelling is part of ASL and is used to spell out English words. You may also have heard that sign language is the same all over the world. In fact, there is no universal sign language.

You may have heard that you have only five senses. In fact, you also have a sense of balance. The semicircular canals in the inner ear tell your brain when your head turns, which helps you stay balanced. When your head moves, fluid in the semicircular canals moves tiny hair cells, which send a signal to your brain. You may think your eyes alone make the picture you see. In fact, the optic nerve carries signals from the eye to the brain, and all your senses send messages to your brain.

Words to know

• transduction: energy conversion
• perception: interpreted sensing
• cochlea: snail-shaped part
• braille: raised dots
• fingerspelling: spelling words