Sensory receptors are specialized neurons that respond to specific types of stimuli, and transduction is the conversion of sensory stimulus energy into an action potential. Hearing and balance are also sensed by mechanoreceptors. In the hair cells, bending opens pore-like channels at the tips of the stereocilia, and when the channels open, chemicals rush into the hair cell and create an electrical signal. Sound waves go into your ears and make your eardrums vibrate, and 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, and hair cells in the cochlea make neural impulses that travel along the auditory nerve to the brain.

Hair cells in the cochlea sit on the basilar membrane and ride the wave made by sound vibrations. The place theory of pitch says that different parts of the basilar membrane respond to different pitches, and hair cells near the wide end of the cochlea detect higher-pitched sounds. Hearing humans can pick up sound frequencies from about 20 to 20,000 hertz. You may have heard that you have only five senses. In fact, you also have a sense of balance, because the vestibular sense helps you keep your balance and body posture. The vestibular organs are fluid-filled and have hair cells similar to the ones in the auditory system, and when your head moves, fluid in the semicircular canals moves tiny hair cells, which send a signal to your brain.

Skin is the body's largest organ, and the epidermis is the top layer of your skin. Your skin has nerve endings that let you feel hot, cold and touch, and nerves in the skin help you feel sensations like hot and cold. Touch starts with touch receptors in your skin, which send nerve impulses to the brain. Meissner's corpuscles respond to pressure and lower-frequency vibrations, and Pacinian corpuscles detect brief pressure and higher-frequency vibrations. Stretch in the skin is sensed by receptors called bulbous corpuscles. How sensitive a patch of skin is depends on the number of receptors per unit area and the distance between them. 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, and 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, and sixty-four combinations are possible using one or more of the six dots. Louis Braille was born in Coupvray, France, on January 4, 1809. 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. ASL is a language completely separate and distinct from English, fingerspelling is part of ASL and is used to spell out English words, and there is no universal sign language.

Rods and cones are photoreceptors that change their membrane potential when light hits them. Rods are so sensitive that a single photon of light can make a rod's connected retinal ganglion cell send an action potential. Cones do not work in dim light, so you cannot see colors in dim light, and in dim light the rods do most of the work, but they give no information about color. Axons from the retinal ganglion cells gather and leave through the back of the eye to form the optic nerve, and the optic nerves from the two eyes meet at a point just below the brain called the optic chiasm. Each eye also has a blind spot, an area of the retina with no light-detecting receptors.

Special smell receptors high in the nose detect odor molecules that travel through the air, and the nose has about 10 million smell receptors. 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 sense 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, and flavor comes from taste and smell working together. Taste buds can detect sweet, sour, bitter, salty and umami (savory) flavors.

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. The lens and the cornea work together to focus light on the retina, and light entering the eye forms an upside-down image on the retina, which the brain turns right-side up. Two eyes together help you judge depth, which is how far away things are. The optic nerve carries signals from the eye to the brain, and the auditory nerve carries signals from the ear to the brain. Your senses send messages to your brain.

Words to know

• mechanoreceptors: bending detectors
• stereocilia: hair tips
• vestibular: balance-related
• corpuscles: skin receptors
• transduction: energy conversion