Name the five senses.
Hearing, sight, taste, touch, and smell. But how do we hear, see, taste, touch and smell? It all has to do, obviously, with the nervous system.
The sensory division of the peripheral nervous system (PNS) includes several sense organs—the eyes, ears, mouth, nose, and skin. Each sense organ has special cells, called sensory receptors, that respond to a particular type of stimulus. For example, the nose has sensory receptors that respond to chemicals, which we perceive as odors. Sensory receptors send nerve impulses to sensory nerves, which carry the nerve impulses to the central nervous system. The brain then interprets the nerve impulses to form a response.
Sight is the ability to sense light, and the eye is the organ that senses light. Light first passes through the cornea of the eye, which is a clear outer layer that protects the eye (see the figure below). Light enters the eye through an opening called the pupil. The light then passes through the lens, which focuses it on the retina at the back of the eye. The retina contains light receptor cells. These cells send nerve impulses to the optic nerve, which carries the impulses to the brain. The brain interprets the impulses and “tells” us what we are seeing.
Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. Sound waves enter the auditory canal and travel to the eardrum (see the figure below). They strike the eardrum and make it vibrate. The vibrations then travel through several other structures inside the ear and reach the cochlea. The cochlea is a coiled tube filled with liquid. The liquid moves in response to the vibrations, causing tiny hair cells lining the cochlea to bend. In response, the hair cells send nerve impulses to the auditory nerve, which carries the impulses to the brain. The brain interprets the impulses and “tells” us what we are hearing.
The ears are also responsible for the sense of balance. Balance is the ability to sense and maintain body position. The semicircular canals inside the ear (see the figure above) contain fluid that moves when the head changes position. Tiny hairs lining the semicircular canals sense movement of the fluid. In response, they send nerve impulses to the vestibular nerve, which carries the impulses to the brain. The brain interprets the impulses and sends messages to the peripheral nervous system. This system maintains the body’s balance by triggering contractions of skeletal muscles as needed.
Taste and Smell
Taste and smell are both abilities to sense chemicals. Like other sense receptors, both taste receptors and odor receptors send nerve impulses to the brain, and the brain “tells” use what we are tasting or smelling.
Taste receptors are found in tiny bumps on the tongue called taste buds (see the figure below). There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty taste is called umami.
Odor receptors line the passages of the nose (see the figure below). They sense chemicals in the air. In fact, odor receptors can sense hundreds of different chemicals. Did you ever notice that food seems to have less taste when you have a stuffy nose? This occurs because the sense of smell contributes to the sense of taste, and a stuffy nose interferes with the ability to smell.
Touch is the ability to sense pressure. Pressure receptors are found mainly in the skin. They are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Some touch receptors sense differences in temperature or pain. How do pain receptors help maintain homeostasis? (Hint: What might happen if we couldn’t feel pain?)
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- Human senses include sight, hearing, balance, taste, smell, and touch.
- Sensory organs such as the eyes contain cells called sensory receptors that respond to particular sensory stimuli.
- Sensory nerves carry nerve impulses from sensory receptors to the central nervous system.
- The brain interprets the nerve impulses to form a response.