Senses

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. (a stimulus is anything that makes an organism or a part of an organism react in some way.)

​

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.

​

​

​

​a layer at the back of the eyeball containing cells that are sensitive to light and that trigger nerve impulses that pass via the optic nerve to the brain, where a visual image is formed.

​

​

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 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 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.

Ability to sense chemicals

​

​

Taste receptors are found in tiny bumps on the tongue called taste buds. There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty taste is called umami. ​

​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.

The ability to sense pressure.​

​

​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?)

​

​