Senses

by Miss Singleton

​SENSES

​​The 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.)

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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​

The ability to sense light.

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​cornea

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clear, protective covering on the outside of the eye.

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​PUPIL

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Opening in the eye through which light passes.

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​LENS

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clear, curved structure near the front of the eye that helps focus light and form images on the retina.

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​RETINA

​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.

​OPTIC NERVE

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Nerve which carries impulses from the eye to the brain.

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​SIGHT: HOW DO WE SEE?

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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

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ability to sense sound.

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​EARDRUM

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membrane that vibrates when sound waves strike it, passing the vibrations to other parts of the ear.

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​Semicircular Canals

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Canals inside the ear; contain fluid which moves when the head changes position; helps to maintain the body’s balance.

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Vestibular Nerve​

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Nerve which carries the impulses to the brain; responds to the position of fluid within the semicircular canals.

auditory canal

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part of the ear where sound waves enter.

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auditory nerve

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This nerve carries impulses from the ear to the brain.

​Balance

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the ability to sense and maintain body position.

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​Cochlea

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fluid-filled, spiral-shaped structure in the inner ear, lined with tiny hair cells that translate sound waves to nerve impulses.

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​HEARING: How do we hear?

​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.

​TASTE AND SMELL

Ability to sense chemicals

​Taste Receptors

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Sends nerve impulses from the taste buds to the brain.

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​Odor Receptors

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Sends nerve impulses from the nose to the brain.

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​Taste Buds

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 & Smell: How do we 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.

​TOUCH

The ability to sense pressure.​

​Pressure Receptors

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Sends nerve impulses from the skin to the brain; especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet.

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​Touch: How do we touch?

​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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