The Senses

The human body has 5 major senses; sight, hearing, smell, taste and touch, that provide the information about the external environment and transmit stimuli to sensory nerves and on to the brain for processing.

The sense organs allow humans to interact with their surroundings.

Lights, sounds, smells, taste and touch from our surrounding environment are received by the sense organs of the eyes, ears, nose, tongue and skin.

Sensory receptors in each of these organs allow the nervous system to collect stimuli from the environment.

The sense organs that collect light from the environment are the EYES.

The human eye is about 2.5 centimeters in diameter.

Most of the eyeball rest in the bony eye socket of the skull. Only about 1/6th of the eye is exposed.

3 pairs of small muscles attach the eye to the eye socket.

Secretions from the tear glands keep the eye moist.

The CONJUNCTIVA, a protective membrane covers the eye.

External parts of the eye include the eyelids, eye lashes and eyebrows.

The eye is made up of many different parts that help to collect, focus and analyze light energy.

The eyeball is divided into 2 chambers that are separated by a lens.

– The front chamber contains the CORNEA, the IRIS and blood vessels.
– The CORNEA focuses the light that enters the eye from the environment.
– The IRIS is the part of the eye that controls the opening to the inner eye. This opening in the inner eye is called the PUPIL.
– The iris can adjust the size that the pupil opens to let in more or less light.
– The pigment in the iris gives the eye its color.
– The chamber is filled with a clear watery fluid called AQUEOUS HUMOR.

– The LENS is transparent and is made of many layers of protein fibers.
– It is about 8 millimeters in diameter.
– The function of the lens is to focus light on the retina at the back of the eye.
– The lens helps the eye adjust the light coming off near or far-away objects.
– The shape of the lens changes; FLATTENS when focusing on DISTANT OBJECTS and THICKENS when focusing on NEAR OBJECTS.
– The ability to bring objects into focus even though they are located at different distances is called ACCOMMODATION.
– Many vision problems are caused by improper functioning of the lenses of the eyes.

– The posterior chamber contains the RETINA, RODS, CONES, and the OPTIC NERVE.
– Light is projected from the lens of the eye to the RETINA, located in the inner eye.
– There are PHOTORECEPTORS found in the retina that convert light energy into nerve impulses that are sent to the brain by way of the OPTIC NERVE.
– 2 types of photoreceptors, CONES and RODS, control the collection of light energy in the retina.
– RODS are good at collecting light energy, even at low levels. They help the eye accommodate in dim light and aid in night vision.
– There are about 120 million rods.
– CONES also collect light, but they can distinguish the different colors of light. They are responsible for color vision.
– There are about 7 million cones.
– The chamber is filled with a jelly-like substance called the VITREOUS HUMOR.

Light enters the eye through the pupil and passes through the cornea, the aqueous humor, the lens and the vitreous humor.

Light reaches the retina, where the signals are set up. These signals are sent to the optic nerve, which carries them to the visual portions of the brain.

The lens turns the image upside down and reverses it from left to right.

The visual centers in the brain correct the inversions and reversals of the lens to make the image right side up.

The eyes collect, focus and send light energy to the BRAIN. The brain interprets and gives meaning to the light energy.

Colors that we can see are really just different wavelengths (or energy amounts) of visible light. The brain associates color with them.

Sound is the movement of air molecules detected by a listening device. The ears detect movement of sound waves in the air.

Ears collect the stimulus of sound in the process of hearing.

The human ear is made up of 3 sections; the OUTER EAR, the MIDDLE EAR and the INNER EAR.

– The outer ear catches sound waves and transports then to the EARDRUM.
– Waves of air molecules first enter the AUDITORY CANAL.
– At the end of the auditory canal is a thin membrane called the EARDRUM or TYMPANUM. The eardrum separates the outer ear from the middle ear.
– The movement of air molecules causes the ear drum to vibrate.

– The middle ear is a small cavity that is filled with air.
– It lies within the skull bone between the outer and inner ear.
– At the bottom of the middle ear is an opening that leads into a canal.
– The canal, called the EUSTACHIAN TUBE, is a passageway that connects the middle ear to the throat.
– The air movement that caused the eardrum to vibrate now causes 3 tiny bones to vibrate.
– The 3 bones are called the HAMMER, ANVIL and STIRRUP, the smallest bones in the body.
– The vibrations move along these bones until they are transferred from the stirrup to the OVAL WINDOW, a membrane covered opening between the middle and inner ear.

– The inner ear is entirely encased in bone.
– In the inner ear is the CHOCHLEA, a fluid-filled structure. The cochlea has many canals that are lined with hair.
– The vibrations from the stirrup cause waves within the fluid found inside the cochlea. These fluid waves trigger very small, fine HAIR in side the cochlea.
– The stimulation of these hairs in the cochlea produces nerve impulses via the AUDITORY NERVE that are sent to the brain for interpretation.

– The ears also help the human body maintain BALANCE.
– Above the cochlea are 3 structures called the SEMICIRCULAR CANALS. These canals contain fluid and are lined with hairs.
– Depending on the position of the body, especially the head, the fluid in these structures will be found in different parts of the canal.
– The hairs lining the inside of these canals will be triggered depending on where the fluid is at any particular moment.
– These hairs then send nerve impulses to the brain, and the brain interprets the position of the body.

– At the base of the middle ear is a tube, the EUSTACHIAN TUBE, that connects the middle ear with the back of the throat.
– This tube maintains atmospheric pressure on both sides of the eardrum.
– If the air pressure on the outside of the eardrum changes, swallowing or opening the mouth either release a buildup of air in the middle ear or sends air into the middle ear to control air pressure balance.
– People often sense this as a “popping” in their middle ear. The popping is the middle ear adjusting to the changing air pressure. This usually happens at high altitudes.

The sense of smell and taste are closely related.

The sense organs for the nose and mouth are designed to collect chemicals from the environment.

Chemical molecules that enter the nose and mouth are collected by receptors in these areas.

The receptors of smell are located in the mucus membranes of the upper part of the nasal cavities.

The receptors in the nose for smell are very small, hair-like nerve endings. When chemicals contact these nerves, impulses are sent to the OLFACTORY NERVE to the brain.

The brain interprets the nerve impulses sent by the various nerve endings as a particular odor.

The sense off smell is closely linked to the sense of taste.

When chemical molecules dissolve in the salvia of the mouth, they trigger TASTE BUDS.

Taste buds are found on the lining of the tongue.


The 5 taste classes are interpreted an certain area of the tongue.

Chemicals that enter the mouth are also interpreted by receptors in the nose. Much of what we taste is really what we smell.

The sense of touch is obtained through millions of nerve endings located throughout the body in the skin.

These nerve endings respond to the stimuli of PRESSURE, PAIN, and TEMPERATURE.

The entire body is covered with nerve endings; however area like the fingers, the toes and the face have more nerves, that are more sensitive to softer stimuli.

The strength of all impulses sent from the skin to the brain are the same.

The strength of a nerve impulse from the skin depends on 2 things.
1. The number of nerve endings in the skin actually stimulated by the touch.
2. The frequency of the impulses that these nerves send to the brain.

This means that the strongest pain of sense of pressure to the skin causes the highest frequency and the greatest number of impulses to be sent to the brain for interpretation and action.