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.


Integumentary System

The Integumentary System consist of the skin and its derivatives; hair, glands, nails and nerve endings.

– Skin covers the body and provides a physical barrier that protects underlying tissues from physical and bacteria invasion, dehydration and ultraviolet light.
– Certain cells of the epidermis play a role in the immune system, warding off invaders.
– Under conditions of high temperature or strenuous exercise, glands produce sweat and the evaporation of the sweat provides a cooling mechanism for the body.
– Under conditions of low temperature, the production of sweat decreases.
– Sweat also contributes to excretion. Along with heat and some water, it removes a small amount of salts and several organic compounds.
– The dermis contains an extensive network of blood vessels that carry up to 10% of the total blood flow in a resting adult.
– Nerve endings that detect temperature, touch, pressure and pain are located in the skin.
– Synthesis of vitamin D begins with the activation of a precursor in the skin by ultraviolet light.
– This compound is modified and becomes CALCITRIOL (the most active form of vitamin D).
– Calcitriol aids in the absorption of calcium in food from the digestive tract into the blood.

The skin consists of different tissues that are joined to perform specific functions.

It is the largest organ of the body, in surface and weight.

The skin is composed of 2 parts; the EPIDERMIS and the DERMIS.
– The epidermis is the outer, thinner portion of the skin. It is usually 4 to 5 layers thick, depending on its location.
– Its deepest layer is a single layer of stem cells capable of continuous cell division.
– It contains the cells that are sensitive to touch.
– Some of the cells produce KERATINOCYTES that keep moving up the skin layers.
– While these cells are moving up to the skin’s surface, MELANIN is taken in by the cells. The cells lose their nucleus and other organelles, they lose their ability to carry on vital metabolic reactions and eventually die.
– Some of the stem cells move down in the skin and become oil and sweat glands and hair follicles.
– The most superficial layer of the epidermis, the STRATUM CORNEUM, is composed of about 30 rows of flat, dead cells, completely filled with KERATIN. This layer is shed continuously and replaced by cells from the lower layers.
– It serves as an effective layer against light, heat waves, bacteria and many chemicals.
– It takes about 2 to 4 weeks for the epidermal cells to make their migration from the lowest layer to the skin’s surface.
– The epidermis is composed of layers of STRATIFIED SQUAMOUS EPITHELIUM (one of the four main types of tissue in the body).
– The epidermis contains 4 types of cells: KERATINOCYTES, MELANOCYTES, LANGERHANS CELLS and MERKEL CELLS.
– Keratinocytes make up 90% of the epidermal cells.
– They produce the protein KERATIN which is a tough, fibrous protein that helps protect the skin and underlying tissue from heat, microbes and chemicals.
– Keratinocytes also release a waterproofing sealant for the skin.
– Melanocytes make up 8% of the epidermal cells.
– They produce the pigment MELANIN, which contributes to skin color and absorbs ultraviolet light.
– Once inside the cells the melanin granules cluster to form a protective veil over the nucleus of the cell on the side of the skin’s surface, this shields the DNA(genetic material) from ultraviolet damage.
– Langerhans Cells make up a small portion of the epidermal cells and participate in the immune responses against microorganisms.
– Merkel Cells are the least numerous of the epidermal cells.
– They are located in the deepest layer of the epidermis, where they are in contact with nerve cells and function in the sensation of TOUCH.

– The dermis is an inner, thicker layer of connective tissue.
– The connective tissue of the dermis has collagen, elastic fibers, blood vessels, nerves, glands and hair follicles embedded in it.
– The combination of collagen and elastic fibers in the dermis provide the skin with STRENGHT and ELASTICITY(the ability to stretch).
– Nerve endings sensitive to COLD are found in and just below the dermis.
– Nerve endings sensitive to HEAT are located in the middle and outer dermis.
– The dermis is attached by fibers to a layer called the SUBCUTANEOUS LAYER, which isn’t considered a part of the skin.
– The subcutaneous layer serves largely as a storage depot for fat and contains larger blood vessels that supply the skin.
– The subcutaneous layer is attached to the underlying tissue.

– MELANIN, CAROTENE and HEMOGLOBIN are the 3 main pigments responsible for skin color.
– Melanin is a brown-black pigment found mostly in the epidermis and causes variations in skin tone from pale yellow to black.
– Since the number of melanocytes is about the same for all races, differences in skin color are due to the amount of pigment the melanocytes produce and disperse to the keratinocytes.
– ALBINISM is an inherited inability of an individual of any race to produce melanin. Melanin is absent in their skin, hair and eyes, they appear white.
– VITILIGO is the partial or complete loss of melanocytes from patches of skin, it produces irregular white spots.

– Hairs are growths of the epidermis.
– Hair occurs all over the body, except on the palms of the hands and the soles of the feet.
– Genetics and hormones determine the thickness and pattern of distribution of hair.
– Normal hair loss is between 70 and 100 hairs per day.
– The main function of hair is PROTECTION. Hair guards the scalp from injury and the sun’s harmful rays.
– Eyebrows and eyelashes protect the eyes from foreign particles. Hairs in the nostrils and ears have a similar function.
– Hair also aids in sensing light touch. Touch receptors associated with hair follicles are activated when a hair moves even slightly.
– Each hair is composed of columns of dead, keratinized cells welded together.
– The SHAFT is the superficial portion of the hair that projects from the surface of the skin.
– The shaft of straight hair is round in cross-section and curly hair is oval.
– The ROOT is the part of the hair that penetrates into the dermis and sometimes the subcutaneous layer.
– The HAIR FOLLICLE surrounds the root.
– Around each hair follicle are nerve endings that are sensitive to touch.
– Smooth muscle (ARRECTOR PILI) is attached to the hair follicle. When these muscles contract under conditions of fright or cold, the hair pulls into an upright position. This is what happens when your “hair stands on end” or you get “Goosebumps”.
– HAIR COLOR is related to the pigment melanin. The melanin is formed by the melanocytes and then passed into the hair.
– Dark colored hair is mostly true melanin.
– Blond or red hair is due to melanin mixing with iron and sulfur.
– Gray hair is the progressive loss of the enzyme responsible for the formation of melanin.
– White hair is the result of the accumulation of air bubbles in the hair shaft.

There are 4 types of glands associated with the skin; oil, sweat, ceruminous and mammary glands.
– Oil glands vary in size depending on where they are located.
– They are larger in the skin of the breast, face, neck and upper chest.
– They are smaller in most of the trunk of the body and the extremities.
– They are absent on the palms of the hands and the soles of the feet.
– Oil glands are usually connected to hair follicles. These glands produce an oily substance (SEBUM) that contains fats, cholesterol, proteins, inorganic salts and pheromones.
– Sebum has several functions including, coating the surface of the hair to keep it from becoming dry and brittle, it prevents excessive evaporation of water from the skin, it keeps the skin soft and supple and it inhibits the growth of bacteria.
– There are 3 to 4 million sweat glands in the human body.
– They empty their secretions on to the skin’s surface
– They fall into 2 categories: ECCRINE and APOCRINE.

– Eccrine glands are simple, coiled, tubular and more common than apocrine glands.
– They begin functioning soon after birth.
– They mainly produce sweat or perspiration. They are found in the skin of the forehead, palms and soles.
– SWEAT is a mixture of water, salts, urea, uric acid, amino acids, sugar, lactic acid and ascorbic acid.
– Its main function is to help regulate body temperature by providing a cooling mechanism. As sweat evaporates, large quantities of heat energy leave the body’s surface. It also plays a small role in eliminating waste.

– Apocrine glands begin functioning at puberty.
– They are simple, coiled, tubular glands. They are located in the subcutaneous layer and their excretory ducts open into the hair follicle.
– Their secretion is more thick and sticky than sweat, due to the addition of lipids and proteins.
– These glands are located in the armpits, pubic region and pigmented regions of the breast.

– Ceruminous glands are modified sweat glands located in the subcutaneous layer of the external ear.
– They produce a waxy substance that acts as a sticky barrier, preventing foreign substances from entering the ear.
– Are also modified sweat glands.

– Nails are plates of tightly packed, hard keratinized cells of the epidermis.
– The NAIL BODY is the visible portion of the nail.
– The NAIL ROOT is buried in a fold of skin.
– The epithelium underneath the nail root is called the NAIL MATRIX and is made up of cells undergoing mitosis to produce nail growth.
– Nails grow at various rates; the average nail grows about 1 millimeter per week.