Touch

While your other four senses (sight, hearing, smell, and taste) are located in specific parts of the body, your sense of touch is found all over. This is because your sense of touch originates in the bottom layer of your skin called the dermis. The dermis is filled with many tiny nerve endings which give you information about the things with which your body comes in contact. They do this by carrying the information to the spinal cord, which sends messages to the brain where the feeling is registered. The nerve endings in your skin can tell you if something is hot or cold. They can also feel if something is hurting you. Your body has about twenty differnt types of nerve endings that all send messages to your brain. However, the most common receptors are heat, cold, pain, and pressure or touch receptors. Pain receptors are probably the most important for your safety because they can protect you by warning your brain that your body is hurt! Some areas of the body are more sensitive than others because they have more nerve endings. Have you ever bitten your tongue and wondered why it hurt so much? It is because the sides of your tongue have a lot of nerve endings that are very sensitive to pain. However, your tongue is not as good at sensing hot or cold. That is why it is easy to burn your mouth when you eat something really hot. Your fingertips are also very sensitive. For example, people who are blind use their fingertips to read Braille by feeling the patterns of raised dots on their paper. To learn more about Braille, click on the word "Braille" located on the left-hand side of this page! picture source:http://library.thinkquest.org/3750/hear/hear.html

Sight

Imperfect Eyesight
Not all people have perfect vision. People who can see things up close, but not far away are considered to be nearsighted. This happens when the light entering the eye focuses on a point in front of the retina. On the other hand, people who can see far away objects but not those that are up close are farsighted. Farsightedness occurs when the light that enters the eye focuses on a point behind the retina. Whether a person is nearsighted or farsighted, glasses or contacts help that person to see things much more clearly!

NEARSIGHTED	FARSIGHTED

SIght

Protection
Because the eye is such an important and complex part of our body, we have many features which protect the eye. The eyebrows are the strips of hair above your eyes which prevent sweat from running into them. Eyelashes help keep the eye clean by collecting small dirt and dust particles floating through the air. The eyelashes also protect the eye from the sun's and other light's glare. The eyelids sweep dirt from the surface of the eye. The eyelid also protects the eye from injury. Tears are sterile drops of clean water which constantly bathe the front of the eye,keeping it clean and moist.

Sense of Sight

From the moment you wake up in the morning to the time you go to sleep at night, your eyes are acting like a video camera. Everything you look at is then sent to your brain for processing and storage much like a video cassette. This is a very simplified explanation, but as you read on, you will discover why the sense of sight is actually considered the most complex of the five senses.How Your Eyes Work Take a moment to locate an object around you. Do you know how you are able to see it? Would you believe that what you are actually seeing are beams of light bouncing off of the object and into your eyes? It is hard to believe, but it is true. The light rays enter the eye through the cornea, which is a thick, transparent protective layer on the surface of your eye. Then the light rays pass through the pupil (the dark circle in the center of your eye) and into the lens. When light rays pass through your pupil, the muscle called the iris (colored ring) makes the size of the pupil change depending on the amount of light that's available. You may have noticed this with your own eye if you have looked at it closely in a mirror. If there is too much light, your pupil will shrink to limit the number of light rays that enter. Likewise, if there is very little light available, the pupil will enlarge to let in as many light rays as it can. Just behind the pupil is the lens and it focuses the image through a jelly-like substance called the vitreous humor onto the back surface of the eyeball, called the retina. The retina, which is the size of your thumbnail, is filled with approximately 150 million light-sensitive cells called rods and cones. Rods identify shapes and work best in dim light. Cones on the other hand, identify color and work best in bright light. Both of these types of cells then send the information to the brain by way of the optic nerve. The amazing thing is, when they send the image to the brain, the image is upside down! It is the brain's job to turn the image rightside up and then tell you what you are looking at. The brain does this in a specific place called the visual cortex.

Sense of Hearing.

Like your other sense organs, your ears are extremely well-designed. In fact, they serve two very important purposes. Do you know what they are? You were probably able to figure out that your ears help you to hear sounds, but what you probably did not know is that your ears also help you to keep your balance.
How You Hear
When an object makes a noise, it sends vibrations (better known as sound waves) speeding through the air. These vibrations are then funneled into your ear canal by your outer ear. As the vibrations move into your middle ear, they hit your eardrum and cause it to vibrate as well. This sets off a chain reaction of vibrations. Your eardrum, which is smaller and thinner than the nail on your pinky finger, vibrates the three smallest bones in your body: first, the hammer, then the anvil, and finally, the stirrup. The stirrup passes the vibrations into a coiled tube in the inner ear called the cochlea.

Hearing

Hearing

Hearing or audition is the sense of sound perception. Hearing is all about vibration. Mechanoreceptors turn motion into electrical nerve pulses, which are located in the inner ear. Since sound is vibrations propagating through a medium such as air, the detection of these vibrations, that is the sense of the hearing, is a mechanical sense because these vibrations are mechanically conducted from the eardrum through a series of tiny bones to hair-like fibers in the inner ear, which detect mechanical motion of the fibers within a range of about 20 to 20,000 hertz, with substantial variation between individuals. Hearing at high frequencies declines with an increase in age. Inability to hear is called deafess. Sound can also be detected as vibrations conducted through the body by tactition. Lower frequencies than can be heard are detected this way.

The Sense of Taste

1. The tongue is the sensory  organ that gives us the sense of taste. It is sensitive to sweet, salty, sour, and bitter tastes.

2. The upper surface of the tongue is rough. This is because it is covered with taste buds.

3. Taste receptors can be found in the taste buds. They responds to various chemicals in food.
     Each type of taste receptor is sensitive to a certain types of taste.

4. a) The tongue has four types of taste receptor.
    b) These are sweet receptors, sour receptors, salty receptors, and bitter receptors.
    c) Each receptor is located in a particular area of the tongue. Thus, each area of the tongue is    
        able to detect a certain type of taste.

5. Chemicals in food dissolved in saliva. => The chemicals stimulate taste receptors in the taste  
    buds => Impulses form the taste receptors are sent along the nerves to the brain. => The brain
    identifies the taste.

Detection of Smell

1. Certain chemical particles present in the air circulate the smells in the air. For example, flowers, food, and perfumes release chemicals into the air.

2. When these chemical particles enter the nasal cavity, they dissolve the mucous lining that coats the smell receptors in the nasal cavity.

3. The dissolved chemicals particles stimulate the smell receptors

4. The smell receptors then send out nerve impulses to the brain. The brains identifies the smell.

5. Structural adaptations of the nose to detect smells:
    a) Smell receptors are located at the root of nasal cavity. This enables the receptors to have     
       maximum exposure to the air that enters the nasal cavity.
    b) The smells receptors are always covered in moist muscus which is secreted by the glands in
        the nasal cavity.

The Sense Of Smell

Structure of The Nose.

1. The nose is the sensory organs for smell.

2. The upper part of the nasal cavity of the nose has many sensory cells or receptors to detect    
    smell.

3. These smell receptors are sensitive to all kinds of the smells.

Sensitve of the skin.

1.  a) When the receptors are stimulated, they send out nerve impulses along the nerves to the              
         brain.
    b) The brain interpprets these impulses as the sensation of touch.
    c) It will then send suitable messages to the effectors as how to react.

2. The skin on different parts of the body has different part of the body has different degrees of  
    sensitivity.
    a) The fingertips, the lips, the tongue, the nose, and the neck are more sensitive to touch.

3. The sensitive of the skin depends on:
    a) the distance between the receptors
    b) the thickness of the epidermis

4. The skin on parts of the body which has thinner epidermis and more receptors is more sensitive  
    to touch.

5. a) Blind poeple use their sense of touch to identify object in their environment.
    b) They use their fingertips to read book in braille.



6. Doctors usually give injections to their patients on their forearms or buttocks because these areas are less sensitive to touch and pain.

Cross section of human skin.

Name	Funtion
Pain receptor	Lie very close to the skin surface to detect even a slight pain.
Heat receptor	Sensitive to heat and is able to detect the temperature when it is hotter than usual.
Touch receptor	Detect any light touches and identify the texture of an object.
Pressure receptor	Lie deep within the skin and detect any forceful pressure against the skin.
Cold receptor	Sensitive to cold and is able to detect the temperature when it is colder than usualy.

The Sense Of Touch

Structure of the human skin.

1. The skin helps regulate body temperature. It is also a sensory organ that detects touch.
2. The skin has five different types of receptors.
    These are:
    a) pain receptors
    b) touch receptors
    c) heat receptors
    d) cold receptors
    e) pressure receptors
3. The receptor in the skin are sensitive to the stimuli of pain, touch, heat, cold, and pressure.
4. a) The pain receptors are situated in the epidermis of the skin.
    b) The touch, heat, and cold receptors are situated in the dermis.
    c) The pressure receptors are found in the fatty layer.