Form 2 Scientist - Sensory Organs Part 1

1.1 Sensory Organs And Their Functions

- Sensory Organs enable us to detect changes that occur in its environment.

- a change in the environment that causes the body to react or respond is stimulus.

- Humans being have five sensory organs. They are skin,nose,tongue,ears and eyes.
- Human also have five different types  of senses. They are  touch, smell, taste, hearing, and sight.
- Table 1.1 shows the relation between the sensory organs

 Skin  -  touch  -  pressure, pain, heat, cold
Nose  -  smell  -  chemicals in the air
Tongue  -  taste  -  chemicals in food
Eyes  -  hearing  -  sound
Ears   -  sight      -  light

The pathway from stimulus to response
- the nervous system of humans consist of the brain, the spinal cord and nerves.
- a receptor is a special sensitive tissue or organ which detects stimuli.
- the nerves carry information from the receptors to the spinal cord and the brain, and from the brain to the     spinal cord and to the muscle or glands.


Stimulis  -  receptors in sensory organs  -  nerves  -  brain  -  nerves  -  effectors/muscle  -  response

1.2 The Sense of Touch

Structure of the Human Skin
- the skin helps to regulate body temperature. It is also a sensory organs that detect touch.
- 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 
- the receptors in the skin are sensitive to the stimuli of pain, touch, heat, cold, pressure.
- these five receptors have different function :
  (a) pain receptors - lie very close to the skin surface to detect even a slightly pain
  (b) touch receptors - detect any light touches and identify the texture of an object.
  (c) heat receptors - sensitive to heat and are able to detect the temperature when it is hotter than usual.
  (d) cold receptors - sensitive to cold and are able to detect the temperature when it is colder than usual.
  (e) pressure receptors - lie deep within the skin and can detect any forceful pressure against the skin.

Sensitivity of the Skin
1. The skin on different parts 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.
   (b) the legs, the elbows, the soles of the feet and the back of the body are less sensitive.
2. The sensitive of the skin depend  on :
    (a) the distance between  the receptors
    (b) the thickness of the epidermis
3.The skin on the part of the body which has thinner epidermis and more receptors is more sensitive to touch.
4. Blind people use their sense of touch to identify object in their environment. They use their fingertips to 'read' books in Braille.
5. Doctors usually give injection to their patients on their forearms or buttocks because these areas are less sensitive to touch and pain.

PMR Alert!!!
- Our skin is 2mm thick, it covers the whole body. It consist of two main layers, the outer epidermis and the inner dermis.
- The epidermis is made up of dead cells, and is waterproof. It is also a protective layer which prevents the entry of bacteria into the body tissues.
- The receptors in the dermis  and the epidermis enable the skin to detect various external stimuli.