F2C1 The World Through Our Senses (part 1)

Sensory organs -- detect stimuli
Stimuli -- changes in the environment that cause our body to react

We have five sensory organs
1. Skin responds to TOUCH
2. Nose responds to SMELL ( also helps in tasting)
3. Tongue responds to TASTE
4. Ears respond to SOUND
5. Eyes respond to LIGHT


The Pathway from detecting a stimulus to producing a response in human beings


STIMULUS--->RECEPTORS(located in sensory organs)--->nerve impulses--->NERVES--->nerve impulses--->BRAIN--->nerve impulses--->NERVES--->nerve impulses--->EFFECTORS(muscles and glands)--->RESPONSE


THE SENSE OF TOUCH
1 Skin detects touch
2 The receptors in the skin are pain receptors, touch receptors, heat receptors, cold receptors, and pressure receptors.

PAIN receptors located at the top to detect the slightest pain.(e.g somebody pinching your face)


TOUCH receptors detects light touches and identifies textures of an object. (e.g touching)

HEAT receptors detects increase in temperature(e.g touching hot stuff)

COLD receptors detects decrease in temperature(e.g touching ice)

PRESSURE receptors detects any forceful pressure against the skin(e.g carrying a heavy object)


Sensitivity of the Skin
1 The skin on different parts of the body has different degrees of sensitivity.
2 The higher the number of receptors, the higher the sensitivity.
3 The thicker the epidermis, the lower the sensitivity.
4 Lips, the neck, the fingertips has high sensitivity
5 Legs, the elbows, the soles of the feet, the back of the body has low sensitivity

THE SENSE OF SMELL
1 The nose detects smell.
2 The nostrils open into a hollow space( the nasal cavity)
3 The nasal cavity is lined with mucus, mucus warms and moistens the air before it enters the lungs.
4 Hairs in the nostrils filter out dust and dirt so that clean air enters the lungs.
5 The roof of the nasal cavity has sensory cells/smell receptors to detect smell.
6 When a person has a cold, he/she cannot detect smells well because too much mucus is being produced and the mucus prevents the chemicals from simulating the smell receptors.


Detection of Smells
1 Chemicals(e.g in food, flowers, poop) present in inhaled air
2 Chemical particles enter the nasal cavity
3 Chemicals dissolves in mucus lining
4 Smell receptors are simulated and send out nerve impulses to the brain.
5 Brain identifies the smell.


THE SENSE OF TASTE
1 The tongue detects taste.
2 The tongue is covered with taste buds and taste buds contains many taste receptors.
3 The chemicals(e.g in food) must dissolve in saliva before they can be stimulated by the taste receptors
4 Different receptors are located at different parts of the tongue.

Areas of the tongue that are sensitive to taste

Detection of Taste

1 When we chew food, chemicals in food dissolves in the saliva

2 Dissolved chemicals stimulate taste receptors to produce nerve impulses

3 Nerve impulses travels to the brain

4 Brain identifies the taste.

Sense of Taste and Smell

The nasal and mouth cavities are connected. Chemicals from the food stimulates the taste receptors and also the smell receptors. This means that taste and smell are detected at the same time so that we can identify the flavours of food. When we have cold, smell cannot be detected by the smell receptors so we cannot detect the flavour of the food so the food becomes tasteless.

THE SENSE OF HEARING

1 The ear detects sound.

Structure of the ear

1. The pinna collects and directs sound waves into the ear canal(a.k.a auditory canal)
2. Ear canal directs sound waves to the ear drum(a.k.a tympanic membrane)
3. Ear drum vibrates and transfer vibrations to the ossicles
4. Ossicles[made up of Malleus(Hammer), Incus(Anvil), Stapes(Stirrup)] amplify the vibraions and transfer them to the oval window(before the cochlea)
5. Oval window transfers the vibrations to the inner ear. Vibrations cause the fluid in the cochlea to vibrate.
6. Vibration of the fluid stimulates the receptors in the cochlea. Nerve impulses are generated
7. Auditory nerves sends the nerve impulses to the brain which then will interpret the impulses as sounds.

Semicircular canals and Eustachian tube are part of the ear but NOT part of the hearing mechanism

Semicircular canals help the body maintain its balance.

Eustachian tube helps balance the air pressure on both sides of the eardrum.

THE SENSE OF SIGHT

1 The eyes detect light to allow us to see

2 The wall of the eyeball is made up of three layers:

1. Sclera (outer layer, that is the white part you see in your eye)
2. Choroid (middle layer, your iris is part of your choroid)
3. Retina (inner layer, has many photoreceptors called cones and rods that are sensitive to light)

Structure of the eye

1. Conjuctiva is a thin and transparent membrane that protects the cornea
2. Ciliary body contracts and relaxes to change thickness of lens. This changes the focal length of lens
3. Cornea is a curved, transparent layer and is a continuation of the sclera
4. Iris controls the size of the pupil and the amount of light that enters the eye
5. Pupil allows light to pass into the eye
6. Lens bend and focuses light to form images on the retina. The thickness of lens can be change to focus near and distant objects. ( When focusing on a near object, lens thickens, when focusing on a distant object lens become thinner.)
7. Aqueous humour helps to refract light and maintain the shape of the eyeball
8. Vitreous humour helps to refract light and maintain the shape of the eyeball
9. Suspensory ligaments hold the lens in place and is connected with the ciliary body.
10. Retina detects light stimuli and sends nerve impulses to the brain
11. Choroid supply nutrients and oxygen to the eye, the choroid has black pigments that absorb light and prevent internal reflection of light.
12. Sclera protects and shapes the eye.
13. Yellow spot (fovea) is the part most sensitive to light
14. Optic nerve sends nerve impulses to the brain
15. Blind spot is the point where optic nerve leaves the eyeball, thus there is no receptor cells, hence, cannot detect light.

Detection of Light
1 Light rays travel from an object and enter the eye through the pupil
2 The light is refracted by the cornea, aqueous humour, eye lens, vitreous humour
3 An inverted smaller image of the real object is formed on the retina
4 The image stimulates the photoreceptors and nerve impulses are sent to the brain. The brain then interprets the inverted image and converts it to an upright image

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