The World Through Our Senses

The Wourld Through Our Senses Sensory organ, senses and stimuli Stimuli – changes that occur in the surroundings. Example : sound, chemical sunstances and light intensity Sensory organs – Organs that detect stimuli Each sensory organ is sensitive to only certain types of stimuli. For example, the eyes are only sensitive to light but not to sound. Sense – The ability of an organism to detect a stimulus Human beings have five sensory organs Senses of smell Senses of touch Senses in humans senses of taste Senses of hearing senses of sight Relation between sensory organ, stimuli, and senses in human beings Sensory organ Stimulus /stimuli detected Sense Eyes Light Sense of light Ears Sound Sense of hearing Nose Smell Sense of smell Skin Touch, pressure, pain, heat and

cold Sense of touch

Tongue Taste (chemicals in Food) Sense of taste Response to stimuli Nerve impulses

Nerves Stimulus Receptor in

sensory organ

Nerves

Respons Nerve impulses

Brain

Effector

Common pathway in detecting a stimulus and producing a response in human beings

Sense of Touch 1. The skin is a sensory organ that gives us our sense of touch. It can detect stimuli such as touch, pain, pressure, heat and cold.

2. Refer to the diagram, skin consists of two main layer, the epidermis and dermis. Below the skin is a layer of fatty tissue. 3. The skin has five different receptor; 3.1 The touch receptor – sensitive to slight pressure 3.2 The pain receptor – sensitive to pain 3.3 The heat receptor – sensitive to heat 3.4 The cold receptor – sensitive to cold 3.5 The pressure receptor – sensitive to heavy pressure 4. The sensitivity of skin depend on; a) How close together the receptor are Example : the lips and fingertips are very sensitive to touch due to the large concentration of touch receptor.

b) How deep the receptor are Example : The receptor with thick epidermis are less sensitive. The epidermis on the neck is thinner than that on the knee. So neck more sensitive than the skin at the knee.

c) The sense of touch is very important to the blind because it helps them to identify object and feel their way around and to reads book in Braille .

Sense of Smell

1. Smells are actually chemicals present in the air. 2. The nose can detect these chemicals. 3. The smell receptors or sensory cells are located at the roof of the nasal cavity.

Refer to diagram. This position allows the smell receptor to have the maximum exposure to the air current.

4. How smells are detected; a) When air is breathed in, the air current passes through the nasal cavity. The chemicals present in the air dissolve in the mucous lining and stimulate the smell receptor. b) These receptors send out nerve impulses to the brain. The brain interprets the message and identify the smells.

Sense of Taste

1. The tongue is the sensory organ related to the sense of taste. 2. It is sensitive to four basic tastes – sweet, salty, sour and bitter. 3. The surface of the tongue is covered with little bumps. These bumps contains

tiny structures called taste buds. Each taste bud contains many taste receptors which are sensitive to chemicals in food.

4. There are four types of taste buds , one for each basic taste. Different areas of the tongue are sensitive to different tastes. Refer to the diagram;

5. How taste detected ; a) When we chew our food, the chemicals of the food dissolve in the salive.

b) The dissolved chemicals stimulate the taste receptors in the taste buds to produce nerve impulses. c) Messages are sent to the brain to be interpreted. This enables us to identify the taste of the food.

6. The nasal cavity and the mouth cavity are connected. This allows the taste and smell of food in the mouth to be detected at the same time.

Sense of Hearing The human ear has three main parts – the outer ear, the middle ear and the inner ear.

Part Structure

Function

Outer ear a) Pinna (ear flap) b) Ear cannal

Shape like funnel. Made of cartilage and skin. A 2.5 cm long, narrow tube lined with hair.

Collects and directs sound waves into the ear canal Directs sound waves to the eardrum

Middle ear a) Eardrum b) Ossicles c) Oval window d) Eustachian tube

A thin , stretched membrane at the end of the ear canal Three small bones A thin , small membrane at the end of the ossicles A narrow tube that connects the middle ear to the back of the throat.

Vibrates when sound waves hit it Amplify vibrations and transfer them from the eardrum to the oval window. Transfers vibrations from the ossicles to the cochlea Equalises the air pressure on both sides of the eardrum, thus preventing itu from damage.

Inner ear a) Cochlea b) Auditory nerve c) Semicircular canals

A called tube filled with fluid and lined with tiny hair-like sensory cells. Nerve fibres that connect the ear to the brain Three semicircular cannals situated at right angles to each other. Contain fluid and sensory cells.

Detects vibrations and converts them into nerve impulses Carries vibrations and converts them into nerve impulses Detect the position and movement of the head to help us keep our balance (Note: They are not involved in the hearing mechanism.

8. How sounds are detected by the ear :

a) The pinna collects sound waves and directs them along the ear canal to the eardrum. b) When the sound waves hit the eardrum, it vibrates at the same frequency as the sound

waves. The vibrations are then transferred to the ossicles.

c) The ossicles amplify for the vibrations about 20 times before transferring them to the oval window.

d) The vibration of the oval windows causes the fluid in the cochlea to move in the

form of waves . The movement of the fluid stimulates the sensory cells or receptors in the cochlea. Nerve impulses are produced.

e) The auditory nerve carries the impulses to the brain.

f) The brain interprets the impulses as sounds.

Sense of Sight

1. The eyes are the sensory organs of sight. They are sensitive to light. 2. The eyelids and eyelashes keep dust and dirt out of the eyes. 3. The eye brows help to prevent sweat from running into the eyes.

4. The wall of eyeball is made up of three layers. (a) Sclere – tough, outer layer (b) Choroid – black, middle layer (c) Retina – sensitive to light , innermost layer

The structure of the human eye and the function of the different parts

Part Stucture Function

Sclera White, fibrous, outer layer that is opaque and tough

Protects and maintains the shape of the eyeball

Choroid Middle layer that contains black pigment and many blood vessels

- The blood vessels supply nutrients and oxygen to the eye. - The black pigment absorbs light and therefore prevents reflection of light inside the eye which would make an image less sharp.

Retina The innermost layer of the eyeball. It contains many photoreceptors known as cones and rods

- Detect light and produces nerve impulses - The cones detect colours in bright light, enabling us to have colour vision -The rods enables us to see in dim light . They detect shades of grey in dim light.

Cornea The curved and transparent layer of the sclera at the front of the eye.

Its curved surface helps to refract (bend) light onto the retina.

Conjunctiva The thin , transparent layer of membrane in front of the cornea

Protects the cornea

Iris The coloured part of the eye that is continous with the choroids. It is made up of muscles

Controls the size of the pupil and thus the amount of light entering the eye.

Pupil The hole in the centre of the iris

Controls the amount of light entering the eye.

Lens A transparent bioconvex and leastic disc.

-It refract and focuses an image onto the retina. -The thickness of the lens is changed to focus near and distant objects.

Ciliary body Make up of strong muscle Contract and relaxes to change the thickness of the lens

Suspensory ligaments

Strong fibres that connect the lens to the ciliary body

Hold the lens in its position

Aqueous humour A watery transparent liquid that fills the space between the cornea and the lens

- Helps to focus the image onto the retina -Helps to maintain the shape of the eyeball -The cornea , the conjunctiva and the lens obtain food and oxygen from the blood vessels in the choroids layer by diffusion through this liquid.

Vitreous humour A thick , jelly-like and transparent substance that fills the space behind the lens

-Helps to focus the image onto the retina -Helps to maintain the shape of the eyeball

Yellow spot(fovea) The most sensitive spot on the retina. It is located directly opposite the pupil

Detect light or image that fall on it

Blind spot A point on the retina where the optic nerve leaves the eye. There are no photoreceptors here.

Images that fall on this spot cannot be detected.

Optic nerve Made up of nerve fibres that connect the retina to the brain.

Carries nerve impulses from the retina to the brain.

5. How wee see 1. When we look at an object, light rays from the object enter each eye through the pupil. The intensity of the light influences the size of the pupil. a) When it is very bright , the size of the pupil decreases to let in less light. b) When it is dim, the size of the pupil increases to let in more light. 2. As the light rays pass through the cornea, the aqueous humour , the lens and vitreous humour, the rays are refracted to form an image on the retina. 3. The image is upside down and smaller than the actual object. 4. The brain interprets the impulses and we can see the object the right way up. The interpretation also allows us to know the shape, the colour , the size and the distance of the object.

5. Eyes have the ability to focus on objects a different distance. Refer to diagram;

Light and Sight Properties of light; a). Light travels in straight lines within the same medium b). Light extremely fast and has speed of 3.0 x 10 8 ms-1 in air. c). Light can be reflacted when it hits a surface d). Light can be refracted when it travels from one transparent medium to another.

1. A beam of light is made up of a bundle of rays. It can be parellal , divergent or convergent. Refer to the diagram.

2. Reflaction of light a) Reflaction of light occurs when light bounces off the surface of an object. Refer to diagram

b) When a parallel beam of light hits a very smooth surface, it is reflected as a parallel beam. Its called ‘regular reflection’. It also produced sharpe image. c) When a parallel beam of light hits a rough surface cause the reflected light to head off in all directions. This type of reflection is called irregular of diffuse reflection. Refer to the diagram;

Defect of vision and ways to correct them

Short-sightness (myopia)

Long-sightness (hypermetropia)

Similarity

Both are caused by the conditions of lens and eyeball

Differences

- Sees near objects clearly - Distance object are blurred

Condition of vision

- Sees distant objects clearly - Near objects are blurred

- The lens is too thick - The eyeball is too long

- The lens is too thin - The eyeball is too short

Causes

Agtigmatism Colur-

blindness

Long-sightedness

Defect of vision Presbyopia

Short-sightdness

Blind spot

Optical illusion

Monocular vision

Limitation of sight

Cannot see tiny objects

Stereoscopic vision

Cannot see very distance object

Diagram 4 : Monocular and stereoscopic vision

Magnifying glass Binoculars

Periscope

Ultrasound scanning

Devises to overcome limitation of sight

Magnetic Resonance Imaging

(MRI) machine x- ray machine

Telescope Stimuli and Responses in Plants a) Tropisms Tropism are growth responses of plants to external stimuli b) Geotropism Geotropism is the growth movement shown by plant in response to gravity c) Phototropism Phototropism is the growth movement shown by plants in response to light d) Hydrotropism Hydrotropism is the growth movement shown by plants in response to water. e) Thigmotropism Thigmotropism is a response by plants to touch or contact with the solid structure

Structure questions

1. Figure 1 shows the cross section of the human ear (a) Name the parts labeled M to Q (b) Which labeled parts of the ear responsible in carrying out the following functions ?

(i) Changing sound vibrations to electrical messages (ii) Amplifying sound vibrations (iii) Helps in balancing by detecting the position of the head

(c) Complete the sequence below to show how sound is detected by the human ear. Pinna Auditory Nerve

2. Figure 2 shows the cross section of the human eye.

(a) label the cornea and retina in Figure 2 (b) The following sequence shows the path of light rays from the object entering

the human eye. Cornea Aqueous humou Pupil R Vitreous humour Retina

(i) Mark R in Figure 2 (ii) State the function of R

(c) A student suffers from a detect of vision due to his slightly long eyeball.

(i) Name the defect of vision the student is suffering from (ii) Mark X along the line EF to show the position of the image that is

formed in his eye (iii) What kind of lense can be used to correct this defect?