Form 2 Chapter 1: THE WORLD THROUGH OUR SENSESPage | 11

The World through Our SensesSensory Organs: Skin (Touch) Nose (Smell) Tongue (Taste) Ear (Hearing) Eye (Sight)Properties of Light: Light travels in a straight line Reflection Refraction Dispersion of white lightSight Mechanism: Formation of image by convex lens Formation of image on retinaLight & SightHearing Mechanism: Frequency PitchProperties of Sound: Reflection AbsorptionSound & HearingLimitation of SensesDefects of Sensory OrgansSight: Stereoscopic Monocular Optical Illusion Blind SpotHearing: Stereoscopic hearing Frequency range Hearing AidsSight: Long-sightedness Short-sightedness Astigmatism CorrectionsHearing: Deafness CorrectionsStimuli & Responses in Plants: Phototropism (light) Geotropism (gravity) Hydrotropism (water) Thigmotropism (move toward) Nastic movement (move run away)

1.1 Sensory Organs

Sensory Organs and Their Stimuli

Humans being have five main sensory organs, which are skin, nose, tongue, ears and eyes. Sense detects stimuli, which are changes in surroundings (around us). Five senses: sight hearing touch taste smell The ability of the sensory organs to detect stimuli is called senses. Organs that detect stimuli called sensory organs. Changes or object that can detect by our senses are called stimuli. All the sensory organs are complete with sensory receptors, which are the nerve endings that can detect stimuli.

StimuliSenseSensory organ

Light(Brightness, colour)SightEye

SoundHearingEar

Touch, heat, cold, pain, pressureTouchSkin

Sweet, sour, bitter, saltyTasteTongue

Smell or odorsSmellNose

The Pathway from Stimulus to ResponseStimulusReceptors (Sensory organ)NervesBrainNervesEffectors (such as muscles)Response (is used to describe nerve impulses)

Our sensory organs are used to detect stimuli (Singular: stimulus). A stimulus is anything that causes a reaction in a living organism, e.g. Pain, heat, sound, chemicals. When a stimulus is detected, receptor cells in the sensory organ will be triggered and generate impulses. These impulses will be transmitted through the nerve and then to the brain. The brain will further process and interpret the impulse signals as specific information than the organism has to respond to. This information is then sent back to the sensory organ through the nerve to provide responses.

1.2 Sense of Touch

The sense of touch is the sense that is sensitive towards the touch stimulus provided by specific objects. Organ involve in the detection of touch stimulus is the skin. Structure of human skin: The skin is the largest organ in our body. The skin divided into three layer epidermis, dermis, fat layer (subcutaneous layer @ adipose layer @ fat layer) Receptors are cell in our body that detected stimuli and turn them into electrical impulses called nerve impulse. Our body has different types of receptors. These are: pain receptor detects pain, very close to the skin and are most numerous touch receptor reacts to light touch heat receptor detects heat or any rise in temperature cold receptor detects cold and any fall in temperature pressure receptor senses strong pressure and locates deep within the skin, texture and weight of an object can be identified The pain receptor is in the epidermis. The touch, heat and cold receptors are in the dermis. The pressure receptors are in the fat layer. Each type of receptor can detect only one particular stimulus. The part or our body contain receptor are called sensory organsParts of the skinFunction

Epidermis and dermis Protect the body from physical injuries Prevent water loss (the skin is water proof) Prevent the entry of bacteria and germs Produce vitamin D

Adipose(fat tissue) Functions as a heat insulator To keep the body warm

Hair Controls body temperature by trapping a layer of air to insulate heat

Sweat glands Controls body temperature by the evaporation of sweat Secrete excrement such as water, urea and mineral salts

The degree of sensitivity of the skin depends on the: Thickness of the epidermis The thinner the epidermis, the more sensitive the skin is to the stimulus Number of receptors present The more receptors found on the skin the more sensitive is that part of the skin. The part of the body that have thin epidermis and many receptors sensitive to touch are lips fingertips behind the ears armpit the back of the neck The part of the body that are not so sensitive to touch are the: elbow knee palm of the hand and sole of the foot buttocks/hip The parts of the body usually chosen by the doctor for injection are along the upper arm and the buttocks. This is because these parts have thick epidermis and fewer receptors. Braille is a code made up of small raised dots on paper. Blind people use their fingertips to read Braille writing on books, signs, currency notes and elevator buttons.

1.3 THE SENSE OF SMELL

The nose is sensory organ for smell. Cells sensitive to smell (smell receptors) are found on the top of the nasal cavity. Structure of the human nose. The human nose has a cavity lined with epithelium tissue. On the surface of the epithelium tissue are the sensory cells known as olfactory cells. The surface of the nasal cavity is moist because of the mucus secreted by the cells of a gland. The nasal cavity has a pair of external opening (nostrils) which have hair to filter dust from the air inhaled through the nose. The nose detects smell in the following way:1. The inside of the nasal cavity produce mucus2. The chemical vapor entering nose during breathing dissolves in mucus. The dissolve chemicals stimulate the smell receptor at the top of the nasal cavity.3. The smell receptor then produce impulse which are sent through the nerves to the brain4. The brain interprets / evaluates the smell

Hot food, flowers and perfumes release chemicals into the airWhen chemicals in the air enter the nose, they dissolve in the mucous lining and stimulate the sensory cells.The sensory cells send out nerve impulses to the brainThe brain receives and interprets the impulses as smells

When we have a cold or flu, a lot of mucus is produce. The smell receptors are surrounded by this thick layer of mucus and very little of chemical vapor gets to the smell receptors. Therefore, the smell receptors do not get stimulated enough to effectively function as a sensory organ of smell. The sensitivity of the nose towards stimuli is influenced by the following factors: The strength of the smell. A stronger smell will be detected by the nose easily compared with a weaker smell. The presence of mucus in the nose. A lot of mucus will reduce the sensitivity of the nose.

1.4 THE SENSE OF TASTE

The tongue is the sensory organ for taste. The surface of the tongue has groups of cells known as taste buds which are sensitive to taste. There are four types of taste buds on the tongue sensitive to sweet, bitter, sour and salty. The taste buds contain many taste receptors. Different parts of the tongue are more sensitive towards a certain taste. The combination of different taste creates different taste sensations The tongue detects taste in the following way:When food is chewed, the chemicals in the food dissolve in the saliva The dissolved chemicals stimulate the taste receptors in the taste budsThe taste receptors send out impulses to the brainThe brain receives and interprets the impulses as taste

Both senses of taste and smell are needed when we taste food When we chew the food, some chemicals from the food enter our nasal passages to stimulate the smell receptors in the nose. When we eat, our tongue gives us the taste and our nose smell of the food. The sense of smell helps the sense of taste .This explains why food dose not taste appetizing when we have a cold or flu because we cannot smell effectively It is easier to drink an unpleasant tasting medicine with the nose pinched because without the sense of smell, the medicine will taste less awful.

1.5 THE SENSES OF HEARING

The ear is the sensory organ that is sensitive to sound stimuli produced by vibrating object. A human ear has three main parts. The outer ear, filled with air. The middle ear, filled with air. The inner ear, filled with liquid/ fluid Function of the different parts of the human ear.

Part of earStructureFunctions

OUTER EARPinna Made of cartilage and skins. Shape like a funnelCollects and directs sound waves into the ear canal

Ear canal/ auditory canalA narrow tube, lined with hairs and earwaxDirects sound waves to the eardrum

Eardrum A thin, stretched membraneVibrates when sound waves hit it

MIDDLE EAROssicles Three small bones Amplify vibrations by about 20 times and transfer them to the oval window

Oval windowA thin membrane attached to the last bone of ossiclesTransfers vibrations of the ossicles to the cochlea

Eustachian tube A narrow tube that connects the middle ear to the back of the throatBalances air pressure on both sides of the eardrum, preventing it from damage (not involved in the hearing mechanism)

INNER EARCochlea A coiled tube, filled with fluid. Contains receptorsDetects vibrations and converts them into nerve impulses

Auditory nerveNerve fibers that connect to the brainCarries nerve impulses to the brain

Semicircular canalsThree semicircular canals situated at right angles to each other Maintain the balance of the body (not involved in the hearing mechanism)

The hearing mechanism

The pinna collects the sound wavesThe sound waves enter the ear and travel through the ear canal towards the eardrum. The eardrum vibrates.The ossicles amplify the vibrations before transferring them to the oval window.The vibrations of the oval window cause the fluid inside the cochlea to move like wavesThe brain interprets the impulses as soundsThe nerve impulses produced by the receptors in the cochlea are carried along the auditory nerve to the brain

Sound wavesPinnaEar canalSound wavesEardrumSound wavesOssiclesVibrations

Oval windowVibrationsCochleaNerve impulsesAuditory nerveVibrationsNerve impulsesBrainSound can be heard

1.6 SENSE OF SIGHT

The eye is the sensory organ of sight that can is sensitive to light. The eyelids and eyelashes keep dust and dirt out of eyes. When we blink, the eyelids wipe tears over the eyes, keeping then moist and clean. Tears contain chemicals that kill bacteria. The layer of the human eye: the sclera (outermost) the choroids (middle) and the retina (innermost)

Structure of the eyeCharacteristicFunction

Sclera- Strong, hard and white layer- Protects the eye- Maintains the shape of the eye

Cornea- Curved transparent layer in front of the sclera- Refracts and focuses the light onto the retina

Choroid- Black pigmented middle layer of the eye- Contains many blood vessels- Absorbs light and prevents reflection of light in the eye- Supplies nutrients and oxygen to the eye

Conjunctiva- Thin transparent membrane in front of the cornea- Protects the frontal part of the cornea

Iris- Coloured part of the eye that is continuous with the choroid- Controls the size of the pupil

Pupil- Opening in the centre of iris- Allow light to enter the eye

Eye lens- Elastic biconvex transparent disc- Refracts and focuses light onto the retina

Ciliary body- Muscular extensions from the choroid- Changes the thickness of the lens through contractions and relaxations

Suspensory ligaments- Strong fibres that connect the ciliary muscles to the eye lens- Hold the eye lens in position

Aqueous humour- Transparent liquids between the cornea and the eye lens- Maintains the shape of the eye- Helps to focus light onto the retina

Vitreous humour- Thick, transparent jelly-like substances inside the eyeball- Maintains the shape of the eye- Refracts light onto the retina

Retina- Contains photoreceptors (such as cones and rods)- Detects light and produces nerve impulses

Optic Nerve- Nerve fibres that connects the retina to the brain- Carries nerve impulses from the retina to the brain

Yellow spot (fovea)- The most sensitive area of the retina- Located directly opposite the pupil- Detects light or images and changes them into nerve impulses

Blind spot- The spot where optic nerve exits the eyeball- Has no photoreceptors- Cannot detect any light or images that fall onto it

Changes in the size of the pupil under different situations The pupil in a normal situation

When a person moves from a bright area into a dark area The pupil enlarges More light enters the eye After a while the eye can see in the dark

When a person moves from a dark area into a bright area The pupil becomes smaller Less light enters the eye The eye is protected from excess light

MECHANISM OF SIGHT

Reflected light rays from an object enter the eye The lens converges and focuses the light rays onto the retina The cornea, aqueous and vitreous humour help to refract and focus the light onto the retinaA real, inverted and diminished/ smaller image is formed on the retinaThe nerve impulse produced by the photoreceptors is sent from the optic nerve to the brainThe brain interprets the impulses as an upright image of the actual size object

The lens in the human eye is a transparent convex lens. An inverted image is focused on the retina. Light rays are refracted by the eye lens. The eye lens focuses the image onto the retina by changing the thickness of the eye lens. The thickness of the lens is changed by the ciliary muscles. Focusing near objects: To focus near objects onto the retina, the ciliary muscles contract. The eye lens becomes thicker. Focusing distant object: To focus distant objects onto the retina, the ciliary muscles relax. The eye lens becomes thinner.

The pathway of light as it enters the eye:

Light rays from an objectCorneaAqueous humourLight raysPupilLens

Vitreous humourRetinaNerve impulsesOptic nerveNerve impulsesBrainAn upright object is seenLight raysLight raysLight raysLight rays

1.7 LIGHT & SIGHT Light is a form of energy. Light travels in straight lines which causes the formation of eclipses. Light cannot travel through opaque objects, thus shadows are formed. Light travels at a speed of ; 300,000,000 meter/second (m/s) in the air. 225,000,000 m/s in water 200,000,000 m/s in glass

Properties of lightReflectionRefractionA light ray bounces off the surface of an objectA light ray bends when it goes through media of different densities

REFLECTION OF LIGHT

When light hits an opaque object, it may be absorbed or reflected (thats mean change direction). Reflection of light occurs when light bounces off the surface of an object. When the reflected light rays enter our eyes, we can see the object. The amount and direction of the reflected light depend on the nature of the surface it hits. Object with very smooth surfaces such as mirrors, sheets of glass and polished metals are good reflectors of light. They reflect light in a regular pattern which called regular reflection. When light falls on irregular surfaces such as paper, cloth and wood, the reflected rays are scattered in all directions which called irregular or diffused reflection. If the light that hits a surface is reflected regularly, sharp images are produced. That is why we can see our image on very smooth surface. Diffused reflection of light enables us to see an object from any angle. A plane mirror reflects light regularly.

The image in a plane mirror is, virtual (cannot be formed on a screen) upright laterally inverted (the left-hand side of the object appears on the right-hand side of the image) the same size as the object at the same distance behind the mirror as the object is in front of the mirror. Applications of the reflection of light in daily life Plane mirrors are used at home and as rear-view mirrors in vehicles Convex mirrors produce a wider scope of view. They are used as security mirrors in shops, blind corner mirrors on roads and wing mirrors in vehicles. Concave mirrors produced magnified images. They are used as dental mirrors, cosmetics mirrors and in microscopes and telescopes. Periscopes are used in submarines to look above the surface while staying submerged. Two plane mirrors are used in a periscope to reflect and change the direction of light A kaleidoscope is a tube made up of mirrors with small pieces of coloured objects. The light from the objects is reflected from one mirror to another, producing beautiful multiple images.

REFRACTION OF LIGHT

Light travels through transparent media like air, water and glass. The speed of light changes when it moves from one medium to another with a different density which causes the light to be refracted (bent). Refraction of light is the bending of light. This happens when light travels from one transparent medium to another which is of a different density. The speed of light decrease when light enters a denser medium. The following shows the three situations of the movement of light rays through two different media.

The following examples show how light travels from one medium to another with differentdensities.

Daily phenomena of refraction of light are shown below:

VARIOUS DEFECT OF VISION

A person who suffers from the defects of vision has unfocused visions because the images are not formed exactly on the retina There are many causes of defects of vision, such as old age, heredity and diseases due to infections. Some defects of vision can be treated and some are permanent Other defects of vision include strabismus (misalignment of the eyes), keratoconus (cone-shaped cornea) and macular degeneration (blindness due to ageing) Comparison between long-sightedness and short-sightednessSimilarityCaused by the condition of the lens and shape of eyeball

Short-sightedness (myopia)DifferencesLong-sightedness (hypermetropia)

Sees near objects clearly but distant objects are blurredCondition of visionSees distant objects clearly but near objects are blurred

Eye lens is too thickEyeball is too longReason for the defectEye lens is too thinEyeball is too short

Light from distant object is focused in front of the retina

Focusing of lightLight from near object is focused behind the retina

Corrected by wearing glasses with concave lenses to diverge light before it enters the eyes.

CorrectionCorrected by wearing glasses with convex lenses to converge light before it enters the eyes.

Astigmatism is another vision defect. Its caused by the cornea and the eye lens that are not evenly curved. As a result, the image formed on the retina is distorted and not clear (hazy/blurred). This defect can be overcome by wearing glasses with cylindrical lenses or contact lens or by surgery.

Colour-blindness Colour-blind people cannot see some or all colours. Most of them cannot distinguish between red and green colours. Most sufferers are men. This defect is due to reduced number of cone cells on the retina or defects in them. It is inherited from the mother who is a carrier for the defect and it cannot be corrected. Presbyopia As we grow older, our eye lenses become less elastic. The ciliary body also loses its ability to contract and relax. As a result, images for near and distant objects are blurred. This defect is corrected by wearing glasses with bifocal lenses.

LIMITATIONS OF SIGHT Our sense of sight has its limitations. For example, we cannot see tiny objects such as bacteria and atoms with naked eyes. We also cannot see very distant objects such as far away planets and stars. And we cannot see through opaque objects. Optical illusion. Optical illusion is a phenomenon where the images that we see are different from reality. Optical illusions are caused by disturbances to the nerve impulses carried to the brain. The brain does not interpret accurately the information sent by the photoreceptors in the eye. This creates a false perception or impression on the objects we see.

Examples of optical illusion.

Blind spot The blind spot is a spot on the retina of the eye that cannot detect light stimulus. The image of the object formed at the blind spot cannot be seen by the eye because there are no light-sensitive cells (photoreceptors) at the blind spot. Close your left eye and stare at the cross mark in the diagram with your right eye. Now slowly move toward the diagram. Keep looking at the cross mark while you move. At a particular distance, the spot will disappear (it will reappear again if you move even closer). The spot disappears because it falls on the optic nerve head, the hole in the photoreceptor sheet.

MONOCULAR & STEREOSCOPIC VISION The stereoscopic vision is vision using both eyes. The overlapping area of the vision of both the left and right eyes is called stereoscopic field Advantages of stereoscopic vision. See three-dimensional pictures of objects. Enables more accurate estimation of distance and position. Animal predators normally have stereoscopic vision so better estimate the distance when getting to pounce on their prey. Monocular vision is a vision using only one eye. The advantage of monocular vision is having a wide vision field. Its disadvantage is that it cannot estimate distance accurately. Animal preys use monocular vision to detect predators coming from all direction.

DEVICES TO OVERCOME LIMITATION OF SIGHT

Optical devices are invented to increase the ability to see minute or very distant objects.

INSTRUMENTFUNCTION

MicroscopeTo see minute objects such as microorganisms.

TelescopeTo see far-off objects such as stars and planets.

BinocularTo see tiny distant objects such as birds in the sky.

Magnifying glassTo see small objects such as insects

PeriscopeIt is used in a submarine to see objects above the water level

X-ray machineTo see internal organs in the body or to see the contents in the passengers luggage at the airport

MRI (Magnetic Resonance Imaging) scannerTo see the bodys interior in order to diagnose medical conditions such as tumours, spinal or soft tissues injuries

Ultrasound scanning deviceTo see the conditions of the foetus in pregnant women

1.8 SOUND AND HEARING

PROPERTIES OF SOUND Sound is a form of energy. Sounds are produced by vibrations. Anything that vibrates in the air produces sounds. When an object vibrates, kinetic energy is changed into sound energy. Sound needs a medium to travel. A medium can be a solid, a liquid or a gas. Sound cannot travel through a vacuum. Sound can be transferred fastest in solids and slowest in gases. This is because the particles in solids and liquids are closer each other compared to the molecules in gases. Vacuum is a space that does not have any particles.

REFLECTION & ABSORBTION Sound can be reflected or absorbed by the surface of an object. The sound reflected repeatedly from one surface is known as echo. Surfaces that are smooth, even and hard are good sound reflectors and produce loud echo. For examples, concrete, plank, metal and mirror Surfaces that are rough, hollow and soft are good sound absorbers and produce weak echo. For examples cloth, sponge, cork, rubber, carpet and cushion. The following methods are used to reduces echoes in concert halls or cinemas: Hanging thick curtains Covering the floor with carpets Installing chairs with soft cushions Lining the walls with acoustic foam The application of the reflection of sound include: Determining the depth of the sea Determining the presence of fish under the sea (by fishermen) and the presence of enemy ships by a submarine (warfare) Determining distances by bats and dolphins to help in their movements.

DEFECTS OF HEARING The most common hearing defect is deafness. Deafness is a hearing defect that is caused by damage to some part of the ear, for example: Tearing of the eardrum Damage to the ossicles Damage to the auditory nerve Damage to the cochlea Damage to the hearing centre of the brain In some cases, the hearing defect can be corrected through surgery or by using a hearing aid. Damaged ossicles can be replaced and torn eardrums can be patched. Some cases of hearing defects cannot be remedied, for example damage to the auditory nerve, cochlea or hearing centre of the brain.

LIMITATION OF HEARING We cannot hear all the sounds around us. Voices that are very soft cannot be heard The range of frequencies of hearing in man is 20 Hz until 20 000 Hz. The following table shows the range of frequencies of hearing of several animals:

ANIMALRANGE of FREQUENCIES

Snake100-800Hz

Frog50-10 000Hz

dog10-50 000Hz

cat60-60 000Hz

bat1000-120 000Hz

grasshopper100-15 000Hz

whale10-50Hz

Different people have different limitations of hearing. For example, old people generally cannot hear as well as young people. Our ear drum become less sensitive to sound as we grow older. To overcome the limitations of hearing, we use The stethoscope Enables doctor to detect the soft heartbeats of patients. Hearing aids Collects sound signals before being sent to the middle ear. Amplifier Boosts weak sound signals.

STEREOPHONIC HEARING Stereophonic hearing is hearing using both ears. The advantages of stereophonic hearing:i. Enables the direction of the source of hearing to be detected more accurately.ii. This is because the ear nearer the source of sound receives sound louder and earlier than the other ear.iii. Animals that have stereophonic sound can detect the presence of preys and predators more quickly.

1.9 STIMULI AND RESPONSE IN PLANTS

Movements of any plant parts towards stimuli are known as tropic movement or tropism. Type of tropism: Growth of plant part towards an external stimulus is known as positive tropism. Growth away from an external stimulus is known as negative tropism. Tropism responses in plants include: Phototropism Geotropism Hydrotropism Thigmotropism move towards to obtain support Nastic movement response to external stimulus

Type of tropismStimulusPositiveNegative

PhototropismLightShootsRoots

GeotropismGravityRootsShoots

HydrotropismWaterRootsShoots

ThigmotropismTouchStems (tendrils)Roots

Nastic movement (seismonasty)Touch

Phototropism Def.: Respond of plant to light +ve phototropism Shoots grow towards light -ve phototropism Roots grow away from light +ve phototropism helps green plants to get light to make food by photosynthesis

Hydrotropism Respond to water by plants +ve hydrotropism Roots grow towards water -ve hydrotropismShoots grow away from water +ve hydrotropism helps the roots to get water Roots are attracted more by water than by gravity.

Geotropism Respond to gravity by plants -ve geotropism Shoots grow upwards against gravity +ve geotropism Roots grow downwards towards gravity +ve geotropism helps the roots to get water and to anchor the plants firmly into the soil for support

Thigmotropism Some plants with soft stems climb up a support when they touch it They climb by twining, by using tendrils or thorn/hooks This respond to touch by plants is called thigmotropism Thigmotropism provides additional support to plants to get sunlight for photosynthesis

Nastic movements A movement made by plants in response to stimuli that may come in any direction The leaves of the mimosa plant close up when they are touched A nastic movement response helps to protect the plant from injury Seismonasty in Venus flytraps helps the plants to get nutrition (food) for survival.