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Chapter 1 The World Through OurSenses

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Our sense organs detect the stimuli and we respond tothem for survival.

Plants also respond to stimuli for their survival.

Human being have five sense organs:-

Sense Organ Sense Stimuli

Skin Touch Pressure, pain, heat, cold

Nose Smell Chemicals

Tongue Taste Chemicals

Ears Hearing Sound

Eyes Sight Light

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The senses organs only receive stimuli. The brain and spinal cord interpret thestimuli received.

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When a stimulus is detected by a sense organ, information (impulse) is sentto the brain by nerves. Impulses travel along sensory nerves to the spinal

cord.

Brain is connected to all parts of the body by nerves. Brain will interpret theimpulses and suitable response will be produced.

Stimulus

The messages that have been interpreted sent along the motor nerves to themuscles or organs.

The muscles or organs will produce suitable responses.

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Skin is the sense organ of touch

Skin responds to the stimuli ofi. Touchii. Painiii. Pressureiv. Cold

v. Heat

Skin consists of two layers :-1. Epidermis outer layer; made up of dead cells &

waterproof; prevents bacteria entering into body tissue2. Dermis inner layer; consists of living cells, blood

capillaries, nerves, sweat glands

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Different parts of the body have different degrees ofsensitivity

Fingertips, lips, tongue, nose & neck = more sensitive

Legs, elbows, soles of feet, back of body = less sensitive

Sensitivity of skin depends ona) Thickness of epidermisb) Number of receptors present

Blind people use the high sensitivity of their fingertipsto read in Braille.

Doctors give injection at the forearms or hips (lesssensitivity to pain).

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Nose is the sense organ of smell

Smell receptors are sensitive to chemical in the air.

Smell receptors are located in the epithelium in theupper nasal cavity.

Glands in the nose secrete mucus.

The mucus keeps the smell receptors moist.

When a person has a cold or flu, a lot of mucus is

produced. The smell receptors are surrounded by thethick layer of mucus and very little of chemical gets tothe receptors. The receptors do not get stimulatedenough to function effectively.

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The inside lining of nasalcavity produces mucus

When air is breathed in, thechemical passes through

nasal cavity dissolves mucus.

The dissolved chemical

stimulates the smellreceptors.

The smell receptors produceimpulses and sent througholfactory nerves to brain

The brain interpret the smell

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Tongue is the sense organ of taste.

Tongue covered with little bumps called taste buds.

Each taste buds contains taste receptors.

Each taste receptor is sensitive to one type of tasteonly.

Four type of taste receptors:-a) Sweet receptors

b) Sour receptors

c) Salty receptors

d) Bitter receptors

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The chemical substances infood/drink dissolve in the

saliva & stimulate tastereceptors in the taste buds

The taste receptors sendimpulses along the sensory

nerves to brain

The brain interpret theimpulses received and

identifies the taste

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Ear is the sense organ of hearing and sensitive tosound stimuli.

Human ear is divided into three parts:-a) Outer ear filled with air

b) Middle ear filled with air

c) Inner ear filled with liquid

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Ossicles

Ovalwindow

Auditory

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Structure Function

OuterEar

Pinna Shape like a funnel and consists of cartilage and skin.Collect sound waves and directs them into the auditorycanal.

Auditory Canal Narrowtube that connects the pinna to the eardrum.Send sound waves to the eardrum (tympanum).

MiddleEar

Eardrum Thin, stretched membrane at the end of the auditory canal.Separates the middle ear from the outer ear.Vibrates at the same frequency as the sound waves thatreach it and transfers the vibrations to the ossicles.

Ossicles Three tiny bones (stirrups, anvil, hammer) that connect theeardrum to the oval window.Amplify the vibrations and transfer them from the eardrum

to the oval window.Oval window Thin membrane at the end of the ossicles, separates the

middle ear from the inner earTransfers the vibration from ossicles to inner ear.

Eustachian tube Narrow tube that connects the middle ear to the back of thethroatBalances the air pressure on both sides of the eardrum.

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Structure Function

InnerEar

Cochlea Spiral-shaped tube that is filled with liquid and lined withsensory receptors.Detects sound vibrations and changes them into nerveimpulses.

Auditory nerves Nerves that connectthe cochlea to the brain to beinterpreted.

Semicircularcanals

Made up of three looped canals that are arranged at right

angles to each other, contain liquid and sensory cells.Help the body to maintain its balance.

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Sound waves Pinna Auditory canal Eardrum

OssiclesOval windowCochleaAuditory nerve

Brain

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Eyes are the sense organ of hearing and respond tolight stimulus.

Human eyes are spherical and located inside sockets inthe skull.

The tear glands produce tears to clean the eyes and killmicroorganisms.

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Object reflects

lightCornea

Aqueous

humourPupil

LensVitreoushumour

RetinaOptic nerve

Brain

The flow chart above shows the route of light rays from an object entering the eye.The image formed on the retina is :

oRealoInverted (but the brain interprets the image as upright)oSmaller than the object (diminished).

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Stereoscopic Vision Monocular Vision

Both eyes at the front of the head. One eye at each side of the head.

Visual fields of the eyes overlap a greatdeal. Images in the overlapped area are

seen in three dimensions.

Visual fields of the eyes do not overlapor overlap only a little. So, most images

are in two dimensions.

Judgement of distance is accurate. Judgementof distance is less accurate.

It has a smaller field of vision. It has a wider field of vision, enablingone to detect its enemies easily.

Most predators have this type of vision. Most preys have this type of vision.

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Defects of the eye occur when the images of objects arenot focused on the retina.

The usual eye defects are :-1. Short-sightedness (myopia)

2. Long-sightedness (presbyopia)

3. astigmatism

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A short-sightedness person can see near objects clearlybut distant objects appear blurred.

The image of a distant object is focused in front of theretina and a blurred image is seen.

The defect may be caused bya) The eyeball being too long

b) The eye lens being too thick The defect can be corrected by wearing glasses

(spectacles) with concave (diverging) lenses.

The concave lenses diverge the light rays before theyenter the eyes and then the light is focused on theretina to form a sharp image.

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A long-sightedness person can see distance objects clearlybut near objects appear blurred.

Long-sightedness occurs when the image of a near object isfocused behind the retina and image seens is blurred.

The defect may be caused bya) The eyeball being too short

b) The eye lens being too thin

The defect can be corrected by wearing glasses(spectacles) with convex(converging) lenses.

The convex lenses converge the light rays before theyenter the eyes and then the light is focused on the retinato form a sharp image.

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The defect is caused by the cornea and the eye lensthat are not evenly curved.

As a result, all the light rays from an object do notfocus at a point on the retina.

Some light rays are focused on the retina while others

are focused either in front of or behind the retina. The image formed on the retina is blurred.

Astigmatism can be corrected by wearing glasses withcylindrical lenses.

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Our sense of sight has its limitations:i. We cannot see objects that are very tiny with our naked

eyesii. We cannot see objects that are very far away (stars,

planets)iii. Optical illusions

Sometimes what we see may not appear to be the realthings. This is because our brain cannot interpretaccurately the information sent by the receptors in theeyes.

Caused by disturbances to the impulses to the brain.

The disturbances maybe due to other objects orconditions around the object.

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Reflection of Light

Light rays are reflected by an opaque surface. Lightcannot pass thru opaque objects.

Transparent objects allow light to pass thru them.

Object with very smooth surfaces, such as mirrors aregood reflectors of light = regular reflection

When light falls on irregular surfaces such as paper,the reflected rays are scattered in all directions =

irregular reflection/ diffused reflection

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Laws of Reflection of Light

The incident ray, the reflected ray and the normal at thepoint of incidence are at the same plane.The angle of incidence = angle of ref lection

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The image of an object formed by a plane mirror is:-

a) Virtual (cannot be formed on a screen)b) Upright

c) The same size as the object

d) Laterally inverted

e) The same distance behind the mirror as the object is infront

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Refraction of Light

Take places when light travels through two transparentmedia with different densities at an angle.

Occurs due to speed of light changes when light travelsfrom a medium to another different density.

Light travels slower when it enter a denser medium.Light travels faster when it enter a medium which is

less dense.

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air

air

glass

glass

air

airglass

glass

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The light bends towards thenormal when it travels from airto glass

The light bends away from thenormal when it travels from

glass to air

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Sound is a form of energy.

Sound is produced by vibrations.

When an object vibrates, its kinetic energy is changedto sound energy.

The transmission of sound requires a medium.

Sound travels thru solids, liquids and gases.

Sound travel fastest thru solids, slowest thru gases.

Sound cannot travel thru a vacuum. No particles invacuum to transmit vibrations.

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Sound can be reflected or absorbed when it reaches a surface.

Surfaces which are hard and smooth reflect sound effectively.(walls, tunnels, glass = good sound reflector)

Surfaces which are soft and rough absorb sound. They are goodabsorbers but poor reflector of sound. (carpets, curtains, softboards)

Echoes = sound reflected from sound Application of reflection of sound :-

a) Determining the depth of seab) Determining the presence of fish or enemys ship (a submarine)

Application of absorption of sound :-a) Halls and big rooms have soft boards on walls, carpets & curtains

to absorb sound and reduce echoesb) Cushion and soft padding in cars absorb the sound from engines.

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Range of audio frequencies of human being is 20 Hz to20000 Hz.

The range of frequencies of man varies according to ageof a person

20000 Hz = ultrasound

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Stereophonic hearing is hearing using both ears.

Enable us to determine the direction of the source of thesound more accurately.

The ear nearer the source of the sound receives the soundearlier and louder than the other ear.

Animals that have stereophonic hearing can detect thepresence of predators more quickly.

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Very soft sounds cannot be heard clearly.

Hearing aids used to amplify weak sounds.

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Stethoscope amplifiesthe heartbeat or thepulse.

The response by plants to a stimulus is called tropism.

Tropism is the growth and movement of a part of aplant towards or away from a certain stimulus.

The parts of a plant which respond to stimuli areshoots, leaves & roots.

Responses in plants :-a) Tropisms Phototropism, geotropism, hydrotropism,

thigmotropism (response towards the stimulus)

b) Nastic movement Seismonastic (response away fromstimulus)

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