Sense Organs Class 10 ICSE Notes
ICSE Class 10 Biology Chapter 11 Sense Organs Notes, Summary, Definition, Diagram. Sense Organs Notes.
Introduction
The sense organ enables us to be aware of the conditions of our external as well as internal environment.
→ Major sense organs in our body – eyes, ears, tongue, nose and skin which are sensitive to light, sound, taste, smell and touch respectively.
→ Senses of balance, body movements, hunger, thirst and pain, etc.
→ Receptors- (sensory cells) – specialised tissue or cell sensitive to a specific stimulus.
i.) Mechanoreceptors – are receptors for touch, pressure of skin due to mechanical change.
ii.) Chemoreceptor – receptors of taste of the tongue and smell of the nose due to chemical influence.
iii.) Photoreceptors – are rods and cones of the retina of eye due to light.
iv.) Thermoreceptors – are heat and cold receptors in skin, due to change in temperature.
v.) Phonoreceptors – receptors for sound/hearing.
1.) The Eyes
a.) Orbits –
Eyes are located in deep socket or orbits.
Each eye is in the form of a ball and can be rotated with the help of six muscles.
b.) Eyelids –
Upper and lower movable eyelids which protect the outer surfaceof eyes.
Each eyelids carries outwardly curved eyelashes which prevent falling of large particles into the eyes.
c.) Eyebrows –
Protective, prevent rain drops into eyes.
d.) Tear glands (lacrimal glands) –
Located upper sideward portion of the orbit.
Six to twelve ducts of gland pour the secretion over the front structure.
Blinking spread the liquid which mainly serves as a lubricant.
Tears keep eye clean by washing away dust particles.
Have antiseptic property due to enzyme lysozyme (kills germs)
e.) Tear ducts –
Drain off liquid into sac .
A nasolacrimal duct conducts the secretion into nasal cavity.
Due to irritation and certain emotions tear glads waters the eyes/tears overflows.
Function of tears –
Lubricate the surface, wash away dust particles, help killing germs, and communicate emotions.
f.) Conjunctiva –
Thin membrane covering the entire front part of eye
Over cornea, it reduced into single layer of transparent epithelium.
Conjunctivitis – outermost layer turns red due to a viral infection.
Structure of eyeball
Composed of 3 concentric layers –
a.) Outer sclerotic layer –
– Made of tough fibrous tissues and white in colour.
– White portion is sclerotic layer visible through conjunctiva.
– Cornea – transparent portion of eye which covers the front region .
– Defective cornea ( opaque or non-functional) can be replaced by a healthy cornea from donated eye.
b.) Middle Choroid layer –
– Richly supplied with blood vessels
– Contain dark blag pigment (melanin) which prevent light rays from reflecting and scattering inside eye
– It expands to form ciliary body
– The smooth muscle in the ciliary body alters the shape of lens.
– Iris also an extension of choroid, partially covering the lens and leaving a circular opening in centre, pupil.
– Iris colour – blue, brown, black, etc.
– Iris contains radial muscles to widen and circular muscles to constrict the pupil.
– The adjustment of the size of pupil regulates the amount of light entering the eye.
c.) Inner Retina –
– Sensitive to light
– Two types of sense cells called rods and cones.
I.) rod cells (inner ends rods-like)
– Sensitive to dim light but do not respond to colour.
– Contain the pigment rhodopsin or visual purple.
– Rod cells are distributed throughout retina.
II.) Cone (inner ends conical)
– Sensitive to bright light and are responsible for colours.
– Contain pigments iodopsin or visual violet.
– The cone cells are confined to the yellow spot.
Yellow Spot – the area of best vision
→ Macula lutea or yellow spot lies at back of eye almost at centre on the horizontal axis of eyeball.
→ contain maximum number of sensory cells and particularly the cones.
→ Region of brightest vision and also colour vision.
Blind Spot – the area of no vision
→ Lateral to yellow spot on nasal side
→ No sensory cells. So, no vision
→ Nerve fibres from all the sensory cells of the retina coverage and bundle together to leave the eyeball in form of optic nerve.
Lens –
→ Transparent, flexible, biconvex crystalline body
→ Found just behind pupil
→ Contains transparent lens fibres
→ Suspendary ligament attaches to ciliary body
→ Ciliary body contains muscles which contract and relax, change the shape of lens for viewing objects at different distances.
Two chambers of the eye –
I.) Aqueous chamber –
→ Between lens and cornea
→ Filled with clear watery liquid (aqueous humour)
– Keeps the lens moist and protects it from physical shock
– Refracts light.
II.) Vitreous chamber –
→ Large cavity of eyeball behind the lens
→ Filled with transparent jelly like thicker fluid (vitreous humour)
– Helps in keeping the shape of eyeballs
– Protects the retina and its nerve endings
How do we see?
1) Entry of light rays – (conjunctiva, cornea, aqueous humour, lens, vitreous humour)
2) Focusing of image- the image in the retina is real and inverted
3) Transmission of nerve Impulse from retina to brain (cerebrum)
4) Interpretation by the brain
Accommodation – the process of focusing the eye to see objects at different distance
For distant vision, the lens is more flattened or thinner
For near vision, the lens becomes more convex or rounded.
→ In normal condition (ciliary relaxed), the lens remains stretched by the suspensory ligaments and it is less convex, suited for viewing distant objects.
→ When we look somewhere (ciliary contracts),pull ciliary body slightly forward. This releases suspensory ligament making it loose and then lens, on account of its elasticity, becomes thicker and more rounded or convex.
Light and dark adaptation –
→ When a person moves from bright light area too dark light, he suffers difficulties to see, but his vision gets improved in awhile. This adaption is called dark adaption.
Changes happens due to regeneration of visual purple of rhodopsin, the pigment of rods, which was broken before due to bright light.
→ When a person moves from dark place to bright light area they experience dazzling effect and eventually get better this adaption is called light adaption. This is due to the visual purple of rods is bleached, reducing their sensitivity and pupil constricts (gets narrow)
Colour vision –
→ Only possible through cones of retina stimulated only by bright light.
Common defects of eye
i.) Near or short-sightedness (myopia) –
→ Near objects can be seen clearly while distant objects appear blurred.
→ Image of distant objects formed in front of retina.
Reasons –
a.) Eye ball lengthened from front to back
b.) Lens is too curved
Correction –
– By suitable concave(diverging) lens
– Power of glasses is mentioned in minus(-)
ii.) Far or long-sightedness (hyperopia/hypermetropia) –
→ Difficulty in seeing near objects
→ Image of near object falls behind the retina
Reason-
a.) Shortening of eyeball from front to back
b.) Lens is too flat
Correction –
– By suitable Convex (converging)lens
– Power of glasses is mentioned in plus (+)
iii.) Astigmatism –
→ Some parts of the object are seen in focus while other blurred
Reason –
a.) Due to uneven curvature of cornea
Corrected –
– Cylindrical lenses
iv.) Presbyopia – (condition affecting older people)
→ Who cannot see near objects clearly.
Reason –
a.) Their lens loses flexibility resulting in a kind of far-sightedness
Corrected –
– By convex lens
v.) Cataract –
→ Lens turns opaque
→ Vision is cut down even to total blindness.
Corrected
– Surgically removing the lens
– Using spectacles with highly convex lenses
– Compensating for missing lens
– Small plastic lens is implanted behind or in front of iris.
vi.) Night-blindness –
→ Person feel difficulty in seeing in dim light as during the night.
Reason –
a.) Due to non-formation of the pigment visual purple of rods.
b.) Only rods function in dim light.
c.) Due to deficiency of vitamin A
Correction –
– By eating vitamin A rich food
vii.) Colour blindness –
→ Some people by birth cannot discriminate between certain colours such as red and green.
Reason –
a.) Due to genetic defect
b.) Mostly males suffer this defect, rarely in females.
viii. Corneal opacities –
→ Cornea of some patients gets scarred and turns opaque (white) and non-functonal.
Reasons –
Can cause anything from minor irritation to vision problems and even blindness.
Corrections –
– Defective cornea can be replaced by healthy cornea from a donated eye.
- ) Squint –
→ Two eyes somehow converge leading to “cross eye”.
→ And opposite condition happens when they diverge “wide eye”
→ Both condition may cause double vision or diplopia.
Correction –
– Surgery and suitable exercise can correct
Stereoscopic (binocular) vision –
Simultaneously focusing of an object in both eyes, and their images by a kind of “overlapping” in the brain gives the 3-D effect.
After-image – the basis of motion pictures
→ When you look at a bright object and closes eyes or when you look at brightly coloured object and then a dark object in the same colour, the sensation of light persists for a short period.
This is known as persistence image or after-image.
→ It lasts for about one-tenth of a second. This principle is based on technique of motion picture is based.
THE EAR – organ for hearing and body balance
Main divisions –
1) Outer ear – consists of projecting part pinna and passage auditorial canal leading to ear drum.
2) Middle ear – contains 3 tiny bones= malleus, incus and stapes or hammer , anvil and stirrup
– Eustachian tube connects the cavity of middle ear with throat.
– 3 bones collectively called ear ossicles
– Handle of hammer bone attached to inner surface of eardrum.
– Opposite end is connected to anvil and joined to stirrup.
– Flat part of stirrup called oval window, a membrane – covere opening leading to inner ear.
– Round window also covered by thin membrane connects the middle and inner ear.
3) Inner ear or membranous labyrinth –
– 3 parts = cochlea , semicircular canals and vestibule
– Cochlea is spiral-shapped and looks likea snail shell.
– Has 2 and have turns.
– Endolymph
– Perilymph
– Middle canal contains areas possessing sensory cells, spiral organ called organ of Corti for hearing.
– The sensory cells lie on the basilar membrane.
– A set of 3 semi-circular canals which are arranged at right angles to each other in different planes so that one is horizontal and other 2 vertical.
– Ampulla wich contain sensory cells for dynamic balance while the body is in motion.
– Vestibule= utriculus and succulus
These parts also contain sensory cells for static balance when the body is stationary as in standing.
Functions of Ear-
A.) Hearing –
– pinna collect sound waves and conducts them through the external auditory canal and then finally strike on ear drum which is set into vibration.
– Eustachian tube equalises air pressure on either side of ear drum allowing it to vibrate freely.
– Vibrating ear drum sets 3 ossicles into vibration
– Vibration of last ossicle (stirrup) is amplified due to lever-like action of first 2 ossicles.
– Vibrating stirrup transmits the vibration to membrane of the oval window which make the fluid in cochlear canal also vibrate.
– Vibrating movements of fluid stimulate the hair-like processes of sensory cells of the cochlea and impulses are transmitted to brain via auditory nerve.
B.) Balance
– The moving fluid in the canals pushes against sensory hair cells sending the nerve impulse through the nerve fibres attached to them, to the brain via the auditory nerve.
– Sensory cells in semicircular canals are concerned with dynamic equilibrium (where the body is moving).
– Utriculus and succulus helps in static balance i.e, positional balance with respect to gravity.
Glossary –
→ Life-like continuous movement on screen is an illusion. And felt like real because of after-images.
→ Our sensory endings can receive only those from 20 to 20000Hz.
→ But dogs can perceive sounds of even higher frequencies.
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