BIO(Sense Organs)

                Sense Organs

               We enjoy the beauty of nature with our

eyes, the melodious music with our ears,

the fragrance of flowers with our nose, the

taste of food with our tongue and feel the

cool breeze with our skin. What do we do

when suddenly bright light falls on our eyes

or a hot utensil is touched by chance? All

these situations show just how our senses

pick up information and react to them.

Our senses aren't just a part of us, they

define us. This is because nothing that we

experience in our life, from the most

important to the most boring, would be

possible without the intricate power of our

senses.

Nothing in the entire universe of

scientific exploration can even come close

to matching the ability of our brain to use

information sensed by our eyes, ears, skin,

tongue, and nose to produce a rich sensory

experience in a matter of milliseconds!

But how much do we know about

our senses?

What do our senses do?

Our senses have several roles to play.

They aid our survival by directing us

towards certain informations of our

environment that are important for us and

influence some activity (called as stimuli).

For example tasty food draw us towards

them and our mouth starts watering. Our

senses also help us to locate mates, seek

shelter, and recognize our friends.

Incidentally, our senses also give us the

opportunity to find pleasure in music, art,

athletics, etc.

There are yet other things that our

senses do. You may have experienced

feeling hurt to see someone in pain.

Usually when we have strong emotional

bonds with someone and when he or she

experiences pain, so do we( not just

emotional ties, we could be influenced by

situations not directly related to us and yet

feel the pain e.g. sympathising and feeling

pain of drought affected people).

How do our senses accomplish all this?

The complete answer is complex, but it

involves one elegantly simple idea that

applies across the sensory system. Our

sensory impressions of the world involve

nerve signals. These play a very important

role in the way we react or respond to

various stimuli or even to same stimuli in

different situations.

For example, flavour preference by our

brain is usually based on our body needs,

like cooked fish. may not smell good to some people.

But if the person is very hungry and has no

other option and particularly if the body has

a need for protein, fish may suddenly smell

good!

Stimuli from the environment around

are received by our body through sense

organs. As we already know, they are the

eyes, ears, nose, tongue and skin. Let's try

to understand the path of receiving a

stimulus and expressing a response

(sensation).

Stimulation to Sensation:

There are certain conditions,

substances etc., in nature that trigger the

process of sensing them by our body. These

are stimulants. Information carried by these

stimulants are picked up by certain organs

called as receptors present in our sense

organs and converted into nerve signals.

These are carried to the brain and processed

to create a sensation. For example, when

reflected light (stimulus) from the surface

of a green leaf and its surroundings reaches

receptors in our eyes, it is converted into

nerve signals. These signals reach the brain

and are interpreted as green coloured shape

against a background. We see this as the

leaf.

 Brain is the centre for all the sensory

activities. It receives information through

sensory nerves that bring nerve signals

from the sense organs and after

interpretation sends off signals through

another type of nerves called as motor

nerves to parts that respond

Activity-1:

Write a few lines about flower in your

note book.

Write about the stimuli and responses

and the sensory and motor functions with

respect to the sense organs involved.

Do you think our sense organs work

together? Why, why not? Give reasons.

All stimuli may not lead to responses.

Only a particular level of stimulus will give

rise to a response. Moreover changes in

stimulus also go unnoticed if they are not

of a particular level.

Activity-2:

Dissolve a pinch of sugar in a glass of

water. Drink a little of this. Does it taste

sugary? Why?

You could try this for different

concentrations of sugar by, adding a quarter

tea spoon full of sugar every time to find

out how much sugar needed to taste

maximum sweetness. This means sugar in

solution has stabilized your sensation.

You may have often noticed while

drinking tea or coffee that if you eat a very

sweet substance in between, your tea or

coffee appears to be less sweet as compared to the sips taken before eating the sweet (Thus, salty snacks go with tea or coffee!). This usually happens because a higher level of the same stimulus masks that of the lower level. 

Our Sense Organs

 As we all know, we have five main

sense organs, the eyes, ears, skin, nose

and tongue. These sense organs have

sensory receptors. Each type of

receptor is highly sensitive to specific

stimulus types.

1. Eye

Vision helps us detect desired targets,

threats, and changes in our physical

environment and to adapt accordingly. So,

how does the visual system accomplish

this? We shall do a few activities and read

the following section to find out about this

Activity-3:

1. Observe the external structure of

your friend's eye, draw the diagram

and label it (you can take the help

of diagram given in this section).

2. Observe the eye ball of your

friend in normal light. Then focus

a beam of torchlight on your

friend's eye.

What is his or her reaction? Why

is it so?

           Human Eye Cross-sectional view

Now ask your friend to keep the eye

closed for around two minutes. Now let

her/him open the eye. Observe the size of

the small black portion in the centre. Ask

your friend to keep her/his eye open

forcibly as you focus the beam of torch

light this time. Observe what happens to the

small dark portion.

What happened to the small dark

portion called the pupil? Guess why

Structure of the Eye

Our eye contains eye lids, eye lashes,

eyebrows and lacrimal glands. A thin layer,

called conjunctiva covers the front portion

of the eye. The eye ball is located in the

eye socket. Only 1/6 portion of the eye ball

is visible to us.

Eye has three main layers. They are

sclerotic layer or sclera, choroid layer and

retina. The outer most thick, tough, fibrous,

non-elastic and white coloured layer is

sclera. The sclera bulges and forms cornea.

The end of sclera connects to the optic

nerve. The second layer is choroid layer.

This layer is black in colour and contains a

lot of blood vessels. It encloses the eye

except the part pupil. The part formed by

the choroid layer around the pupil is iris.

Radial and circular muscles are present in

the iris. Biconvex Lens is present

immediately behind the pupil which is

attached to the ciliary muscles and

suspensory ligaments.

The lens divides the inner eye ball as

aqueous chamber and Vitreous chamber.

Aqueous chamber is filled with water like

fluid whereas vitreous chamber is filled

with jelly like fluid.

Retina contains the cells, called rods

and cones. The area of no vision, called

blind spot and the area of the best vision,

called yellow spot are present in the retina.

The yellow spot is also called Macula or

Fovea.

Functioning of the Eye:

The Visual Sensation

You might think of the eye as a sort of

“video camera” that the brain uses to make

motion pictures of the world. Like a

camera, the eye gathers light through a

convex lens, focuses it, and forms an image

in the retina at the back of the eye. The lens,

turns the image left to right and upside down

(you may have studied in the chapter on light

that we get an inverted /upside down image

through a convex lens). This visual reversal

may have influenced the very structure of

the brain, which tends to maintain this

reversal in its sensory processing regions.

Thus, most information from the sense

organs crosses over to the opposite side

of the brain. Likewise, “maps” of the body

in the brain’s sensory areas are typically

reversed and inverted. But while a digital

camera simply forms an electronic image,

the eye forms an image that gets extensive

further processing in the brain.

 The unique characteristic of the eye

that makes it different from other sense

organs, lies in its ability to take the

information from light waves, then

transform the characteristics of light into

neural signals that the brain can process.

This happens in the retina,

the lightsensitive layer of cells at the back of the

eye that acts much like the light-sensitive

chip in a digital camera. As with a camera,

things can go wrong. For example, the

lenses of those who are “nearsighted” focus

images short of (in front of) the retina; in

those who are “farsighted,”the focal point

extends behind the retina. Either way,

images are not sharp without corrective

lenses.

              Cones and rods

Making the fine distinctions necessary

for colour vision is the job of the nearly

seven million cones containing the pigment

iodopsin that come into play in brighter

light. Each cone is specialized to detect the

light waves we sense either as blue, red, or

absorb light energy and respond by creating

nerve impulses.

But why are there two sorts of

photoreceptors? Our eyes function

sometimes in near darkness and sometimes

in bright light. These two types of

processors involving distinct receptor cell

types named for their shapes have evolved

for this purpose.

Nearly 125 million tiny rods containing

the pigment rhodopsin “are present in our

eyes, they detect low intensities of light at

night, though they cannot make the fine

distinctions that give rise to our sensations

of colour.

Eye protection:

Each eye is protected by eyelids, eye

lashes, eye brows and lacrimal or tear

glands. A thin membrane covers the front

part of the eye. This membrane is called

conjunctiva. The conjunctiva is made up of

transparent epithelium. It is also a

protective cover to the eye. Whenever

unwanted substances come in contact with

this layer, the lacrimal glands are stimulated

to wash the substance out of the eye. The

fluids that are filled in the eyeball (vitreous

and aqueous chambers) protect the lens and

other parts of the eye from mechanical

shocks. Cornea is the clean window in the

sclera in front of the Iris. It protects the

eye from direct exposure to light.

Eye: Some structures that bring

about adjustments

The Iris is a muscular structure which

adjusts the size of the pupil which is nothing

but a gap between the iris in front of the

lens. Adjustments are made depending on

light intensity.

Cilliary muscles and suspensor

ligaments are capable of adjusting the focal

length of the eye lens.

Activity-5:

1. Observe the Iris and its surroundings of your friend’s eye.

    Can you find the pupil?

2. Observe the colours and patterns in the iris of your friend' s eyes.

     Is there any difference from one another? Select a minimum of           ten members and note the result. Use a hand lens for close                  observation. Record your observations in your notebook.

Activity-6:

1. Enter into a dark room from a very

    bright place. What happens?

2. Sit in a dark room for some time. Then

    go into a bright lit room. What happens

    discuss your experiences in the class

    room?

   Do you know that the impression of an

   image stays in the retina for about 1/16 of

   a second. If the still images of an object

   are flashed at the rate faster than 16 per

   second, the eye receives it as moving. That

   is how we see movies.

Diseases and defects of the eye:

The main diseases and defects of the eye are - Night blindness, Xeropthalmia, myopia (near sightedness), Hypermetropia

(far sightedness), glaucoma, cataract and colour blindness. Some persons may have eye defects by birth due to various reasons.

Ask your teacher about these eye defects and write one or two sentences for each in your notebook.

Ear:

Apart from hearing ear helps in maintaining the equilibrium of our body. Do you know

by which bone your ears made of? Observe the following picture to know the inner parts of

ear?

1. Outer ear (Pinna)

2. Auditory canal

3. Ear drum

4,5,6. Semicircular canals

7. Cochlea

8 . Vestibular nerve

9. Cochlear nerve

10. Eustachian tube

11. Ear Ossicles (Utriculus)

External ear:

It is the visible part of the ear on either

side of our head. It is a flap like structure,

called the Pinna. It leads to the ear canal.

The pinna is crumpled and made up of

cartilage.

Have you ever observed wax like

substance in your ear? Do you know

where does it comes from.?

Pinna has ceruminous (wax producing)

and sebaceous glands (oil producing).

These help to keep the ear cannal lubricated

prevent the dust and other particles from

entering into the ear canal. The ear canal

is also called Auditory Meatus. A thin layer,

called tympanum or ear drum is present at

the end of the auditory meatus. It is present

in between the external and middle ear. It

is in the shape of a cone. Its narrow area

connects to the first bone malleus of the

middle ear.

Middle ear:

Middle ear plays an important role in

amplifying the vibrations received on the

tympanum membrane. The chain of three

bones, Malleus, Incus and Stapes helps to

do the same. Oval window is a membrane,

covered ending of the middle ear which

opens into the inner ear through round

window.

Internal ear or Inner ear:

Internal ear consist of bony labyrinth

enclosing the membranous labyrinth. The

membranous labyrinth consists of

Vestibule, three semicircular canals and

cochlea. The anterior part of the vestibule

is Sacculus and the posterior part is

Utriculus. Nerve fibers from them form

vestibular nerve.

The semicircular canals are connected

to the vestibule and filled with endolymph.

Vestibule and semilunar circles together

form vestibular apparatus. It maintains the

equilibrium of the body, pertaining to the

posture and balance of the body.

Functions of the ear:

To collect and transform vibrations produced by sound to nerve

impulses to be carried to the brain for processing. To maintain balance or

equilibrium:

Ask your teacher in what way ears

maintain balance.

Nose:

Structure of nose

Our external nose has two nostrils.

They lead to the nasal cavity. Nasal septum

divides the nasal cavity into two halves. The

nasal cavity is lined with mucous membrane

and small hairs. Olfactory receptors are

present in the mucous membrane.

Smell and our Nose:

Smell serves a protective function by

sensing the odour of possibly dangerous

food or, for some animals, the scent of a

predator. We humans seem to use the sense

of smell primarily in connection with taste

to locate and identify foods, avoid spoiled

foods etc. Human beings use the sense of

smell in much limited manner as compared

to other animals.

The Smell or Olfactory sensation

Smell of certain flowers like Artabotrys

(Sampenga) and certain fruits like Jack fruit

(Panasa) are liked by some people but not

by others. How do you come to know

something smells good or bad?

Biologically, the sense of smell, or

olfaction, begins with chemical events in

the nose. There, odour (in the form of

airborne chemical molecules) interact with

receptor proteins associated with

specialized nerve cells. These cells,

incidentally, are the body’s only nerve cells

that come in direct contact with the outside

environment. Receptors present at the base

of the skin lining, the inner walls of the

nose, are highly sensitive to odour

chemicals. These odour chemicals can be

complex and varied. For example, freshly

brewed coffee owes its scent to as many

as 600 volatile compounds (substances that

reach gaseous state quickly as they have low

boiling points.)

Tongue

Structure of the tongue

Our tongue is made up of voluntary

muscles. It contains about 10 thousand

taste buds. The taste buds are located in the

walls of the papillae.

Taste and our tongue

Like smell, taste is also a sense based

on identifying chemicals in food and the

texture of it. But the similarity doesn’t

end there: The senses of taste and smell

have a close and cooperative working

relationship. So many of the subtle

distinctions you may think of as flavours

really come from odours. (Much of the

“taste” of an onion is odour, not flavour.

And when you have a cold, you’ll notice that

food seems tasteless because your nasal

passages are blocked.)

Skin:

The sense of touch had received

supreme importance in the sphere of

senses from ancient time. The organ

involved is our skin

Skin and touch:

Skin is the outer most covering of our body. It regulates the body temperature and eliminates certain waste material through

sweat. It is the sense organ of touch. The sense of touch is done by the cutaneous receptors. It is the largest organ of all. It provides the first level of protection to the body.