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“Why did the driver apply brakes whenhe saw just a wild cat?”, responded astudent while reading the news.

Don't you too make sudden responses tounexpected situations? Enlist such

situations.

� The sudden withdrawal of legs whilestepping on fire accidentally.

�

You have already learnt about consciousresponses to the changes in thesurroundings. But responses need notalways be conscious. Analyse the sciencearticle given below and list out, in order,the activities that take place in the bodywhen sudden responses are made.Arrange the enlisted data as a flow chartin your Science diary.

Reflexes - What? How?

Reflex actions are sudden, involuntary reactions of the body in response to stimuli. Theyoccur without the involvement of the conscious areas of the brain. Therefore, all reflexactions are unconscious actions. Reflex actions occur both from the brain and spinalcord which are the parts of the central nervous system.

BIG TRAGEDY AVERTED

July15. The Neelimala road, an accident-prone area, with hairpin curves and steep slopes, is again in the news. This time theaccident occurred when a wild cat jumped in front of a bus fullof passengers. Seeing the cat unexpectedly before him, thedriver applied the brake suddenly. The bus ran out of controland was about to fall into a precipice when it hit against a mud-wall. A great disaster was thus averted.

Fig - 2.1. Spinal cord

central canal white matterdorsal root

ventral root grey matter

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BIOLOGY

However, these actions are mainly formed from the spinal cord. The vertebral columnprotects the spinal cord just as the skull protects the brain. Like the brain, the spinal cordtoo has a covering of meninges. The central canal and the space between the layers ofmeninges are filled with the cerebrospinal fluid. When sudden responses are required,instructions are given by the spinal cord itself. Impulses reach the brain only after that.Subsequently conscious responses are made. Reflex arc is the path of impulses in thereflex action.

Reflexes formed from the brain are called cerebral reflexes. Blinking of the eye is acerebral reflex.

Reflexes may also be developed through constant practice and experience. Theapplication of sudden brakes by the driver and salivation at the thought of tasty food areexamples. Such responses are called conditioned reflexes.

Fig. 2.2. Reflec arc

The hand iswithdrawn by theaction of the muscle

The fingertipunknowinglytouches a thorn.The receptorsthere becomeexcited.

Sensory neuroncarries impulses tothe spinal cord.

Motor neuron

carries the messagesfrom the spinal cord tothe effector muscles.

Inter neuron

connects the sensoryneuron and the motorneuron.

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BIOLOGY

List the changes in the physiologicalactivities of the boy running away fromthe dog.

� Rate of heart beat increases� Breathing rate increases�

What might be the changes in the bodyafter he finds relief?

.....................................................................

Do you know why such changes take placein the body?

In critical situations, reflex actions serve as a defence mechanism also. In such situations,the nervous system as a whole becomes more alert. As a result physiological activitiesmay also change.

Look at the following cartoon

The Mastermind of Unconscious Activities

Our nervous system can be divided into two. The Central Nervous System (CNS) andthe Peripheral Nervous System (PNS). CNS consists of the brain and the spinal cord.Conscious decisions required for physiological activities are mainly formed in the brain.However, in emergency situations immediate decisions are taken by the spinal cord.Nerves connecting the central nervous system to the organs of the body constitute theperipheral nervous system.

There are many activities which occur beyond our conscious area. Such activities arecontrolled by the Autonomous Nervous System (ANS) which is a part of the peripheralnervous system. The autonomous nervous system is composed of the SympatheticSystem and the Parasympathetic System. The sympathetic and parasympatheticsystems supply nerve fibres to most of the internal organs.

Observe the Figure, compare the functions of the sympathetic and parasympatheticsystems and prepare notes. Complete Table.2.1.

Vow....!

Alas!

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BIOLOGY

Haven't you understood that the sympathetic system equips the body to face criticalsituations? After overcoming the crisis, the parasympathetic system bringsphysiological activities back to normal state.

We have learnt the role of the nervous system and the functions of its different parts insensing the changes taking place in the surroundings and responding to themaccordingly. But changes take place every moment not only in the external environmentbut in the internal environment too. All these are internal stimuli. Some of them aregiven below.

Actions of the sympathetic system Actions of the parasympatheticwhen the boy runs away from the dog system when the boy finds relief.

Table-2.1

Secretion of salivadecreases

Tracheadilates

Functions of theStomach slow

down

Hormone secretionincreases

Peristalsis slowsdown

Urinary bladderregains the original

relaxed state

Rate of heart beatdecreases

Parasympathetic systemSympathetic system

Fig. - 2.3 Autonomous nervous system-Actions

Pupil dilates Pupil constricts

Secretion of salivaincreases

Rate of heartbeat increases

Trachea constricts

Functions of thestomach is stimulated

Hormone secretiondecreases

Peristalsis is enhanced

Urinary bladdercontracts

spinal cord

Sympathetic ganglia

Glycogen is convertedinto glucose

Glucose is convertedinto glycogen; secretionof bile is enhanced

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BIOLOGY

� Variation in blood pressure� Change in water content� Change in CO2 levelReceptors capable of perceiving suchstimuli are present in the internal organsand muscles. For example, chemoreceptorsand baroreceptors. Excitation of thesereceptors leads to the generation ofimpulses which in turn will cause thecorresponding organs to act accordingly.

So far we have learned about the role ofthe nervous system in perceiving stimulifrom the surroundings and from withinthe body, and making responses. How dothe different parts of the nervous systemact in a co-ordinated manner? Analyze thefollowing illustration based on theindicators, and complete the flow chartgiven below.

Indicators

« What are the parts of the central nervous system?« What are the parts included in the peripheral nervous system?« How does the brain communicate with organs?« How does the spinal cord communicate with organs?

Illustration-2.1

Brain

Organs

Cranial nerves (12 pairs)

Spinal nerves (31pairs)

Spinal cord

Cen

tral

Ner

vous

Syst

em

Peri

pher

alN

ervo

us S

yste

m

Nervous system

Peripheralnervous system

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BIOLOGY

The health of the nervous system isessential for mental and physical well-being. The health of the nervous systemmay get disrupted due to several reasons.You know about diseases like stroke andparalysis. But there are many otherconditions that also adversely affect thenervous system.

Alzheimer diseaseThis disease is caused by a continuousdegeneration of neurons in the brain. Ithappens due to the accumulation of aninsoluble protein in the form of plaque innervous tissues. The chief symptom of thedisease is the complete loss of memory.Even the memory regarding the day, dateand details about oneself like where oneis, who one's family members are, etc. arelost. As a result, the working of the mindtopples. The patient becomes unaware ofhis action or his behaviour towards others.Gradually this leads to death. Even thoughthis disease is more common among agedpeople, it may also affect people below60. In this condition the patient needs thepresence and attention of others. Let'shope that medical science will soon beable to conquer this disease which is athreat to individual health andcommunity health as a whole.

Parkinson diseaseThis disease is caused by thedegeneration of specific ganglions in thebrain. It results from the deficiency of aneurotransmitter called dopamine. Thesymptoms of this disease include tremordue to the irregular movement ofinvoluntary muscles, uncontrolled flow ofsaliva and the loss of balance of the body

due to the defective co-ordination ofmuscular activities. Patients develop anaversion to life and also depression.Though treatment using dopamine ispossible a complete cure can not beexpected.

EpilepsyThe disease is caused by the irregularityin the electric impulse in the brain. Fits dueto frequent and uncontrolled muscularcontractions and unconsciousness are thesymptoms. During the fits that may last forseconds or minutes, clenching of the teeth,biting of the tongue and frothy dischargefrom the mouth are common. Today,epilepsy is treated effectively withmedicines. However, medicines are to betaken regularly.

A complete cure through treatment is notpossible for several diseases affecting thenervous system. Those who nurse suchpatients are to be extremely careful andconsiderate. They must develop anattitude of mind that will enable them tolook after the patients patiently, takinginto account their helpless and patheticcondition. Only then we can attainhumaneness, the noble quality that ahigher state of brain entitles.

Discuss how we should behave withpeople who suffer the pangs of variousdiseases. Prepare a note and record it inthe Science diary.

The mechanisms to act according to thechanges that take place every moment inthe surroundings and within the body areseen in other animals too. Analyse thegiven descriptions and form inferencesabout them.

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BIOLOGY

In Unicellular Organisms

In unicellular organisms, plasma membrane and cytoplasmplay a major role in perceiving stimuli and emitingresponses. Chlamydomonas and euglena have specificparts known as eyespot to detect the presence of light.

Seeing through sound too

Bats are not blind. They move during night not by seeing butsensing the obstacles in their way. They produce soundwaves of high frequency. Since these sound waves cannotbe heard by us they are called ultrasonic sound. These wavesstrike the objects ahead and bounce back. Analysing thisecho, the bat can interpret the exact position, distance, natureetc. of the object.

Eyes - how many?

The eye of insects is made up of thousands of minute eyescalled ommatidia. On the outer surface of eachommatidium, there is a lens with six sides. Behind it isfound a transparent conical structure below whichphotosensitive (light sensitive) cells are seen. The imageformed in the eyes of insects is erect. Impulses from eachommatidium reach the brain. The brain integrates thousandsof images and makes vision possible.

Eye Spot

Fig - 2.4

Fig - 2.5

Fig - 2.6

Fig - 2.7

Nervenet

Hydra is a multicellular organism having a simple structure. Inhydra nerve cells are extended all over the body like a net. Itis through this nervenet that communication between the bodyand the external environment and elicitation of responses takeplace.

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BIOLOGY

Why hear?

Snakes do not have the power to hear. When the snake-charmer plays the blow pipe (makudi) they dance notbecause they hear the music, but to the movements of thepipe. Snakes can sense the vibrations on the ground. Theyfrequently extend the tongue to sense the smell. Olfactoryparticles that stick on to the tongue are received by thereceptors in a special organ called Jacobson’s organsituated in the roof of the mouth and the smell is detected.The highly sensitive thermoreceptors in viper, python etc.,help them to detect the presence of other animals at night.

The living world is full of many such wonders. Collect information about them andprepare a magazine on the communication systems in animals. Whatever the diversitiesbe, responding to changes is the chief attribute of life. This attribute is common to allorganisms. This is one of the similarities that binds organisms with one another.

Fig.2.8.

Can't see, but ...

Even though plants cannot see things, they are able to sensethe presence of light and respond to it. In plants the greenpigment chlorophyll is synthesized in the presence of light.This phenomenon is called photomorphogenesis. You havelearnt about the tip of the stem growing towards light. Lightinfluences flowering and in some plants seed germinationalso. Plants are equipped for such responses by the protein

pigment called phytochrome.Fig.2.9.

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Follow up Activities1) Analyse the following situations and answer the following questions.

� Unknowingly touched fire� Experienced pain� Withdrew the hand� Applied antiseptic on burnt part(a) Enlist the stimuli and responses(b) Which is the conscious response?(c) Is it after experiencing pain that the hand is withdrawn? Justify your opinion.(d) List sequentially the parts through which impulses pass while experiencing

pain.(e) Illustrate reflex arc.

2) "The sympathetic system and the parasympathetic system act like the acceleratorand brake of a vehicle". – Evaluate whether this statement is completely true andnote down your opinion.

3) Analyse the following statements.a) Other organisms do not have the ability to receive stimuli as man.b) Even though nerves are absent, unicellular organisms can respond to stimuli.