Control and Coordination: Class 10 Science · Chapter 6

You are reading on the bus home when a mosquito settles on your wrist. You feel the prick, your other hand swats it, and your eyes never leave the page. A moment later the driver brakes hard, and without a thought you grip the seat in front. In a few seconds your body noticed two changes around it and answered each one with exactly the right movement.

A change that a living thing can detect is a stimulus (उद्दीपन; plural stimuli), and what it does about the change is its response. The response has to suit the stimulus. You duck a cricket ball flying at your face, but you reach out for one lobbed gently to you. Choosing the right response is control. Making the eyes, brain, arms and legs act together, in the right order, is coordination.

Movement is the response we notice most. Some movements come only from growth: over a few days a sunflower seedling on a windowsill leans towards the glass, and a gourd creeper winds itself up a fence wire. Others have nothing to do with growth: a gecko darting at a moth, a dog wagging its tail, your own eyelids blinking. Both kinds are controlled. In a body made of many cells, special tissues do this work. This chapter looks first at the two systems animals use, nerves and hormones, and then at how plants manage without any nerves at all.

6.1 Animals – Nervous System

Every response begins with noticing a change. In animals, noticing, carrying messages and deciding are done by nervous tissue, and the final action by muscle tissue; you met both in Class 9.

Detecting is the job of receptors (ग्राही): nerve cells whose tips are built to respond to one kind of change. The eye's receptors respond to light, the nose's olfactory receptors to smell and the tongue's gustatory receptors to taste (Table 6.1). Most receptors are grouped in the sense organs, but the skin has them spread all over it.

The basic unit of nervous tissue is the nerve cell, or neuron (तंत्रिका कोशिका). Look at its parts in Fig. 6.1:

• Dendrites: short, branching threads around the cell body. Their tips pick up information, either from a stimulus or from another neuron.
• Cell body: the widest part, which holds the nucleus and keeps the cell alive.
• Axon: one long fibre that carries the signal away from the cell body. A single axon running down your leg can be about a metre long.
• Nerve endings: the branched tip of the axon, where the message is handed on to the next cell.

A message changes form on its journey: inside a neuron it is electrical, but between neurons it is chemical. Follow one message through Fig. 6.1 and Fig. 6.2:

1. When a dendrite tip is stimulated, chemical changes there turn the stimulus into a tiny electrical signal, an electrical impulse.
2. Like a ripple running along a rope, the impulse travels on, through the cell body and all the way down the axon.
3. Neighbouring neurons never quite touch. When the impulse arrives at the nerve endings, the cell lets out a small amount of chemicals into the narrow gap between the two cells. This gap is the synapse.
4. On the far side, the chemicals lock onto the next neuron's dendrite. That triggers a brand-new electrical impulse, and the relay goes on. The whole crossing takes well under a thousandth of a second.
5. At the end of the chain, the same kind of junction hands the message to a muscle cell or a gland cell, which then acts.

6.1.1 What happens in Reflex Actions?

Walk barefoot across a lawn and step on a thorn: your foot jerks up before you have even said 'ouch'. A speck of dust flies at your eye and you blink. The doctor taps just below your kneecap and your leg kicks forward by itself. Each is a reflex action (प्रतिवर्ती क्रिया): a quick, automatic response to a stimulus, made without our thinking about it.

Why not let the brain decide? Because deciding takes time. A thought is not one signal but a storm of them, bouncing among huge numbers of neurons. For the thorn, the pain signal would first have to climb more than a metre of nerve to the head. The brain would weigh it up, and then a command would have to travel all the way back to the leg. Meanwhile the thorn keeps pressing in.

The body's answer is a short cut. Inside the spinal cord, the incoming 'pain' nerve is wired almost directly to the outgoing nerve that works the leg muscles, so the command is given right there, without waiting for the brain. The spinal cord suits this job because every nerve from the trunk and limbs passes through it anyway. This shortest route from stimulus to response is called a reflex arc (प्रतिवर्ती चाप, Fig. 6.3). It works like a school gate guard with standing orders: if a fire breaks out, he opens the gate at once and informs the principal afterwards.

The brain is not left out. The message from the receptor also travels on up the spinal cord, so a moment after your foot has jerked away you feel the pain and know what happened.

In the history of life, quick wired responses came first. A jellyfish has only a loose net of nerves and no brain at all, yet it pulls in its tentacles the instant they are touched. Big thinking brains appeared much later, and the animals that have them kept their reflex arcs, because in an emergency nothing is faster.

6.1.2 Human Brain

The brain and the spinal cord together make up the central nervous system (CNS), the body's control room. Reports arrive there from every organ, and the CNS combines them before it responds. The brain is the main coordinating centre. The spinal cord, besides running reflexes, is the highway that carries messages up to the brain and commands back down.

Most of what you do in a day is not reflex. Kicking a football, pouring a glass of water or switching on a fan are voluntary actions: you decide to do them. So muscles take orders along two routes: straight from the spinal cord in a reflex, or from the brain when you have made a decision.

Nerves link the CNS to the rest of the body. Twelve pairs of cranial nerves leave the brain, and thirty-one pairs of spinal nerves leave the spinal cord. Together they form the peripheral nervous system (PNS), the network of cables between the CNS and the sense organs, muscles and glands.

Different parts of the brain do different jobs. The brain has three main regions: the fore-brain, the mid-brain and the hind-brain (Fig. 6.4).

The fore-brain is the thinking part; its largest portion is the cerebrum. Watch it at work. You smell something sweet frying, hear a sizzle and see golden coils in a kadhai. Each sense delivers its impulses to its own sensory area of the cerebrum: one area handles sight, another hearing, another smell, and so on. Association areas then combine the three reports and check them against memory: jalebis! Having made sense of it all, the fore-brain chooses what to do, and its motor areas send orders to the voluntary muscles that reach your hand out for a plate.

Not everything the fore-brain senses comes from the eyes, ears or nose. The feeling that tells you to stop after the third paratha comes from a centre for hunger and fullness in the hypothalamus, at the base of the fore-brain. The hypothalamus also helps control thirst and body temperature, and, as you will see in Section 6.3, it directs the release of many hormones.

Other actions carry on whether we attend to them or not. Your heart keeps beating through the night, your stomach churns your lunch, and you swallow and breathe while fast asleep. Deciding cannot easily start or stop them. They are involuntary actions, and most are run by the mid-brain and the hind-brain.

The mid-brain makes quick, automatic adjustments of the eyes and head, such as changing the size of the pupils or turning your head towards a sudden bang.

The hind-brain has three parts. The medulla controls involuntary actions such as the heartbeat, blood pressure, the flow of saliva, swallowing, coughing, sneezing and vomiting. The pons relays messages between brain regions and helps the medulla regulate breathing. The cerebellum makes voluntary movements smooth and precise and keeps the body's posture and balance. Threading a needle, writing neatly, balancing on one foot and standing steady in a moving bus all depend on it.

6.1.3 How are these Tissues protected?

Brain tissue is soft, not much firmer than set curd, and easily injured. It is housed in the cranium, the rounded part of the skull. Between the bone and the brain lie protective membranes with fluid between them. When the head is jolted, the fluid cushions the brain, much as water protects an egg floating in a sealed jar. The spinal cord has its own armour. Press along the middle of your back, from the neck downwards, and you can count separate knobs of bone. They belong to the vertebral column (backbone): a stack of ring-shaped bones through which the spinal cord passes (last step of Fig. 6.4).

6.1.4 How does the Nervous Tissue cause Action?

Every movement you make, from a blink to a sprint, ends with muscle cells doing one simple thing: getting shorter. A muscle is fixed to bones at both ends, so when its cells shorten (contract), the muscle pulls the bones closer together and your arm bends or your leg swings. The nervous system's part is to tell the right muscles when to do this.

Inside each muscle cell lie bundles of protein threads of two kinds, overlapping like the fingers of two loosely interlocked hands. A nerve impulse arriving at the junction between nerve ending and muscle sets off a chemical change. The proteins alter their shape and pull the threads further into each other, just as interlocked fingers slide deeper. The threads stay the same length, but the cell as a whole gets shorter (Fig. 6.5).11The two main muscle proteins are actin and myosin. You will meet them in Class 11.

6.2 Coordination in Plants

Plants cannot run from danger or walk to food, and they have neither nerves nor muscles. Yet they are far from still. Speeded-up films show shoots nodding, tendrils circling and leaves opening and closing. Brush a leaf of chhui-mui (the touch-me-not, Mimosa pudica) and its leaflets snap shut within a second or two; a seedling on a windowsill takes days to lean towards the glass. These are the two kinds of plant movement, set side by side below.

6.2.1 Immediate Response to Stimulus

The folding of a chhui-mui leaf is powered by water. At the base of each leaflet, and where the leaf stalk joins the stem, sits a small swollen cushion of cells. Full of water, these cells are firm, like a well-pumped tyre, and they hold the leaf open. A touch makes some of them lose water quickly; they go limp, so the leaflets fold together and the stalk sags (Fig. 6.6). Within half an hour or so water flows back, the cells firm up and the leaf reopens. So a plant's 'motor' is water pressure inside cells, not the protein threads of muscle.

Now touch just the tip of one leaflet. The folding spreads to leaflets you never touched, so news of the touch must have travelled. Plants pass such signals on by electrical and chemical changes from one cell to the next. Unlike animals, though, they have no special tissue for the job: ordinary cells relay the message.

6.2.2 Movement Due to Growth

A bitter-gourd (karela) vine cannot stand up on its own, so it climbs. Its tendrils, thin threads near the leaves, wave slowly about until one touches something. Then uneven growth takes over: cells on the side touching the support almost stop lengthening, while those on the outer side keep growing. A thread that grows faster on its outer side must curl inwards, so within a day or so the tendril has wrapped itself tightly round the support (Fig. 6.7).

Tendrils are quick by plant standards. Most growth responses take days, and we notice them only when we look back and find the plant pointing somewhere new. When a stimulus such as light or gravity sets the direction in which a plant part grows, the response is a tropism (अनुवर्तन). Growth towards the stimulus is positive; growth away from it is negative.

Growth in response to light is phototropism. Shoots grow towards light (positive) and roots grow away from it (negative). A money plant on a windowsill soon leans out towards the glass, and seedlings in a dark storeroom stretch towards the strip of light under the door. Bending towards light gives the leaves more sunlight for photosynthesis, while growing away from it keeps the roots in the dark soil, where water and minerals are.

Light is not the only signal. Sow a gram seed upside down and its root still bends round to grow down, while its shoot bends round to grow up. This response to gravity is geotropism (Fig. 6.9). Roots are positively geotropic and shoots negatively geotropic. A seedling laid on its side in a dark box shows this clearly: with no light to steer it, the root tip still curves down and the shoot tip curves up.

Water and chemicals can steer growth too. Growth towards water is hydrotropism: roots near a leaking pipe or a garden tap spread towards the wet soil, and tree roots sometimes crack drains to reach the water inside. Growth in response to a chemical is chemotropism. A well-known case is the pollen tube, which grows from a pollen grain down to the ovule, guided by chemicals from the ovule. You will meet it in Chapter 7.

Line up the plant responses by speed. A chhui-mui leaf folds in about a second; a young sunflower takes a whole day to swing its head from east to west; a shoot needs several days to bend towards a window. The faster a response has to be, the faster the message behind it must travel.

Electrical impulses win on speed, and animals use them wherever speed matters. But a wired system has drawbacks, set out in the comparison below: it reaches only the cells that nerves connect to, and a nerve cell must pause to reset after every impulse. Many jobs, such as steering growth over weeks, need a message that reaches every cell and keeps acting for a long time. For these, living things use chemical messengers called hormones. A hormone spreads out from the cells that release it, and only cells carrying matching receptor molecules respond to it.

Plant hormones

In plants, hormones are the main way of coordinating growth, development and responses to light, gravity and touch. A plant hormone is usually made in one place and reaches the cells it acts on by diffusion, sometimes some distance away.

The best-known plant hormone is auxin (ऑक्सिन). It is made in the shoot tip and moves into the region just below it, where it makes cells stretch. Now we can explain Activity 6.2. When light falls on a shoot from one side, auxin moves away from the light to the shaded side. Cells on the shaded side, with more auxin, grow longer than cells on the lit side. A stem whose one side lengthens more than the other has to curve, and it curves towards the shorter side, the one facing the light (Fig. 6.10).

Three more hormones complete the list (Table 6.6). Gibberellins, like auxin, make stems grow longer: sugarcane growers spray them for taller canes, and grape growers for bigger berries. Cytokinins make cells divide, which is why there is plenty of them wherever cells multiply fast, in developing fruits and seeds; coconut water is a rich natural source.

Abscisic acid works the other way, as a brake on growth. It builds up when a plant is stressed, for example by drought. It slows growth, its effects include the wilting and ageing of leaves, and it keeps seeds and buds dormant until conditions improve.

6.3 Hormones in Animals

Animals use chemical messages too. Imagine walking down a quiet lane at dusk when a dog starts barking and rushes at you. Within seconds your heart is thumping, you are breathing fast, and your legs feel ready to sprint. You do not yet know whether you will run or stand your ground and shoo the dog away. Either way, your body needs extra energy, quickly, from many organs working together.

Getting ready involves the heart, the lungs, the blood vessels and nearly every muscle, far more than a few nerves could reach in time. So within moments a chemical alarm goes out through the blood. Two adrenal glands, each sitting like a cap on top of a kidney, pour the hormone adrenaline into the bloodstream, and the blood delivers it everywhere. Only certain target organs respond (Fig. 6.12):

• Heart: it beats faster and harder, pumping more oxygen-rich blood to the muscles.
• Blood vessels: the small arteries that supply the gut and the skin narrow, because the muscles in their walls tighten. Blood goes instead to the skeletal muscles, which is why a scared face can look pale.
• Breathing: the diaphragm and rib muscles work harder, so you breathe faster and take in more oxygen.

Digestion can wait; escape cannot. This emergency response is often called fight or flight.

Hormones such as adrenaline are made by endocrine glands (अंतःस्रावी ग्रंथियाँ). These glands have no ducts: they pour their hormones straight into the blood. (Glands such as the salivary glands, which empty through a duct, are a different kind.) The endocrine glands and their hormones make up the endocrine system, the second way, after the nervous system, in which the body is controlled and coordinated.

Plant hormones steer a shoot towards light. Animal hormones do not make us grow towards anything, but they do control where and how much we grow. As a child grows up, the arms, legs and organs keep the right sizes for one another: the two legs end up almost exactly the same length, and the heart grows in step with the rest of the body. Hormones help to keep this careful plan.

Iodine and thyroxin

In the 1950s, health surveys in the Himalayan foothills found that many people had a swelling at the front of the neck. The cause lay in their food: the soil and water there contain very little iodine. The thyroid gland, in the front of the neck, needs iodine to make the hormone thyroxin. Thyroxin sets the pace at which the body's cells use carbohydrates, proteins and fats (its metabolism), and so helps the body grow in a balanced way.

Without enough iodine the thyroid cannot make enough thyroxin, and it may swell. The swelling shows as a lump at the front of the neck, just where Fig. 6.13 places the thyroid. This condition is goitre. To prevent it, almost all salt sold in India today is iodised.

Growth hormone and puberty

The pituitary gland, a pea-sized gland hanging below the brain, makes growth hormone, which drives the growth of bones and muscles through childhood and the teenage years. If a child makes too little of it, growth slows and the child may remain very short, a condition called dwarfism. Too much in childhood can make a person unusually tall (gigantism).

Compare a class photo from Class 5 with one from Class 9. In between came a growth spurt, new hair in the armpits, and changes in voice and body shape: the changes of puberty. They are triggered by sex hormones that start flowing in the early teens: testosterone from the testes in boys and oestrogen from the ovaries in girls. These hormones also prepare the body for reproduction, which you will study in Chapter 7.

Insulin and blood sugar

More than 10 crore people in India live with diabetes, according to an ICMR study published in 2023. In diabetes the level of sugar (glucose) in the blood stays too high, and over the years this damages the eyes, kidneys, nerves and heart. The hormone that normally brings blood sugar down is insulin, made by the pancreas. Some diabetic patients make too little insulin, so doctors prescribe insulin injections along with a careful diet.

Feedback keeps hormones in check

Hormones act in tiny amounts, and too much can be as harmful as too little. So the body must control how much of each hormone is released, and when. It does this with a feedback mechanism (पुनर्भरण क्रियाविधि): the result that a hormone produces itself adjusts how much more of that hormone is released.

Blood sugar shows how this works (Fig. 6.14). After a meal of rice and dal, glucose from digestion enters the blood and its level rises. Cells in the pancreas sense the rise and release more insulin. Insulin helps body cells take up glucose and the liver store it, so the level falls. As it returns to normal, the pancreas senses that too and cuts down its insulin. The level stays within a narrow, safe range. An automatic controller on a rooftop water tank works in the same way: when the water drops, the pump switches on, and when the tank is full, the controller switches it off.