Control and Coordination

Chapter 5 told you that movement is a sign of life. This chapter asks a sharper question: how does an organism decide to move? A living thing moves in response to a change in its surroundings. Such a change that the organism can detect is called a stimulus: heat, light, touch, a chemical, gravity. The movement that follows is the response. Because a response must be exactly matched to the stimulus (you pull your hand from a hot pan, you do not pull it from a cold one), the body needs a system that detects the change, decides on the response and then makes the right part of the body act. That system is what we call control and coordination.

In animals, two kinds of tissue do this job together. Nervous tissue carries information quickly as electrical impulses, and muscular tissue produces the actual movement. Alongside the nerves, animals also use chemical messengers called hormones, which travel in the blood and give slower but longer-lasting control. Plants have no nervous system or muscles at all, yet a sensitive plant folds its leaves the moment you touch it and every seedling sends its root down and its shoot up. Plants manage this by chemical signals and by changing the shape or growth rate of their cells.

The examiner likes NCERT exercise question 9, which asks why an organism needs such a system at all. The two-line answer is: to receive stimuli from the environment and from inside the body, and to produce a suitable, coordinated response so that all organs work together for the survival of the organism.

Information about the environment first has to be picked up. This is done by receptors, which are specialised tips of nerve cells sitting in our sense organs. Two receptors are named in NCERT and are asked almost every year: gustatory receptors on the tongue detect taste, and olfactory receptors in the nose detect smell. The skin has receptors for touch, heat and pain; the eye for light; the ear for sound.

The unit of the nervous system is the neuron (nerve cell). It has three parts you must be able to draw and name. The dendrites are short, branched fibres that receive information. The cell body (cyton) contains the nucleus and cytoplasm. The axon is the single longest fibre; it carries the impulse away from the cell body and ends in fine branches called nerve endings.

Now the working. Information picked up at the dendritic tip sets off a chemical reaction that creates an electrical impulse. This impulse travels along the dendrite to the cell body, then along the axon to its end. At the end of the axon the electrical impulse causes the release of certain chemicals. These chemicals cross the tiny gap between the axon end of one neuron and the dendrite of the next, called the synapse, and start a fresh electrical impulse in the next neuron. In the same way, the impulse is finally passed from a neuron to a muscle cell or a gland, which then acts.

A favourite 3-mark question (CBSE 2019) asks why the signal crosses a synapse only from the axonal end of one neuron to the dendritic end of the next and not in reverse. The answer: the chemicals are released only at the axon end, and the receptors for those chemicals are present only on the dendrite of the next neuron. So the impulse can travel in one direction only.

A reflex action is a sudden, automatic, involuntary response to a stimulus, made without conscious thought. Touching a flame, sneezing, blinking when something comes near the eye, the knee jerk: all are reflexes. The whole point of a reflex is speed. If the signal had to go up to the brain, be thought about and then come back, your hand would be burnt by the time you moved it.

The pathway a reflex follows is the reflex arc. Learn the sequence exactly as the marking scheme wants it: Receptor (heat receptor in the skin) → sensory neuron (afferent) carries the impulse to the spinal cord → a relay neuron (interneuron) inside the spinal cord passes it to a motor neuron (efferent) → the motor neuron carries the impulse to the effector (muscle of the arm), which contracts and pulls the hand away. Nerves from all over the body meet in a bundle in the spinal cord on their way to the brain, so the spinal cord is the natural place for such short-cut connections. The reflex arc is therefore formed in the spinal cord, although information about the event also goes on to the brain, which is why you become aware of the pain a moment later.

Two comparisons recur in papers. Reflex action versus walking (NCERT in-text): a reflex is involuntary, very fast and controlled by the spinal cord; walking is voluntary, slower and controlled by the brain (cerebrum decides, cerebellum coordinates). Reflex action versus involuntary action (NCERT exercise 10): both happen without conscious control, but a reflex is a response to an external stimulus and is handled mainly by the spinal cord, whereas an involuntary action such as heartbeat or digestion runs continuously without an external stimulus and is controlled by the mid-brain and hind-brain (medulla).

NCERT also points out that reflex arcs evolved in animals because the thinking part of the brain is not fast enough for emergencies, and that even in animals with a complex brain the arcs stay because they remain more efficient for quick responses.

The spinal cord handles reflexes, but thinking, remembering and most voluntary actions need the brain. Together, the brain and spinal cord form the central nervous system (CNS). The CNS talks to the rest of the body through the peripheral nervous system (PNS), which is made of cranial nerves arising from the brain and spinal nerves arising from the spinal cord. The 2016 two-marker asked exactly this: name the two components of the PNS.

The brain has three main regions. The fore-brain is the largest part and its main structure is the cerebrum, the thinking part. It has separate sensory areas that receive information from the receptors for hearing, smell, sight and so on; association areas that put this information together with what is already stored and decide what to do; and motor areas that send the decision to the muscles. The sensation of feeling full after a meal comes from a centre associated with hunger, also in the fore-brain.

The mid-brain and the hind-brain together control many involuntary actions. The hind-brain has three parts you must know. The medulla controls involuntary actions such as blood pressure, salivation, vomiting and heartbeat. The cerebellum is responsible for the precision of voluntary actions and for maintaining posture and balance; walking in a straight line, riding a bicycle or picking up a pencil all depend on it. The pons lies between the medulla and the mid-brain and helps regulate breathing.

A 2023 two-marker listed four activities and asked for the brain part: posture and balance (cerebellum), heartbeat (medulla), thinking (cerebrum), blood pressure (medulla). A 2018 three-marker did the same with feeling full (fore-brain), vomiting (medulla), picking up a pencil and riding a bicycle (cerebellum). Practise these mappings until they are automatic; they are the cheapest marks in the chapter.

Nervous tissue is delicate, and the brain and spinal cord are so important that the body gives them the best protection it has. The brain sits inside a bony box, the skull (cranium). Inside the skull, the brain is contained in a fluid-filled balloon: the cerebrospinal fluid surrounds the brain and acts as a shock absorber, so that a knock on the head does not transmit the jolt straight to the soft tissue. The spinal cord runs down inside the vertebral column, or backbone, a chain of bones that guards it the way the skull guards the brain.

This section is small but it produces reliable 1-mark and 2-mark questions: how is the brain protected, how is the spinal cord protected, what is the function of the cranium and the vertebral column. Write both the bony covering and the fluid when asked about the brain; many students mention only the skull and lose a mark.

The section also connects to NCERT exercise question 7 on spinal-cord injury. Since sensory impulses travel up the spinal cord to the brain and motor impulses travel down from the brain to the muscles, an injury to the spinal cord disrupts both: reflex actions whose arcs are formed at or below the injured level are lost, and voluntary signals from the brain cannot reach the muscles below the injury, so the person may lose sensation and movement in those parts.

Detecting a stimulus and deciding on a response is useless unless something actually moves. The final step of nervous control is a muscle. When a nerve impulse reaches a muscle, the muscle fibre must move, and the only way a cell can move is by changing its shape. NCERT explains it simply: muscle cells contain special proteins that change both their shape and their arrangement inside the cell in response to the electrical impulse arriving from the nerve. The new arrangement of these proteins makes the muscle cell shorter. A whole muscle made of such cells therefore contracts, pulls on the bone it is attached to, and the limb moves.

This is the answer to the second half of the 2020 five-marker about a cheetah chasing prey: seeing the prey, the cheetah's nervous system sends impulses to the leg muscles; the muscle proteins change shape and arrangement, the cells shorten, and the legs move. At the same time adrenaline (Section 6.3) raises the heart rate and diverts blood to the muscles so that they receive more oxygen and glucose.

The same idea distinguishes animal movement from plant movement, a comparison asked in NCERT exercise 12 and in papers. In our legs, movement is due to muscle contraction driven by nerve impulses and controlled by the brain; the cell does not change its water content. In the sensitive plant, there are no nerves or muscles; the leaf cells change shape by gaining or losing water. Keep this section in mind when you reach 6.2.1.

Plants have no nervous system and no muscles, yet they respond to their surroundings. NCERT opens with two examples. Touch the leaves of the sensitive plant, Mimosa pudica (chhui-mui), and they fold up and droop within seconds. Plant a seed, and however you lay it, the root grows down and the shoot grows up. These are two different kinds of movement. The Mimosa movement is independent of growth; the seedling movement is due to growth. The chapter treats them separately.

Take the immediate response first. The Mimosa leaf moves at the point of touch, so something must carry the information from the touched cells to the cells that move. Plants use electrical-chemical means to pass information from cell to cell, but they have no specialised tissue for conducting it, unlike our nerves. And to move, the plant cannot use muscle proteins. Instead the cells at the base of the leaflets change the amount of water in them. Losing water makes the cells shrink, gaining water makes them swell, and this change in shape folds or opens the leaf. The response is quick, does not depend on the direction of the stimulus, and does not involve growth.

This gives the standard comparison asked as an NCERT in-text question and in CBSE 2023: movement of Mimosa leaves versus movement of a shoot towards light. Mimosa: caused by touch, not directional, not a growth movement, occurs by change in water content of cells, rapid. Shoot towards light: caused by light, directional (towards the stimulus), a growth movement, occurs by unequal growth due to auxin, slow.

Movements like the Mimosa response that do not depend on the direction of the stimulus are called nastic movements; the Mimosa touch response is a thigmonastic movement. Directional growth movements are tropisms, which come next.

Some plant movements are really just differential growth: one side of an organ grows faster than the other, so the organ bends. A tendril of a pea plant is the simplest case. When the tendril touches a support, the side in contact grows more slowly than the side away from it, so the tendril curls around the support. Directional growth movements in response to a stimulus are called tropic movements or tropisms. Growth towards the stimulus is positive, away from it negative.

Learn the four tropisms with the examples the examiner expects. Phototropism, response to light: shoots are positively phototropic, roots negatively. Geotropism (gravitropism), response to gravity: roots grow downward (positive), shoots upward (negative). Hydrotropism, response to water: roots grow towards water. Chemotropism, response to a chemical: growth of the pollen tube towards the ovule is the standard example.

How does a plant know which side to grow on? Through plant hormones (phytohormones), chemical compounds made in one part of the plant that diffuse to another and regulate growth. The board's favourite explanation is phototropism. Auxin is synthesised at the shoot tip and helps cells grow longer. When light falls on one side of the shoot, auxin diffuses towards the shady side. The higher concentration of auxin there makes the cells on the shady side elongate more than those on the lit side, so the shoot bends towards the light. Write these four steps and you have the 3-mark answer.

Four hormones are named in NCERT. Auxin: cell elongation, phototropism, coiling of tendrils. Gibberellins: help in growth of the stem. Cytokinins: promote cell division, so they are found in high concentration in fruits and seeds where cells divide rapidly. Abscisic acid: the growth inhibitor; its effects include wilting of leaves. A 2019 Delhi question asked you to name the hormone for growth (gibberellin), cell division (cytokinin), inhibition (abscisic acid) and elongation (auxin), so keep each one attached to its single job.

NCERT also asks you to design an experiment for hydrotropism. Take two trays of moist soil with seedlings. In one, bury a porous clay pot filled with water near one side. After a few days the roots in that tray are seen growing towards the pot, while the roots in the control tray grow straight down.

Nerves are fast but they have limits: an impulse reaches only the cells connected to nervous tissue, and a cell cannot fire impulses continuously; it must reset before the next one. So animals also use chemical communication. A cell releases a chemical, called a hormone, into the blood; it is carried to the whole body and acts on target cells that can detect it. Hormones are made by endocrine glands (ductless glands, which pour their secretion directly into the blood). This is the answer to the 2020 question on why chemical communication is better than electrical impulses.

Adrenaline from the adrenal glands is the model example. When you are frightened or excited, adrenaline is secreted into the blood and prepares the body to fight or run: the heart beats faster so more oxygen reaches the muscles; the small arteries of the digestive system and skin contract, diverting blood to the skeletal muscles; and the breathing rate rises because the diaphragm and rib muscles contract faster. The squirrel-in-danger (CBSE 2020) and scary-situation (CBSE 2023) questions want exactly these three changes.

Other hormones you must know with gland, function and deficiency effect. Growth hormone from the pituitary regulates growth and development; deficiency in childhood causes dwarfism. Thyroxine from the thyroid regulates carbohydrate, protein and fat metabolism for balanced growth; the gland needs iodine to make it, so iodine deficiency causes goitre, seen as a swollen neck, which is why iodised salt is advised. Testosterone from the testes in males and oestrogen from the ovaries in females bring about the changes of puberty. Insulin from the pancreas regulates blood sugar; too little insulin lets blood sugar rise, causing diabetes, which is why some patients are treated with insulin injections.

Finally, timing and amount are controlled by feedback. When blood sugar rises, the cells of the pancreas detect it and secrete more insulin; as sugar falls, insulin secretion is reduced. This 3-mark feedback question (CBSE 2019) is asked in many disguises.