Control and Coordination

1. Why control and coordination is needed

In the previous chapter we studied life processes that keep an organism alive. Here we ask a different question: how does a living body react to its environment? We tend to link movement with being alive — a cat running, children swinging, buffaloes chewing cud. These movements are a response to a change in the surroundings.

A stimulus is any change in the environment to which an organism reacts (light, heat, touch, sound, smell, gravity). The reaction is the response. Each kind of stimulus needs a different, appropriate response — when we talk to a friend in class we whisper, we do not shout. So responding correctly requires two things working together:

• Control — recognising the event correctly and choosing the right response.
• Coordination — making different parts of the body work together smoothly to carry out that response.

2. Animals — the nervous system & receptors

In animals, control and coordination are provided by nervous and muscular tissues. Touching a hot object is an urgent, dangerous situation — we must detect it and respond fast.

All information from the environment is detected by the specialised tips of certain nerve cells called receptors. These are usually located in our sense organs:

• Gustatory receptors — detect taste (tongue).
• Olfactory receptors — detect smell (nose).
• Photoreceptors — light (eye); phonoreceptors — sound/hearing & balance (inner ear); thermoreceptors — heat; tactile receptors — touch (skin).

Why blocking the nose dulls taste (Activity 6.1): much of what we call "taste" is actually smell. When the nose is blocked (or during a cold) the olfactory receptors cannot work, so food seems tasteless even though gustatory receptors still work.

3. The neuron — structure and how the impulse travels

Nervous tissue is an organised network of nerve cells, or neurons, specialised for conducting information as electrical impulses.

Describing Figure 6.1(a) — structure of a neuron (in words): imagine a star-shaped cell body (cyton) containing the nucleus and cytoplasm. From it sprout many short, branched threads called dendrites — these receive information. One very long fibre, the axon, leads away from the cell body and ends in fine branches called nerve endings. The flow is: dendrite → cell body → axon → nerve ending.

Path of a nerve impulse:

• 1. Information is picked up at the dendritic tip, where it sets off a chemical reaction that creates an electrical impulse.
• 2. The impulse travels from the dendrite to the cell body, then along the axon to its end.
• 3. At the axon's end the electrical impulse triggers the release of tiny amounts of chemicals (neurotransmitters).
• 4. These chemicals cross the synapse (the gap) and start a fresh electrical impulse in the dendrite of the next neuron.

A similar junction, the neuromuscular junction [Figure 6.1(b)], finally delivers the impulse from a neuron to a muscle cell or gland, causing it to act.

Answer key for the three labelled points (NCERT): (i) information is acquired at the dendrite; (ii) it travels as an electrical impulse along the cell body and axon; (iii) it is converted into a chemical signal at the axon ending / synapse.

4. Reflex action and the reflex arc

"Reflex" means a sudden, automatic response done without thinking — pulling the hand back from a flame, blinking, the mouth watering, jumping out of the way of a bus.

Why a reflex is needed: thinking is a complex activity involving many neurons and takes time. If we had to consciously think "this is hot, I should move my hand," we might get burnt before we acted. To save time, the nerve that detects the stimulus is connected to the nerve that moves the muscle by a short-cut pathway — the reflex arc. The best place for this connection is where the nerves first meet — in the spinal cord.

Describing Figure 6.2 — reflex arc (in words): a hand reaches toward a hot pan. Receptors (heat/pain) in the skin detect the heat and start an impulse. A sensory neuron carries it to the spinal cord (part of CNS). Inside the spinal cord a relay (inter) neuron passes it straight to a motor neuron. The motor neuron carries the command to the effector = muscle in the arm, which contracts and pulls the hand away. Meanwhile a copy of the information also travels up to the brain (so we feel the pain afterwards), but the protective movement has already happened.

5. The human brain — parts and functions

The spinal cord does more than reflexes; the real thinking happens in the brain, the main coordinating centre of the body. Together the brain + spinal cord form the central nervous system (CNS), and the cranial + spinal nerves form the peripheral nervous system (PNS), which links the CNS to the rest of the body.

Voluntary actions (writing, talking, moving a chair) are decided by the brain, which then sends messages to muscles. The brain has three major regions: fore-brain, mid-brain and hind-brain.

Describing Figure 6.3 — the human brain (in words): the brain sits inside the cranium (skull). The large, wrinkled, dome-shaped front and top part is the cerebrum (the fore-brain). Below and behind it is the small mid-brain. At the lower back sits the hind-brain, made of the cerebellum (a small wrinkled ball), the pons and the medulla, which continues downward as the spinal cord. Below the fore-brain lie the hypothalamus and the pituitary gland.

• Fore-brain (cerebrum): the main thinking part. It has sensory areas that receive impulses for hearing, smell, sight, etc.; association areas that interpret and combine this information with stored memory; and motor areas that control voluntary muscles. It is also the seat of hunger (a separate centre makes us feel full).
• Mid-brain & hind-brain: control many involuntary actions over which we have no thinking control (e.g. change in pupil size, salivation, blood pressure).
• Medulla (hind-brain): controls involuntary actions like blood pressure, salivation and vomiting (also heartbeat, breathing).
• Cerebellum (hind-brain): responsible for precision of voluntary actions and maintaining posture and balance — walking in a straight line, riding a bicycle, picking up a pencil.

6. Protection of nervous tissue, and how it causes action

Protection: the brain is so delicate that the body protects it in a bony box (the cranium/skull), inside which it floats in a fluid-filled balloon (cerebrospinal fluid) that absorbs shocks. The spinal cord is protected by the vertebral column (backbone) — the hard, bumpy structure felt down the middle of the back.

How nervous tissue causes action: nervous tissue collects information, processes it, decides, and conveys the decision to muscles. When an action must be performed, a nerve impulse reaches the muscle, and the muscle fibre contracts. Muscle cells have special proteins that change both their shape and arrangement in response to the electrical impulse, giving the cell a shorter form — this is how a muscle contracts and moves a body part.

7. Coordination in plants — two kinds of movement

Plants have no nervous system and no muscles, yet they respond to stimuli. They show two types of movement:

• Movement independent of growth — fast, e.g. the leaves of Mimosa pudica (chhui-mui, the touch-me-not / sensitive plant) folding instantly when touched. Plant cells change shape by changing the amount of water in them (swelling or shrinking). Information is sent by electrical-chemical means from cell to cell, but there is no specialised conducting tissue as in animals.
• Movement dependent on growth — slow, directional, e.g. a seedling's shoot growing up and root growing down, or a tendril coiling round a support.

Tendril example: climbers like the pea plant cling using tendrils sensitive to touch. When a tendril touches a support, the side in contact grows less than the side away from the support; the unequal growth makes the tendril coil around the object.

8. Tropic (directional) movements in plants

• Phototropism — response to light. Shoots bend towards light (positively phototropic); roots bend away from light (negatively phototropic). Figure 6.5: a seedling near a window bends its shoot toward the light.
• Geotropism (gravitropism) — response to gravity. Roots grow downward (positively geotropic), shoots grow upward / away from earth (negatively geotropic). Figure 6.6 shows a potted plant whose shoot grew up (negatively geotropic) and root grew down (positively geotropic).
• Hydrotropism — response to water; roots grow towards a source of water.
• Chemotropism — response to a chemical; e.g. the growth of the pollen tube towards the ovule during fertilisation.

Speed comparison: the touch response of the sensitive plant is very quick; sunflowers turning with day/night is slower; growth-related (tropic) movements are slowest of all.

9. Plant hormones (phytohormones)

Since plants have no nerves, they use chemical communication. Electrical impulses have limits (they reach only nerve-connected cells, and a cell needs time to reset before firing again). So plants release hormones — chemicals made at one place that diffuse to the area where they act, reaching all cells slowly and steadily.

• Auxin — made at the shoot tip; helps cells grow longer (elongate). How phototropism works: when light falls on one side, auxin diffuses to the shady side; the higher auxin there makes those cells grow longer, so the shoot bends towards the light.
• Gibberellins — like auxin, help in the growth of the stem.
• Cytokinins — promote cell division; found in high concentration where cells divide rapidly, such as in fruits and seeds.
• Abscisic acid (ABA) — inhibits growth; causes effects such as wilting of leaves and closing of stomata (a stress hormone).

So auxin, gibberellin and cytokinin promote growth, while abscisic acid inhibits it.

10. Hormones in animals (the endocrine system)

Animals also use chemical signals. A scared squirrel must prepare to "fight or flight." A nerve signal alone would reach only some tissues, so instead a chemical — the hormone adrenaline — is released into the blood and reaches the whole body.

Endocrine glands are ductless — they pour their hormones straight into the blood. Describing Figure 6.7 (in words): in the human body the glands are arranged top to bottom — pineal gland and pituitary gland (with the hypothalamus) in the brain; thyroid gland and parathyroid glands in the neck; thymus in the chest; adrenal glands capping the kidneys; the pancreas near the stomach; and the sex glands — testes in males, ovaries (and uterus) in females.

Adrenaline response (fight-or-flight): secreted from the adrenal glands into the blood; it makes the heart beat faster (more oxygen to muscles), diverts blood away from skin and digestive system to skeletal muscles (by contracting small arteries there), and raises the breathing rate (diaphragm and rib muscles contract). All this prepares the body to act fast.

11. Important animal hormones (Table 6.1)

This table is a frequent board question — learn gland, hormone and function together.

• Growth hormone — gland: pituitary — stimulates growth in all organs. Deficiency in childhood → dwarfism; excess → gigantism.
• Thyroxin — gland: thyroid — regulates carbohydrate, protein and fat metabolism for balanced growth. Needs iodine; iodine deficiency → goitre (swollen neck). This is why iodised salt is advisable.
• Insulin — gland: pancreas — regulates blood sugar. Too little insulin → high blood sugar → diabetes.
• Testosterone — gland: testes — male sex hormone; brings about changes at puberty in boys.
• Oestrogen — gland: ovaries — development of female sex organs, regulates the menstrual cycle and puberty changes in girls.
• Adrenaline — gland: adrenal — prepares the body for emergencies (fight-or-flight).
• Releasing hormones — gland: hypothalamus — stimulate the pituitary to release its hormones (e.g. growth-hormone-releasing factor).

12. Feedback mechanism

Hormones must be secreted in precise amounts at the right time. This is controlled by a feedback mechanism.

Example: when blood sugar rises, the pancreas detects it and secretes more insulin; as the sugar level falls, insulin secretion is reduced. This keeps blood sugar steady.

13. NCERT in-text questions (page 105) — answered

Q1. Difference between a reflex action and walking. A reflex action is a sudden, automatic, involuntary response controlled by the spinal cord (no thinking, very fast), e.g. pulling the hand off a flame. Walking is a voluntary action that is consciously decided and controlled by the brain.

Q2. What happens at the synapse between two neurons? The electrical impulse reaching the axon ending triggers the release of chemicals (neurotransmitters). These cross the synaptic gap and start a new electrical impulse in the dendrite of the next neuron. The synapse also makes the impulse travel in one direction only.

Q3. Which part of the brain maintains posture and equilibrium of the body? The cerebellum (part of the hind-brain).

Q4. How do we detect the smell of an agarbatti (incense stick)? Its molecules are detected by the olfactory receptors in the nose, which send an impulse to the olfactory area of the fore-brain, where the smell is interpreted.

Q5. What is the role of the brain in reflex action? The reflex itself is completed by the spinal cord (the reflex arc) for speed. The information also travels up to the brain, so the brain becomes aware of the event (e.g. feels the pain) afterwards, but is not needed to produce the quick response.

14. NCERT in-text questions (page 108) — answered

Q1. What are plant hormones? Chemical compounds made in one part of a plant that diffuse to other parts to help coordinate growth, development and responses to the environment (e.g. auxin, gibberellin, cytokinin, abscisic acid).

Q2. How is the movement of leaves of the sensitive plant different from the movement of a shoot towards light? The sensitive plant's leaf movement is quick, independent of growth, caused by a change of water in the cells, and is non-directional (nastic) — the same fold whichever side is touched. The shoot bending toward light is slow, growth-dependent and directional (tropic) — it depends on the direction of the stimulus.

Q3. Give an example of a plant hormone that promotes growth. Auxin (also gibberellin or cytokinin).

Q4. How do auxins promote the growth of a tendril around a support? When the tendril touches a support, auxin accumulates on the side away from the support. That side grows faster (elongates more) than the side in contact, so the tendril bends and coils around the support.

Q5. Design an experiment to demonstrate hydrotropism. Take a trough of moist soil and plant a seedling in the middle. On one side place a small porous pot of water; keep the other side dry. After a few days the roots grow towards the side with water, showing hydrotropism. (Control: a similar trough with water on neither side, where roots simply grow straight down.)

15. NCERT in-text questions (page 111) — answered

Q1. How does chemical coordination take place in animals? Through hormones secreted by endocrine glands directly into the blood. The blood carries each hormone to its target organ, where it produces a specific effect; the amounts are controlled by feedback.

Q2. Why is the use of iodised salt advisable? Iodine is needed by the thyroid gland to make thyroxin, which regulates metabolism. Iodine deficiency causes goitre (a swollen neck) and poor growth, so iodised salt supplies the needed iodine.

Q3. How does our body respond when adrenaline is secreted into the blood? The heart beats faster (more oxygen to muscles), blood is diverted from skin and digestive system to skeletal muscles, and the breathing rate rises — preparing the body for fight or flight.

Q4. Why are some patients of diabetes treated by giving injections of insulin? In such patients the pancreas does not produce enough insulin, so blood sugar stays high. Insulin injections restore the hormone and bring the blood sugar level back to normal.

16. NCERT Exercises (page 112) — fully solved

Q1. Which of the following is a plant hormone? (a) Insulin (b) Thyroxin (c) Oestrogen (d) Cytokinin. Answer: (d) Cytokinin — the others are animal hormones.

Q2. The gap between two neurons is called a: (a) dendrite (b) synapse (c) axon (d) impulse. Answer: (b) synapse.

Q3. The brain is responsible for: (a) thinking (b) regulating the heart beat (c) balancing the body (d) all of the above. Answer: (d) all of the above — cerebrum thinks, medulla regulates heartbeat, cerebellum balances.

Q4. Function of receptors; what if they don't work? Receptors detect specific stimuli from the environment (taste, smell, light, heat, sound, touch) and start the nerve impulse. If they are damaged, the body cannot detect changes in surroundings, so responses are delayed or absent — e.g. one might not feel pain, smell smoke, or react to danger, which can be harmful.

Q5. Draw the structure of a neuron and explain its function. (Draw: cell body with nucleus, branched dendrites, long axon ending in nerve endings.) Function: dendrites receive the stimulus and generate an impulse; the cell body and axon conduct it; at the nerve ending chemicals are released across the synapse to pass the signal to the next cell — so the neuron receives, conducts and transmits information.

Q6. How does phototropism occur in plants? When light falls on one side of a shoot, the hormone auxin diffuses to the shady side. The higher auxin there makes those cells grow longer, so the shoot bends towards the light (positive phototropism in shoots; roots bend away).

Q7. Which signals will get disrupted in case of a spinal cord injury? The spinal cord carries reflex action signals and the signals travelling between the body and the brain. So a spinal cord injury disrupts reflexes and the communication of impulses to and from the brain for the parts below the injury (loss of sensation and voluntary movement).

Q8. How does chemical coordination occur in plants? Plants have no nervous system; they use hormones. A hormone made at one site (e.g. auxin at the shoot tip) diffuses to other parts and controls their growth, movement and development — e.g. auxin, gibberellin, cytokinin promote growth, abscisic acid inhibits it.

Q9. What is the need for a system of control and coordination in an organism? A multicellular body has many organs that must work together. Control and coordination let the body detect changes in the environment and produce the right response in the right organs at the right time — protecting the body and keeping its internal activities balanced.

Q10. How are involuntary actions and reflex actions different? Both are automatic and not under conscious control. But reflex actions are sudden, very fast responses to a stimulus controlled by the spinal cord (e.g. withdrawing the hand from a flame), whereas involuntary actions are ongoing internal processes controlled by the mid-brain and hind-brain (medulla) — heartbeat, breathing, digestion — and are not necessarily triggered by an external stimulus.

Q11. Compare and contrast nervous and hormonal mechanisms.

• Nervous: information travels as electrical impulses along neurons; very fast; effect is short-lived; reaches only nerve-connected cells; response is precise/localised.
• Hormonal: information travels as chemicals through the blood; slow but longer-lasting; reaches the whole body; widespread effect, controlled by feedback.
• Common: both bring about control and coordination, and both work by carrying information from one part of the body to another.

Q12. Difference between movement in a sensitive plant and movement in our legs. The sensitive plant's movement is not under any nervous control and involves no muscles; cells move by changing their water content, and it is independent of growth. Movement of our legs is a voluntary action controlled by the brain, carried out by muscles in response to nerve impulses.

17. Common mistakes to avoid

• Saying the brain controls reflex action — it is the spinal cord; the brain only becomes aware afterwards.
• Mixing up gland and hormone — insulin is from the pancreas (not thyroid); thyroxin is from the thyroid.
• Writing that shoots are negatively phototropic — shoots bend towards light (positive); only roots are negative.
• Confusing geotropism directions — roots positive (down), shoots negative (up).
• Saying the sensitive plant has muscles or nerves — it has neither; its leaves move by water changes.
• Calling the synapse a part of the neuron — it is the gap between two neurons.
• Forgetting that auxin moves to the shady side in phototropism (a very common slip).

18. Quick revision checklist

• Stimulus → receptor → nervous system / hormones → response.
• Neuron parts: dendrite (receives) → cell body → axon → nerve ending; synapse uses chemicals.
• Reflex arc: receptor → sensory neuron → spinal cord → motor neuron → effector.
• Brain: fore-brain = cerebrum (thinking), hind-brain = cerebellum (balance) + medulla (heartbeat, BP, breathing).
• CNS = brain + spinal cord; PNS = cranial + spinal nerves.
• Tropisms: photo (light), geo (gravity), hydro (water), chemo (chemical) — positive or negative.
• Plant hormones: auxin, gibberellin, cytokinin (promote) ; abscisic acid (inhibits).
• Animal hormones: growth (pituitary), thyroxin (thyroid, needs iodine), insulin (pancreas), testosterone (testes), oestrogen (ovaries), adrenaline (adrenal). Feedback keeps amounts correct.