Control and Coordination
Why This Matters
Touch a hot pan and your hand jerks back before you even feel the pain. See a ball flying at your face and you blink without deciding to. Smell food and your mouth waters on its own. Your body is constantly sensing the world and responding — and, crucially, responding appropriately: you whisper to a friend in class, you don’t shout.
That last point is the whole chapter. Movement in response to the environment isn’t random — it’s controlled and coordinated. The right change triggers the right response, at the right speed. Pulling your hand off a flame must be instant; growing toward sunlight can take days.
Living bodies manage this with two systems working together: a fast, wired nervous system (electrical signals) and a slower, body-wide hormonal system (chemical messengers). Plants, with no nerves or muscles at all, pull off control and coordination using only chemicals and clever growth. This chapter is how living things keep it all in order.
The Big Idea
Organisms survive by sensing changes (stimuli) and responding with the right action. Animals use a fast electrical system (nerves) for quick responses and a slower chemical system (hormones) for body-wide, sustained changes. Plants use only chemicals and directional growth.
Why two systems? Electrical impulses are lightning-fast but only reach cells that nerves connect to, and a nerve cell needs a moment to “reset” before firing again. Chemical messengers (hormones) are slower but travel in the blood to every cell and can act steadily over time. Fast-and-local vs slow-and-everywhere — between them they cover every situation.
Hold that contrast in mind and the neuron, the reflex, the brain and every hormone in this chapter fall into place.
Let’s Break It Down
The neuron and the nerve impulse
The nervous system is built from nerve cells called neurons. Information flows through one in a fixed direction:
- Receptors (in sense organs — tongue, nose, ear, skin, eye) detect the stimulus. (Gustatory = taste; olfactory = smell.)
- The signal becomes an electrical impulse at the dendrite tip → travels to the cell body → along the axon.
- At the axon’s end, the impulse releases chemicals that cross a tiny gap, the synapse, starting a fresh impulse in the next neuron — or telling a muscle to act.
Reflex actions: the shortcut
When you touch something hot, waiting for the brain to “think it through” would be too slow — you’d get burnt. So the body uses a shortcut called a reflex arc: the sensory nerve connects to the motor nerve right in the spinal cord, skipping the conscious brain. The message also travels up to the brain, but the response has already happened.
Path: receptor (skin) → sensory neuron → relay neuron (spinal cord) → motor neuron → effector (muscle). Reflexes evolved because thinking isn’t fast enough — and they’re still faster even now that we can think.
The human brain
The brain + spinal cord = central nervous system (CNS). Nerves to the rest of the body form the peripheral nervous system. The brain has three main regions:
| Region | Main jobs |
|---|---|
| Fore-brain (cerebrum) | Thinking, deciding; receiving & interpreting sight, smell, hearing; voluntary actions; hunger centre |
| Mid-brain | Some involuntary actions (e.g. reflexes of the eye) |
| Hind-brain — cerebellum | Precision & balance: walking straight, riding a cycle, posture |
| Hind-brain — medulla | Involuntary vital actions: heartbeat, breathing, blood pressure, salivation, vomiting |
The delicate brain sits inside the bony skull (cranium), cushioned by fluid; the spinal cord is protected by the vertebral column (backbone).
How muscles move: when an impulse reaches a muscle, special proteins inside the muscle cells change shape and arrangement, so the cells shorten — that contraction is the movement.
Why is a reflex action faster than a deliberate, thought-out action like moving a chair?
A reflex uses a short reflex arc in the spinal cord: the sensory neuron connects almost directly to the motor neuron, so the response happens without waiting for the brain to think. A deliberate action needs signals to travel up to the brain, be processed and decided on, then sent back — which takes longer.
Which part of the brain keeps you balanced while riding a bicycle?
The cerebellum (in the hind-brain) controls precision of voluntary movements and balance/posture — exactly what’s needed to stay upright on a cycle. The medulla runs involuntary things like heartbeat.
Coordination in plants
Plants have no nerves and no muscles, yet they respond. They do it in two ways:
- Immediate movement (no growth): the touch-me-not (chhui-mui / Mimosa) folds its leaves when touched. The leaf moves at a spot away from where it was touched, so a signal must travel — plants use electrical-chemical signals cell-to-cell, then cells change shape by gaining or losing water (swelling / shrinking).
- Movement due to growth (tropism): directional growth toward or away from a stimulus. It’s slow but permanent.
| Tropism | Stimulus | Example |
|---|---|---|
| Phototropism | Light | Shoot bends toward light; root away from it |
| Geotropism | Gravity | Root grows down (toward gravity); shoot grows up |
| Hydrotropism | Water | Roots grow toward water |
| Chemotropism | Chemicals | Pollen tube grows toward the ovule |
Plant hormones coordinate this growth, made in one place and diffusing to where they act:
- Auxin (made at the shoot tip) makes cells grow longer. When light hits one side, auxin moves to the shady side, so that side grows more and the shoot bends toward the light.
- Gibberellins — help stems grow.
- Cytokinins — promote cell division (high in fruits and seeds).
- Abscisic acid — inhibits growth (causes wilting). The one “stop” signal among these.
Hormones in animals
Animals add a chemical (endocrine) system on top of nerves. Endocrine glands release hormones straight into the blood, which carries them to target organs.
Imagine a squirrel facing danger: it must get ready to fight or run. Nerves alone can’t prepare the whole body, so the adrenal glands release adrenaline into the blood — heart beats faster (more oxygen to muscles), breathing speeds up, blood is diverted from the gut/skin to the skeletal muscles. The body is primed in seconds.
| Hormone | Gland | Main role |
|---|---|---|
| Growth hormone | Pituitary | Growth of the body (too little in childhood → dwarfism) |
| Thyroxin | Thyroid (needs iodine) | Controls metabolism of carbs/proteins/fats; iodine lack → goitre |
| Insulin | Pancreas | Lowers blood sugar (too little → diabetes) |
| Adrenaline | Adrenal glands | Prepares body for emergency (fight or flight) |
| Testosterone / Oestrogen | Testes / Ovaries | Changes at puberty; male / female development |
Feedback keeps it precise. Hormones must be released in just the right amount. If blood sugar rises, the pancreas senses it and makes more insulin; as sugar falls, it makes less. This self-correcting loop is a feedback mechanism.
A person's neck is swollen (goitre). The most likely cause is:
The thyroid needs iodine to make thyroxin. Without enough iodine the thyroid enlarges, causing goitre (swollen neck) — which is exactly why we use iodised salt.
Common Mistakes
The brain controls reflex actions — that's how they're so fast.
The brain controls most actions, so it seems it must control reflexes too.
Reflexes are handled by the SPINAL CORD through a reflex arc, NOT by the conscious brain — that's exactly why they're so fast. The signal does reach the brain, but the response has already happened.
Nerve impulses travel both ways along a neuron.
A wire carries current both ways, so a neuron should too.
Within the pathway, information flows ONE way: dendrite → cell body → axon → synapse → next cell. The synapse acts like a one-way valve (chemicals are released on one side, received on the other).
A plant bends toward light because the lit side grows faster.
It seems the side facing the sun would be the active, growing one.
It's the opposite: auxin moves to the SHADY side, so the shady side grows LONGER, bending the shoot toward the light. The growth happens away from the light.
Hormones act instantly, just like nerves.
Both are 'messengers', so they seem similar.
Nerves are fast but local (only reach connected cells); hormones are SLOWER but travel in the blood to reach all cells and act in a sustained way. They're complementary systems, not the same.
Quick Check
The gap between two neurons where chemicals carry the signal across is called:
The tiny gap between two neurons is the synapse. The electrical impulse reaches the axon end, releases chemicals that cross the synapse, and trigger a new impulse in the next neuron.
Which of these is a plant hormone?
Cytokinin is a plant hormone (it promotes cell division). Insulin, thyroxin and oestrogen are all animal hormones.
Practice Problems
These are written by Curriv and are completely free. Try before revealing.
Easy
Name the three main parts of the human brain and one job of each.
- Fore-brain (cerebrum): thinking, deciding, and processing the senses (sight, smell, hearing).
- Mid-brain: controls some involuntary actions (such as reflexes of the eye).
- Hind-brain: the cerebellum controls balance and precise movement; the medulla controls involuntary actions like heartbeat and breathing.
Why is the use of iodised salt advisable?
The thyroid gland needs iodine to make the hormone thyroxin, which controls the body’s metabolism. If our diet lacks iodine, we may develop goitre (a swollen neck). Iodised salt supplies the iodine and prevents this.
Medium
How does our body respond when adrenaline is secreted into the blood?
Adrenaline (from the adrenal glands) prepares the body for an emergency (“fight or flight”):
- the heart beats faster, supplying more oxygen to the muscles,
- breathing rate increases (diaphragm and rib muscles contract more),
- blood is diverted away from the digestive system and skin toward the skeletal muscles.
Together these ready the body to act quickly.
How is the movement of a touch-me-not (sensitive plant) leaf different from a shoot bending toward light?
- The sensitive plant’s leaf folds quickly and involves no growth — cells change shape by gaining/losing water (swelling/shrinking). It is a temporary, reversible movement.
- A shoot bending toward light is slow and is caused by growth (the hormone auxin makes the shady side grow longer). It is a permanent, directional (tropic) movement.
Challenge
Compare nervous and hormonal control in animals — give two clear differences.
- Speed & duration: nerve impulses are very fast but short-lived; hormonal action is slower but longer-lasting.
- Reach: nerves only reach cells they are physically connected to; hormones travel in the blood and can reach all cells of the body.
(Also: nerves use electrical impulses, hormones use chemical messengers.) Together they give the body both quick local responses and slow body-wide ones.
Explain, using insulin, how a feedback mechanism keeps a hormone at the right level.
Insulin (from the pancreas) lowers blood sugar. The amount released is controlled by a feedback loop:
- When blood sugar rises (e.g. after a meal), the pancreas detects it and releases more insulin, which brings the sugar down.
- As blood sugar falls, the pancreas senses that and releases less insulin.
So the hormone level rises and falls automatically with need, keeping blood sugar steady. (In diabetes this control fails, so insulin may be given by injection.)
Summary
- Control and coordination let an organism sense changes (stimuli) and respond appropriately, using the nervous system (fast, electrical) and hormones (slower, chemical).
- A neuron carries info one way: receptor → dendrite → cell body → axon → synapse → next cell. The synapse passes the signal chemically.
- A reflex action uses a reflex arc in the spinal cord for instant responses, bypassing conscious thought.
- The brain = fore-brain (thinking, senses, voluntary action), mid-brain, and hind-brain (cerebellum = balance; medulla = heartbeat, breathing). Muscles move when proteins inside them shorten the cell.
- Plants coordinate without nerves: quick water-driven movements (touch-me-not) and slow tropic growth (photo-, geo-, hydro-, chemotropism), guided by hormones — auxin (bending toward light), gibberellins, cytokinins, and growth-inhibiting abscisic acid.
- Animal hormones (endocrine system): adrenaline (emergency), thyroxin (metabolism, needs iodine), insulin (blood sugar), growth hormone, testosterone/oestrogen (puberty). Feedback keeps each at the right level.
What’s Next
You’ve seen how a body keeps itself running (Chapter 5) and how it senses and controls everything (this chapter). But no individual lives forever — so how does life itself continue? In Chapter 7: How do Organisms Reproduce?, you’ll see how living things make more of their own kind: asexual and sexual reproduction, the flower, the human reproductive system, and why reproduction is the bridge that carries life from one generation to the next.