Coordination and Response
Organisms survive by detecting changes and responding to them. A change is detected, a message is sent, and something happens. Coordination is how that message gets from the place that detected the change to the place that acts on it.
Humans do this in two ways:
- The nervous system — fast, electrical, short-lived, targeted
- The endocrine (hormonal) system — slower, chemical, longer-lasting, widespread
Almost every question in this topic is about one of those two, or about how they keep internal conditions steady (homeostasis).
1. The essential vocabulary
Get these five right and most of the topic follows.
| Term | Definition |
|---|---|
| Stimulus | A change in the internal or external environment that triggers a response |
| Receptor | A specialised cell or organ that detects a stimulus |
| Effector | A muscle or gland that carries out the response |
| Response | The change produced by the effector |
| Synapse | The gap between two neurones |
Two of these are routinely confused:
A synapse is a gap, not a process. If a question asks “what is a synapse?”, the answer is the junction/gap between two neurones. The process of a message crossing it is synaptic transmission. Answering one when the question asked for the other loses the mark.
Effectors are muscles or glands. Only those two. If your answer names something else, check it again.
2. The nervous system
The nervous system has two divisions:
| Division | Contains |
|---|---|
| Central nervous system (CNS) | Brain and spinal cord |
| Peripheral nervous system (PNS) | All the nerves outside the brain and spinal cord |
(The 0610 syllabus does not require you to know the individual parts of the brain — cerebrum, medulla and so on. Nor does it require nodes of Ranvier, Schwann cells or the somatic/autonomic split. Those belong to other specifications and to A Level.)
The structure of a neurone
A neurone is a specialised cell — its structure is adapted to its job of conducting electrical impulses.
| Part | Function |
|---|---|
| Dendrites | Receive impulses from the previous neurone and carry them towards the cell body |
| Cell body | Contains the nucleus |
| Axon | Carries the impulse away from the cell body; long, so impulses travel over long distances |
| Myelin sheath | A fatty layer that insulates the axon, stopping the impulse leaking out and speeding up transmission |
| Axon terminals | Pass the impulse on to the next neurone |
How to tell dendrites from axons in a diagram: dendrites branch and end in fine points; axons end in rounded knobs (the terminals). If an arrow showing the direction of the impulse is given, use it — impulses always travel towards the cell body along dendrites and away from it along the axon.
Nerve impulses are electrical, not chemical. The myelin sheath works as an insulator, in exactly the way plastic coating works on an electrical wire.
The three types of neurone
| Type | Carries impulses | Cell body position |
|---|---|---|
| Sensory | From receptor → CNS | In the middle, off to one side of the fibre |
| Relay | Within the CNS, from sensory to motor | In the middle, with many short extensions all round |
| Motor | From CNS → effector | At one end, with one long axon |
Identifying them in an exam diagram: cell body at one end with a single long axon = motor. Cell body in the middle with many similar-length branches all round = relay. Cell body part-way along, with a long dendrite on one side = sensory.
3. The reflex arc
A reflex action is a means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors.
Its two examinable features are that it is:
- Rapid
- Automatic / involuntary — it does not involve conscious thought
The reflex arc is the pathway the impulse takes. Learn it as a fixed six-step sequence:
stimulus → receptor → sensory neurone → relay neurone (in CNS) → motor neurone → effector → response
Worked through with the classic example of touching a hot object:
- Stimulus — heat
- Receptor — temperature/pain receptors in the skin detect it
- Sensory neurone — carries the impulse to the spinal cord
- Relay neurone — inside the CNS, passes the impulse from the sensory to the motor neurone
- Motor neurone — carries the impulse out to the effector
- Effector — the muscle in the arm contracts, pulling the hand away
The impulse goes to the spinal cord rather than the brain because that route is shorter and therefore faster. The brain is informed, but only afterwards — which is why you notice the pain after your hand has already moved.
Voluntary vs involuntary
| Voluntary | Involuntary (reflex) | |
|---|---|---|
| Conscious thought? | Yes | No |
| Speed | Slower | Rapid |
| Coordinated by | Brain | Usually spinal cord |
| Example | Deciding to pick something up | Pulling your hand from a hot object; the pupil reflex |
A useful nuance: skeletal muscles can be involved in both. Pulling your hand away from something hot is involuntary but uses skeletal muscle. Don’t assume “involuntary” always means smooth muscle.
4. The synapse
Impulses are electrical along a neurone, but neurones do not touch. At the gap, the signal is carried chemically.
Synaptic transmission — the five-step answer
- The impulse arrives at the axon terminal of the presynaptic neurone
- Vesicles containing neurotransmitter move to and fuse with the membrane
- The neurotransmitter is released into the synapse (the gap)
- It diffuses across the synapse
- It binds to receptor molecules on the postsynaptic neurone, generating a new impulse
Step 3 is the one students omit. Saying the neurotransmitter goes “to the next neurone” without saying it is released into the synapse loses a mark. The whole process is named after that gap — mention it.
Note also that transmission is one-way: neurotransmitter is only released on one side and only received on the other, which is why impulses cannot travel backwards.
Drugs and the synapse — a recurring question type
Questions often show a synapse with and without a drug, and ask you to explain the effect. The reasoning chain is always the same, and you get a mark per link:
- Fewer neurotransmitter molecules released → fewer diffuse across the synapse → fewer bind to receptors on the postsynaptic neurone → the impulse is slower or does not pass at all
Or, if the drug is shown occupying the receptors:
- The drug blocks the receptor molecules → neurotransmitter cannot bind → no impulse is generated in the next neurone → the response is delayed or does not happen
5. Sense organs and the eye
A sense organ is a group of receptor cells responding to specific stimuli.
The eye is the only sense organ examined in detail.
Structure and function
| Structure | Function |
|---|---|
| Cornea | Refracts (bends) light — it does most of the focusing |
| Sclera | Tough outer coat; protects the eye and maintains its shape |
| Conjunctiva | Thin transparent membrane that keeps the eye moist and lubricated |
| Iris | The coloured part; controls the size of the pupil, and so how much light enters |
| Pupil | The hole through which light enters — not a structure, just a gap |
| Lens | Focuses light onto the retina; changes shape for near and distant objects |
| Ciliary muscles | Contract and relax to change the shape of the lens |
| Suspensory ligaments | Hold the lens in place and transmit the pull of the ciliary muscles |
| Retina | Contains the light-sensitive cells (rods and cones) |
| Optic nerve | Carries impulses to the brain |
| Choroid | Layer containing blood vessels supplying the eye; black pigment absorbs stray light |
| Fovea (yellow spot) | Region of the retina with the highest concentration of cones — sharpest, most detailed image |
| Blind spot | Where the optic nerve leaves the retina — no rods or cones, so no image can be formed there |
Two corrections worth making explicitly:
The cornea refracts light — it does not block refraction. It is the eye’s main refracting surface, bending light inwards so that it can be focused on the retina.
The blind spot is not the edge of your peripheral vision. It is a specific small area where the optic nerve leaves the eye, containing no light-sensitive cells. The limit of how far you can see to the side is a different thing entirely.
Also worth knowing: the image formed on the retina is upside down. The brain interprets it the right way up.
Rods and cones
| Rods | Cones | |
|---|---|---|
| Sensitive to | Low light intensity | Bright light |
| Colour? | No — grey/black-and-white only | Yes |
| Types | One | Three — sensitive to red, green and blue |
| Where | Spread across the retina | Concentrated at the fovea |
This is why colours are hard to distinguish at night — only the rods are working.
The pupil reflex
The iris contains two sets of muscles that work as an antagonistic pair: when one contracts, the other relaxes.
| Light | Circular muscles | Radial muscles | Pupil |
|---|---|---|---|
| Bright | Contract | Relax | Constricts (gets smaller) — less light enters, protecting the retina |
| Dim | Relax | Contract | Dilates (gets wider) — more light enters |
A memory hook that survives exam pressure: C for Contract, C for Circular — in bright light the circular muscles contract.
Contract means get shorter, not bigger. Students regularly reason that “the circular muscle contracts, so it gets bigger, so the pupil gets bigger.” The opposite is true: the circular muscle shortens, tightening like a drawstring around the pupil and making the hole smaller.
This is a reflex action — it is rapid and automatic, and you cannot choose to stop it.
Accommodation
Accommodation is the change in shape of the lens to focus on near or distant objects.
| Looking at | Ciliary muscles | Suspensory ligaments | Lens |
|---|---|---|---|
| A near object | Contract | Slacken (loosen) | Becomes thicker / more curved — refracts light more |
| A distant object | Relax | Become tight | Becomes thinner / flatter — refracts light less |
The logic: when the ciliary muscle contracts, the ring of muscle gets smaller, so the ligaments are no longer pulled tight, so the lens is released and springs into a fatter shape.
Do not confuse accommodation with the pupil reflex. Accommodation changes the shape of the lens in response to distance. The pupil reflex changes the size of the pupil in response to light intensity. Different muscles, different stimuli, different structures.
6. Hormones and the endocrine system
A hormone is a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.
All four parts of that definition can earn marks: chemical, gland, blood, target organ.
Hormones travel everywhere in the blood, but only affect cells with complementary receptors that the hormone can bind to. That is why a hormone has specific targets despite circulating throughout the body.
Glands you should know
| Gland | Hormone | Main effect |
|---|---|---|
| Adrenal glands | Adrenaline | Prepares the body for action |
| Pancreas | Insulin and glucagon | Regulate blood glucose |
| Testes | Testosterone | Male secondary sexual characteristics |
| Ovaries | Oestrogen, progesterone | Female secondary sexual characteristics; menstrual cycle |
Adrenaline — the “fight or flight” hormone
Released from the adrenal glands when the body is frightened, stressed or in danger. Its effects all serve one purpose: preparing the muscles for vigorous activity.
| Effect | Why |
|---|---|
| Increased heart / pulse rate | Delivers oxygen and glucose to muscles faster |
| Increased breathing rate and depth | Takes in more oxygen for respiration |
| Increased blood glucose concentration | More glucose available for respiration (the liver converts glycogen to glucose) |
| Pupils dilate | More light enters the eye |
| Blood diverted to muscles, away from the digestive system | Muscles get the oxygen and glucose they need; digestion can wait |
Answers about adrenaline score best when each effect is linked to increased respiration in muscles, rather than just listed.
Nervous vs hormonal coordination
This comparison is examined constantly.
| Nervous system | Endocrine system | |
|---|---|---|
| Message type | Electrical impulse | Chemical (hormone) |
| Transported by | Neurones | The blood |
| Speed | Very fast (fractions of a second) | Slower |
| Duration of effect | Short-lived | Long-lasting |
| Area affected | Localised — one specific area | Widespread — may affect several organs |
| Example | Reflex actions | Growth, puberty, blood glucose control |
(Adrenaline is the exception that proves the rule — it acts quickly for a hormone.)
7. Homeostasis
Homeostasis is the maintenance of a constant internal environment.
Body conditions such as temperature, blood glucose concentration and water content must be kept within narrow limits, because the enzyme-controlled reactions in cells only work properly within those limits.
Negative feedback
Almost all homeostatic control works by negative feedback:
- A condition deviates from its set point
- The change is detected
- Corrective mechanisms are activated, acting in the opposite direction
- The condition returns to the set point
- The corrective mechanisms are switched off
If a level rises, mechanisms switch on to lower it; if it falls, mechanisms switch on to raise it. Always the opposite direction — hence negative.
Control of blood glucose
The pancreas both detects the change and releases the hormone.
When blood glucose is too high (for example, after a meal):
pancreas detects it → releases insulin → liver converts glucose → glycogen for storage → blood glucose falls
When blood glucose is too low:
pancreas detects it → releases glucagon → liver converts glycogen → glucose → blood glucose rises
Glycogen and glucagon are not the same word. Glycogen is the stored carbohydrate in the liver. Glucagon is the hormone. They are also both different from glucose. This is comfortably the most common spelling-and-meaning error in the whole topic — and glucagon is a hormone, never an enzyme.
Type 1 diabetes is a condition in which the pancreas cannot produce enough insulin, so blood glucose cannot be regulated. It is treated with insulin injections, together with careful diet and exercise. Symptoms include extreme thirst, tiredness, blurred vision and weight loss.
Control of body temperature
Normal human body temperature is about 37 °C. Temperature receptors in the skin and in the brain detect changes.
When too hot:
- Sweating — water in sweat evaporates from the skin, and evaporation takes heat energy from the body, cooling it
- Vasodilation — the arterioles supplying the skin capillaries widen, so more blood flows through the capillaries near the skin surface, and more heat is lost by radiation
- Hairs lie flat, so no insulating layer of air is trapped
When too cold:
- Shivering — rapid, involuntary muscle contraction; respiration in the muscles releases heat
- Vasoconstriction — the arterioles supplying the skin capillaries narrow, so less blood flows near the surface and less heat is lost
- Hairs stand upright, trapping a layer of air, which is a good insulator
- Sweating stops
A correction that matters: blood vessels do not move closer to or further from the skin surface. This is a very widespread misconception. What changes is the diameter of the arterioles supplying the skin capillaries, and therefore how much blood flows through them. Writing that the vessels “move up” or “sink down” will not score, and describes something that does not happen.
Note also that sweating only cools you when the sweat evaporates. Sweat running off the skin has no cooling effect — the mark is for evaporation.
8. Tropisms
A tropism is a growth response of a plant, in which the direction of growth is determined by the direction of the stimulus.
| Tropism | Stimulus | Shoots | Roots |
|---|---|---|---|
| Phototropism | Light | Positive — grow towards light | Negative |
| Gravitropism (geotropism) | Gravity | Negative — grow upwards | Positive — grow downwards |
How phototropism works — the full-mark answer
The chemical involved is auxin. The sequence:
- Auxin is made in the shoot tip
- Auxin stimulates cell elongation
- Light causes unequal distribution of auxin — it moves to the shaded side
- So there is a higher concentration of auxin on the shaded side
- The cells on the shaded side elongate more, so that side grows longer
- The shoot therefore bends towards the light
Start with where auxin is made. “Auxin is produced in the shoot tip” is a mark on its own and is the point students most often skip, jumping straight to “auxin moves to the shaded side.”
Use “stimulates cell elongation.” Not “makes it grow”, not “encourages growth”. Elongation is the term.
Why this is an advantage: the shoot grows towards the light, so the leaves absorb more light for photosynthesis, producing more glucose for growth and giving a higher yield.
9. Mistakes that cost marks
Saying a synapse is the transmission of an impulse. A synapse is the gap. The process is synaptic transmission.
Forgetting that the neurotransmitter is released into the synapse.
Saying “contract” means “get bigger.” Muscles get shorter when they contract.
Mixing up the pupil reflex and accommodation. Light intensity → pupil size. Distance → lens shape.
Confusing glycogen, glucagon and glucose.
Saying blood vessels move towards or away from the skin in temperature regulation. They change diameter.
Saying sweating cools you without mentioning evaporation.
Omitting that auxin is made in the shoot tip.
Saying the blind spot is the edge of your vision. It is where the optic nerve leaves the retina.
Naming something other than a muscle or gland as an effector.
Writing “it’s more accurate” when asked why an investigation should be repeated. The answer wanted is to identify anomalous results.
Frequently asked questions
What is the difference between the nervous and endocrine systems? The nervous system sends electrical impulses along neurones — fast, short-lived and localised. The endocrine system sends chemical hormones in the blood — slower, longer-lasting and more widespread.
Why does a reflex go through the spinal cord rather than the brain? Because it is a shorter pathway and therefore faster. Speed is the whole point of a reflex.
What is the difference between a receptor and an effector? A receptor detects the stimulus. An effector — a muscle or gland — carries out the response.
Which muscles contract in bright light? The circular muscles of the iris contract, and the radial muscles relax, so the pupil constricts.
What happens to the lens when you look at something close? The ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes thicker and more curved.
Does insulin raise or lower blood glucose? It lowers it, by causing the liver to convert glucose into glycogen. Glucagon raises it.
Why do plants bend towards light? Auxin made in the shoot tip accumulates on the shaded side, where it stimulates cells to elongate more. The shaded side grows longer, so the shoot bends towards the light.
Quick revision checklist
- I can define stimulus, receptor, effector, response and synapse
- I know what the CNS and PNS each contain
- I can label a neurone and give the function of each part
- I can identify sensory, relay and motor neurones from a diagram
- I can write the reflex arc as a six-step sequence
- I can state the two features of a reflex action
- I can describe synaptic transmission, including release into the synapse
- I can explain how a drug blocking receptors affects a response
- I can label the eye and give the function of every structure
- I know the pupil reflex in both directions, with the correct muscles
- I can describe accommodation for near and distant objects
- I know the difference between rods and cones, and what the fovea and blind spot are
- I can define a hormone in a way that earns all the marks
- I can list the effects of adrenaline and link each to respiration in muscles
- I can compare nervous and hormonal coordination across all five rows
- I can define homeostasis and explain negative feedback
- I can describe blood glucose control in both directions without confusing glycogen and glucagon
- I can describe temperature control, using evaporation and arteriole diameter correctly
- I can explain phototropism starting from where auxin is made
These notes cover topic 14 of the Cambridge IGCSE Biology (0610) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across many one-to-one IGCSE Biology lessons, with particular attention to the errors students make most often and the wording examiners reward.
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