Coordination and Response
Summary: Organisms detect stimuli and produce responses through two coordination systems: the nervous system (fast, electrical impulses along neurones) and the endocrine system (slower, chemical hormones in the blood). The nervous system involves CNS (brain + spinal cord), sensory/motor/relay neurones, reflex arcs, and synapses. Key hormones include adrenaline (“fight or flight”), insulin and glucagon (blood glucose regulation). The eye detects light and can accommodate (change lens shape) for near and far vision. Tags: igcse biology nervous-system hormones reflex-arc eye homeostasis insulin Created: 2026-07-16 Last Updated: 2026-07-16
1. Nervous vs Hormonal Control
| Feature | Nervous System | Endocrine (Hormonal) System |
|---|---|---|
| Nature of signal | Electrical impulses | Chemical messengers (hormones) |
| Speed | Very fast (milliseconds) | Relatively slow (seconds to days) |
| Pathway | Along neurones (specific fixed paths) | Through the bloodstream (general, to all cells) |
| Target | Specific cells (muscles, glands) at synapse | Specific target cells with complementary receptors |
| Duration | Short-lived (impulse stops quickly) | Longer-lasting (hormones persist in blood) |
| Examples | Reflex actions, voluntary movements | Blood glucose control, puberty, adrenaline response |
2. The Nervous System
Components
| Structure | Function |
|---|---|
| Central Nervous System (CNS) | Brain and spinal cord. Integrates sensory information and coordinates responses |
| Peripheral Nervous System (PNS) | All nerves outside the CNS. Carries impulses between the CNS and the rest of the body |
| Sensory neurones | Carry impulses from receptors (sense organs) to the CNS |
| Motor neurones | Carry impulses from the CNS to effectors (muscles or glands) |
| Relay neurones (intermediate/connector neurones) | Found in the CNS. Connect sensory neurones to motor neurones |
Neurones
A neurone is a nerve cell specialised for transmitting electrical impulses.
| Part of Neurone | Function |
|---|---|
| Dendrites | Branched endings that receive impulses from other neurones or receptors |
| Cell body | Contains the nucleus and most cytoplasm |
| Axon | Long extension that carries impulses away from the cell body |
| Myelin sheath | Fatty layer around the axon that insulates it and speeds up impulse transmission |
| Synaptic knobs (axon terminals) | Swollen endings of the axon that pass impulses to the next neurone or effector |
The Reflex Arc
A reflex is a rapid, automatic, involuntary response to a stimulus. It does not involve conscious thought (brain) — the pathway bypasses the brain to save time, increasing the chance of survival.
Pathway of a reflex arc:
- Stimulus (e.g. heat from a flame) detected by a receptor
- Receptor (e.g. skin pain receptor) generates an impulse
- Sensory neurone carries the impulse to the spinal cord (CNS)
- Relay neurone in the CNS passes the impulse from sensory to motor neurone
- Motor neurone carries the impulse from the CNS to an effector
- Effector (e.g. biceps muscle) contracts → response (hand pulls away)
Key features of reflexes:
- Involuntary — no conscious decision
- Stereotyped — always the same response to a given stimulus
- Fast — bypasses the brain (only goes through spinal cord or brainstem)
- Protective — protects the body from harm (e.g. blinking, coughing, withdrawing from pain)
Synapses
A synapse is the junction between two neurones (or between a neurone and an effector).
How a synapse works:
- An electrical impulse arrives at the presynaptic knob
- This triggers the release of neurotransmitter (a chemical, e.g. acetylcholine) from vesicles into the synaptic cleft (the gap)
- The neurotransmitter diffuses across the cleft
- It binds to receptors on the postsynaptic membrane
- This generates a new electrical impulse in the postsynaptic neurone
Functions of synapses:
- Ensure one-way transmission of impulses (neurotransmitter is only released on one side)
- Allow integration — a postsynaptic neurone can receive inputs from many presynaptic neurones
- Can be involved in learning and memory
3. The Endocrine System
The endocrine system consists of glands that secrete hormones directly into the bloodstream.
| Hormone | Source (Gland) | Target | Effects |
|---|---|---|---|
| Adrenaline | Adrenal glands (above kidneys) | Multiple organs (heart, liver, muscles) | “Fight or flight” response: increases heart rate, increases blood pressure, dilates pupils, increases blood flow to muscles, stimulates liver to release glucose (from glycogen) for respiration |
| Insulin | Pancreas (beta cells in islets of Langerhans) | Liver and muscles | Lowers blood glucose: stimulates cells to take up glucose; stimulates liver and muscles to convert glucose → glycogen (glycogenesis) |
| Glucagon | Pancreas (alpha cells in islets of Langerhans) | Liver | Raises blood glucose: stimulates liver to convert glycogen → glucose (glycogenolysis) and release it into the blood |
| ADH (antidiuretic hormone) | Pituitary gland (in brain) | Kidney collecting ducts | Increases water reabsorption — reduces urine volume (see Excretion in Humans) |
| FSH (follicle stimulating hormone) | Pituitary gland | Ovaries (females), testes (males) | Stimulates egg development and oestrogen secretion (females); stimulates sperm production (males) |
| LH (luteinising hormone) | Pituitary gland | Ovaries, testes | Triggers ovulation (females); stimulates testosterone production (males) |
| Oestrogen | Ovaries | Uterus, pituitary | Repairs and thickens uterine lining; controls secondary sexual characteristics in females |
| Progesterone | Ovaries (corpus luteum) | Uterus | Maintains uterine lining for implantation of embryo |
| Testosterone | Testes | Male reproductive organs | Controls sperm production and secondary sexual characteristics in males |
Blood Glucose Regulation (Negative Feedback)
Blood glucose concentration must be kept within narrow limits because:
- Glucose is the main substrate for respiration — cells need a constant supply
- High glucose damages cells; low glucose means insufficient energy for brain function
When blood glucose is too HIGH:
- Detected by pancreas (beta cells)
- Pancreas secretes insulin
- Insulin stimulates: liver and muscles to convert glucose → glycogen; cells to take up more glucose from blood
- Blood glucose concentration falls back to normal
- Pancreas reduces insulin secretion
When blood glucose is too LOW:
- Detected by pancreas (alpha cells)
- Pancreas secretes glucagon
- Glucagon stimulates liver to convert glycogen → glucose, released into blood
- Blood glucose concentration rises back to normal
- Pancreas reduces glucagon secretion
Diabetes — Type 1
Type 1 diabetes is an autoimmune disease where the immune system destroys the beta cells of the pancreas, resulting in little or no insulin production.
Consequences:
- Blood glucose rises to dangerous levels after eating (hyperglycaemia)
- Glucose appears in urine (kidneys cannot reabsorb all of it)
- Symptoms: excessive thirst, frequent urination, weight loss, fatigue
Management:
- Regular insulin injections (or insulin pump)
- Carefully controlled diet (managing carbohydrate intake)
- Regular exercise
- Regular blood glucose monitoring
4. The Eye — Structure and Accommodation
Structure of the Eye
| Structure | Description and Function |
|---|---|
| Cornea | Transparent outer layer at the front of the eye. Refracts (bends) light as it enters the eye — provides most of the eye’s focusing power |
| Iris | Coloured ring of muscle. Controls the size of the pupil, regulating how much light enters the eye |
| Pupil | Hole in the centre of the iris. Light enters through the pupil |
| Lens | Transparent, flexible, biconvex structure behind the iris. Changes shape for accommodation (focusing on near and far objects) |
| Ciliary muscles | Ring of muscle around the lens. Contract/relax to change the shape of the lens |
| Suspensory ligaments | Fibres connecting the ciliary muscles to the lens. Transmit tension from ciliary muscles to the lens |
| Retina | Light-sensitive layer at the back of the eye. Contains rods (detect light intensity, black and white, work in dim light) and cones (detect colour, work in bright light). Fovea is the area of the retina with the highest concentration of cones — sharpest vision |
| Optic nerve | Bundle of sensory neurones carrying impulses from the retina to the brain |
| Sclera | Tough, white outer coat. Protects the eyeball |
| Choroid | Dark layer between sclera and retina. Contains blood vessels supplying the retina; dark pigment absorbs light (prevents internal reflection) |
| Blind spot | Point where the optic nerve leaves the retina. No rods or cones — no vision at this point |
Accommodation — Focusing on Near and Far Objects
The lens changes shape to focus light onto the retina.
| Near Vision (close object) | Far Vision (distant object) | |
|---|---|---|
| Light rays from object | Diverging | Parallel |
| Need | More refraction (bending) | Less refraction |
| Ciliary muscles | Contract | Relax |
| Suspensory ligaments | Slacken (reduced tension) | Tighten (pulled taut) |
| Lens shape | Fat/rounded (more convex) | Thin/flattened (less convex) |
| Refractive power | Higher | Lower |
Pupil Reflex
The iris adjusts pupil size to control the amount of light entering the eye.
| Bright Light | Dim Light | |
|---|---|---|
| Circular muscles in iris | Contract | Relax |
| Radial muscles in iris | Relax | Contract |
| Pupil size | Constricts (smaller) | Dilates (larger) |
| Purpose | Prevents damage to retina from too much light | Allows more light in to improve vision |
Related Notes
- Cell Structure and Organisation — Nerve cells (neurones) as specialised cells
- Movement Into and Out of Cells — Active transport in nerve impulse transmission (Na+/K+ pump)
- Excretion in Humans — ADH role in osmoregulation by the kidneys
- Reproduction — FSH, LH, oestrogen, progesterone in the menstrual cycle
- Human Nutrition — Blood glucose; diabetes management
- IGCSE-Bio-Index — Full IGCSE Biology index
Sources
- BBC Bitesize GCSE Biology — Nervous system / Hormones and the endocrine system, BBC (free educational resource)
- OpenStax Biology 2e — Chapter 35: The Nervous System / Chapter 37: The Endocrine System, Rice University (free, CC BY 4.0)
- Cambridge IGCSE Biology 0610 — Topic 14: Coordination and Response, Cambridge Assessment International Education
- CK-12 Biology for High School — Nervous System and Endocrine System chapters, CK-12 Foundation (free, CC BY-NC 3.0)
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”The brain is part of every reflex” | The reflex arc bypasses the conscious brain — impulses travel to the spinal cord/brainstem and straight back out. The brain is informed afterwards |
| ”Hormones travel in nerves” | Hormones travel in the bloodstream. Nerve impulses travel along neurones. They are completely different systems |
| ”Insulin raises blood glucose” | Insulin lowers blood glucose (stimulates glucose → glycogen). Glucagon raises blood glucose (stimulates glycogen → glucose) |
| “The lens does all the focusing in the eye” | The cornea does most of the refracting (fixed focus). The lens provides fine adjustment (accommodation) |
| “For near vision, the ciliary muscles relax” | For near vision, ciliary muscles contract (suspensory ligaments slacken), allowing the lens to become more rounded. For far vision, ciliary muscles relax |
| ”All neurones look the same” | Sensory, motor, and relay neurones have different structures — they are specialised for their roles in the reflex arc |