Neurons, receptors & nerve impulses
A mosquito lands on your arm and you slap it before you even think. Messages travelled through nerve cells at lightning speed.
A stimulus is a change around or inside us. Receptors (in the eye, ear, nose, tongue, skin) detect it. The neuron is the nerve cell that carries the message as an electrical nerve impulse. At a synapse the impulse is passed to the next neuron by a chemical.
Equation / formula : Stimulus → receptor → dendrite → cell body → axon → synapse → next neuron
In the figure : The impulse always runs one way: dendrite end to axon end, then jumps the synapse by a chemical.
Neurons, receptors & nerve impulses
In the app you can move this figure with your finger.
Open in the appRule 1 : Receptors and their organs — Each sense organ has receptors for one kind of stimulus: light in the eye, sound in the ear, smell in the nose, taste on the tongue, touch and heat in the skin.
- Easy : Smelling hot tea: olfactory receptors in the nose pick up the smell.
- Medium : Burning a fingertip is sensed by heat receptors in the skin, which start the impulse.
- For exams : Receptors convert the stimulus into an electrical impulse; this is why a damaged receptor means no sensation even if the nerve is fine.
Rule 2 : Parts of a neuron and the synapse — The dendrites collect the message, the cell body holds the nucleus, the long axon carries the impulse away. At the synapse the axon end releases a chemical that crosses the gap and starts a new impulse in the next neuron.
- Easy : Think of a relay race: the baton (chemical) is handed from one runner (neuron) to the next.
- Medium : A neuron can be as long as a metre (e.g., from toe to spinal cord) because the axon is very long.
- For exams : Impulses cannot go backwards because the chemical is released only from the axon end and received only by dendrites.
Nerve impulses are fast because they are electrical; hormones, being chemical, are slower.
Saying the dendrite sends the impulse out. The dendrite receives; the axon sends.
Receptor senses, dendrite takes in, axon sends out, synapse passes on. Memory clue : "DA-S": Dendrite Accepts, Axon Sends, Synapse hands over.
Spot the difference
| Point | Dendrite | Axon |
|---|---|---|
| Job | receives impulse | sends impulse away |
| Length | short, branched | long, single |
Practice questions
- 1What is a stimulus? Give an example.
- 2Name the receptor-containing organ for smell.
- 3What is a synapse?
- 4Which part of a neuron receives the impulse?
- 5Which part carries the impulse away from the cell body?
- 6Why does an impulse travel in one direction only?
- 7How does the impulse cross the gap at a synapse?
- 8If the receptors in the skin of a finger are damaged but the nerve is healthy, will the person feel touch? Explain.
Show answers (try them yourself first)
- 1A change we can detect, e.g., a pin-prick or a sound.
- 2Nose.
- 3The tiny gap where one neuron passes the impulse to the next by a chemical.
- 4Dendrite.
- 5Axon.
- 6The chemical is released only at the axon end and received only by dendrites.
- 7A chemical released from the axon end diffuses across and starts a new impulse.
- 8No; receptors start the impulse, so without them no impulse begins.
Reflex action & reflex arc
You touch a hot pan and pull your hand back before feeling the pain. The spinal cord decided, not the brain.
A reflex action is a quick, automatic response to a stimulus without thinking. The pathway it follows is the reflex arc: receptor → sensory neuron → spinal cord (relay neuron) → motor neuron → effector (muscle or gland).
Equation / formula : Receptor → sensory neuron → spinal cord → motor neuron → effector
In the figure : The short loop through the spinal cord saves precious time, so the hand moves before the brain is even aware.
Rule 1 : Reflex vs voluntary action — Reflexes are automatic and fast and are handled by the spinal cord. Voluntary actions (writing, kicking a ball) are decided by the fore-brain after thinking.
- Easy : Blinking at a sudden flash is a reflex; choosing a pen to write is voluntary.
- Medium : Heartbeat and breathing are involuntary but are not always reflexes, since they are not a quick answer to a stimulus.
- For exams : Reflex arcs avoid the delay of thinking; this matters most in animals with simple nervous systems and in danger.
Rule 2 : Three kinds of neurons — The sensory neuron brings the message in, the relay (connecting) neuron links the two sides inside the spinal cord, and the motor neuron carries the order out to the effector.
- Easy : Sensory = in, motor = out.
- Medium : If the sensory neuron is cut, the impulse never reaches the cord; if the motor neuron is cut, the muscle never gets the order.
- For exams : The effector is a muscle or a gland; in the hot-pan reflex it is the arm muscle.
The spinal cord is the centre of reflex actions; the brain is informed afterwards.
Saying the brain controls a reflex. The spinal cord does; the brain only receives the news.
Reflex = fast, automatic, spinal cord; voluntary = thinking, fore-brain. Memory clue : "RSSME": Receptor, Sensory, Spinal cord, Motor, Effector.
Spot the difference
| Point | Reflex action | Voluntary action |
|---|---|---|
| Speed | very fast | slower |
| Control by | spinal cord | fore-brain |
Practice questions
- 1Define reflex action.
- 2Give two examples of reflex actions.
- 3Name the effector in the hot-pan reflex.
- 4Write the reflex arc in order.
- 5What does the relay neuron do?
- 6Why is a reflex arc useful in danger?
- 7Is sneezing voluntary or reflex? Why?
- 8Differentiate reflex and voluntary action in two points.
Show answers (try them yourself first)
- 1A quick automatic response to a stimulus without thinking.
- 2Pulling the hand from a hot object; blinking at a flash.
- 3The arm muscle.
- 4Receptor → sensory neuron → spinal cord → motor neuron → effector.
- 5Connects sensory and motor neurons inside the spinal cord.
- 6It saves time because the brain need not decide.
- 7Reflex; it happens automatically without thinking.
- 8Reflex: fast, spinal cord. Voluntary: slower, fore-brain, thought-out.
Human brain & its protection
Riding a cycle needs thinking, balance, and a steady heartbeat all at once. Different parts of the brain share the work.
The brain and spinal cord form the central nervous system; nerves from them form the peripheral nervous system. The brain has three regions: fore-brain, mid-brain and hind-brain (cerebellum, pons, medulla).
Equation / formula : Fore-brain: thinking, senses, hunger · Cerebellum: balance · Medulla: involuntary actions
In the figure : The skull holds the brain, the backbone holds the cord, and fluid inside acts as a cushion.
Rule 1 : Parts of the brain — The fore-brain is the largest part and is the thinking centre with sensory, association and motor areas. The cerebellum keeps balance and makes movement precise. The medulla runs involuntary actions like blood pressure, saliva and vomiting.
- Easy : Threading a needle needs the cerebellum; feeling full after a meal is the fore-brain.
- Medium : A person who cannot walk straight (as if drunk) may have damage to the cerebellum.
- For exams : Damage to the medulla is dangerous because it controls life-supporting functions like breathing and heartbeat.
Rule 2 : Protection of brain and cord — The brain is inside the bony skull and floats in a fluid-filled bag that absorbs shocks. The spinal cord is inside the vertebral column (backbone).
- Easy : A helmet adds extra protection to the skull.
- Medium : A spinal-cord injury can cut the signals between the brain and the body parts below the injury.
- For exams : Muscle cells move because special proteins in them change shape on receiving a nerve impulse, making the cell shorter.
Cranial nerves come from the brain, spinal nerves from the cord; together they link the centre with the body.
Saying the cerebellum controls heartbeat. That is the medulla; the cerebellum handles balance.
Fore-brain thinks, cerebellum balances, medulla keeps vital functions going. Memory clue : "F-C-M": Fore = Fikar (thought), Cerebellum = Cycle (balance), Medulla = Machine (heart, breath).
Spot the difference
| Point | Cerebellum | Medulla |
|---|---|---|
| Controls | balance, precise movement | BP, saliva, vomiting |
| Part of | hind-brain | hind-brain |
Practice questions
- 1Name the three regions of the brain.
- 2Which part controls balance?
- 3Name the structure that protects the brain.
- 4What do the cranial nerves do?
- 5What is the role of the association area?
- 6Why is the spinal cord inside the backbone?
- 7Which part is likely damaged if a person loses hunger sensation?
- 8How does a muscle cell shorten when an impulse arrives?
Show answers (try them yourself first)
- 1Fore-brain, mid-brain, hind-brain.
- 2Cerebellum.
- 3Skull and the fluid-filled bag.
- 4Carry messages between the brain and head and body parts.
- 5Compares new information with stored memory and interprets it.
- 6For protection; it is a delicate message pathway.
- 7Fore-brain (hunger centre).
- 8Special proteins change shape and the cell becomes shorter.
Coordination in plants
A houseplant on a window ledge slowly leans toward the light, and a touch-me-not folds its leaves in a second. Plants respond without nerves.
Tropism is growth of a plant part in a direction set by a stimulus: phototropism (light), geotropism (gravity), hydrotropism (water), chemotropism (chemicals). Plant hormones such as auxin, gibberellin, cytokinin and abscisic acid coordinate growth.
Equation / formula : Shoot: toward light, away from gravity · Root: away from light, toward gravity and water
In the figure : In every tropism the plant bends by unequal growth of cells on two sides.
Rule 1 : Quick vs slow movement — The touch-me-not folds its leaves at once because cells at the leaf base lose water and shrink; no growth is needed. Tropisms are slow because they depend on growth.
- Easy : Touch-me-not folds in seconds; a shoot takes hours to bend toward light.
- Medium : Plants pass information from cell to cell by electrical-chemical signals; they have no nerves.
- For exams : Tendrils coil because the side touching the support grows slower than the far side.
Rule 2 : Plant hormones — Auxin (made at the shoot tip) makes cells grow longer; when light falls from one side, auxin moves to the shaded side so that side grows longer and the shoot bends toward light. Gibberellins help stem growth, cytokinins promote cell division, abscisic acid slows growth and causes wilting.
- Easy : Auxin = "grow longer"; ABA = "stop and wilt".
- Medium : Cytokinin is plentiful in fruits and seeds, where cells divide rapidly.
- For exams : Plants need growth inhibitors too, so that growth is regulated and not endless.
Plant hormones are made away from where they act and spread by diffusion; there is no blood or nerve.
Saying auxin moves to the lighted side. It moves to the shaded side.
Shoot: +light, −gravity. Root: −light, +gravity, +water. Auxin goes to shade. Memory clue : "AGCA": Auxin grows, Gibberellin stems, Cytokinin divides, ABA arrests.
Spot the difference
| Point | Touch-me-not folding | Phototropism |
|---|---|---|
| Speed | very fast | slow |
| Needs growth? | no (water loss) | yes (unequal growth) |
Practice questions
- 1What is tropism?
- 2Name the hormone made at the shoot tip.
- 3Which tropism makes roots grow down?
- 4Name a plant hormone that promotes cell division.
- 5Why does the touch-me-not fold its leaves quickly?
- 6Explain why a shoot bends toward light.
- 7How do tendrils coil round a support?
- 8Why are growth inhibitors like ABA needed?
Show answers (try them yourself first)
- 1Directional growth of a plant part in response to a stimulus.
- 2Auxin.
- 3Geotropism.
- 4Cytokinin.
- 5Cells at the leaf base lose water and shrink; no growth needed.
- 6Auxin moves to the shaded side, making those cells longer, so the shoot bends toward light.
- 7The side touching the support grows slower than the far side.
- 8To regulate growth and prevent endless growth.
Hormones in animals
Before an exam your heart pounds and palms sweat. A hormone, adrenaline, has been released into your blood.
Hormones are chemical messengers made by endocrine glands (no ducts), released into the blood and acting on target organs. They work slower than nerves but last longer. The amount is kept right by feedback control.
Equation / formula : Gland → hormone in blood → target organ · High blood sugar → more insulin → sugar falls → less insulin
In the figure : Each gland sends its hormone into the blood; only the cells with matching receptors respond.
Rule 1 : Adrenaline, thyroxin, insulin — Adrenaline (adrenal glands) speeds up heartbeat and breathing and sends blood to the muscles. Thyroxin (thyroid) needs iodine and regulates metabolism; lack of iodine causes goitre. Insulin (pancreas) lowers blood sugar; too little causes diabetes.
- Easy : Iodised salt prevents goitre.
- Medium : A diabetic person may take insulin injections because the body does not make enough to control sugar.
- For exams : Adrenaline prepares many organs at once for emergency: faster heart, faster breathing, less blood to digestion and skin.
Rule 2 : Growth, puberty and feedback — The pituitary gland (base of the brain) makes growth hormone: too little gives dwarfism, too much gives gigantism. Testosterone (testes) and oestrogen (ovaries) bring the changes of puberty. Feedback keeps each hormone in the right amount.
- Easy : Voice change in boys and the start of periods in girls are puberty changes.
- Medium : After a sweet meal, blood sugar rises, so the pancreas releases more insulin; when sugar falls, it releases less.
- For exams : Hormones act only on target cells that have the matching receptors, though the blood carries them everywhere.
Nerves give fast, short-lived control; hormones give slower but longer-lasting control. Many actions need both.
Saying thyroxin is made by the pituitary, or insulin by the liver. Thyroxin: thyroid; insulin: pancreas.
Adrenal – adrenaline; thyroid – thyroxin; pancreas – insulin; pituitary – growth. Memory clue : "A-T-I-G": Adrenal-Adrenaline, Thyroid-Thyroxin, Insulin-pancreas, Growth-pituitary.
Spot the difference
| Point | Nervous control | Hormonal control |
|---|---|---|
| Carrier | nerve (electrical) | blood (chemical) |
| Speed / duration | fast, short | slow, long |
Practice questions
- 1What is a hormone?
- 2Which gland makes insulin and what does it do?
- 3Which element is needed for thyroxin?
- 4Name the disease caused by iodine deficiency.
- 5Which hormone prepares the body for an emergency?
- 6What happens if growth hormone is low in childhood?
- 7How does feedback control insulin level?
- 8Differentiate nervous and hormonal control in two points.
Show answers (try them yourself first)
- 1A chemical messenger made by an endocrine gland and carried by blood.
- 2Pancreas; lowers blood sugar.
- 3Iodine.
- 4Goitre.
- 5Adrenaline.
- 6Dwarfism.
- 7High sugar → more insulin; as sugar falls, less insulin.
- 8Nerve: electrical, fast, short. Hormone: chemical in blood, slow, long-lasting.