Nervous System & Hormones (Years 9–11) — Science

How your body senses and responds — fast nerves, slower hormones, and homeostasis.

Touch something scalding hot and your hand pulls back before you have even thought about it. That is your body's control systems at work, keeping you safe and steady without you noticing.

Why this matters: Your insides need to stay within a narrow safe range — body temperature, blood sugar, water levels. Keeping these steady is homeostasis, and it is run by two messaging systems: fast nerves and slower hormones.

The nervous pathway from stimulus to response

The nervous system: fast and electrical

Nerves carry electrical signals at high speed. Every response follows the same pathway:

Stimulus — a change in the surroundings (heat, light, sound).
Receptor — a cell that detects the stimulus (e.g. in the skin, eye, ear).
CNS — the central nervous system, made of the brain and spinal cord, which processes the signal.
Effector — a muscle or gland that carries out the response.
Response — the action itself.

A reflex is a shortcut version of this: fast, automatic, and protective. Pulling your hand off a hot pan happens through the spinal cord alone, before your brain has fully registered the pain — that speed is what stops you getting badly burnt. Blinking when something flies at your eye and your knee jerking when tapped are reflexes too. Because they skip the slow "thinking" step in the brain, they buy you precious fractions of a second.

A reflex arc: skin → spinal cord → muscle, bypassing the brain.

Hormones: slow and chemical

Hormones travel in the blood (not nerves) to reach target organs.

Think of the two systems like sending a message. A nerve signal is a phone call — instant, direct, over in a second. A hormone is a letter posted in the blood — it takes longer to arrive but its effect lingers. Hormones are chemical messengers made by glands and carried in the blood. For example, insulin from the pancreas lowers blood sugar; adrenaline prepares the body for "fight or flight".

Comparing the two

A nerve signal is a quick spike; a hormone builds slowly and lingers.

Feature	Nervous	Hormonal
Signal type	Electrical	Chemical
Carried by	Nerves	Blood
Speed	Very fast	Slower
How long it lasts	Brief	Longer-lasting

Worked reasoning

Worked example. Put a reflex in order: effector, receptor, CNS.
Ask: "what happens first?" Something must be detected (receptor), then processed (CNS), then acted on (effector).
Correct order: receptor → CNS → effector. The response always comes last, never before the signal is processed.

Common mix-up: Thinking hormones travel along nerves. They do not. Hormones are dissolved in the blood and reach every part of the body; nerves carry only electrical signals. Getting the two carriers muddled is the classic exam slip.

Memory hook: Nerves = Now (fast, electrical); Hormones = Hang around (slow, chemical, in the blood).