The Control Centre

The Control Centre 🧠

Touch something hot and your hand is gone before you even feel the pain. That split-second escape is your **nervous system** at work. Learn how you detect the world, how the signal travels, and why a reflex is faster than a thought.

Wired for response 🕸️

Your **nervous system** lets you react to your surroundings and coordinate your behaviour. At its heart is the **central nervous system (CNS)** — the **brain** and **spinal cord**. The CNS connects to the body through **neurones** (nerve cells) that carry electrical impulses very quickly.

Who does what?

From stimulus to response ➡️

Put those parts together and you have the pathway of any response: **stimulus → receptor → sensory neurone → CNS → motor neurone → effector → response**

Which neurone?

Which neurone carries impulses **from a receptor to the CNS**?

• The sensory neurone
• The motor neurone
• The relay neurone
• The effector

Reflexes: no thinking required ⚡

Some responses are **reflexes** — they are **automatic** and **rapid**, and don't involve the **conscious** part of the brain. Because they skip conscious thought, reflexes are **fast** — which makes them ideal for **protecting** the body (pulling away from heat, the pupil shrinking in bright light).

The reflex arc 🔄

A reflex travels a short-cut called the **reflex arc**. It runs through the spinal cord, using a **relay neurone** instead of the conscious brain: **stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response**

Trace the reflex

An interactive activity.

Label the arc

An interactive activity.

Why so fast?

Why is a reflex action **faster** than a normal, conscious reaction?

• It does not involve the conscious part of the brain, so the impulse takes a shorter route
• The impulse is much stronger
• It uses no motor neurone
• Because you have practised it

Leaping the gap 🔌

Neurones don't quite touch — there is a tiny gap between them called a **synapse**. When an electrical impulse reaches the gap, it triggers the release of **chemicals** (neurotransmitters). These **diffuse across** the synapse and set off a **new electrical impulse** in the next neurone.

Crossing the synapse

How does a nerve impulse get across the gap (synapse) between two neurones?

• Chemicals are released and diffuse across the gap, triggering a new impulse
• The electrical impulse jumps straight across as a spark
• The neurones touch and pass it directly
• The blood carries it across

Testing reactions (RP7)

In the ruler-drop test, a person catches the falling ruler after it has dropped a **shorter** distance than before. What does that show?

• Their reaction time is faster
• Their reaction time is slower
• They have stronger muscles
• Nothing about reaction time

In the exam 🎓

Wired up. Grade-9 habits for the nervous system: • Sequence the reflex arc precisely: **stimulus → receptor → sensory → relay → motor → effector → response**. • A reflex is fast and involuntary **because it bypasses the conscious brain**. • At a **synapse**, transmission is **chemical** — neurotransmitters **diffuse** across the gap to trigger the next impulse.