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. Five words carry the whole topic.

Who does what?

From stimulus to response

Put those parts together and you have the pathway of any response. A stimulus is picked up by a _____. A _____ neurone carries the impulse to the CNS, which coordinates the response. A _____ neurone then carries the impulse out to an _____, which brings about the 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**: **automatic**, **rapid**, and not involving the **conscious** part of the brain. That makes them ideal for **protecting** the body, like pulling away from heat or the pupil shrinking in bright light. A reflex takes a short-cut called the **reflex arc**. Follow it on the diagram: it runs through the **spinal cord**, using a **relay neurone** rather than travelling up to the conscious brain.

Trace the reflex

Put the stages of a reflex arc into the correct order.

• A stimulus is detected by a receptor
• A sensory neurone carries the impulse to the spinal cord
• A relay neurone passes it on within the CNS
• A motor neurone carries the impulse to the effector
• The effector brings about the response

Label the arc

Label the parts of this reflex arc (the flow runs from the receptor, up to the spinal cord, and out to the effector).

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**, called neurotransmitters. These **diffuse across** the synapse and set off a **new electrical impulse** in the next neurone.

Crossing the synapse

Two neurones do not touch. The gap between them is a _____. When an impulse arrives, _____ are released, and they _____ across the gap. That sets off a brand-new _____ impulse in the next neurone.

Testing reactions (RP7)

A student does the ruler-drop test five times. Their reaction times are 0.18 s, 0.22 s, 0.20 s, 0.19 s and 0.21 s. What is the mean reaction time, in seconds?

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. • **RP7**: repeat the drop and take a **mean**. A shorter drop distance means a **faster** reaction time.