The nervous system
The nervous system lets the body detect and respond quickly to changes. It has two parts: the central nervous system (CNS) — the brain and spinal cord — and the peripheral nervous system — the nerves connecting the CNS to the rest of the body.
Key idea
The pathway of a message is: stimulus → receptor → sensory neurone → CNS → motor neurone → effector → response.
Types of neurone
A neurone is a nerve cell that carries electrical impulses. There are three types:
- Sensory neurone — carries impulses from a receptor to the CNS.
- Relay (intermediate) neurone — found in the CNS, links neurones together.
- Motor neurone — carries impulses from the CNS to an effector (a muscle or gland).
The synapse and reflexes
Neurones do not touch. The tiny gap between them is a synapse. When an impulse arrives, a chemical called a neurotransmitter is released, diffuses across the gap, and starts a new impulse in the next neurone.
A reflex action is an automatic, rapid response that protects the body and does not need conscious thought by the brain. The nerve pathway it follows is a reflex arc.
Worked example
You touch a hot object. Heat is the stimulus; skin receptors detect it; a sensory neurone carries the impulse to the spinal cord; a relay neurone passes it to a motor neurone; the effector muscle contracts and pulls your hand away — all before you feel the pain.
Remember
- Reflexes are fast because the message does not have to travel to the conscious brain first.