The nervous system and reflexes · Year 9
The nervous system lets us react to our surroundings and coordinate our behaviour. Receptors detect changes, nerve impulses carry information to the brain and spinal cord, and these send impulses to muscles and glands. Reflexes are automatic and very fast, which protects us from harm.
Key facts
- A stimulus is a change in the environment. Receptors detect it; the coordinator (the central nervous system: brain and spinal cord) processes the information; effectors (muscles or glands) bring about a response.
- Neurones carry electrical impulses. Sensory neurones carry them from receptors to the central nervous system; relay neurones pass them on inside it; motor neurones carry them to effectors.
- A synapse is the gap between two neurones. The impulse makes the first neurone release a chemical, which diffuses across the gap and starts an impulse in the next neurone.
- A reflex is a fast, automatic response that does not involve the conscious part of the brain. Reflex arc: receptor → sensory neurone → relay neurone → motor neurone → effector.
- Muscles respond by contracting; glands respond by releasing (secreting) chemicals such as hormones.
- Reaction time can be measured with the ruler-drop test: the shorter the distance fallen, the faster the reaction. Time = √(2 × distance ÷ g).
- Speed of an impulse = distance ÷ time. A fatty myelin sheath makes impulses travel much faster.