The nervous system and reflexes

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.

Worked examples

Example 1

You touch a hot plate and pull your hand away. Describe the reflex arc.

  1. Receptors in the skin detect the heat (the stimulus).
  2. A sensory neurone carries the impulse to the spinal cord.
  3. A relay neurone passes it to a motor neurone, across synapses.
  4. The motor neurone carries it to a muscle in the arm, which contracts and pulls the hand away.

Answer: temperature receptors in the skin → sensory neurone → relay neurone in the spinal cord → motor neurone → arm muscle contracts

Example 2

A nerve impulse travels 0.9 m in 12 ms. Calculate its speed.

  1. Change the time into seconds. 12 ms = 0.012 s
  2. Speed = distance ÷ time. 0.9 ÷ 0.012 = 75 m/s

Answer: 75 m/s

Example 3

In the ruler-drop test, a ruler fell 18 cm before it was caught. Use t = √(2d ÷ g), with g = 9.8 m/s², to find the reaction time.

  1. Change cm into m. 18 cm = 0.18 m
  2. Put the numbers in. t = √(2 × 0.18 ÷ 9.8) = 0.19 s

Answer: 0.19 s

Example 4

Explain how an impulse crosses a synapse.

  1. The impulse reaches the end of the first neurone.
  2. It releases a chemical into the gap.
  3. The chemical diffuses across and binds to receptors on the next neurone.
  4. This starts a new electrical impulse.

Answer: a chemical is released, diffuses across the gap and binds to receptors on the next neurone, starting a new impulse