The Reflex Arc and Synapses · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Pull away before you think

Science · The Human Body · ages 14-15
Name ______________________   Date ____________

  1. Which part senses the heat of the pan?

    • The muscle in the arm that pulls
    • The receptor in the skin
    • The brain in the skull

  2. A hand touches a hot pan. What is the correct order of the arc?

    • Receptor, sensory, relay, motor, effector
    • Effector, motor, relay, sensory, receptor
    • Sensory, receptor, motor, relay, effector

  3. A reflex needs a conscious decision before the muscle moves.

    Circle one:   True   False

  4. Why does a chemical synapse slow the signal down?

    • The gap stops the signal fully and kills it
    • Neurones run out of energy there
    • Chemicals take time to cross the gap

  5. What crosses a chemical synapse between two neurones?

    • An electric wire like a cable
    • A chemical messenger
    • A blood cell

  6. Why does this withdrawal reflex protect better than a decided response?

    • It is stronger than any decision
    • It remembers past burns
    • It is faster, skipping the brain in this arc

  7. At a chemical synapse, signals travel electrically inside a neurone and chemically between neurones.

    Circle one:   True   False

  8. Lena traces a withdrawal arc but puts the relay before the receptor. What is wrong?

    • Nothing, that order works
    • The receptor must sense first, before any relay
    • The effector should sit in the middle of the arc
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Pull away before you think W1-mt_evlgCVxTnK-s1

  1. The receptor in the skin · Receptors in the skin sense the heat and start the signal.
  2. Receptor, sensory, relay, motor, effector · Sense first, decide in the cord, then act: receptor through to effector.
  3. False · The loop skips the conscious brain, which is what makes it fast.
  4. Chemicals take time to cross the gap · At a chemical synapse, chemical crossing takes time, while electrical travel inside is fast.
  5. A chemical messenger · At a chemical synapse, one neurone releases a chemical messenger that crosses the gap.
  6. It is faster, skipping the brain in this arc · This withdrawal reflex skips the conscious brain to trade thought for speed and limit burn damage.
  7. True · Action potentials run inside, and chemical messengers bridge each chemical gap.
  8. The receptor must sense first, before any relay · Nothing can be relayed before it is sensed, so the receptor comes first.
Worksheet · LightMySky