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InkFlaskCambridge IGCSE
Contains the answers

Answers · Nerves and reflexes

Reflexes — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. AQ1 · Practice

Options as numbered on the pupil sheet: 1 — sensory neurone; 2 — motor neurone; 3 — relay neurone.

Model answer. X has its cell body on a short side branch part way along: a sensory neurone. Y ends on a muscle and has its cell body at one end: a motor neurone. Z lies inside the spinal cord, where it links other neurones: a relay neurone.

A full answer includes

  • I matched X to sensory neurone: its cell body is on a side branch part way along.
  • I matched Y to motor neurone: it ends on a muscle.
  • I matched Z to relay neurone: it lies inside the spinal cord.
  • I did not name a neurone by its length.

Also accept

  • X sensory; Y motor; Z relay

Watch for

  • “X is a motor neurone.” — X has its cell body on a side branch part way along, the sign of a sensory neurone. The neurone that ends on a muscle (Y) is the motor neurone.

2. AQ2 · Practice

Model answer. A sensory neurone carries impulses from a receptor to the CNS. A motor neurone carries them from the CNS to an effector; a relay neurone links neurones inside the CNS.

A full answer includes

  • I chose a sensory neurone.
  • I did not choose a motor neurone: it carries impulses out of the CNS.

Also accept

  • sensory neurone

Watch for

  • “A motor neurone.” — A motor neurone carries impulses out of the CNS to an effector.
  • “A relay neurone.” — A relay neurone lies inside the CNS and links other neurones. It does not reach the receptor.

3. AQ3 · Practice

Model answer. Row B. The receptor, in the skin of the sole, detects the heat. A sensory neurone carries an impulse to the spinal cord, where a relay neurone passes it to a motor neurone. The effector, a muscle in the leg, contracts and lifts the foot. The brain is not on the path.

A full answer includes

  • I chose row B.
  • I checked that the receptor is in the skin of the sole.
  • I checked that the relay neurone is in the spinal cord.
  • I checked that the effector is a muscle in the leg.
  • I did not choose row A, which swaps the receptor and the effector.

Also accept

  • B

Watch for

  • “Row A.” — The muscle makes the response, so it is the effector. The receptor is where the heat is detected: the skin of the sole.
  • “Row C.” — In this reflex the relay neurone is in the spinal cord. The impulse does not need to reach the brain before the foot lifts.
  • “Row D.” — A relay neurone lies inside the CNS. The neurone running along the leg from the sole is the sensory neurone.

4. AQ4 · Practice

Options as numbered on the pupil sheet: 1 — Motor neurone; 2 — Effector; 3 — Receptor; 4 — Relay neurone; 5 — Sensory neurone.

Model answer. A receptor detects the stimulus; a sensory neurone carries the impulse to the CNS; a relay neurone passes it on; a motor neurone carries it to the effector, which responds.

A full answer includes

  • I put the receptor first.
  • I put the sensory neurone second.
  • I put the relay neurone third.
  • I put the motor neurone fourth.
  • I put the effector last.
  • I did not put the motor neurone before the sensory neurone.

Also accept

  • receptor; sensory; relay; motor; effector

Watch for

  • “receptor; motor neurone; relay neurone; sensory neurone; effector.” — The sensory neurone comes first: it carries the impulse in from the receptor.

5. AQ5 · Practice

Model answer. The knee jerk and the sneeze are automatic and rapid: you do not decide to make them, so they are reflex actions. Kicking a football towards the goal and putting up your hand are actions you decide to make, so they are not reflexes.

A full answer includes

  • I chose the knee kicking forward when the tendon is tapped.
  • I chose sneezing when dust gets into the nose.
  • I did not choose kicking a football or putting up a hand: you decide to do those.

Also accept

  • the knee jerk; the sneeze

Watch for

  • “You kick a football towards the goal.” — You decide when and where to kick, so it is not automatic: not a reflex.
  • “You put up your hand when the teacher asks a question.” — Putting up your hand is a choice you make after thinking about the question: not a reflex.

6. AQ6 · Practice

Model answer. Effectors are muscles, which contract, and glands, such as the salivary glands, which release a substance. A bone moves only because a muscle pulls it, so the muscle is the effector. A motor neurone carries the impulse to the effector; it does not make the response itself.

A full answer includes

  • I chose a muscle.
  • I chose a gland.
  • I did not choose a bone: it is moved by a muscle.
  • I did not choose the motor neurone: it carries the impulse to the effector.

Also accept

  • a muscle; a gland

Watch for

  • “A bone in the arm.” — The bone moves, but only because a muscle contracts and pulls it. The muscle is the effector.
  • “A motor neurone.” — A motor neurone carries the impulse out to the effector. It does not make the response itself.

7. AQ7 · Practice

Model answer. A sensory neurone carries impulses from a receptor into the CNS, and its cell body is on a short side branch part way along the fibre. A motor neurone carries impulses from the CNS out to an effector, and its cell body is at one end, inside the CNS.

A full answer includes

  • I wrote that a sensory neurone carries impulses from a receptor to the CNS.
  • I wrote that a motor neurone carries impulses from the CNS to an effector.
  • I wrote that a sensory neurone's cell body is on a side branch part way along.
  • I wrote that a motor neurone's cell body is at one end, inside the CNS.
  • I did not write that one is long and the other short: both are long.

Also accept

  • direction: receptor to CNS vs CNS to effector; cell body side branch vs at one end

Watch for

  • “A sensory neurone is long and a motor neurone is short.” — Both are long. The short one is the relay neurone.

8. AQ8 · Practice

Model answer. In the reflex arc the sensory neurone from the skin of the elbow passes the impulse to a relay neurone in the spinal cord, which passes it straight to the motor neurone and the arm muscle. The impulse does not have to reach the brain first. Other neurones carry impulses up to the brain, which arrive a moment later, so you feel the pain after the arm has moved.

A full answer includes

  • I wrote that the relay neurone linking the sensory and motor neurones is in the spinal cord.
  • I wrote that the impulse reaches the arm muscle without going to the brain first.
  • I wrote that impulses reach the brain a moment later, so the pain is felt afterwards.
  • I did not write that the brain works very fast and moves the arm first.

Also accept

  • relay neurone in spinal cord; no need to reach brain; brain informed afterwards

Watch for

  • “The brain works very fast, so it moves the arm first.” — The response does not need the brain at all: the relay neurone is in the spinal cord.

9. AQ9 · Practice

Model answer. A reflex action is an automatic and rapid response: you do not decide to make it. It integrates and coordinates a stimulus with the response of effectors, which are muscles or glands.

A full answer includes

  • I wrote automatic: no decision is made.
  • I wrote rapid.
  • I wrote that it links a stimulus with the response of effectors.
  • I wrote that the effectors are muscles or glands.
  • I did not write that any fast movement is a reflex.

Also accept

  • automatic; rapid; stimulus to response of effectors (muscles or glands)

Watch for

  • “A reflex is any fast movement.” — A fast movement you decide to make, like a catch, is not a reflex. A reflex is automatic.

10. AQ10 · Practice

Model answer. A receptor in the skin of the finger detects the spine. An impulse travels along a sensory neurone to the spinal cord, where a relay neurone passes it to a motor neurone. The motor neurone carries the impulse to the effector, a muscle in the arm, which contracts and pulls the hand away.

A full answer includes

  • I wrote that a receptor in the skin of the finger detects the spine.
  • I wrote that a sensory neurone carries an impulse to the spinal cord.
  • I wrote that a relay neurone in the spinal cord passes it to a motor neurone.
  • I wrote that the motor neurone carries it to a muscle in the arm, the effector.
  • I wrote that the muscle contracts and pulls the hand away.
  • I did not send the impulse to the brain first.

Also accept

  • receptor in skin; sensory neurone to spinal cord; relay neurone; motor neurone; arm muscle contracts

Watch for

  • “The impulse goes from the finger to the brain, then to the arm.” — In this reflex the relay neurone is in the spinal cord; the brain is not on the path.

11. AT1 · Apply

Model answer. The receptor still detects the heat and the sensory neurone still carries an impulse to the spinal cord. But the relay neurone links the sensory neurone to the motor neurone, so without it no impulse reaches the motor neurone or the arm muscle: the reflex does not happen.

A full answer includes

  • I predicted that the reflex does not pull the hand away.
  • I wrote that the impulse still reaches the spinal cord along the sensory neurone.
  • I wrote that it cannot pass on to the motor neurone.
  • I wrote that no impulse reaches the effector muscle.
  • I did not write that the hand moves away faster along a shorter path.

Also accept

  • no reflex; impulse cannot pass from sensory to motor neurone; muscle gets no impulse

Watch for

  • “The hand moves away faster because the path is shorter.” — Without the relay neurone there is no path from the sensory to the motor neurone at all.

12. AT2 · Apply

Model answer. (a) Receptors in the lining of the windpipe detect the crumb; the effectors are the muscles used in breathing, in the chest, which contract hard to force air out. (b) Because the reflex is automatic and rapid, the crumb is pushed out at once, before it can block the airway or reach the lungs.

A full answer includes

  • (a) I suggested receptors in the lining of the windpipe.
  • (a) I suggested the breathing muscles of the chest as the effectors.
  • (b) I wrote that the reflex is automatic and rapid.
  • (b) I wrote that the crumb is forced out before it can block the airway.
  • (a) I did not give the chest muscles as the receptors.

Also accept

  • (a) receptors in windpipe lining; breathing (chest) muscles; (b) rapid, automatic: clears the airway quickly

Watch for

  • “(a) The receptor is the chest muscles.” — Muscles are effectors. The receptors detect the crumb, so they are where the crumb touches.

13. AT3 · Apply

Model answer. (a) Its cell body is in the CNS and its fibre ends on an effector, so it is a motor neurone. (b) The gland is the effector. (c) A gland responds by releasing a substance.

A full answer includes

  • (a) I wrote a motor neurone.
  • (b) I wrote that the gland is the effector.
  • (c) I wrote that the gland releases (secretes) a substance.
  • (a) I did not call it a sensory neurone because it is long.

Also accept

  • (a) motor neurone; (b) effector; (c) the gland releases a substance

Watch for

  • “(a) A sensory neurone, because it is long.” — Sensory neurones are long too, but they carry impulses into the CNS. One that ends on a gland is a motor neurone.