Answers · Nerves and reflexes

Nerves and reflexes — final check key

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

1. RK1 · Final check

Model answer. Row A. Along the neurones the message travels as electrical impulses; a chemical carried in the blood (row B) is a hormone. The decision to brake is made in the brain, part of the CNS; the nerves of the PNS (row C) only carry impulses to and from it. Linking the eyes with the muscles of both hands, so that they act together at the right moment, is coordination; regulation (row D) keeps a function such as body temperature at the right level.

A full answer includes

I chose row A.
I checked that along neurones the message travels as electrical impulses.
I checked that the decision to brake is made in the brain, part of the CNS.
I checked that linking the eyes with the muscles of both hands is coordination.
I did not choose row D: keeping a function at the right level is regulation, which is not what happens here.

Also accept

Watch for

“Row B.” — A chemical carried in the blood is a hormone. Along neurones the message is an electrical impulse.
“Row C.” — The nerves of the PNS carry impulses to and from the CNS. The decision to brake is made in the brain.
“Row D.” — Regulation keeps a function, such as body temperature, at the right level. Making the eyes and both hands act together at the right moment is coordination.

2. RK2 · Final check

Options as numbered on the pupil sheet: 1 — regulation; 2 — coordination.

Model answer. Coordination links receptors with many effectors so that they act together at the right moment: reading the music while both hands play, and watching the rope while the legs jump in time. Regulation keeps a body function at the right level as conditions change: the skin blood vessels narrow in the cold and widen again when you are warm, which keeps body temperature steady; the heart beats faster for a moment when you stand up, which keeps enough blood reaching the brain.

A full answer includes

I sorted playing a tune on a keyboard into coordination.
I sorted skipping with a rope into coordination.
I sorted the skin blood vessels in a cold wind into regulation.
I sorted the heart beating faster on standing up into regulation.
I did not sort a case into coordination just because muscles act in it.

Also accept

keyboard coordination; skipping coordination; skin blood vessels regulation; heart on standing regulation

Watch for

“The heart beating faster is coordination, because the heart is a muscle.” — Muscles act in both kinds of case. Here the heart rate changes to keep the blood supply to the brain at the right level: regulation.
“Skipping is regulation, because it keeps the same rhythm.” — Keeping a rhythm is the eyes, legs and arms acting together at the right moment: coordination. Regulation keeps a body function, such as body temperature, at the right level.

3. RK3 · Final check

Model answer. The 40 cm catch is far from the others, which lie between 12 and 15 cm, so it is anomalous and is left out of the mean. From the table the other four times are 0.16, 0.17, 0.17 and 0.16 s. Their total is 0.66 s, so the mean is 0.66 ÷ 4 = 0.165 s.

Checked automatically

Anomalous catch: 40 cm (exact value)
- Common wrong answer 15 cm: 15 cm is the slowest of the four close catches, but it fits their spread. The anomalous catch is far from all of them.
Mean of the other four: 0.165 s (within ±0.006 s)
- Common wrong answer 0.19 s: That keeps the anomalous 40 cm catch. Leave it out and use the other four.
- Common wrong answer 13.5 cm: That is the mean distance in cm. Turn each catch into a time.
- Common wrong answer 0.66 s: That is the total of the four times. Divide by 4.

A full answer includes

I identified the 40 cm catch as anomalous.
Method: I turned 12, 14, 15 and 13 cm into 0.16, 0.17, 0.17 and 0.16 s.
Method: I added them, 0.66 s, and divided by 4.
Answer: I wrote 0.165 s (0.16 s or 0.17 s also fits).
I did not keep the 40 cm catch in the mean.

Also accept

40 cm; 0.165 s (0.16 to 0.17 s)

Watch for

“0.19 s, using all five catches.” — Keeping the anomalous 40 cm catch pulls the mean away from the pupil's usual reaction time. Leave it out.
“13.5 cm.” — That is the mean distance. The question asks for a time: turn each catch into a time first.

4. AK1 · Final check

Model answer. Row B. U ends on a muscle, an effector, and its cell body is inside the CNS: a motor neurone. V is short and lies wholly inside the spinal cord: a relay neurone. W runs from the skin, where the receptors are, and its cell body is on a side branch: a sensory neurone. The impulse goes W → V → U, from the skin to the muscle.

A full answer includes

I chose row B.
I checked that U ends on a muscle, an effector: a motor neurone.
I checked that V lies wholly inside the spinal cord: a relay neurone.
I checked that W has its cell body on a side branch and starts in the skin: a sensory neurone.
I did not name the neurones in the order of their letters.

Also accept

Watch for

“Row A.” — The letters are not in the order of the path. U ends on a muscle, so it carries impulses out of the CNS: a motor neurone. W, with its cell body on a side branch, brings impulses in from the skin.
“Row C.” — A cell body inside the spinal cord does not make a neurone a relay neurone: U leaves the cord and ends on a muscle, so it is a motor neurone. V never leaves the cord, so it cannot reach a muscle.
“Row D.” — A cell body on a side branch is the sign of a sensory neurone, not a relay neurone. V, wholly inside the spinal cord, is the relay neurone.

5. AK2 · Final check

Model answer. (a) The receptor detects the heat, and impulses travel along the sensory neurone to the relay neurone in the spinal cord. But the motor neurone is cut, so no impulse reaches the effector, the arm muscle, and the reflex does not pull the hand away. (b) The receptor, the sensory neurone and the neurones that carry impulses up the spinal cord to the brain all still work, so the heat is still felt.

A full answer includes

(a) I predicted that the reflex does not pull the hand away.
(a) I wrote that no impulse can reach the effector, the arm muscle, along the cut motor neurone.
(b) I predicted that the heat is still felt.
(b) I wrote that the receptor and the sensory neurone still work, and impulses still reach the brain.
(a) I did not write that the hand still moves, only more slowly.

Also accept

(a) no: no impulse reaches the arm muscle; (b) yes: the sensory side still reaches the brain

Watch for

“(b) The heat is not felt either, because the reflex arc is broken.” — Only the motor side is cut. Impulses from the receptor still reach the spinal cord and travel up it to the brain.

6. AK3 · Final check

Model answer. When vapour from a cut onion reaches the eyes, the tear glands release tears automatically: the glands are the effectors, and the response is releasing a substance. In blinking, the knee jerk and the narrowing of the pupil, something moves, so the effectors are muscles: the eyelid muscles, a thigh muscle and muscles in the iris.

A full answer includes

I chose: tears flow when vapour from a cut onion reaches the eyes.
I checked that the tear glands release a substance, tears.
I did not choose a reflex in which something moves: its effectors are muscles.

Also accept

tears flowing when onion vapour reaches the eyes

Watch for

“You blink when a moth flies close to your eyes.” — The eyelid closes because muscles contract. A gland would release a substance.
“Your lower leg swings forward when a doctor taps just below your kneecap.” — The leg moves because a thigh muscle contracts. A gland would release a substance.
“The pupils of your eyes get smaller in bright light.” — The pupil gets smaller because muscles in the iris contract. A gland would release a substance.

7. AK4 · Final check

Model answer. (a) P is a sensory neurone. (b) Its cell body is on a short side branch part way along the fibre, and it runs from fine endings in the skin, where a receptor is, to the spinal cord, part of the CNS: it carries impulses in.

A full answer includes

(a) I wrote a sensory neurone.
(b) I gave: its cell body is on a short side branch part way along.
(b) I gave: it runs from fine endings in the skin to the spinal cord.
(a) I did not call it a motor neurone because it is long.

Also accept

(a) sensory neurone; (b) cell body on a side branch part way along; from the skin (receptor) to the spinal cord (CNS)

Watch for

“(a) A motor neurone, because it is long.” — A motor neurone is long too, but its cell body is at one end and it ends on an effector. P has its cell body on a side branch and runs from the skin into the spinal cord.

8. SK1 · Final check

Model answer. The sensory neurone and the relay neurone do not touch. They meet at a synapse: a junction between two neurones, with a tiny gap between them. The impulse cannot simply flow on; the message has to cross the synapse.

A full answer includes

I wrote that the two neurones are not joined: they meet at a synapse.
I wrote that a synapse is a junction between two neurones.
I wrote that there is a tiny gap between the two neurones at the synapse.
I did not write that the impulse flows on through joined cells.

Also accept

not joined; a synapse, a junction between two neurones with a tiny gap

Watch for

“The neurones are joined end to end, like two pieces of wire.” — Neurones never join: where two meet there is a synapse, with a tiny gap between them.

9. SK4 · Final check

Model answer. Row C. Vesicles containing neurotransmitter are in the end of the neurone the impulse arrives along; the receptor proteins are on the membrane of the next neurone; the synaptic gap is a narrow space between them, and nothing joins the two neurones. Row A swaps the two neurones, row B puts the receptor proteins inside the vesicles, and row D joins the neurones with a tube.

A full answer includes

I chose row C.
I checked that the vesicles are in the neurone the impulse arrives along.
I checked that the receptor proteins are on the next neurone's membrane.
I checked that the gap is a narrow space, with nothing joining the neurones.
I did not choose row A, which swaps the two neurones.

Also accept

Watch for

“Row A.” — The vesicles are in the neurone the impulse arrives along, the sending one; the receptor proteins are on the next neurone.
“Row B.” — The vesicles hold neurotransmitter. The receptor proteins are on the membrane of the next neurone, where the neurotransmitter binds.
“Row D.” — Nothing joins the two neurones: the gap is a narrow space that the neurotransmitter diffuses across.

10. SK2 · Final check

Model answer. (a) No impulse is stimulated in the next neurone. (b) The arriving impulse still stimulates the vesicles to release neurotransmitter into the gap. But the molecules are broken down before they can diffuse across, so none bind with the receptor proteins on the next neurone, and no impulse is stimulated in it: the message stops at this synapse.

A full answer includes

(a) I predicted that no impulse is stimulated in the next neurone.
(b) I wrote that the vesicles still release neurotransmitter into the gap.
(b) I wrote that the molecules are broken down before they can diffuse across the gap.
(b) I wrote that nothing binds with the receptor proteins on the next neurone.
(b) I did not write that the impulse crosses the gap by itself.

Also accept

(a) no impulse; (b) released, broken down before diffusing across, nothing binds

Watch for

“(a) An impulse still starts, because the impulse crosses the gap by itself.” — An impulse cannot cross the gap. Only neurotransmitter binding with the receptor proteins starts a new impulse.

11. SK3 · Final check

Model answer. Only neurone 3 gets a new impulse. The impulse travels both ways along neurone 2. At the 2-to-3 synapse, neurone 2 is the sending side: it has vesicles, so neurotransmitter is released, crosses the gap and binds with receptor proteins on neurone 3. At the 1-to-2 synapse, neurone 2 is the receiving side: it has receptor proteins but no vesicles there, and neurone 1 has no receptor proteins, so nothing crosses back to neurone 1.

A full answer includes

I chose: neurone 3 only.
I explained that at the synapse between 2 and 3, neurone 2 has the vesicles.
I explained that at the same synapse, neurone 3 has the receptor proteins.
I explained that at the synapse between 1 and 2, neurone 2 has receptor proteins but no vesicles, so nothing is released towards neurone 1.
I did not choose neurone 1 just because the impulse travels both ways along neurone 2.

Also accept

neurone 3 only

Watch for

“Neurone 1 and neurone 3.” — The impulse travels both ways along neurone 2, but each synapse only lets it through from the side with vesicles. At the 1-to-2 synapse, neurone 2 has none.
“Neurone 1 only.” — Neurone 2 receives from neurone 1: at that synapse it has receptor proteins, not vesicles. It sends to neurone 3.
“Neither of them.” — At the 2-to-3 synapse, neurone 2 has vesicles, so the impulse does cross to neurone 3.