Answers · Nerves and reflexes

How fast can you react? — answer key

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

1. RQ1 · Practice

Model answer. The central nervous system is the brain and the spinal cord. The nerves that run out to the arms, legs and body are the peripheral nervous system (PNS), and the backbone is bone that protects the spinal cord.

A full answer includes

I chose: the brain and the spinal cord.
I did not include any nerves: nerves outside the brain and spinal cord are the PNS.

Also accept

the brain and the spinal cord

Watch for

“The brain, the spinal cord and the arm nerves.” — The nerves in the arms are outside the brain and spinal cord, so they are part of the PNS.
“The brain only, with nothing else.” — The spinal cord is part of the CNS too, together with the brain.
“The spinal cord and the backbone.” — The backbone is bone that protects the spinal cord; it is not part of the nervous system. And the brain is missing.

2. RQ2 · Practice

Model answer. Messages travel along neurones as electrical impulses. No substance moves along the neurone: the impulse is an electrical signal passing along the cell.

A full answer includes

I chose: as electrical impulses.
I did not choose a chemical diffusing along the cell: nothing moves along a neurone.

Also accept

electrical impulses

Watch for

“As a chemical diffusing along the cell.” — Diffusion is far too slow to carry a message a metre in a fraction of a second. Along a neurone the message is an electrical impulse.
“As a liquid flowing along inside the cell.” — Nothing flows along a neurone. The message is an electrical impulse passing along the cell.
“As a hormone carried along inside the cell.” — Hormones are chemicals carried in the blood, not inside neurones. Along a neurone the message is an electrical impulse.

3. RQ3 · Practice

Options as numbered on the pupil sheet: 1 — PNS; 2 — CNS.

Model answer. The brain and the spinal cord make up the CNS. Every nerve outside them is part of the PNS: the nerve in the leg, and also the nerve that runs from the tongue to the brain. That nerve joins the brain, but it lies outside it.

A full answer includes

I sorted the brain into the CNS.
I sorted the spinal cord into the CNS.
I sorted the nerve in the leg into the PNS.
I sorted the nerve from the tongue to the brain into the PNS.
I did not put the tongue nerve in the CNS just because it joins the brain.

Also accept

brain CNS; spinal cord CNS; leg nerve PNS; tongue nerve PNS

Watch for

“The spinal cord is PNS.” — The spinal cord is part of the CNS, together with the brain.
“A nerve that joins the brain is part of the CNS.” — The nerve lies outside the brain, so it is part of the PNS. Only the brain and the spinal cord are the CNS.

4. RQ4 · Practice

Options as numbered on the pupil sheet: 1 — sound-sensitive cells in the ear; 2 — muscles in the arm and hand; 3 — the sound of the buzz.

Model answer. The stimulus is the change that is detected: the sound of the buzz. The receptors that detect it are sound-sensitive cells in the ear. The effectors that make the response are the muscles in the arm and hand.

A full answer includes

I matched the stimulus to the sound of the buzz.
I matched the receptor to the sound-sensitive cells in the ear.
I matched the effector to the muscles in the arm and hand.
I did not match the receptor to the muscles: muscles make the response.

Also accept

stimulus the buzz; receptor ear cells; effector arm and hand muscles

Watch for

“The receptor is the muscles in the hand.” — The muscles carry out the response, so they are the effectors. The receptors detect the stimulus: here, in the ear.

5. RQ5 · Practice

Model answer. Beside 30 cm the table gives 0.25 s, so the reaction time for this catch is 0.25 s.

Checked automatically

Reaction time: 0.25 s (within ±0.005 s)
- Common wrong answer 0.30 s: Centimetres are not hundredths of a second. Read the table: 30 cm is 0.25 s.

A full answer includes

I found 30 cm in the distance column.
I wrote 0.25 s.
I did not write 0.30 s: centimetres are not hundredths of a second.

Also accept

0.25 s

Watch for

“0.30 s.” — That turns 30 cm into 0.30 s, as if 1 cm were 0.01 s. Read the time from the table instead: 0.25 s.

6. RQ6 · Practice

Model answer. The total is 0.20 + 0.22 + 0.19 + 0.21 + 0.18 = 1.00 s. There are five times, so the mean is 1.00 ÷ 5 = 0.20 s.

Checked automatically

Mean reaction time: 0.20 s (within ±0.005 s)
- Common wrong answer 1.00 s: That is the total. Divide by 5.

A full answer includes

Method: I added the five times: 1.00 s.
Method: I divided the total by 5.
Answer: I wrote 0.20 s.
I did not stop at the total, 1.00 s.

Also accept

0.20 s

Watch for

“1.00 s.” — That is the total of the five times. Divide it by 5 to get the mean: 0.20 s.

7. RQ7 · Practice

Model answer. A nerve is not one cell: it is a bundle of many neurones wrapped together. Some of them are sensory neurones, carrying impulses from receptors in the hand towards the CNS. Others are motor neurones, carrying impulses from the CNS out to the muscles of the hand. Each neurone carries its own impulses, so the nerve as a whole carries impulses both ways.

A full answer includes

I wrote that a nerve is a bundle of many neurones.
I wrote that some are sensory neurones, carrying impulses from receptors in the hand towards the CNS.
I wrote that some are motor neurones, carrying impulses from the CNS to muscles in the hand.
I did not write that one neurone carries impulses both ways at once.

Also accept

a nerve is many neurones; sensory ones carry impulses in, motor ones carry impulses out

Watch for

“A nerve is one long cell that carries impulses both ways.” — A nerve is a bundle of many neurones. Different neurones in it carry impulses in different directions.

8. RQ8 · Practice

Model answer. Row A. A sensory neurone carries impulses from the touch receptors in the foot along a nerve in the leg to the spinal cord, and they travel up the spinal cord to the brain. The brain decides to take the stone out. Impulses travel down the spinal cord and out along motor neurones to the muscles of the arm and hand, the effectors.

A full answer includes

I chose row A.
I checked that impulses from the foot reach the spinal cord first, then travel up it to the brain.
I checked that the brain makes the decision to reach down.
I checked that a motor neurone carries the impulses out to the hand muscles.
I did not choose row B: a nerve from the foot does not run straight to the brain.

Also accept

Watch for

“Row B.” — The nerves of the leg join the spinal cord. Impulses from the foot reach the brain by travelling up the spinal cord.
“Row C.” — Deciding to reach down is a choice, and choices are made in the brain. Impulses only pass through the spinal cord on their way.
“Row D.” — A sensory neurone brings impulses in from a receptor. Impulses travel out to muscles along a motor neurone.

9. RQ9 · Practice

Model answer. From the table the times are 0.22, 0.23, 0.21, 0.25 and 0.21 s. Their total is 1.12 s, so the mean is 1.12 ÷ 5 = 0.224 s, which is 0.22 s to 2 decimal places.

Checked automatically

Mean reaction time: 0.22 s (within ±0.005 s)
- Common wrong answer 25 s: That is the mean distance in cm, not a time. Use the table for each catch.
- Common wrong answer 0.23 s: That is the time for the mean distance, 25 cm. Turn each catch into a time first, then find the mean of the five times.
- Common wrong answer 1.12 s: That is the total of the five times. Divide by 5.

A full answer includes

Method: I read five times from the table: 0.22, 0.23, 0.21, 0.25 and 0.21 s.
Method: I added them, 1.12 s, and divided by 5.
Answer: I wrote 0.22 s, to 2 decimal places.
I did not find the mean distance first and turn that into a time.

Also accept

0.22 s

Watch for

“25 cm.” — That is the mean distance. The question asks for a time: turn each catch into a time first, then find their mean.

10. RQ10 · Practice

Model answer. At the start Q's thumb is already level with the 5 cm mark. So the mark Q reads after a catch is 5 cm more than the distance the rule really fell while Q reacted. The table turns a bigger distance into a longer time, so every one of Q's times is too long, and P and Q cannot be compared fairly. Both must start with the 0 cm mark level with the top of the thumb.

A full answer includes

I wrote that Q's thumb starts at the 5 cm mark, not at 0 cm.
I wrote that every mark Q reads is 5 cm more than the distance the rule really fell.
I wrote that the table turns a bigger distance into a longer time.
I did not write that Q simply reacts more slowly.

Also accept

Q reads 5 cm too much each time, so the table gives a longer time

Watch for

“Q reacts more slowly, so the times are longer.” — The times are too long because of how the rule was held, not because of Q's reactions: each reading includes 5 cm the rule never fell.

11. RQ11 · Practice

Model answer. Regulation means keeping a body function at the right level as conditions change. In the sun, receptors detect the rise in temperature. Impulses travel to the CNS and out to the sweat glands, the effectors, which release more sweat and cool the body. Back inside, the change is detected again and the glands release less. The nervous system keeps body temperature at the right level: that is regulation.

A full answer includes

I wrote that receptors detect the rise in temperature.
I wrote that impulses travel to the CNS.
I wrote that impulses travel out from the CNS to the sweat glands, the effectors.
I wrote that the glands release more sweat in the heat and less when it is cool again.
I wrote that this keeps body temperature at the right level: regulation.
I did not write that the sweat glands decide by themselves.

Also accept

receptors detect the temperature; impulses via the CNS to the sweat glands; more sweat, then less; body temperature kept at the right level

Watch for

“The sweat glands decide by themselves to make more sweat.” — The glands are effectors: they respond to impulses from the nervous system, which links the receptors to them.

12. RT1 · Apply

Model answer. (a) The 31 cm catch is anomalous: it is far from all the others, which lie between 14 and 18 cm. The 18 cm catch is the slowest of the rest, but only by a little, so it fits their spread and stays in. (b) The repeat takes the place of the anomalous catch. From the table the times are 0.17, 0.19, 0.19, 0.17 and 0.18 s. Their total is 0.90 s, so the mean is 0.90 ÷ 5 = 0.18 s.

A full answer includes

(a) I identified the 31 cm catch as anomalous.
(a) I wrote that 31 cm is far from the others, which are all between 14 and 18 cm.
(a) I wrote that 18 cm is only a little slower than the rest, so it fits their spread.
(b) Method: I used 15, 18, 17, 14 and 16 cm: 0.17, 0.19, 0.19, 0.17 and 0.18 s.
(b) Method: I added them, 0.90 s, and divided by 5.
(b) Answer: I wrote 0.18 s.
I did not keep the 31 cm catch in the mean.

Also accept

(a) 31 cm; 18 cm fits the spread of the others; (b) 0.18 s

Watch for

“(a) The 18 cm catch is anomalous too, because it is the slowest of the rest.” — Being the slowest is not enough. 18 cm is close to the others; only a catch far from all of them is anomalous.
“(b) 0.19 s, keeping the 31 cm catch.” — Keeping an anomalous catch pulls the mean away from the pupil's usual reaction time. Use the repeat instead.

13. RT2 · Apply

Model answer. (a) The stimulus is the moving ball, detected by light-sensitive cells in the eye, and the effectors are the muscles of the legs, arms and body. (b) The dive needs many muscles to contract together, in the right order and at the right moment; the nervous system links the receptors to all these effectors, which is coordination.

A full answer includes

(a) I wrote the stimulus: the ball moving towards the goal.
(a) I wrote a receptor: light-sensitive cells in the eye.
(a) I wrote the effectors: muscles of the legs, arms and body.
(b) I wrote that many muscles must act together.
(b) I wrote that they must act at the right moment.
(b) I wrote that the nervous system links the receptors in the eye to all these effectors.
(a) I did not give the leg muscles as the receptor.

Also accept

(a) ball; eye; muscles; (b) links receptors to many effectors acting together

Watch for

“(a) The receptor is the leg muscles.” — The muscles are effectors. The eyes detect the ball, so they hold the receptors.

14. RT3 · Apply

Model answer. Row C. F's mean is 1.13 ÷ 5 = 0.226 s and G's mean is 0.97 ÷ 5 = 0.194 s, so over five catches G is faster. F's first catch, 0.17 s, is far from F's other four, which are 0.23 to 0.25 s: one catch can differ from a pupil's usual time by chance, so it is not a fair way to rank two people. The data suggest that G reacts faster.

A full answer includes

I chose row C.
I checked that G's mean time, 0.194 s, is shorter than F's, 0.226 s.
I checked that one catch can differ from a pupil's usual time by chance.
I did not choose row A: one catch each is not enough to rank two people.

Also accept

Watch for

“Row A.” — A single catch can differ from a pupil's usual time by chance. Over all five catches, G's mean time is shorter.
“Row B.” — The first catch is not the fairest one: it is only one catch. A mean of several catches is a better estimate.
“Row D.” — Not every one of G's times is shorter: F's first catch, 0.17 s, beats all of them. Compare the means instead.

15. RT4 · Apply

Options as numbered on the pupil sheet: 1 — PNS; 2 — CNS.

Model answer. Sort each part by where it lies. The cell body of the motor neurone is inside the spinal cord, so it is in the CNS; its long fibre runs down the leg, outside the spinal cord, so it is in the PNS. The fibre from the skin of the arm is in a nerve outside the CNS, so it is in the PNS. A short neurone wholly inside the brain is in the CNS.

A full answer includes

I sorted the cell body inside the spinal cord into the CNS.
I sorted the long fibre in the leg into the PNS.
I sorted the fibre running from the skin of the arm into the PNS.
I sorted the short neurone wholly inside the brain into the CNS.
I did not sort a whole neurone by one end: each part goes where it lies.

Also accept

cell body in the spinal cord CNS; fibre in the leg PNS; fibre from the arm skin PNS; short neurone in the brain CNS

Watch for

“The fibre in the leg is CNS, because its cell body is in the spinal cord.” — The fibre lies outside the brain and spinal cord, in a nerve of the leg, so that part is in the PNS.
“The fibre from the skin of the arm is CNS, because it carries impulses to the spinal cord.” — Where a fibre carries impulses does not decide. It lies outside the brain and spinal cord, so it is in the PNS.