Nerves and reflexes — return questions key

1. RR1 · Return

Model answer. Along neurones the message is an electrical impulse. It passes along a nerve in the arm, part of the peripheral nervous system, to the spinal cord, part of the central nervous system together with the brain.

A full answer includes

• (1) I wrote electrical.
• (2) I wrote nerve.
• (3) I wrote peripheral.
• (4) I wrote central.
• I did not write that the spinal cord is part of the peripheral nervous system.

Also accept

• electrical; nerve; peripheral; central

Watch for

• “(4) peripheral” — The spinal cord is part of the central nervous system, together with the brain.
• “(1) chemical” — Along a neurone the message is electrical.

2. RR2 · Return

Model answer. Impulses from receptors in the foot travel along nerves in the leg (the PNS) to the spinal cord, then up the spinal cord (the CNS) to the brain. The cut stops them on the way up, so they cannot reach the brain and the touch is not felt.

A full answer includes

• I chose: impulses from the foot cannot reach the brain, so the touch is not felt.
• I did not choose the leg nerves as a route straight to the brain: they reach the CNS at the spinal cord.

Also accept

• impulses cannot reach the brain, so the touch is not felt

Watch for

• “Impulses travel along the leg nerves straight to the brain, so it is felt.” — The nerves of the leg join the spinal cord, and impulses then travel up the cord to the brain. The cut is on that route.
• “Nothing changes, because the spinal cord is part of the PNS.” — The spinal cord is part of the CNS, and impulses from the legs travel up it to the brain.
• “The foot stops making impulses, so its receptors no longer work.” — The receptors and the leg nerves are not damaged: they still make and carry impulses. The impulses are stopped at the cut.

3. RR3 · Return

Model answer. Pupil B's catches are 15, 17, 16 and 16 cm, and one of 33 cm: that catch is far from all the others, so it is anomalous. Pupil A's slowest catch, 22 cm, is only a little beyond the rest and fits their spread. Pupils C and D are slower or faster overall, but each of their catches lies close to the others, so none is anomalous.

A full answer includes

• I chose Pupil B: the 33 cm catch is far from the others, which are 15 to 17 cm.
• I did not choose Pupil A: 22 cm is the slowest of A's catches, but it fits their spread.
• I did not choose a pupil just because all their catches are slow or all are fast.

Also accept

• Pupil B

Watch for

• “Pupil A, because the 22 cm catch is the slowest.” — Being the slowest catch is not enough. 22 cm is close to A's other catches, 18 to 21 cm, so it fits the spread.
• “Pupil C, because all the catches are slow.” — A pupil can be slow every time. Anomalous means one catch that does not fit the same pupil's other catches; C's are all close together.
• “Pupil D, because the catches are much faster than the others.” — An anomalous catch is compared with the same pupil's other catches, not with other pupils. D's catches are all close together.

4. RR4 · Return

Model answer. Shivering starts when the body gets cold and stops once it is warm again, so it keeps body temperature at the right level: that is regulation. Threading a needle and catching a ball are coordination: receptors linked with many muscles so that they act together. A breathing rate that rises and stays high is not being kept at the right level.

A full answer includes

• I chose shivering: it starts when the body is cold and stops once it is warm.
• I did not choose an example of coordination, such as eyes and hands working together.

Also accept

• shivering starts in the cold and stops once warm

Watch for

• “Your eyes and both hands work together to thread a needle.” — That is coordination: receptors and many muscles linked to act together. Regulation keeps a function at the right level.
• “Your breathing rate rises in a run and stays high all evening.” — Regulation would bring the breathing rate back down once the body no longer needs it high.
• “You catch a ball that a friend throws towards you.” — Catching links the eyes with the muscles of the hands: coordination of a response, not keeping a function at the right level.

5. AR1 · Return

Model answer. A relay neurone, inside the CNS, links sensory neurones and motor neurones.

A full answer includes

• I chose a relay neurone.
• I did not choose the sensory or the motor neurone: they are the two being linked.

Also accept

• relay neurone

Watch for

• “A sensory neurone.” — The sensory neurone brings the impulse in. The link inside the CNS is a relay neurone.
• “A motor neurone.” — The motor neurone carries the impulse out to the effector. The link inside the CNS is a relay neurone.

6. AR2 · Return

Model answer. Light-sensitive cells in the eye detect the fly. An impulse travels along a sensory neurone into the brain, where a relay neurone passes it to a motor neurone. The motor neurone carries the impulse out to the muscles of the eyelid, the effectors, which contract and close the eye. The arc passes through the brain, but no decision is made, so it is still a reflex.

A full answer includes

• (1) I wrote eye.
• (2) I wrote sensory.
• (3) I wrote relay.
• (4) I wrote motor.
• (5) I wrote muscles.
• I did not write that the eyelid holds the receptors: the eyelid muscles are the effectors.

Also accept

• eye; sensory; relay; motor; muscles

Watch for

• “(2) motor” — The neurone that carries the impulse in from the eye is the sensory neurone. The motor neurone carries it out to the eyelid.
• “(5) glands” — Glands release substances. Closing the eye is a movement, so the effectors are muscles.

7. AR3 · Return

Model answer. The knee jerk is a reflex: it is automatic, so no decision is made, and the impulse takes a short path through the spinal cord along only a few neurones. The catch needs the brain to decide to act, and the impulses travel a longer path, from the eye to the brain and down to the hand.

A full answer includes

• I chose: no decision is made, and the impulse passes along fewer neurones.
• I did not choose faster neurones in the leg: the difference is the path and the decision.

Also accept

• no decision is made, and the impulse passes along fewer neurones

Watch for

• “Neurones in the leg carry impulses faster than those in the arm.” — The difference is not the speed along each neurone, but the short path and the missing decision.
• “A tap is a stronger stimulus than a falling rule, so the response comes sooner.” — A stronger stimulus does not remove the decision. The knee jerk is quick because it is automatic and its path is short.
• “The leg kicks every day, so the knee jerk is better practised.” — Practice does not turn a decided action into a reflex. The knee jerk is automatic from the start: no decision is made.

8. SR1 · Return

Model answer. A synapse is a junction between two neurones. In this chain the sensory neurone meets the relay neurone, so there is a synapse there (and another where the relay neurone meets the motor neurone). A cell body, the middle of a fibre and a nerve leaving the spinal cord are all parts of single neurones or bundles of them, not junctions between two neurones.

A full answer includes

• I chose: where the end of the sensory neurone meets the relay neurone.
• I did not choose a place on one neurone: a synapse is a junction between two neurones.

Also accept

• where the sensory neurone meets the relay neurone

Watch for

• “The cell body of the sensory neurone, on its side branch.” — The cell body is part of one neurone. A synapse is where two neurones meet.
• “Half way along the fibre of the motor neurone.” — That is the middle of one neurone. A synapse is where the end of one neurone meets the next.
• “Where a nerve leaves the spinal cord.” — A nerve is a bundle of neurones; where it leaves the cord no neurone meets another. A synapse is a junction between two neurones.

9. SR2 · Return

Options as numbered on the pupil sheet: 1 — the space that neurotransmitter molecules diffuse across; 2 — bind with neurotransmitter molecules on the next neurone; 3 — hold neurotransmitter molecules until an impulse arrives.

Model answer. Vesicles in the end of the first neurone hold neurotransmitter molecules until an impulse arrives. The synaptic gap is the space the molecules diffuse across. Receptor proteins on the membrane of the next neurone bind with the neurotransmitter molecules.

A full answer includes

• I matched vesicles to holding neurotransmitter until an impulse arrives.
• I matched the synaptic gap to the space crossed by diffusion.
• I matched receptor proteins to binding with neurotransmitter on the next neurone.
• I did not match the vesicles to binding: the vesicles are in the first neurone.

Also accept

• vesicles hold; gap crossed by diffusion; receptor proteins bind

Watch for

• “The vesicles bind the neurotransmitter on the next neurone.” — The vesicles are in the first neurone and release neurotransmitter. The receptor proteins on the next neurone bind it.

10. SR4 · Return

Model answer. When the neurotransmitter binds with the receptor proteins, an impulse is stimulated in the next neurone, and it travels on along it.

A full answer includes

• I chose: an impulse is stimulated in the next neurone.
• I did not choose the neurotransmitter travelling along the next neurone.

Also accept

• an impulse is stimulated in the next neurone

Watch for

• “The vesicles release neurotransmitter.” — That happens first, when the impulse arrives at the first neurone.
• “The neurotransmitter diffuses back across the gap.” — Nothing crosses back: the message goes one way only, into the next neurone.
• “The neurotransmitter travels along the next neurone.” — The neurotransmitter only crosses the gap. Along the next neurone the message is a new electrical impulse.

11. SR3 · Return

Model answer. A synapse is one-way because only one side has vesicles and only the other has receptor proteins. If both neurones had both, either side could release neurotransmitter and either side could bind it, so impulses could cross both ways. A wider gap or a stronger impulse does not change which side can send, and swapping the two sides only reverses the one direction.

A full answer includes

• I chose: both neurones had vesicles of neurotransmitter and receptor proteins.
• I did not choose a wider gap or a stronger impulse: they do not change which side can send.

Also accept

• both neurones with vesicles and receptor proteins

Watch for

• “The synaptic gap were made twice as wide.” — A wider gap only makes diffusion take longer. The direction is set by which side has vesicles and which has receptor proteins.
• “The impulse arriving at the synapse were stronger.” — However strong the impulse, a side with no vesicles has nothing to release.
• “The vesicles were in the next neurone and the receptors on the first.” — Then impulses would cross from the next neurone to the first only: still one direction.