OCR A Level Biology A: Nervous system, reflexes, heart control and muscle (H420) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 5.1.5(g) to (l) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025. Read the study guide first, and use the revision notes for recall. The OCR A Level Biology hub and the printable checklist show the rest of the course.

Questions

1. (a) State the division of the nervous system (central, somatic or autonomic) to which each belongs: (i) the cerebellum, (ii) a motor neurone to the biceps, (iii) a neurone from the brain stem that slows the heart, (iv) the spinal cord. [4] (b) State one structural difference between the motor pathway to a skeletal muscle and the motor pathway to cardiac muscle. [1]

2. Three patients each have damage to one part of the brain. Name the part most likely to be damaged and explain the symptom. (a) Patient A understands speech and can walk, but staggers and misses a cup when reaching for it. [2] (b) Patient B has a body temperature that no longer returns to normal after being in a cold room. [2] (c) Patient C stops breathing after a head injury, although the lungs and the diaphragm are undamaged. [2]

3. A reflex hammer taps the patellar tendon of five volunteers. The time from the tap to the start of muscle activity (the latency) is recorded. In a second test, the same volunteers press a button when a light flashes.

Volunteer	Knee jerk latency / ms	Button press / ms
1	38	212
2	41	236
3	36	198
4	44	241
5	42	223

(a) Calculate the mean latency for each test. [2] (b) Calculate how many times longer the button press takes than the knee jerk, using the means. [1] (c) Explain why the knee jerk is faster. [2] (d) Explain the survival value of reflex actions. [2]

4. A photomicrograph of skeletal muscle has a scale bar of 2.0 µm that measures 30 mm. A relaxed sarcomere measures 34.5 mm and a contracted one 27 mm. (a) Calculate the magnification. [2] (b) Calculate the percentage decrease in sarcomere length on contraction. [3] (c) State two features that would distinguish cardiac muscle from this skeletal muscle in a stained section. [2]

5. Data supplied for one myofibril are in the table. Each thin filament is 0.9 µm long and the A band is 1.5 µm in every state.

Sarcomere length / µm	I band / µm	H zone / µm
3.2
2.6
2.0
1.8

(a) Complete the table, using sarcomere = A band + I band and H zone = sarcomere − 2 × thin filament length. [2] (b) State what the constant A band shows about the filaments. [1] (c) Explain the changes in I band and H zone with the sliding filament model. [2]

6. A motor neurone fires. Give a sequenced account, with reasons, of the events that end with the sarcomeres of a skeletal muscle fibre getting shorter. [8]

7. A muscle holds 3.6 mmol of ATP and 18.0 mmol of creatine phosphate per kg, and uses ATP at 2.4 mmol per kg per second. (a) Calculate how long the ATP alone would last, and how long ATP plus creatine phosphate would last, assuming one creatine phosphate makes one ATP. [3] (b) Explain how creatine phosphate maintains the supply of ATP, and why it is not enough for a 400 m race. [2]

8. Explain how heart rate increases at the start of exercise and returns to resting level afterwards. [6]

9. Five volunteers had their pulse counted for 30 s at rest, immediately after 3 minutes of exercise, and 5 minutes later.

Volunteer	Rest	After exercise	5 minutes later
A	36	84	46
B	33	79	42
C	39	90	51
D	31	75	40
E	35	83	45

(a) Convert the resting counts to beats per minute and find the mean. [2] (b) The mean rate after exercise is 164.4 beats per minute. Calculate the percentage increase from rest. [2] (c) Suggest two ways to make the investigation more reliable. [2] (d) Describe what the 5-minute data show. [1]

10. In an experiment, liver cell extracts were exposed to adrenaline and the cAMP produced was measured.

Adrenaline / nmol dm⁻³	0	1	10	100	1000
cAMP / µmol per g	0.5	2.1	7.9	15.2	15.8

(a) Outline the steps linking adrenaline arriving at a liver cell to cAMP being made. [3] (b) Describe and suggest an explanation for the trend between 100 and 1000 nmol dm⁻³. [2] (c) Explain why adrenaline needs a second messenger. [2]

Answers

1. (a) (i) central [1]; (ii) somatic [1]; (iii) autonomic [1]; (iv) central [1]. (b) Skeletal: one motor neurone from the CNS to the muscle, whereas cardiac has two neurones with a synapse in a ganglion [1]. [5] Examiner insight: name the division exactly as asked; “nervous system” or “brain” is too vague to answer a “which division” question.

2. (a) Cerebellum [1]; it coordinates movement and balance [1]. (b) Hypothalamus [1]; it monitors temperature and coordinates the response [1]. (c) Medulla oblongata [1]; it controls the breathing rate through autonomic centres [1]. [6] Examiner insight: each part needs a named function linked to the symptom; naming the region without the function leaves a mark unclaimed.

3. (a) Reflex: 201 ÷ 5 = 40.2 ms [1]; button: 1110 ÷ 5 = 222 ms [1]. (b) 222 ÷ 40.2 = 5.5 times [1]. (c) Few synapses, with a direct monosynaptic link [1]; no conscious processing in the cerebrum [1]. (d) Rapid and automatic, so the body avoids harm before it is consciously aware [1]; innate, so no learning is needed and posture or balance is protected [1]. [7] Examiner insight: “faster” needs a reason that refers to the pathway; show the sums for the means, so that a slip in arithmetic does not hide a correct method.

4. (a) 2.0 µm = 0.0020 mm [1]; 30 ÷ 0.0020 = ×15 000 [1]. (b) Relaxed 34.5 ÷ 15 000 = 0.0023 mm = 2.3 µm [1]; contracted 27 ÷ 15 000 = 1.8 µm [1]; (2.3 − 1.8) ÷ 2.3 × 100 = 21.7% [1]. (c) Branched fibres [1]; intercalated discs (and a single central nucleus) [1]. [7] Examiner insight: convert to one unit before dividing; magnification has no units, so a bare number with a stray unit loses clarity.

5. (a) I band: 1.7, 1.1, 0.5, 0.3 [1]; H zone: 1.4, 0.8, 0.2, 0.0 [1]. (b) The myosin filaments do not shorten [1]. (c) Actin filaments slide towards the centre between the myosin filaments [1]; so the overlap increases, which shortens the I band and H zone [1]. [5] Examiner insight: say that the filaments slide, not shorten; an answer gains most when it links the change in each band to the movement of actin.

6. Calcium channels open in the nerve ending and acetylcholine is released [1]; it diffuses across the cleft to receptors on the sarcolemma, which depolarises [1]; the depolarisation travels into the T tubules [1]; the sarcoplasmic reticulum releases calcium ions [1]; calcium attaches to troponin, shifting tropomyosin off the actin binding sites [1]; myosin heads attach, forming cross-bridges [1]; the heads tilt (power stroke) and drag actin towards the centre, shortening the sarcomere [1]; fresh ATP binds so the heads let go, and its hydrolysis resets them for another cycle [1]. [8] Examiner insight: in a long answer, present the steps in sequence and use the correct terms; mixing up troponin and tropomyosin is a common way to lose credit.

7. (a) ATP alone: 3.6 ÷ 2.4 = 1.5 s [1]; total 3.6 + 18.0 = 21.6 mmol [1]; 21.6 ÷ 2.4 = 9.0 s [1]. (b) Creatine phosphate donates its phosphate to ADP, making ATP rapidly [1]; the store runs out within seconds, so a long race depends on respiration [1]. [5] Examiner insight: give the unit with each time; for the explanation, state both what creatine phosphate does and why the store is limited.

8. Exercise raises carbon dioxide, which lowers blood pH [1]; chemoreceptors detect this and signal the medulla [1]; the medulla sends impulses along the sympathetic nerve to the SAN, releasing noradrenaline [1]; adrenaline from the adrenal glands also raises the rate [1]; afterwards, carbon dioxide falls and the medulla sends impulses along the vagus nerve [1]; acetylcholine at the SAN slows the heart to resting level [1]. [6] Examiner insight: name the nerve and the transmitter for each direction of change; the answer must cover both the increase and the return.

9. (a) Rates: 72, 66, 78, 62, 70 [1]; mean 348 ÷ 5 = 69.6 beats per minute [1]. (b) (164.4 − 69.6) ÷ 69.6 × 100 [1] = 136% [1]. (c) Same exercise type and duration for all [1]; repeat and take a mean, or use a heart rate monitor instead of counting [1]. (d) The rate is still above resting for every volunteer, so recovery is not complete [1]. [7] Examiner insight: scale the 30 s count to a minute before averaging; a percentage change is divided by the original value.

10. (a) The hormone attaches to its receptor on the cell-surface membrane [1]; adenylyl cyclase is switched on [1]; it turns ATP into cAMP [1]. (b) cAMP stays almost constant, rising only from 15.2 to 15.8 [1]; receptors or adenylyl cyclase are saturated, so more adrenaline has no effect [1]. (c) Adrenaline is water-soluble and cannot cross the membrane [1]; cAMP carries the signal inside and amplifies it [1]. [7] Examiner insight: quote figures when describing a trend; the explanation needs a limiting factor such as saturation, not the word “plateau” alone.

Where marks are usually lost

• Naming “the brain” where a part such as the cerebellum or medulla oblongata is needed.
• Describing a reflex without a link between the short pathway and the speed.
• Using relay neurones in the knee jerk, which is monosynaptic.
• Not converting units (mm to µm) in magnification questions.
• Dividing percentage change by the new value.
• Writing “the filaments shorten”, or that the A band changes.
• Missing ATP in cross-bridge detachment.
• Saying adrenaline enters the cell.

Next steps

• Review the revision notes and the study guide.
• Visit the OCR A Level Biology hub and the printable checklist.
• Take any of the free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

OCR, A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025: section 5.1.5(g) to (l), Plant and animal responses, with the mathematical skills and practical activity groups cited in the specification for these statements.