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

These are condensed notes for 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. For full explanations and fuller worked examples, read the study guide. Test yourself with the practice questions, tick off topics on the printable checklist, and see the OCR A Level Biology hub for the rest of the course. The free diagnostics show which topics need most work.

Organisation of the nervous system (5.1.5(g))

Structural	Made of
Central nervous system (CNS)	Brain plus spinal cord
Peripheral nervous system (PNS)	Nerves outside the CNS (sensory in, motor out)

Functional (within PNS)	Somatic	Autonomic
Effector	Skeletal muscle	Cardiac muscle, smooth muscle, glands
Control	Voluntary	Involuntary
Pathway	One motor neurone	Two neurones, ganglion between
Divisions	None	Sympathetic (activity) and parasympathetic (rest)

The brain (5.1.5(h))

Part	Function
Cerebrum	Thought, memory, language, sensory interpretation, voluntary movement
Cerebellum	Movement coordination, balance, posture
Medulla oblongata	Heart rate, breathing rate, blood pressure (autonomic)
Hypothalamus	Monitors blood composition and temperature; controls the pituitary
Pituitary gland	Releases hormones (anterior and posterior lobes differ)

Reflexes (5.1.5(i))

Reflex arc order: stimulus, receptor, sensory neurone, then a relay neurone in most reflexes (absent in the knee jerk), motor neurone, effector.

Knee jerk method in steps

1. A tap on the patellar tendon stretches the quadriceps.
2. Stretch receptors fire and impulses enter the spinal cord along a sensory neurone.
3. A direct synapse with a motor neurone (monosynaptic).
4. The motor neurone stimulates the quadriceps, which contracts.
5. The leg extends.

Survival value: fast (few synapses, no conscious decision), automatic, innate, protective, and helps maintain posture and balance.

Reflex versus voluntary: a reflex is involuntary, fast and stereotyped; a voluntary response involves the cerebrum, is slower and can be varied.

Nervous and hormonal coordination (5.1.5(j))

	Nervous	Hormonal
Signal	Impulse, transmitter	Hormone in blood
Speed	Fast	Slower
Duration	Short	Longer
Target	Specific	Widespread

Fight or flight: threat, hypothalamus, sympathetic nerves, adrenal glands release adrenaline. Effects: higher heart rate and stroke volume, deeper breathing, wider pupils, glycogen to glucose, blood diverted to muscles.

Cell signalling in outline

1. Adrenaline (first messenger) binds to a surface receptor.
2. Adenylyl cyclase is activated.
3. Adenylyl cyclase makes cAMP, the second messenger, from ATP.
4. cAMP then triggers enzymes within the cell and the response follows, with amplification.

Heart rate (5.1.5(k))

Factor	Pathway	Effect
Sympathetic nerve	Medulla, then accelerator nerve to SAN, noradrenaline	Faster
Parasympathetic (vagus)	Medulla, then vagus nerve to SAN, acetylcholine	Slower
Adrenaline	Blood to SAN	Faster
Raised blood carbon dioxide (lower pH)	Chemoreceptors to medulla	Faster

The SAN is the pacemaker: the heart is myogenic.

Rate from a pulse count: beats counted ÷ time (s) × 60. A count of 25 beats in 20 s is 25 × 3 = 75 per minute. Rate from an ECG: 60 ÷ time between R peaks (s).

Muscle (5.1.5(l))

	Skeletal	Involuntary (smooth)	Cardiac
Striated	Yes	No	Yes
Nuclei	Many	One	Usually one
Shape	Long, unbranched	Spindle	Branched, intercalated discs
Fatigue	Yes	Slowly	Does not

Sarcomere regions: A band = full myosin length (constant on contraction). I band = actin only (shortens). H zone = myosin only (shortens). Sarcomere = A band + I band.

Skeletal muscle fibre features: many nuclei, sarcolemma folded into T tubules, sarcoplasmic reticulum storing calcium ions, many mitochondria, and myofibrils made of actin (thin, with troponin and tropomyosin) and myosin (thick, with heads that act as ATPase).

Reading a stained section or photomicrograph

You see	It is
Long parallel fibres, regular cross-striations, several nuclei at the edge	Skeletal muscle, longitudinal section
Rounded polygons packed together, nuclei at the edge	Skeletal muscle, transverse section
Branched fibres with dark lines across them, one central nucleus	Cardiac muscle (the dark lines are intercalated discs)
Short spindle-shaped cells, no striations, one central nucleus	Involuntary (smooth) muscle

Under a light microscope, draw what you see, label the nuclei and striations, and show a scale bar or magnification.

Sarcomere reminder: with an A band of 1.5 µm and thin filaments of 0.9 µm, a sarcomere of 2.4 µm has an I band of 2.4 − 1.5 = 0.9 µm and an H zone of 2.4 − 1.8 = 0.6 µm. When the sarcomere shortens to 1.8 µm, the I band is 0.3 µm and the H zone is 0 µm.

Neuromuscular junction in steps

1. Action potential opens calcium channels in the neurone terminal.
2. Vesicles release acetylcholine.
3. It binds to receptors on the sarcolemma, causing depolarisation.
4. Acetylcholinesterase removes it.

Sliding filament in steps

1. Calcium ions flood out of the sarcoplasmic reticulum.
2. They attach to troponin, and tropomyosin shifts.
3. Myosin heads latch onto actin as cross-bridges.
4. The power stroke pulls actin inwards.
5. ATP binds, the head detaches, and ATP hydrolysis resets it.

ATP supply: respiration; creatine phosphate donates phosphate to ADP, giving ATP quickly but only for seconds.

Worked reminders

• Actual length = image length ÷ magnification. At ×5000, 12 mm on the image is 12 ÷ 5000 = 0.0024 mm = 2.4 µm.
• Magnification from a scale bar: a bar of 3 µm (0.003 mm) drawn 45 mm long gives 45 ÷ 0.003 = ×15 000.
• Percentage change = change ÷ original × 100: a pulse from 70 to 133 is (133 − 70) ÷ 70 × 100 = 90%.
• Creatine phosphate alone: 12 mmol at 3 mmol per second lasts 12 ÷ 3 = 4 s.

Monitoring heart rate: practical reminders

• Measure pulse at rest, immediately after exercise and at set times during recovery; count for at least 15 s and scale up.
• Keep exercise type, duration and intensity the same, use the same person and repeat to find a mean.
• An ECG sensor or heart rate monitor gives a continuous trace and removes counting error.
• Use healthy, consenting volunteers who may stop at any time.
• The recovery time is the time taken for the rate to return to the resting value.

Extended answer frame: sliding filament

Write it in this order: action potential, calcium ion release, troponin and tropomyosin, cross-bridge, power stroke, ATP and detachment, repeat. Then state what happens to the A band, I band and H zone. Mention the source of ATP (respiration and creatine phosphate) if the question asks about sustained or sudden activity.

Short answer reminders

• Why is a reflex quicker than a voluntary action? Fewer synapses, no processing in the cerebrum, and a short pathway.
• Why does adrenaline need a receptor? Target cells respond only if they have the receptor for that hormone.
• Why can the heart beat without nerves? The SAN is myogenic; nerves and hormones only change the rate.
• Why do muscle fibres have many mitochondria? Contraction and calcium pumping use large amounts of ATP.
• Why is the muscle fibre multinucleate? It is a long cell formed by fused cells, so many nuclei control protein synthesis along its length.

Must-know distinctions

• Somatic (skeletal muscle, voluntary) versus autonomic (smooth and cardiac muscle, glands).
• Sympathetic (noradrenaline, speeds heart) versus parasympathetic (acetylcholine, slows heart).
• First messenger (adrenaline, outside) versus second messenger (cAMP, inside).
• Cerebrum (thought) versus cerebellum (coordination, balance).
• Filaments slide; they do not shorten.

Quick self-test

1. State the two parts of the central nervous system.
2. Which part of the autonomic nervous system speeds the heart?
3. Name the brain region that coordinates movement and balance.
4. Why is the knee jerk faster than a response that needs thought?
5. Name the second messenger that adrenaline causes to be made.
6. Which part of a sarcomere stays the same length during contraction?
7. What is the role of calcium ions in contraction?
8. What does creatine phosphate do?
9. A 20 s count gives 25 beats. What is the rate per minute?
10. Which structure lies between the neurone and the muscle fibre at a neuromuscular junction?
11. Which enzyme ends the stimulation of a muscle fibre by acetylcholine?
12. A heart rate rises from 64 to 144 beats per minute. What is the percentage increase?

Answers

1. Brain and spinal cord.
2. The sympathetic division (accelerator nerve).
3. Cerebellum.
4. Few synapses, no conscious processing and a short pathway.
5. Cyclic AMP (cAMP).
6. The A band.
7. They bind to troponin, which moves tropomyosin and exposes the binding sites on actin.
8. It gives its phosphate to ADP to make ATP rapidly.
9. 25 × 3 = 75 beats per minute.
10. The synaptic cleft (with acetylcholine crossing it).
11. Acetylcholinesterase.
12. (144 − 64) ÷ 64 × 100 = 125%.

Where marks are usually lost

• Writing “the brain” or “the nervous system” where a named region or division is needed.
• Saying the medulla “controls the heart” without saying it changes the rate through the SAN.
• Describing the knee jerk with a relay neurone, when it is monosynaptic.
• Saying adrenaline “enters the cell” or that cAMP is the first messenger.
• Writing that filaments or the A band shorten.
• Leaving out ATP for detachment of myosin and for pumping calcium ions back.
• Forgetting units, or not converting mm to µm before using the magnification.
• Giving percentage change relative to the new value instead of the original.

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.