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

This guide covers 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. It sits in Module 5, Communication, homeostasis and energy, and teaches how the nervous system is organised, what the parts of the brain do, how reflexes work, how nervous and hormonal signals combine in fight or flight, how heart rate is controlled and how skeletal muscle contracts. Plant responses (5.1.5(a) to (f)) are outside this unit. Module 5 content can be assessed in Biological processes (Component 01) and in Unified biology (Component 03).

Use it with the revision notes and the practice questions. The OCR A Level Biology hub and the printable checklist cover the rest of the course, and the free diagnostics show where to start. Magnification and percentage skills are practised in Foundations in Biology.

What this unit covers

Spec reference	What you must be able to do
5.1.5(g)	Describe the structural (central and peripheral) and functional (somatic and autonomic) organisation of the mammalian nervous system
5.1.5(h)	Describe the gross structure of the human brain and the functions of the cerebrum, cerebellum, medulla oblongata, hypothalamus and pituitary gland
5.1.5(i)	Explain reflex actions, including the knee jerk reflex and the survival value of reflexes
5.1.5(j)	Explain coordination by the nervous and endocrine systems, including fight or flight and, in outline, adrenaline, adenylyl cyclase and cyclic AMP
5.1.5(k)	Explain the effects of hormones and nervous mechanisms on heart rate
5.1.5(l)	Describe muscle structure, compare muscle types, interpret stained sections and photomicrographs, and explain the sliding filament model, including ATP and creatine phosphate

5.1.5(g) Organisation of the nervous system

Structurally, the central nervous system (CNS) is made up of the brain and the spinal cord. The peripheral nervous system (PNS) is all the nerves outside them: sensory neurones carrying impulses in and motor neurones carrying impulses out.

Functionally, the PNS has two parts.

Feature	Somatic nervous system	Autonomic nervous system
Control	Voluntary (and reflex)	Involuntary
Effectors	Skeletal muscle	Cardiac muscle, smooth muscle and glands
Pathway	One motor neurone from CNS to effector	Two neurones with a synapse in a ganglion between them
Transmitter at effector	Acetylcholine	Acetylcholine or noradrenaline

The autonomic system has two divisions with opposite effects on many organs. The sympathetic division prepares the body for activity; the parasympathetic division is dominant at rest.

5.1.5(h) The human brain

Gross structure means what you can see and name on a whole or sectioned brain.

Part	Position	Functions
Cerebrum	Two large, folded hemispheres filling the upper skull	Conscious thought, memory, language, interpreting sensory input, starting voluntary movement
Cerebellum	Beneath the back of the cerebrum	Coordinates movement, balance and posture
Medulla oblongata	Where the brain joins the spinal cord	Autonomic control of heart rate, breathing rate and blood pressure
Hypothalamus	Underside of the brain, above the pituitary	Monitors blood composition and temperature; coordinates responses; makes hormones passed to the pituitary
Pituitary gland	Hangs from the hypothalamus	Releases hormones (the anterior lobe and posterior lobe release different ones)

The folding of the cerebral cortex increases surface area, so more neurones fit inside the skull. Link each part to a specific function in answers.

5.1.5(i) Reflex actions

A reflex is a rapid, automatic, involuntary response to a stimulus. The pathway is a reflex arc, running from stimulus through receptor, sensory neurone, an optional relay neurone and motor neurone to the effector and its response (stimulus to effector, in that order). Impulses do not need to reach the conscious part of the brain before the response starts.

The knee jerk reflex

1. A tap on the patellar tendon stretches the quadriceps muscle, and stretch receptors in the muscle detect this.
2. Impulses travel along a sensory neurone into the spinal cord.
3. It synapses directly with a motor neurone (a monosynaptic arc, with no relay neurone).
4. The motor neurone carries impulses to the quadriceps.
5. The quadriceps contracts and the lower leg extends.

Survival value

Reflexes are fast because there are few synapses and no decision-making delay. They are innate, so no learning is needed, and they protect the body (withdrawing a hand from a hot object, blinking). The knee jerk reflex helps maintain posture and balance by adjusting muscle tension when a muscle is stretched.

Worked example: why a withdrawal reflex is slower than the knee jerk

Data supplied: impulses travel at 60 m/s in the neurones. Each synapse (including a neuromuscular junction) adds a delay of 0.7 ms. In the knee jerk, the total neurone length is 1.0 m with two junctions (the spinal synapse and the neuromuscular junction). In a foot withdrawal reflex, the total length is 1.8 m with three junctions (two spinal synapses with a relay neurone, and the neuromuscular junction).

1. Knee jerk conduction time = 1.0 ÷ 60 = 0.0167 s = 16.7 ms.
2. Add delays: 16.7 + (2 × 0.7) = 18.1 ms.
3. Withdrawal: 1.8 ÷ 60 = 30.0 ms; add (3 × 0.7) = 2.1 ms, giving 32.1 ms.
4. The withdrawal is slower because the pathway is longer and has an extra synapse.

5.1.5(j) Nervous and hormonal coordination

Nervous	Hormonal
Electrical impulses plus transmitters	Hormones carried in the blood
Fast	Slower
Short-lived effect	Longer-lasting effect
Specific, localised effectors	Widespread; any cell with the receptor

Fight or flight

The hypothalamus detects a threat, and the sympathetic division stimulates the adrenal glands to release adrenaline into the blood. Effects: heart rate and stroke volume rise, breathing deepens, pupils dilate, glycogen is broken down so blood glucose rises, and blood is diverted to muscles. Nervous impulses start the response and adrenaline sustains it. The survival value is that the body is ready for sudden action.

Cell signalling (outline only)

Adrenaline is water-soluble and cannot cross the cell-surface membrane, so it acts as a first messenger.

1. Adrenaline binds to a receptor on the outside of the target cell’s membrane.
2. The receptor switches on the enzyme adenylyl cyclase, which sits on the inner face of the membrane.
3. Adenylyl cyclase turns ATP into cyclic AMP (cAMP), the second messenger.
4. cAMP then triggers enzymes within the cell, which produce the response (in liver cells, glycogen breakdown).

One adrenaline molecule makes many cAMP molecules, so the signal is amplified.

5.1.5(k) Control of heart rate

The heart is myogenic: the sinoatrial node (SAN) starts each beat without nervous stimulation. Nervous and hormonal signals change its rate.

• The medulla oblongata contains the cardiac control centre. Chemoreceptors detect a fall in blood pH (more carbon dioxide) during exercise, and pressure receptors detect blood pressure.
• The sympathetic accelerator nerve releases noradrenaline at the SAN and speeds the heart up.
• The parasympathetic vagus nerve releases acetylcholine at the SAN and slows it down.
• Adrenaline from the adrenal glands also acts on the SAN and raises heart rate.

Worked example: pulse before, during and after exercise (monitoring physiological functions)

Pulse counts over 15 s: rest 18, immediately after exercise 42, two minutes later 27.

1. Rates per minute: 18 × 4 = 72, 42 × 4 = 168, 27 × 4 = 108.
2. Percentage increase = (168 − 72) ÷ 72 × 100 = 133%.
3. Recovery in two minutes = 168 − 108 = 60 beats per minute, which is 60 ÷ 96 = 62.5% of the rise.

An ECG sensor gives the same information. At a chart speed of 25 mm/s, an R-to-R distance of 21 mm is 21 ÷ 25 = 0.84 s, so heart rate = 60 ÷ 0.84 = 71 beats per minute. Control exercise intensity and time, use the same volunteer, and repeat for reliability. Use healthy, consenting volunteers.

5.1.5(l) Muscle

Types of muscle

Feature	Skeletal	Involuntary (smooth)	Cardiac
Control	Voluntary	Involuntary	Involuntary
Striations	Yes	No	Yes
Cell shape	Long, unbranched, cylindrical	Short, spindle-shaped	Branched, joined by intercalated discs
Nuclei	Many, at the edge	One, central	Usually one, central
Contraction	Fast, fatigues	Slow, sustained	Rhythmic, myogenic, does not fatigue

Interpreting stained sections and photomicrographs

In a longitudinal section, skeletal muscle shows parallel fibres with regular cross-striations (alternating dark and light bands) and several nuclei at the edge of each fibre. In a transverse section, fibres appear as rounded polygons with nuclei at the edge. Cardiac muscle shows branching and darker intercalated discs. To find an actual size, use actual = image ÷ magnification, after converting to the same units (1 mm = 1000 µm).

Structure of a skeletal muscle fibre

A fibre contains many myofibrils, each made of repeating sarcomeres between Z lines. Thin actin filaments (with tropomyosin and troponin) attach to the Z lines; thick myosin filaments, with heads, lie between them. The fibre is wrapped in the sarcolemma, which folds into T tubules, and it is full of sarcoplasmic reticulum and mitochondria.

Region	Contains	Appearance
A band	The whole length of the myosin filaments	Dark
I band	Actin only	Light
H zone	Myosin only (centre of A band)	Lighter band within A band

Neuromuscular junction

1. An action potential arrives at the motor neurone terminal, and calcium ion channels open.
2. Vesicles of acetylcholine fuse with the membrane and release it into the cleft.
3. Acetylcholine binds to receptors on the sarcolemma, sodium ions enter and the sarcolemma depolarises.
4. Acetylcholinesterase breaks acetylcholine down, ending the stimulus.

The depolarisation then passes along the sarcolemma and into the T tubules.

The sliding filament model

1. Calcium ions leave the sarcoplasmic reticulum and attach to troponin.
2. Tropomyosin shifts position, uncovering the myosin-binding sites on actin.
3. Myosin heads (still carrying ADP and phosphate) attach to actin as cross-bridges.
4. The heads tilt, pulling actin towards the centre of the sarcomere (the power stroke) and releasing ADP and phosphate.
5. ATP binds to the head, which detaches from actin. Myosin ATPase hydrolyses the ATP and the head returns upright, ready to bind again.

ATP is needed for detachment and for pumping calcium ions back into the sarcoplasmic reticulum during relaxation. Filaments do not shorten: they slide. So the I band and H zone shorten, the sarcomere shortens, and the A band stays the same length.

Worked example: sarcomere change from a micrograph

At ×10 000, a relaxed sarcomere is 26 mm long on the image and a contracted one is 21 mm. Data supplied: the A band is 1.5 µm, and each thin filament is 0.9 µm.

1. Relaxed: 26 ÷ 10 000 = 0.0026 mm = 2.6 µm. Contracted: 21 ÷ 10 000 = 2.1 µm.
2. Percentage decrease = (2.6 − 2.1) ÷ 2.6 × 100 = 19%.
3. I band = sarcomere − A band: 1.1 µm relaxed, 0.6 µm contracted.
4. H zone = sarcomere − (2 × 0.9): 0.8 µm relaxed, 0.3 µm contracted.

ATP supply and creatine phosphate

Muscle holds little ATP. Creatine phosphate in the sarcoplasm transfers its phosphate to ADP, making ATP at once. It lasts only seconds and is rebuilt at rest using ATP from respiration. Longer activity needs aerobic respiration in mitochondria, with anaerobic glycolysis adding ATP in intense exercise.

Worked example (data supplied): muscle holds 4.5 mmol ATP and 13.5 mmol creatine phosphate per kg, and uses 1.5 mmol ATP per kg per second.

1. ATP alone lasts 4.5 ÷ 1.5 = 3.0 s.
2. With creatine phosphate: (4.5 + 13.5) ÷ 1.5 = 12.0 s, four times as long.

Common errors

• Saying the filaments shorten, or that the A band shortens.
• Confusing the cerebellum (movement, balance) with the cerebrum (thought).
• Writing that a reflex “does not involve the brain” when impulses still reach it afterwards.
• Treating adrenaline as entering the cell, rather than acting through cAMP.

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.