IB DP Biology: Nervous system, brain, spinal cord and reflexes (DP Biology) – Revision Notes

These revision notes condense statements C3.1.1 to C3.1.10 of the IB Biology guide, first assessment 2025, for DP Biology. All ten are SL and HL, with no HL-only content, and they apply to the examination sessions that follow that first assessment. For full explanations read the study guide, then test yourself with the practice questions. The IB DP Biology hub, the printable checklist and the free diagnostics help you track progress.

Key definitions

Term	Meaning
System integration	Coordination of component parts so that they perform an overall function together
Emergent property	A feature of the whole that its separate parts do not have
Central nervous system	Brain and spinal cord
Sensory neuron	Carries impulses from a receptor cell to the central nervous system
Motor neuron	Takes impulses out of the central nervous system towards a muscle or gland
Interneuron	Links neurons inside the central nervous system
Nerve	A bundle of nerve fibres in a protective sheath
Reflex	A rapid, involuntary response; no decision by the cerebral hemispheres is needed to start it
Effector	The muscle or gland that produces the response

C3.1.1 and C3.1.2: hierarchy and integration

Order from small to large: cell, tissue, organ, body system, organism. Each level is a subsystem of the next. Integration between them produces emergent properties, for example:

• a cardiac muscle cell cannot pump blood, but the heart can;
• a hunting animal’s success depends on senses, brain, muscles, lungs and circulation working together;
• the loss of one subsystem (for example the blood supply) disables others that depend on it.

C3.1.3: nervous vs endocrine and the blood system

Feature	Nervous	Endocrine
Message	Electrical impulses, then a chemical across synapses	Hormone in the blood
Reaches	Specific target by specific neurons	Any cell with a receptor
Speed	Very fast	Slower
Effect lasts	Briefly	Longer

Blood as a transport link, with examples:

• oxygen: lungs to muscle;
• carbon dioxide: muscle to lungs;
• glucose (chemical energy): intestine and liver to brain;
• urea: liver to kidney;
• heat: deep organs to skin;
• hormones: gland to target cell.

C3.1.4: the brain

• It combines several inputs (sight, sound, touch, body position, memory) into one coordinated output.
• It supports learning and memory: experience alters connections between neurons, so later responses improve.
• You do not need slow-acting neurotransmitter detail.

C3.1.5: the spinal cord

Conscious process	Unconscious process
Involves awareness	No awareness needed
Decision by the cerebral hemispheres	Integrated without that decision
Example: choosing to lift a cup	Example: withdrawing a hand from a hot object

Spinal cord section:

• grey matter in the middle: cell bodies, interneurons and synapses;
• white matter outside: myelinated axons running up and down;
• dorsal root: sensory fibres in; dorsal root ganglion holds the cell bodies of sensory neurons;
• ventral root: motor fibres out.

C3.1.6 and C3.1.7: input and output

Method in steps for any pathway question:

1. Name the receptor and the stimulus it detects.
2. Sensory neuron carries impulses to the CNS (spinal cord, then possibly up to the cerebral hemispheres).
3. The CNS integrates the input (grey matter, or cerebral hemispheres).
4. Motor neurons carry impulses out (from the cerebral hemispheres, down the cord, out by the ventral root).
5. The effector, a skeletal muscle, is stimulated to contract at the neuromuscular junction.
6. Relaxation or reverse movement needs the antagonistic muscle to contract.

The left hemisphere mainly controls the right side of the body, and the right hemisphere the left.

C3.1.8: nerves

• A protective sheath surrounds the bundle.
• Fibres are cut across, so axons appear as circles.
• Myelinated fibres show a ring (myelin) round the axon; unmyelinated fibres are smaller and lack a clear ring.
• Many nerves are mixed: sensory and motor fibres lie side by side, carrying impulses in opposite directions.

Magnification: image size ÷ actual size (same units). Actual size: image size ÷ magnification.

Worked reminder: scale bar 18 mm on the image equals 90 µm. Magnification = 18 ÷ 0.090 = ×200. A fibre bundle 30 mm wide on the image is 30 ÷ 200 = 0.15 mm = 150 µm.

C3.1.9: pain reflex arc

Order	Part	Note
1	Pain receptor	Free sensory nerve ending in the skin of the hand
2	Sensory neuron	Enters cord by the dorsal root
3	Interneuron	One only, in the grey matter
4	Motor neuron	Leaves by the ventral root
5	Effector	Skeletal muscle contracts to withdraw the hand

Key points:

• the response is involuntary and fast, as the pathway is short and the brain is bypassed;
• pain is felt afterwards because other interneurons signal the cerebral hemispheres;
• speed = distance ÷ time, and time = distance ÷ speed.

Worked reminder: a neuron 0.80 m long conducting at 40 m/s takes 0.80 ÷ 40 = 0.020 s = 20 ms.

C3.1.10: the cerebellum

• Sits at the back of the brain, below the cerebral hemispheres.
• Coordinates skeletal muscle contraction (timing and force) and balance.
• Gives a general control of the movements of the whole body.
• Damage gives clumsy, jerky movement and poor balance; strength is not lost.

Reasoning patterns to rehearse

Explain why a reflex is fast. Give three linked points: few neurons and only two synapses, so little delay; no decision by the cerebral hemispheres is needed to start it; and myelinated fibres conduct quickly.

Interpret damage. Match the lost function to the structure:

Observation	Likely region	Reasoning
Muscles strong but movements jerky and balance poor	Cerebellum	Contraction still possible; coordination lost
No voluntary movement in the legs, yet the foot still withdraws from a pinprick	Spinal cord injured above the leg segments	The reflex arc below the injury is intact; the link to the cerebral hemispheres is broken
Cannot feel the pinprick but the foot withdraws	Pathway to the cerebral hemispheres interrupted	Feeling needs the signal to reach the cerebral hemispheres

Compare two nerve fibres. Use the same quantity for each: “the myelinated fibre conducts at 56 m/s and the unmyelinated at 1.4 m/s, so the myelinated fibre is 40 times faster”. Quote numbers from the data and state the factor.

Percentage change. (new value minus original value) ÷ original value × 100. Worked reminder: a reaction time falls from 0.20 s to 0.17 s, so the change is (0.17 − 0.20) ÷ 0.20 × 100 = −15%.

Write the route in order. In any explanation of a response, name each structure and say what it does: receptor detects, sensory neuron carries in, integrating centre processes, motor neuron carries out, effector contracts.

Must-know distinctions

• Sensory vs motor: toward the CNS vs away from it.
• Dorsal vs ventral root: sensory in, motor out.
• Grey vs white matter: cell bodies and synapses vs myelinated axons.
• Nerve vs neuron: a bundle vs a single cell.
• Reflex vs voluntary: spinal and automatic vs decided by the cerebral hemispheres.
• Cerebral hemispheres vs cerebellum: initiate voluntary movement and consciousness vs coordinate and balance it.

Quick self-test

1. List the five levels of organisation from cell to organism.
2. Define an emergent property.
3. State two differences between nervous and endocrine signalling.
4. Name two materials carried by blood between organs and where each goes.
5. What is the difference between a conscious and an unconscious process?
6. Which root carries sensory fibres into the spinal cord?
7. Where does the interneuron of the pain reflex lie?
8. What is the effector in the pain withdrawal reflex of the hand?
9. A sensory neuron is 0.90 m long and conducts at 45 m/s. How long does conduction take?
10. A nerve image is ×200 and a fibre measures 12 mm on the image. What is the actual width in µm?
11. State the general role of the cerebellum.
12. Why can a person feel pain after the hand has already moved away?

Answers

1. Cell, tissue, organ, body system, organism.
2. A feature of the whole that its separate parts do not have.
3. Nervous: impulses, very fast, specific target, brief. Endocrine: hormones in blood, slower, longer lasting (any two).
4. For example oxygen from lungs to muscle, and glucose from the intestine or liver to the brain.
5. Conscious involves awareness and a decision by the cerebral hemispheres. Unconscious needs neither.
6. The dorsal root.
7. Inside the grey matter of the cord.
8. Skeletal muscle, for example in the arm.
9. 0.90 ÷ 45 = 0.020 s = 20 ms.
10. 12 ÷ 200 = 0.060 mm = 60 µm.
11. Coordinates skeletal muscle contraction and maintains balance.
12. Other interneurons pass impulses up to the cerebral hemispheres, which takes extra time and comes after the reflex began.

Where marks are usually lost

• Writing “brain” for the integrating centre of a reflex.
• Leaving out the receptor, or not naming it as a free sensory nerve ending in the skin.
• Giving pathway components in the wrong order.
• Swapping dorsal and ventral roots.
• Saying that the interneuron lies in the white matter.
• Describing hormones as “fast” or nerves as “long lasting” with no comparison.
• Omitting units or failing to convert mm to µm when using magnification.
• Naming a transported substance without saying where it goes from and to.
• Saying that the cerebellum “controls” movement without mentioning coordination and balance.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Section C3.1, Integration of body systems, statements C3.1.1 to C3.1.10.