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

This study guide covers the first ten statements of section C3.1 (Integration of body systems), C3.1.1 to C3.1.10, in the IB Biology guide, first assessment 2025, for DP Biology. These statements are listed for both SL and HL, so nothing here is HL only. It applies to the examination sessions that follow that first assessment. The later statements of C3.1, on hormones, feedback and plant growth regulation, are taught on other pages. Use this guide with the revision notes and the practice questions. The IB DP Biology hub, the printable checklist and the free diagnostics sit alongside it.

What this unit covers

Statement	What you must be able to do	Tier
C3.1.1	Explain why coordination between parts is needed for a system to do its job	SL and HL
C3.1.2	Describe the levels of organisation from cell upwards as nested subsystems; link integration to emergent properties	SL and HL
C3.1.3	Compare nervous and hormonal signalling; show how blood moves materials and energy between organs	SL and HL
C3.1.4	Describe the brain as an organ that combines several inputs and supports learning and memory	SL and HL
C3.1.5	Explain the spinal cord as a centre for unconscious processes; distinguish conscious from unconscious	SL and HL
C3.1.6	Explain input from receptors to the spinal cord and cerebral hemispheres via sensory neurons	SL and HL
C3.1.7	Explain output from the cerebral hemispheres to muscles via motor neurons	SL and HL
C3.1.8	Interpret a transverse section of a nerve	SL and HL
C3.1.9	Explain the pain reflex arc with one interneuron	SL and HL
C3.1.10	Outline the cerebellum’s general role in movement and balance	SL and HL

C3.1.1 and C3.1.2: integration and the hierarchy of life

A system is a set of parts that together do something none could do alone. System integration means the parts are coordinated, so the output of one fits the needs of another. Without coordination, a muscle could contract with no oxygen on the way, or a heart could beat at one rate while the body needed another.

Multicellular animals are built as a hierarchy of subsystems:

• Cell: the basic unit, for example a neuron or a cardiac muscle cell.
• Tissue: similar cells working together, for example nervous tissue.
• Organ: several tissues forming one structure with a function, for example the spinal cord.
• Body system: organs working towards a shared function, for example the nervous system.
• Organism: all the systems integrated into one living whole.

Each level is itself a system, and each is integrated with the others. Integration gives rise to emergent properties: features of the whole that no single part has. A single heart muscle cell cannot pump blood, and a heart alone cannot keep a person alive. A sprinting animal that spots, chases and catches prey shows emergent behaviour that depends on sensing, deciding, moving, breathing and circulating all being coordinated.

C3.1.3: nervous signals, hormonal signals and transport

Two signalling systems link organs. Blood links them physically.

Feature	Nervous system	Endocrine system
Signal	Electrical impulses along neurons, chemical across synapses	Hormones secreted into the blood
Route	Along specific neurons to specific targets	Carried round the body by blood plasma
Speed	Very fast	Slower, as blood must carry the hormone
Duration	Short, stops when impulses stop	Often longer lasting
Target	Muscle or gland connected by neurons	Any cell with the matching receptor

Hormones act only on cells with receptors for them, so a hormone travelling everywhere still affects selected targets.

The blood system also moves materials and energy. Examples: oxygen from the lungs to working muscle; carbon dioxide the other way; glucose from the small intestine and liver to the brain; urea from the liver to the kidneys; hormones from glands to target cells; and heat from deep organs to the skin. Moving glucose is moving chemical energy. If circulation stops, organs that depend on each other are cut off.

C3.1.4: the brain as an integration organ

The brain receives input from many receptors at once and combines it. Catching a thrown ball needs sight (position, speed), hearing, signals from muscles and joints, and memory of earlier catches. The cerebral hemispheres process these together and send a single coordinated output.

The brain also supports learning and memory. Experience changes the connections between neurons, so some pathways are used more easily next time. You are not required to know slow-acting neurotransmitters or other fine detail. You need the idea that the brain integrates several inputs and stores information to improve later responses.

C3.1.5: the spinal cord and unconscious processes

A conscious process involves awareness and usually a decision by the cerebral hemispheres, such as choosing to pick up a pen. An unconscious process happens without awareness or choice. The spinal cord is an integrating centre for some of these. It receives sensory input, processes it in its grey matter and sends a motor output, often before the brain is informed.

A transverse section of the cord shows:

• Grey matter in the centre (roughly butterfly-shaped), holding neuron cell bodies, interneurons and synapses.
• White matter around it, made of myelinated axons that carry impulses up and down.
• A dorsal root on each side, bringing sensory fibres in. A swelling on it, the dorsal root ganglion, holds the sensory neuron cell bodies.
• A ventral root on each side, carrying motor fibres out.

C3.1.6 and C3.1.7: input and output

Sensory neurons carry impulses from receptor cells (such as photoreceptors, thermoreceptors, mechanoreceptors and pain receptors) to the central nervous system. Some inputs reach the spinal cord only. Others are passed up the cord to the cerebral hemispheres, where they are experienced consciously.

Motor neurons carry impulses from the central nervous system to effectors. For voluntary movement, neurons from the motor area of the cerebral hemisphere pass down the spinal cord. They link, via synapses, to motor neurons that leave through the ventral root and run in nerves to skeletal muscle. At the neuromuscular junction, the neuron stimulates the muscle fibre to contract. Muscles can only be stimulated to contract, so relaxing and movement in the other direction depend on an antagonistic muscle contracting.

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

C3.1.8: nerves

A nerve is a bundle of nerve fibres (axons) wrapped in a protective connective tissue sheath. Many nerves are mixed: they contain both sensory and motor fibres, travelling in opposite directions within the same cable. In a transverse section you can see:

• the outer protective sheath;
• many round fibre cross-sections;
• myelinated fibres, each with a myelin layer round the axon, appearing as a ring;
• unmyelinated fibres, smaller with little or no ring.

Worked example: a nerve section

A micrograph of a transverse nerve section has a scale bar 25 mm long that represents 100 µm. The nerve is 70 mm across on the image. In a sampled area, 48 of 80 fibres are myelinated.

1. Magnification = image length ÷ actual length = 25 mm ÷ 0.100 mm = ×250.
2. Actual diameter = 70 mm ÷ 250 = 0.28 mm = 280 µm.
3. Myelinated percentage = 48 ÷ 80 × 100 = 60.0%, so 40.0% are unmyelinated.

C3.1.9: the pain reflex arc

A reflex is a rapid, involuntary response to a stimulus. The pain withdrawal reflex of the hand follows this pathway:

Stage	Structure	Location and action
1. Receptor	Free sensory nerve endings (pain receptors)	In the skin of the hand; stimulated by damaging heat, pressure or chemicals
2. Sensory neuron	Sensory neuron	Impulse travels to the spinal cord via the dorsal root; cell body in the dorsal root ganglion
3. Interneuron	One interneuron	In the grey matter; passes the signal by synapses
4. Motor neuron	Motor neuron	Cell body in the grey matter; axon leaves by the ventral root
5. Effector	Skeletal muscle (for example in the arm)	Contracts, pulling the hand away

Synapses lie between sensory neuron and interneuron, and between interneuron and motor neuron. The reflex is quick because the pathway is short and the brain is not needed to start the response. Other interneurons pass the signal up to the cerebral hemispheres, so you feel the pain afterwards.

Worked example: timing a reflex

Data supplied: the sensory neuron is 0.70 m long and conducts at 56 m/s. The motor neuron is 0.65 m long and conducts at 52 m/s. The measured time from stimulus to muscle response is 26.0 ms.

1. Sensory conduction = 0.70 ÷ 56 = 0.0125 s = 12.5 ms.
2. Motor conduction = 0.65 ÷ 52 = 0.0125 s = 12.5 ms.
3. Total conduction = 25.0 ms, leaving 26.0 − 25.0 = 1.0 ms for the synapses.
4. With two synapses: 1.0 ÷ 2 = 0.5 ms each, if equal and the interneuron’s conduction time is ignored.

If the same 0.70 m were an unmyelinated fibre at 1.4 m/s, the time would be 0.70 ÷ 1.4 = 0.50 s. That is 40 times slower than 12.5 ms, which shows why myelinated fibres suit fast reflexes.

A voluntary response to the same stimulus, such as deliberately pressing a button, might take 0.200 s. Against a 0.026 s reflex, 0.200 ÷ 0.026 ≈ 7.7, so the reflex is about 7.7 times faster. The voluntary route passes through the cerebral hemispheres with extra synapses.

C3.1.10: the cerebellum

The cerebellum lies at the back of the brain, below the cerebral hemispheres. Your required understanding is general: it coordinates skeletal muscle contraction and maintains balance. It receives information about body position, balance and the plan for movement. It adjusts the timing and force of contractions so movements are smooth and accurate. Damage typically causes jerky, uncoordinated movement and loss of balance, while muscles can still contract because motor pathways remain.

Common errors

• Saying the brain controls reflexes. The spinal cord integrates them.
• Placing the interneuron in the white matter. It lies in grey matter.
• Mixing up the roots: sensory fibres enter by the dorsal root, motor fibres leave by the ventral root.
• Saying a nerve is a single neuron. It is a bundle of fibres.
• Writing that hormones “go only to” their target. They travel to all tissues in the blood but act only where receptors exist.
• Saying that muscles are stimulated to relax.
• Claiming the cerebellum starts voluntary movement. It coordinates it.
• Forgetting to convert units when calculating magnification.

Next steps

Test yourself with the practice questions, then condense with the revision notes. To find weak spots, take a free 10-minute diagnostic.

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.