IB DP Biology: Neural signalling: neurons, action potentials and synapses (DP Biology)

This study guide covers section C2.2 (Neural signalling) of the IB Biology guide, first assessment 2025, for DP Biology. Statements C2.2.1 to C2.2.7 are SL and HL. Statements C2.2.8 to C2.2.16 are additional higher level and are marked HL only. It applies to the examination sessions that follow the first assessment in 2025. Use it with the revision notes and the practice questions. See also the IB DP Biology hub, the printable checklist and the free diagnostics.

What this unit covers

Section	What you must be able to do	Tier
C2.2.1	Describe neurons: cell body, axon, dendrites	SL and HL
C2.2.2	Explain how pumping builds the negative resting potential	SL and HL
C2.2.3	Explain a nerve impulse as an electrical event	SL and HL
C2.2.4	Compare impulse speeds; use correlation and R²	SL and HL
C2.2.5	Describe chemical synapses and one-way transmission	SL and HL
C2.2.6	Explain neurotransmitter release, including calcium	SL and HL
C2.2.7	Explain an excitatory postsynaptic potential, using acetylcholine	SL and HL
C2.2.8	Explain depolarization and repolarization with voltage-gated channels	HL only
C2.2.9	Explain propagation by local currents	HL only
C2.2.10	Interpret oscilloscope traces	HL only
C2.2.11	Explain saltatory conduction	HL only
C2.2.12	Explain effects of neonicotinoids and cocaine	HL only
C2.2.13	Explain inhibitory postsynaptic potentials	HL only
C2.2.14	Explain summation	HL only
C2.2.15	Explain the perception of pain	HL only
C2.2.16	Explain consciousness as an emergent property	HL only

C2.2.1 Neurons

A neuron is a cell with a cell body (cytoplasm and a nucleus) from which elongated nerve fibres project. An axon is a single long fibre. Dendrites are several shorter fibres. Electrical impulses travel along these fibres. A fibre can be very long: the axon of a motor neuron to your foot runs from the spinal cord to the muscle.

C2.2.2 The resting potential

A neuron that is not sending an impulse is at its resting potential. The inside of the membrane is negative relative to the outside, at about -70 mV in many neurons. Because the two sides carry different charges, the membrane is polarized, and the difference is the membrane potential.

The sodium-potassium pump uses energy from ATP to move 3 sodium ions out of the cell and 2 potassium ions in for each cycle. This builds two concentration gradients: sodium is high outside, potassium is high inside. The inside is negative for three reasons:

• Each pump cycle removes one more positive charge than it brings in.
• Open potassium channels let K⁺ leak out down its gradient, and few sodium channels are open to let Na⁺ leak back.
• Large negatively charged molecules inside the cell cannot cross the membrane.

Worked example. A pump runs 600 cycles. Sodium ions out: 600 × 3 = 1800. Potassium ions in: 600 × 2 = 1200. Net positive charge moved out: 1800 - 1200 = 600 charges. The pump makes the inside more negative as well as building both gradients.

C2.2.3 The nerve impulse

A nerve impulse is an action potential: a brief reversal of the membrane potential that passes along the fibre. Positively charged ions cross the membrane during the impulse, which is why the signal counts as electrical. It is propagated, which means it is regenerated along the fibre rather than fading with distance.

C2.2.4 Speed of nerve impulses

Three comparisons matter:

• A giant axon such as the squid’s has a wide diameter and low resistance to the flow of ions. It carries impulses faster than narrow non-myelinated fibres of about 1 µm diameter. Typical figures are tens of metres per second for the squid axon and about 1 m s⁻¹ for narrow unmyelinated fibres.
• Myelinated fibres conduct faster than non-myelinated fibres of similar diameter.
• Within non-myelinated fibres, speed rises with diameter.

Worked example (speed). An impulse crosses 0.60 m of myelinated fibre in 8.0 ms. Speed = distance ÷ time = 0.60 ÷ 0.0080 = 75 m s⁻¹. A non-myelinated fibre takes 0.50 s for the same distance: 0.60 ÷ 0.50 = 1.2 m s⁻¹. The myelinated fibre is about 63 times faster.

Correlation and R²

The guide asks you to describe positive and negative correlations and to use the correlation coefficient r, which runs from -1 to +1. A value near +1 is a strong positive correlation, near -1 a strong negative one and near 0 means little linear relationship. The coefficient of determination, R², is r squared. It shows how much of the variation in the dependent variable is explained by the variation in the independent variable. The guide notes that conduction speed is positively correlated with axon diameter and negatively correlated with animal size.

Worked example. Fictional data for non-myelinated fibres:

Diameter (µm)	2	5	10	20	40	80
Speed (m s⁻¹)	0.9	1.4	2.1	2.8	4.4	6.1

Calculation of r from these six pairs gives r = 0.979. R² = 0.979² = 0.959, so about 96% of the variation in speed is explained by variation in diameter. The remaining 4% comes from other factors such as temperature and measurement error. A strong correlation shows an association, and does not prove that diameter causes the change.

C2.2.5 Synapses

A synapse is a junction between two neurons, or between a neuron and an effector cell such as a muscle fibre. This unit is limited to chemical synapses. The presynaptic and postsynaptic membranes are separated by a narrow synaptic cleft. A signal passes in only one direction because neurotransmitter is released from the presynaptic side and its receptors are on the postsynaptic side.

C2.2.6 Release of neurotransmitter

1. An action potential depolarizes the presynaptic membrane.
2. Voltage-gated calcium channels open and Ca²⁺ diffuses into the presynaptic neuron.
3. Calcium acts as a signalling chemical inside the neuron. It causes vesicles of neurotransmitter to move to the membrane and fuse with it.
4. The neurotransmitter is released by exocytosis into the cleft.

C2.2.7 The excitatory postsynaptic potential

Acetylcholine is the example. It is used at many kinds of synapse, among them neuromuscular junctions.

1. The neurotransmitter diffuses across the cleft.
2. It binds to transmembrane receptors on the postsynaptic membrane.
3. The receptor is part of a channel, which opens. Positive ions enter, mainly sodium.
4. The postsynaptic membrane depolarizes slightly. This small change is an excitatory postsynaptic potential (EPSP).
5. The neurotransmitter is then broken down (acetylcholine is split by an enzyme) or removed, which ends the signal.

C2.2.8 and C2.2.9 The action potential (HL only)

Axon membranes contain voltage-gated sodium channels and voltage-gated potassium channels.

1. Resting: both channel types are closed. The membrane is at about -70 mV.
2. Stimulus: a small depolarization opens a few sodium channels. If the membrane reaches the threshold potential (about -55 mV in many neurons), many more sodium channels open. Below threshold there is no action potential, so the response is all-or-nothing.
3. Depolarization: sodium ions diffuse in, so the inside becomes positive (about +30 to +40 mV).
4. Repolarization: sodium channels close and potassium channels open. K⁺ diffuses out and the inside becomes negative again, briefly overshooting the resting value before the pumps and leak channels restore it.

Propagation. Sodium ions entering at one point spread sideways inside the axon, and outside the axon they move toward the depolarized region. These local currents depolarize the next stretch of membrane to threshold, so its sodium channels open. The region behind is repolarizing and cannot fire again at once, so the impulse moves in one direction.

C2.2.10 Oscilloscope traces (HL only)

A trace plots membrane potential (mV) against time (ms).

Feature	Cellular event
Flat line near -70 mV	Resting potential
Slow rise to about -55 mV	Depolarization to threshold
Steep rise to a peak	Sodium channels open
Steep fall	Potassium channels open, sodium closed
Dip below -70 mV	Potassium channels still open

Worked example. A trace shows peaks at 5, 25 and 45 ms, each from -70 mV to +35 mV. Interval between peaks = 20 ms = 0.020 s. Frequency = 1 ÷ 0.020 = 50 impulses per second. Peak height above rest = 35 - (-70) = 105 mV. Every peak is the same size, which shows the all-or-nothing nature of the action potential. A stronger stimulus gives a higher frequency, not a larger peak.

C2.2.11 Saltatory conduction (HL only)

In a myelinated fibre the myelin sheath insulates the axon, so ion movement across the membrane happens only at the gaps called nodes of Ranvier. Ion pumps and channels are clustered at these nodes. The action potential jumps from node to node. Because the membrane at each node is depolarized once per internode and not along the whole length, impulses are faster. For a speed of 60 m s⁻¹ and nodes 1.2 mm apart, the time per internode is 1.2 × 10⁻³ ÷ 60 = 2.0 × 10⁻⁵ s = 20 µs.

C2.2.12 Exogenous chemicals (HL only)

• Neonicotinoids are pesticides that bind to acetylcholine receptors in insects and block normal synaptic transmission. They are not broken down by acetylcholinesterase, so the insect’s nervous system is disrupted.
• Cocaine blocks the reuptake of the neurotransmitter dopamine into the presynaptic neuron. Dopamine stays in the cleft, so the postsynaptic neuron is stimulated for longer.

C2.2.13 and C2.2.14 Inhibition and summation (HL only)

An inhibitory neurotransmitter opens channels that make the postsynaptic membrane hyperpolarized, more negative than rest. This is an inhibitory postsynaptic potential (IPSP). A postsynaptic neuron adds up the effects of many EPSPs and IPSPs. Only if the total depolarization reaches threshold does it fire.

Worked example. Resting -70 mV, threshold -55 mV. Three presynaptic neurons each give an EPSP of +6 mV: -70 + 18 = -52 mV. This reaches threshold, so an action potential fires. Add one IPSP of -4 mV: -52 - 4 = -56 mV. This is below threshold, so there is no action potential.

C2.2.15 Pain (HL only)

Free nerve endings in the skin carry channels for positively charged ions. Intense heat, acid or capsaicin (from chilli peppers) opens them. Cations flow in, the threshold potential is reached, and impulses travel along neurons to the brain, which is where pain is perceived.

C2.2.16 Consciousness (HL only)

Consciousness is an emergent property: it arises from the interaction of vast numbers of neurons in the brain, and is not a property of any single neuron.

Common errors

• Saying the pump makes the inside negative “by pumping out negative ions”. It moves positive ions.
• Writing that sodium ions enter during repolarization. Potassium leaves.
• Saying a stronger stimulus makes a bigger action potential.
• Forgetting that calcium enters the presynaptic neuron.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Section C2.2 Neural signalling.