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

These notes condense section C2.2 (Neural signalling) of the IB Biology guide, first assessment 2025, for DP Biology. C2.2.1 to C2.2.7 are SL and HL; C2.2.8 to C2.2.16 are HL only. They suit the sessions that follow the first assessment in 2025. For full explanations read the study guide, then test yourself with the practice questions. Also see the IB DP Biology hub, the printable checklist and the free diagnostics.

Key terms

Term	Meaning
Neuron	Cell with a cell body, one axon and several dendrites
Axon	Single long fibre that conducts impulses away from the cell body
Dendrite	One of several short fibres
Polarized	Different charge on the two sides of the membrane
Membrane potential	The voltage difference across the membrane
Resting potential	Membrane potential of an inactive neuron, about -70 mV
Threshold potential	Depolarization (about -55 mV) at which sodium channels open (HL only)
Action potential	Brief reversal of membrane potential that is propagated
Synapse	Junction between neurons, or neuron and effector cell
EPSP	Small depolarization of the postsynaptic membrane
IPSP	Hyperpolarization of the postsynaptic membrane (HL only)

C2.2.1 to C2.2.3 in steps

Resting potential

1. The sodium-potassium pump uses ATP.
2. 3 Na⁺ out, 2 K⁺ in per cycle.
3. Sodium is high outside; potassium is high inside.
4. K⁺ leaks out through open channels; few Na⁺ channels are open.
5. Negatively charged molecules stay inside.
6. Result: inside negative, about -70 mV.

A nerve impulse is electrical because positive ions move across the membrane. It is regenerated along the fibre, so it does not fade.

C2.2.4 Speed

Comparison	Faster	Reason
Giant axon vs narrow non-myelinated	Giant axon	Wide diameter, low resistance to ion flow
Myelinated vs non-myelinated	Myelinated	Impulse jumps between nodes (HL only explanation)
Wider vs narrower fibre	Wider	Same reason as giant axon

Speed = distance ÷ time. Example: 0.45 m in 6.0 ms = 75 m s⁻¹. The same distance in 0.30 s = 1.5 m s⁻¹.

Correlation

• r near +1: strong positive. r near -1: strong negative. r near 0: weak or none.
• R² = r². It is the fraction of variation in the dependent variable explained by the independent variable.
• r = 0.979 gives R² = 0.959, so about 96% is explained.
• Conduction speed is positively correlated with axon diameter and negatively correlated with animal size.
• Correlation does not show cause.

C2.2.5 to C2.2.7 Synapses

Method in steps: transmission across an excitatory synapse

1. Action potential reaches the presynaptic membrane and depolarizes it.
2. Voltage-gated Ca²⁺ channels open; Ca²⁺ enters.
3. Calcium signals vesicles to fuse with the membrane.
4. Neurotransmitter is released by exocytosis.
5. It diffuses across the synaptic cleft.
6. It binds to transmembrane receptors on the postsynaptic membrane.
7. Channels open, positive ions enter, giving an EPSP.
8. The neurotransmitter is removed or broken down.

One direction only: release is presynaptic and the receptors are postsynaptic. Acetylcholine is the example, found at many synapses including neuromuscular junctions. Only chemical synapses are required.

C2.2.8 to C2.2.10 Action potential and traces (HL only)

Phase	Channels	Ion movement	Membrane potential
Resting	Both voltage-gated types closed	Pump and leak only	About -70 mV
Depolarization	Voltage-gated Na⁺ open (after threshold)	Na⁺ in	Rises to about +30 mV
Repolarization	Na⁺ close, K⁺ open	K⁺ out	Falls below threshold, then below rest
Recovery	K⁺ close	Pump restores gradients	Back to -70 mV

Local currents. Na⁺ entering diffuses sideways inside the axon and toward the active region outside. The next section reaches threshold. The section behind is repolarizing, so the impulse goes one way.

Reading a trace

1. Find the baseline: the resting potential.
2. Find the peak: the top of depolarization.
3. Measure the interval between two peaks in ms.
4. Impulses per second = 1000 ÷ interval in ms.

Example: peaks 20 ms apart give 1000 ÷ 20 = 50 per second. Peak size stays constant: all-or-nothing. A stronger stimulus raises frequency only.

C2.2.11 Saltatory conduction (HL only)

• Myelin insulates the axon.
• Nodes of Ranvier hold clusters of pumps and channels.
• The action potential jumps from node to node, so it is faster.

C2.2.12 to C2.2.14 Chemicals and integration (HL only)

Chemical	Effect
Neonicotinoid	Blocks synaptic transmission at acetylcholine receptors in insects
Cocaine	Blocks reuptake of dopamine, so it stays in the cleft
Inhibitory neurotransmitter	Hyperpolarizes the postsynaptic membrane (IPSP)

Summation. Add all EPSPs (positive) and IPSPs (negative) to the resting potential. If the total reaches threshold, an action potential fires; otherwise it does not. Example: -70 + 6 + 6 + 6 = -52 mV (fires). With an IPSP of -4: -56 mV (does not fire).

C2.2.15 and C2.2.16 Pain and consciousness (HL only)

• Free nerve endings in the skin have cation channels that open to high temperature, acid or capsaicin.
• Positive ions enter, threshold is reached, impulses travel to the brain, and pain is perceived there.
• Consciousness is an emergent property of interacting neurons, not of one neuron.

Worked reminders

Pump arithmetic. For 600 cycles: 600 × 3 = 1800 Na⁺ out; 600 × 2 = 1200 K⁺ in; net 600 positive charges out.

Time per internode (HL only). Speed 60 m s⁻¹, nodes 1.2 mm apart: 1.2 × 10⁻³ ÷ 60 = 2.0 × 10⁻⁵ s = 20 µs.

Amplitude (HL only). Peak at +35 mV, rest at -70 mV: 35 - (-70) = 105 mV.

Depolarization needed to reach threshold (HL only). From -70 mV to -55 mV is 15 mV of depolarization.

Comparing speeds. 75 ÷ 1.2 = 62.5, so the myelinated fibre in the example conducts about 63 times faster.

Command terms for this unit

• Describe the structure or the sequence: give the features or steps in order.
• Explain a change: give the cause using ions, channels or pumps, not just the outcome.
• Compare speeds: state both and use comparative words such as faster or slower.
• Interpret a trace: link each part of the line to a cellular event.
• Outline: give the main points without detail.
• Calculate: show the working and include units.

Memory chain for the action potential (HL only)

Rest, then stimulus, then threshold, then sodium in, then peak, then potassium out, then undershoot, then recovery by the pump. If you can say the chain in order and name the channel and ion at each link, you can answer almost any question on this section.

Must-know distinctions

• EPSP and action potential: an EPSP is small and graded; an action potential is all-or-nothing.
• Depolarization and hyperpolarization: depolarization moves toward zero or positive; hyperpolarization moves further negative.
• Pump and channel: the pump uses ATP against a gradient; a channel allows diffusion down a gradient.
• Neonicotinoid and cocaine: one blocks receptors, the other blocks reuptake.

Quick self-test

1. What is the name of the single long fibre of a neuron?
2. How many sodium and potassium ions does the pump move per cycle, and in which directions?
3. Name the ion that enters the presynaptic neuron and triggers vesicle fusion.
4. An impulse travels 0.72 m in 9.0 ms. Calculate the speed.
5. A data set gives r = -0.90. Calculate R² and state what it means.
6. State the change in membrane potential that defines an IPSP (HL only).
7. A trace has peaks 8 ms apart. Calculate impulses per second (HL only).
8. Resting potential is -70 mV and threshold -55 mV. Three EPSPs of +4 mV arrive together. Does the neuron fire? (HL only)
9. State where pumps and channels cluster in a myelinated fibre (HL only).
10. Name the neurotransmitter used as the example at neuromuscular junctions.

Answers

1. The axon.
2. 3 Na⁺ out and 2 K⁺ in.
3. Calcium (Ca²⁺).
4. 0.72 ÷ 0.0090 = 80 m s⁻¹.
5. R² = (-0.90)² = 0.81, so 81% of the variation in the dependent variable is explained by the independent variable.
6. Hyperpolarization: the membrane becomes more negative than rest.
7. 1000 ÷ 8 = 125 per second.
8. -70 + 12 = -58 mV, which is below -55 mV, so it does not fire.
9. At the nodes of Ranvier.
10. Acetylcholine.

Where marks are usually lost

• Writing “sodium goes in” without saying through voltage-gated channels and down its gradient.
• Describing the pump as moving sodium and potassium in the same direction.
• Leaving out calcium entry when explaining neurotransmitter release.
• Saying the impulse is “an electric current” like a wire. It is movement of ions.
• Treating a bigger stimulus as a bigger action potential.
• Mixing up repolarization (potassium out) with depolarization (sodium in).
• Quoting R² without saying what it means for the two variables.
• Forgetting units: m s⁻¹, mV, ms.
• Claiming correlation proves cause.

Official syllabus

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