7 higher-level sections hidden.
Educerie · IB Diploma · Biology
Theme C Interaction and interdependence · C2.2 Neural signalling
What you must be able to do
| You must be able to | Level | What it looks like in the exam |
|---|---|---|
| Describe the structure of a neuron: cell body, axon, dendrites | SL, HL | Label or annotate a diagram (2 to 3 marks) |
| Explain how pumping Na⁺ and K⁺ sets up a negative resting potential | SL, HL | "Explain how a resting potential is maintained" (3 to 4 marks) |
| Explain why a nerve impulse is electrical and how it moves along a fibre | SL, HL | Short answer, 2 marks |
| Compare impulse speeds: squid giant axons, small non-myelinated fibres, myelinated fibres | SL, HL | Paper 1B data, or "suggest why…" (2 marks) |
| Describe correlations and use r and R² | SL, HL | Paper 1B: "Evaluate the strength of the relationship" (2 to 3 marks) |
| Explain synapses as one-way junctions and the release of neurotransmitter, including Ca²⁺ | SL, HL | "Explain how a nerve impulse passes across a synapse" (4 to 6 marks) |
| Explain how acetylcholine generates an excitatory postsynaptic potential | SL, HL | Part of the synapse question, or Paper 1A |
| Explain depolarization and repolarization with voltage-gated channels and a threshold | HL only | Annotate a trace (3 to 4 marks) |
| Explain propagation by local currents | HL only | 2 to 3-mark short answer |
| Interpret oscilloscope traces and count impulses per second | HL only | Paper 1B calculation with the trace |
| Explain saltatory conduction at nodes of Ranvier | HL only | "Explain why myelinated fibres conduct faster" (3 marks) |
| Explain the effects of neonicotinoids and cocaine on synapses | HL only | 2 to 4-mark short answer |
| Explain inhibitory postsynaptic potentials and summation | HL only | "Explain how a postsynaptic neuron integrates inputs" (4 marks) |
| Outline how free nerve endings detect pain; consciousness as an emergent property | HL only | Short answer, or a linking point in Section B |
Before you start
You need membrane transport from B2.1: diffusion of ions through channels, down a concentration gradient, and active transport by pumps, which uses ATP to move ions against one. You also need chemical signalling in C2.1 if you are HL, because the acetylcholine receptor there is the one at work here.
1The idea in one paragraph
A neuron is a cell that carries electrical signals. It spends energy pumping sodium ions out and potassium ions in, so that the inside of its membrane sits at about −70 mV: the resting potential. A nerve impulse is a brief reversal of that voltage, caused by positive ions rushing through the membrane, and the reversal travels along the nerve fibre like a flame along a fuse. At the end of the fibre is a synapse, a gap the electricity cannot jump. There the impulse makes the neuron release a chemical, a neurotransmitter, which diffuses across the gap and changes the voltage of the next cell. Electrical along the fibre, chemical across the gap: that is the whole of neural signalling.
2Neurons
Neurons are the cells of the nervous system that carry electrical impulses. Figure 1 shows a motor neuron, the kind that carries impulses to a muscle.
- The cell body contains the nucleus and most of the cytoplasm. It keeps the cell alive: proteins, including the channels and pumps, are made here.
- Nerve fibres are long, thin extensions of cytoplasm projecting from the cell body. Their length varies enormously, from a fraction of a millimetre to more than a metre.
- Dendrites are many shorter fibres that receive signals from other neurons and carry impulses towards the cell body.
- The axon is a single long fibre that carries impulses away from the cell body to the axon terminals, where it meets the next cell.
Many axons are wrapped in a myelin sheath, layers of membrane from other cells, with small gaps called nodes of Ranvier. Section 5 explains what the myelin is for.
3The resting potential
A membrane potential is a difference in voltage across a membrane, caused by an uneven distribution of charge. A membrane with a potential across it is polarized. In a neuron that is not sending an impulse, the inside of the membrane is negative compared with the outside, at about −70 millivolts. That is the resting potential. Figure 2 shows how the cell makes it.
Step 1: pumping builds two concentration gradients. The sodium–potassium pump is a protein in the membrane that uses energy from ATP to move ions against their concentration gradients: three Na⁺ out of the cell and two K⁺ in, each cycle. Over time this makes Na⁺ concentrated outside and K⁺ concentrated inside.
Step 2: the inside ends up negative. Three reasons, and a 3-mark question wants all three.
- The pump moves more positive charge out than in. Three positive ions leave for every two that enter.
- The membrane is more permeable to K⁺ than to Na⁺. At rest some potassium channels are open, so K⁺ diffuses back out down its gradient, carrying positive charge out. Very few sodium channels are open, so Na⁺ leaks back in only slowly and cannot balance it. This is the biggest of the three.
- The cytoplasm contains negatively charged proteins and other large anions that cannot leave the cell.
Resting potential: the Na⁺/K⁺ pump uses ATP to move 3 Na⁺ out and 2 K⁺ in; K⁺ then leaks out, so the inside sits at about −70 mV.
The gradients are a store of energy, like water held behind a dam. A nerve impulse is what happens when the channels open and the ions are allowed to flow.
4Nerve impulses are action potentials
A nerve impulse is an action potential: a rapid, temporary reversal of the membrane potential at one point on the fibre, from about −70 mV to about +30 mV and back, in a couple of milliseconds. It happens in two movements of ions:
- Sodium channels open and Na⁺ diffuses in, down its concentration gradient. The inside becomes positive: the membrane is depolarized.
- Sodium channels close, potassium channels open, and K⁺ diffuses out. The inside becomes negative again: the membrane is repolarized.
A nerve impulse is electrical because it is a movement of positively charged ions across the membrane. It is not electrons flowing along a wire. The action potential at one point triggers an action potential at the next point along, so the impulse is propagated along the fibre from the cell body to the terminals. HL section 8 explains how the channels are opened and closed.
5How fast? Speed, size and myelin
Impulses travel at very different speeds in different fibres, and two features of a fibre make the difference.
Diameter. A wider axon has less resistance to the flow of ions along its inside, so each action potential triggers the next point sooner. Squid have giant axons about half a millimetre across, hundreds of times wider than a typical small fibre, and conduct at around 25 m s⁻¹. A small non-myelinated fibre about 1 µm across conducts at about 1 m s⁻¹. The squid's giant axon runs to the muscles that squirt water for a fast escape, and speed is the point.
Myelination. A myelinated fibre conducts far faster than a non-myelinated fibre of the same diameter. A human motor fibre only about 20 µm across, but myelinated, conducts at about 100 m s⁻¹: four times faster than the squid's giant axon at one twenty-fifth of the width. Vertebrates solved the need for speed with insulation instead of size, which is how they can pack many fast fibres into one thin nerve.
| Fibre | Diameter | Myelin | Approximate speed |
|---|---|---|---|
| Small human fibre, e.g. carrying slow pain | about 1 µm | no | about 1 m s⁻¹ |
| Squid giant axon | about 500 µm | no | about 25 m s⁻¹ |
| Human motor fibre | about 20 µm | yes | about 100 m s⁻¹ |
Describing a correlation. The guide wants you to handle data like this with numbers. Two variables are correlated when one changes consistently as the other changes. In a positive correlation both rise together; in a negative correlation one falls as the other rises. A scatter graph shows it; the correlation coefficient, r, measures how closely the points follow a straight line. It runs from −1 to +1:
- r = +1: a perfect positive straight line; r = −1: a perfect negative one.
- r close to 0: no linear relationship.
- The closer r is to +1 or −1, the stronger the correlation. Values beyond about ±0.7 are usually called strong; this is a convention, not a rule.
The coefficient of determination, R², is r squared. It tells you what proportion of the variation in the dependent variable is explained by variation in the independent variable. Figure 3 uses invented data from eight non-myelinated axons.
Panel (b) is a lesson in itself. The relationship between diameter and travel time is strong and obvious to the eye, but it is a curve, and r measures only how well a straight line fits. A low r does not always mean a weak relationship. And even a very high r shows association, not cause: to conclude that diameter causes the change in speed you need a mechanism, which here is the lower resistance of a wider axon.
6Synapses and the release of neurotransmitter
A synapse is a junction between two neurons, or between a neuron and an effector cell such as a muscle fibre or a gland cell. The guide deals only with chemical synapses. The two cells do not touch: between them is the synaptic cleft, about 20 nm wide, which an action potential cannot cross. The signal is carried over as a chemical. Figure 4 follows it.
- An action potential arrives at the axon terminal and depolarizes the presynaptic membrane.
- Depolarization opens voltage-gated calcium channels, and Ca²⁺ diffuses into the terminal, down its concentration gradient.
- Inside the neuron, calcium acts as a signalling chemical: it causes vesicles of neurotransmitter to move to the presynaptic membrane and fuse with it, releasing the neurotransmitter into the cleft by exocytosis.
- The neurotransmitter diffuses across the cleft.
- It binds to receptors on the postsynaptic membrane (section 7).
- The neurotransmitter is removed quickly, so the signal is brief. Acetylcholine is broken down in the cleft by the enzyme acetylcholinesterase; other neurotransmitters are taken back up into the presynaptic neuron.
Why the signal goes one way. Only the presynaptic neuron has vesicles of neurotransmitter, and only the postsynaptic membrane has receptors for it. A signal can pass from the first to the second and never back. That is why a nervous system has direction.
7The excitatory postsynaptic potential
The guide's example neurotransmitter is acetylcholine. It is used at many kinds of synapse in the body, including every neuromuscular junction, where a motor neuron meets a skeletal muscle fibre.
When acetylcholine binds its receptors in the postsynaptic membrane, the receptors, which are ion channels, open. Positively charged sodium ions diffuse into the postsynaptic cell, and the inside of its membrane becomes less negative. This small depolarization is an excitatory postsynaptic potential (EPSP). "Excitatory" because it pushes the cell towards firing an action potential of its own. If the depolarization is large enough, the postsynaptic cell fires: a neuron sends its own impulse onwards, or a muscle fibre contracts. Acetylcholinesterase then breaks the acetylcholine down into acetate and choline, the channels close, and the synapse is ready for the next impulse.
8HLDepolarization and repolarization
SL students can skip to section 15. HL students: this is the section most often examined as an annotated trace.
The channels that make an action potential are voltage-gated: they open or close in response to the membrane potential itself. Figure 5 is the trace an oscilloscope draws of the membrane potential at one point on an axon as one action potential passes.
A. Resting potential, −70 mV. Voltage-gated channels are closed.
Threshold. Something makes the membrane a little less negative: a local current from the region next door (section 9), or an EPSP. If the potential reaches the threshold potential, about −55 mV, voltage-gated sodium channels open. Below threshold they stay shut and nothing happens. This is why an action potential is all or nothing: it either happens in full or not at all, and a stronger stimulus does not make a bigger one.
B. Depolarization. With sodium channels open, Na⁺ diffuses in, down its concentration gradient and attracted by the negative inside. The inflow of positive charge makes the inside less negative, which opens more sodium channels, and the potential shoots up.
C. Peak, about +30 mV. The inside is now positive. The sodium channels close (they inactivate) and cannot reopen for a moment.
D. Repolarization. Voltage-gated potassium channels open, a fraction of a millisecond after the sodium ones. K⁺ diffuses out, down its concentration gradient, carrying positive charge out, and the inside becomes negative again.
E. Overshoot. Potassium channels close slowly, so the potential briefly falls below −70 mV, to about −80 mV, before returning to rest.
Only a tiny fraction of the ions cross the membrane in one action potential, so the concentration gradients barely change. The sodium–potassium pump restores them continuously, which is why neurons use so much ATP.
9HLPropagation by local currents
An action potential at one point starts an action potential at the next. The link is local currents: diffusion of sodium ions sideways, along the axon. Figure 6 shows it.
- Inside the axon, the region that has just depolarized has a high concentration of Na⁺. Those ions diffuse along the axon into the neighbouring region, which is still at rest.
- Outside the axon, the depolarized region has lost Na⁺, so Na⁺ diffuses from the neighbouring resting region towards it.
Both movements make the neighbouring region's inside less negative. When it reaches the threshold, its own voltage-gated sodium channels open and it fires. Then it sends local currents on to the next region, and so on.
Why only forwards? The region behind has just fired, and its sodium channels cannot reopen for a moment (the refractory period). So the impulse moves away from where it started and never turns back.
10HLSaltatory conduction
In a myelinated fibre the myelin sheath is an electrical insulator: ions cannot cross the membrane where it is covered. Ion channels and pumps are clustered at the nodes of Ranvier, the gaps between sections of myelin, and the membrane there is exposed. Figure 7 shows the result.
Action potentials happen only at the nodes. Local currents flow along the inside of the axon, under the myelin, from one node to the next, about a millimetre away, and trigger an action potential there. The impulse jumps from node to node: saltatory conduction, from the Latin for "leap". The slow part of propagation is opening channels and moving ions, and saltatory conduction does that at a node every millimetre instead of continuously along every micrometre of membrane. Impulses are much faster, and because far fewer ions cross the membrane, the pump has less to do and less ATP is used.
11HLReading oscilloscope traces
An oscilloscope plots voltage against time. With one electrode inside an axon and one outside, the trace is the membrane potential. Read a trace in this order:
- Read the axes: membrane potential in mV, time in ms.
- Find the resting potential: the flat line before the spike.
- Find the threshold: the kink where the gentle rise becomes a steep one.
- Match each phase to its ions: rising, Na⁺ in; falling, K⁺ out; dip below resting, K⁺ channels slow to close.
The guide also asks you to measure the number of impulses per second, the frequency. A stronger stimulus does not give a bigger action potential, but it does give more of them per second, so frequency is how a neuron signals intensity. Figure 8 shows a trace with six action potentials.
Count intervals, not peaks: six peaks enclose only five gaps. Measuring across several intervals and dividing is more accurate than measuring one.
12HLInhibition and summation
Not every neurotransmitter excites. An inhibitory neurotransmitter, such as GABA at many synapses in the brain, opens channels that let negative chloride ions in or positive potassium ions out. The inside of the postsynaptic membrane becomes more negative than resting: it is hyperpolarized. This is an inhibitory postsynaptic potential (IPSP), and it moves the cell further from threshold.
A neuron in the brain or spinal cord may receive synapses from hundreds or thousands of presynaptic neurons, some excitatory and some inhibitory. One EPSP on its own is usually far too small to reach the threshold. What decides whether the postsynaptic neuron fires is the sum of all its inputs at that moment: summation. Figure 9 shows it.
- Several EPSPs arriving together, from different presynaptic neurons or in quick succession from one, add up. If the total depolarization reaches threshold, the postsynaptic neuron fires an action potential.
- IPSPs arriving at the same time subtract from the total and can stop it reaching threshold.
The consequence is all or nothing: the postsynaptic neuron fires a full action potential or none. Summation is how a single neuron makes a decision, weighing many inputs into one output.
13HLChemicals from outside the body
Exogenous chemicals are ones that come from outside the body, and many of them act at synapses. The guide names two, with opposite effects. Figure 10 shows both.
Neonicotinoids are pesticides. They bind to acetylcholine receptors in the postsynaptic membranes of insect synapses, binding much more strongly to insect receptors than to mammalian ones. Unlike acetylcholine, they are not broken down by acetylcholinesterase, so they stay bound. Normal transmission by acetylcholine is blocked, and the insect is paralysed and dies. They are effective against crop pests, but they also harm pollinating insects such as bees, which is why their use outdoors has been restricted in some countries.
Cocaine acts at synapses that use dopamine, including those in the brain's reward pathways. Normally dopamine is removed from the cleft by reuptake: transporter proteins in the presynaptic membrane pump it back into the neuron. Cocaine binds to these transporters and blocks reuptake. Dopamine builds up in the cleft and keeps binding postsynaptic receptors, so the postsynaptic neurons are stimulated for longer than normal. This produces the drug's euphoria and contributes to addiction.
14HLPain, and consciousness
Pain begins at neurons with free nerve endings in the skin: the ends of sensory neurons, with no special receptor cell around them. Their membranes contain channels for positively charged ions that open in response to a harmful stimulus, such as a high temperature, acid, or chemicals like capsaicin, the compound that makes chilli peppers hot. (Capsaicin opens the same channels as heat, which is why chilli feels like burning.) Positive ions enter, the nerve ending depolarizes, and if the threshold is reached, action potentials travel along the sensory neurons to the brain. The pain is perceived in the brain, not in the skin. The nerve ending only reports; the experience is made centrally.
Consciousness, the awareness of ourselves and our surroundings, is not a property of any single neuron. No neuron is aware. It is an emergent property: one that arises from the interactions of billions of individual neurons, connected through trillions of synapses, and that cannot be found by studying any one part. It is the clearest example in biology of the theme of this whole unit: interaction produces something the parts alone do not have.
15Where marks are lost
Saying the pump makes the resting potential by itself. The pump builds the concentration gradients; the leak of K⁺ out through open channels is the biggest reason the inside is negative. A full answer names both.
Getting the pump numbers backwards. Three sodium ions out, two potassium ions in. Write "3 Na⁺ out, 2 K⁺ in" exactly.
Describing the impulse as electrons flowing. A nerve impulse is the movement of positively charged ions, Na⁺ in and K⁺ out, across the membrane.
Saying the neurotransmitter "carries the impulse" across the cleft. The action potential stops at the presynaptic membrane. The neurotransmitter diffuses across and causes a new change in potential on the other side.
Forgetting calcium. Every synapse answer needs Ca²⁺: channels open on depolarization, Ca²⁺ diffuses in, and that causes vesicles to release neurotransmitter.
Saying myelin speeds the impulse along "because it conducts". Myelin insulates. It speeds the impulse because action potentials only happen at the nodes, so the impulse jumps between them.
Reading r as the gradient. r measures how closely points fit a straight line, not how steep the line is. A shallow line with every point on it has r = 1.
Counting peaks instead of intervals. Six action potentials span five intervals. Frequency = number of intervals ÷ the time they take.
16Draw it right
- Neuron: label cell body with nucleus, dendrites, axon, myelin sheath, node of Ranvier, axon terminals. Show the axon as one fibre, far longer than the dendrites.
- Resting potential: show the pump with its numbers (3 Na⁺ out, 2 K⁺ in, ATP used) and a potassium channel with K⁺ leaving. Mark inside negative.
- Synapse: vesicles only in the presynaptic terminal, receptors only on the postsynaptic membrane, calcium channels in the presynaptic membrane, the cleft labelled. Number the steps.
- Scatter graph: independent variable on x, dependent on y, both with units; a line of best fit if asked; r or R² written on the graph.
- Action potential trace (HL): axes membrane potential (mV) against time (ms). Mark resting −70 mV, threshold about −55 mV, peak about +30 mV, and the overshoot below resting. Annotate depolarization with "Na⁺ channels open, Na⁺ in" and repolarization with "K⁺ channels open, K⁺ out".
- Summation (HL): a threshold line; small EPSP bumps that do not cross it; a summed rise that crosses it and fires; an IPSP drawn below resting.
17Try it
Marks in brackets. Answers and marker's notes are at the end.
Q1. Which is the main reason the inside of a resting neuron is negative? 1 mark
A. Chloride ions are pumped into the axon.
B. Potassium ions diffuse out through open potassium channels.
C. Sodium ions diffuse in through open sodium channels.
D. The pump moves two sodium ions out for every three potassium ions in.
Q2. Explain how a resting potential is established and maintained in a neuron. 4 marks
Q3. The table gives approximate conduction speeds in five fibres. (Illustrative values.)
| Fibre | Diameter (µm) | Myelinated | Speed (m s⁻¹) |
|---|---|---|---|
| Squid giant axon | 500 | no | 25 |
| Crab leg axon | 30 | no | 5 |
| Human slow pain fibre | 1 | no | 1 |
| Human motor fibre | 15 | yes | 90 |
| Frog motor fibre | 12 | yes | 30 |
(a) Calculate the time, in ms, for an impulse to travel 0.9 m along the human motor fibre. 1 mark
(b) Compare the squid giant axon with the human motor fibre, and explain the difference in speed. 3 marks
(c) For all five fibres, the correlation coefficient between diameter and speed is r = −0.08. Explain why this does not show that diameter has no effect on speed. 2 marks
Q4. Explain how a nerve impulse is transmitted across a synapse that uses acetylcholine. 6 marks
Q5 (HL). An oscilloscope trace shows seven action potentials. The first peak is at 4 ms and the last at 64 ms.
(a) Calculate the frequency of impulses. 2 marks
(b) Explain the changes in membrane potential during one action potential. 4 marks
Q6 (HL). Explain how a postsynaptic neuron that receives both excitatory and inhibitory synapses determines whether it fires an action potential. 4 marks
18In one breath
A neuron has a cell body, many short dendrites and one long axon, often myelinated. The Na⁺/K⁺ pump uses ATP to move 3 Na⁺ out and 2 K⁺ in; K⁺ leaks out through open channels, and large negative proteins stay inside, so the resting potential is about −70 mV. An impulse is an action potential, a brief reversal as Na⁺ enters and then K⁺ leaves, propagated along the fibre; it is electrical because positive ions move. Wider axons conduct faster (squid giant axons), and myelinated ones much faster again; describe such data with r (−1 to +1, how well a straight line fits) and R² (the proportion of variation explained). At a synapse, depolarization opens Ca²⁺ channels, Ca²⁺ triggers exocytosis of neurotransmitter, it diffuses across the cleft and binds receptors; acetylcholine opens Na⁺ channels and makes an EPSP, then acetylcholinesterase removes it. One way only, because only one side has vesicles and only the other has receptors. HL: at threshold (−55 mV) voltage-gated Na⁺ channels open (depolarization to +30 mV), then K⁺ channels open (repolarization, overshoot to −80 mV); local currents of Na⁺ inside and outside bring the next region to threshold; myelin makes the impulse jump node to node; frequency = intervals ÷ time. Neonicotinoids block insect acetylcholine receptors; cocaine blocks dopamine reuptake. IPSPs hyperpolarize; EPSPs and IPSPs summate, all or nothing. Free nerve endings open cation channels to heat, acid or capsaicin, and pain is perceived in the brain; consciousness emerges from neurons interacting.
Answers
Q1. B. A is not the mechanism; C describes depolarization, not rest; D has the numbers reversed (3 Na⁺ out, 2 K⁺ in). B only.
Q2. The sodium–potassium pump in the membrane uses energy from ATP to pump 3 Na⁺ out of the neuron for every 2 K⁺ pumped in, by active transport against their concentration gradients. This builds a high concentration of Na⁺ outside and of K⁺ inside. More positive charge leaves than enters. The membrane is more permeable to K⁺ than to Na⁺, because some K⁺ channels are open at rest, so K⁺ diffuses out, taking positive charge with it. Negatively charged proteins remain inside. The inside is therefore negative, at about −70 mV. 1 each for any four of: pump uses ATP / active transport; 3 Na⁺ out and 2 K⁺ in; concentration gradients established; K⁺ diffuses out through open channels / membrane more permeable to K⁺; negative proteins inside; −70 mV or inside negative. Reversed pump numbers score 0 for that point.
Q3. (a) 0.9 ÷ 90 = 0.01 s = 10 ms. value with unit. (b) The human motor fibre conducts faster (90 m s⁻¹ against 25 m s⁻¹, about 3.6 times faster) even though its diameter is far smaller (15 µm against 500 µm). This is because it is myelinated: action potentials occur only at the nodes of Ranvier and the impulse jumps between them (saltatory conduction), whereas in the non-myelinated squid axon an action potential must occur along every part of the membrane. 1 for a comparison using both speed and diameter, 1 for identifying myelination, 1 for explaining how myelin increases speed. Describing the two separately with no comparison is capped at 2. (c) Speed is affected by a second variable, myelination, as well as diameter. Myelinated fibres are fast despite being narrow, so the points do not lie close to one straight line, and r is near zero. Within the non-myelinated fibres alone, speed rises with diameter. (Also accept: r measures only linear relationships; five points is a very small sample.) 1 for identifying myelination as a confounding variable, 1 for explaining why this lowers r, or for noting the positive trend within one group.
Q4. The action potential arrives at the presynaptic terminal and depolarizes the membrane. Voltage-gated calcium channels open and Ca²⁺ diffuses into the terminal. Calcium causes vesicles containing acetylcholine to fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft by exocytosis. Acetylcholine diffuses across the cleft and binds to receptors on the postsynaptic membrane. The receptors open, Na⁺ diffuses in, and the postsynaptic membrane is depolarized (an EPSP); if threshold is reached, an action potential is generated. Acetylcholinesterase breaks down acetylcholine in the cleft, ending the signal. 1 each for depolarization of the presynaptic membrane, Ca²⁺ entry, exocytosis of vesicles, diffusion across the cleft, binding to postsynaptic receptors, Na⁺ entry and depolarization, breakdown by acetylcholinesterase; maximum 6. "The impulse jumps across the gap" scores 0.
Q5 (HL). (a) Seven peaks give 6 intervals. Time = 64 − 4 = 60 ms. Interval = 60 ÷ 6 = 10 ms. Frequency = 1000 ÷ 10 = 100 impulses per second. M1 for 6 intervals over 60 ms, A1 for 100 s⁻¹. 7 ÷ 60 ms ≈ 117 s⁻¹ scores M0 A0. (b) At rest the potential is about −70 mV. When threshold (about −55 mV) is reached, voltage-gated Na⁺ channels open and Na⁺ diffuses in, depolarizing the membrane to about +30 mV. Na⁺ channels then close and voltage-gated K⁺ channels open; K⁺ diffuses out, repolarizing the membrane. K⁺ channels close slowly, so the potential briefly overshoots below the resting potential before returning to −70 mV. 1 each for threshold opening Na⁺ channels, Na⁺ in causing depolarization, K⁺ channels open and K⁺ out causing repolarization, overshoot or hyperpolarization. Values are not required but must be correct if given.
Q6 (HL). Excitatory neurotransmitters cause small depolarizations of the postsynaptic membrane (EPSPs). Inhibitory neurotransmitters cause hyperpolarization (IPSPs), for example by letting Cl⁻ in. A single EPSP is usually below threshold. The postsynaptic neuron sums all the EPSPs and IPSPs arriving at the same time, or in quick succession (summation). If the net depolarization reaches the threshold, an action potential is fired; if not, no action potential occurs: the response is all or nothing. 1 each for EPSP as depolarization, IPSP as hyperpolarization, summation of inputs, all or nothing at threshold.
Educerie · written from the published IB Diploma Programme Biology guide, first assessment 2025, section C2.2 Neural signalling. Original text, examples and questions. Diagrams drawn by Educerie. Last reviewed 25 September 2026.
Check your understanding
The main ideas of this note. Tick each one you could do now, in an exam, without looking back up. Anything you cannot tick yet is the part to read again.