A Level Biology - How to Solve: Synaptic Transmission and Action Potential Graphs

How to Solve: Synaptic Transmission and Action Potential Graphs

Complete Guide For GCSE/A-Level Biology & Neuroscience (AQA, Edexcel, OCR, IB)

Introduction

"Mastering synaptic transmission and action potential graphs can earn you 12-15% of your A-Level Biology paper—and it’s the difference between a C and an A. Why? Because examiners love testing how neurons ‘talk’ to each other, and if you can read those squiggly voltage graphs, you’ll ace questions on drugs, diseases, and even AI brain models. Let’s break it down so you never lose marks again."

WHAT YOU NEED TO KNOW FIRST

Before diving in, ensure you understand:
1. Resting membrane potential – Neurons sit at -70 mV due to sodium-potassium pumps.
2. Ion channels – Voltage-gated (Na⁺, K⁺) and ligand-gated (neurotransmitter receptors).
3. Depolarisation vs. hyperpolarisation – Positive shift (Na⁺ in) vs. negative shift (K⁺ out/Cl⁻ in).

If any of these are fuzzy, pause and review them first.

KEY TERMS & FORMULAS

Key Terms (MEMORISE THESE)

Formulas (GIVEN ON EXAM SHEET – but know how to use them!)

1. Nernst Equation (for equilibrium potential of an ion) [ E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]{outside}}{[ion] \right) ]}
2. (E_{ion}) = Equilibrium potential (mV)
3. (R) = Gas constant (8.31 J/mol·K)
4. (T) = Temperature (K)
5. (z) = Ion charge (+1 for Na⁺/K⁺, -1 for Cl⁻)
6. (F) = Faraday’s constant (96,485 C/mol)

7. For A-Level, you’ll usually just compare given values (e.g., (E_{Na} = +60 mV), (E_{K} = -90 mV)).

8. Goldman-Hodgkin-Katz (GHK) Equation (for resting potential) [ V_m = \frac{RT}{F} \ln \left( \frac{P_{Na}[Na^+]{out} + P_K[K^+][Cl^-]} + P_{Cl{in}}{P[Na^+]{in} + P_K[K^+] \right) ]} + P_{Cl}[Cl^-]_{out}

9. (P) = Permeability of the ion (how easily it crosses the membrane).
10. For GCSE/A-Level, you won’t calculate this—just know resting potential is a balance of Na⁺ and K⁺ leak channels.

STEP-BY-STEP METHOD

How to Read an Action Potential Graph

Step 1: Label the axes - X-axis = Time (ms) - Y-axis = Membrane potential (mV)

Step 2: Identify the resting potential - Flat line at -70 mV = neuron at rest.

Step 3: Find the threshold - Look for the point where the graph starts rising sharply (usually -55 mV).

Step 4: Trace depolarisation - Rapid upstroke from -55 mV to +40 mV = Na⁺ channels open, Na⁺ rushes in.

Step 5: Locate the peak - Highest point (~+40 mV) = Na⁺ channels inactivate, K⁺ channels open.

Step 6: Trace repolarisation - Downstroke from +40 mV back to -70 mV = K⁺ rushes out.

Step 7: Check for hyperpolarisation - If the graph dips below -70 mV = K⁺ channels close slowly (refractory period).

Step 8: Identify the refractory period - Absolute refractory period = No AP possible (Na⁺ channels inactivated). - Relative refractory period = AP possible but harder (some Na⁺ channels reset).

Step 9: Count the number of APs (if multiple) - Each "spike" = one action potential.

How to Solve Synaptic Transmission Questions

Step 1: Identify the synapse type - Chemical synapse (most common) = neurotransmitters cross a gap. - Electrical synapse (rare) = ions flow directly through gap junctions.

Step 2: List the steps of chemical transmission
1. AP arrives at presynaptic terminal.
2. Voltage-gated Ca²⁺ channels open → Ca²⁺ rushes in.
3. Ca²⁺ causes synaptic vesicles to fuse with membrane → neurotransmitter released (exocytosis).
4. Neurotransmitter binds to receptors on postsynaptic membrane.
5. Ligand-gated ion channels open → ions flow in/out → EPSP or IPSP.
6. Neurotransmitter is reuptaken or broken down (e.g., acetylcholine by acetylcholinesterase).

Step 3: Determine if the synapse is excitatory or inhibitory - Excitatory (EPSP) = Na⁺/Ca²⁺ in → depolarisation (e.g., glutamate). - Inhibitory (IPSP) = Cl⁻ in or K⁺ out → hyperpolarisation (e.g., GABA).

Step 4: Apply summation rules - Temporal summation = One presynaptic neuron fires rapidly → EPSPs add up. - Spatial summation = Multiple presynaptic neurons fire at once → EPSPs add up. - IPSPs subtract from EPSPs (e.g., 3 EPSPs + 1 IPSP = net depolarisation).

Step 5: Decide if threshold is reached - If net depolarisation ≥ -55 mV → AP fires in postsynaptic neuron. - If net hyperpolarisation → No AP.

WORKED EXAMPLES

Example 1 – Basic: Reading an Action Potential Graph

Question: The graph below shows an action potential. Label: a) Resting potential b) Threshold c) Depolarisation d) Repolarisation e) Hyperpolarisation

Graph:

Membrane Potential (mV)
  +40 |       /\
      |      /  \
  0   |     /    \
      |    /      \
-55   |___/        \___
      |   \        /
-70   |    \      /
      |     \____/
-90   |
      ------------------- Time (ms)

Answer:
1. Resting potential = Flat line at -70 mV.
2. Threshold = Point where graph starts rising sharply (-55 mV).
3. Depolarisation = Upstroke from -55 mV to +40 mV.
4. Repolarisation = Downstroke from +40 mV to -70 mV.
5. Hyperpolarisation = Dip below -70 mV (not shown here, but if present, it would be the small undershoot).

What we did and why: - We labelled key points on the graph to show we understand the sequence of ion movements. - Examiners always ask for these labels—memorise them!

Example 2 – Medium: Synaptic Summation

Question: Neuron A fires three times in quick succession onto Neuron B. Each EPSP depolarises Neuron B by 5 mV. Neuron C fires once onto Neuron B, causing an IPSP of -10 mV. Will Neuron B fire an action potential? (Resting potential = -70 mV, threshold = -55 mV)

Answer:
1. Temporal summation from Neuron A: - 3 EPSPs × 5 mV = +15 mV total.
2. Spatial summation from Neuron C: - 1 IPSP × -10 mV = -10 mV.
3. Net change = +15 mV (EPSP) - 10 mV (IPSP) = +5 mV.
4. New membrane potential = -70 mV + 5 mV = -65 mV.
5. Threshold check = -65 mV < -55 mV → No AP fired.

What we did and why: - We added EPSPs and subtracted IPSPs to find the net effect. - We compared the result to threshold to decide if an AP fires. - Common mistake: Forgetting to subtract IPSPs—always check for inhibitory inputs!

Example 3 – Exam-Style: Drug Effects on Synaptic Transmission

Question: A new drug blocks voltage-gated Ca²⁺ channels in presynaptic neurons. Explain how this affects: a) Neurotransmitter release b) The likelihood of an action potential in the postsynaptic neuron

Answer: a) Neurotransmitter release:
1. AP arrives at presynaptic terminal → voltage-gated Ca²⁺ channels should open.
2. Drug blocks Ca²⁺ channels → Ca²⁺ cannot enter.
3. Without Ca²⁺, synaptic vesicles cannot fuse with the membrane.
4. No neurotransmitter is released into the synaptic cleft.

b) Likelihood of postsynaptic AP:
1. No neurotransmitter → no EPSPs/IPSPs in postsynaptic neuron.
2. No depolarisation → membrane potential stays at -70 mV.
3. Threshold (-55 mV) is not reached → no AP fires.

What we did and why: - We linked the drug’s effect to each step of synaptic transmission. - We predicted the outcome based on missing Ca²⁺. - Exam trap: Examiners love asking about drugs/toxins—always think: "Which step does this block?"

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP

"Okay, last-minute cram? Here’s the non-negotiable stuff:
1. Action potential graph? Label resting (-70 mV), threshold (-55 mV), depolarisation (Na⁺ in), repolarisation (K⁺ out), and hyperpolarisation (dip below -70 mV).
2. Synaptic transmission? AP → Ca²⁺ in → vesicles release neurotransmitter → EPSP (Na⁺ in) or IPSP (Cl⁻ in/K⁺ out) → summation → threshold check.
3. Drugs/toxins? Block Na⁺ channels (no AP), block Ca²⁺ (no neurotransmitter), block reuptake (more neurotransmitter).
4. Summation? Add EPSPs, subtract IPSPs. If net ≥ -55 mV → AP fires.
5. Refractory period? Absolute = no AP. Relative = harder AP.

Now go draw an AP graph from memory—if you can label it, you’ve got this!"