Neurone Structure & Resting Potential Revision Notes | A Level AQA Biology

1. Why Neurone Structure and Resting Potential Matter

Every thought, reflex, and muscle contraction depends on a single electrical trick: a neurone holding a charge difference across its membrane, ready to fire. Understanding how that charge is built and maintained is the foundation for everything that follows in nervous coordination.

This lesson covers two tightly linked ideas. First, the physical structure of a myelinated motor neurone — the hardware. Second, the resting potential — the electrical state the neurone maintains when it is not firing. Get these right and action potentials, synaptic transmission, and reflex arcs all fall into place.

2. Structure of a Myelinated Motor Neurone

Picture a long cable running from your spinal cord to a muscle in your leg. That cable is a motor neurone, and its design is precisely matched to its job: carrying signals quickly over long distances away from the central nervous system (CNS) to an effector.

The cell body (soma) contains the nucleus and most of the metabolic machinery. Branching from it are short, highly branched dendrites that receive incoming signals from other neurones. The axon is the long projection that carries the impulse away from the cell body — in a motor neurone this can be over a metre long.

The nodes of Ranvier are not just gaps — they are the only sites where ion exchange can occur along a myelinated axon. This means the action potential effectively leaps from node to node rather than travelling continuously along the membrane. The result is conduction speeds of up to 120 m/s in large myelinated fibres, compared with under 2 m/s in unmyelinated ones.

• Cell body (soma): contains nucleus, rough endoplasmic reticulum, and ribosomes for protein synthesis
• Dendrites: short, branched extensions that receive signals from other neurones
• Axon: long projection carrying impulses away from the cell body towards the effector
• Myelin sheath: lipid-rich insulating layer formed by Schwann cells wrapped around the axon
• Nodes of Ranvier: gaps between adjacent Schwann cells where the axon membrane is exposed
• Axon terminals (synaptic knobs): the end of the axon where neurotransmitters are released at a synapse
• Motor end plate: the specialised junction between the axon terminal and the muscle fibre

A key distinction to hold onto: motor neurones carry impulses away from the CNS (efferent), whereas sensory neurones carry impulses towards the CNS (afferent). The structural differences reflect this — motor neurones have the cell body at one end of the axon, while sensory neurones have a cell body positioned to the side of a long axon running between receptor and CNS.

3. Setting the Scene: Ion Distribution Across the Membrane

Now that the hardware is clear, the question becomes: how does a neurone hold a charge difference across its membrane? The answer lies in the unequal distribution of ions on either side — and the membrane's selective control over which ions can cross.

At rest, the inside of the neurone is negatively charged relative to the outside. The key players are sodium ions ($\text{Na}^+$) and potassium ions ($\text{K}^+$). At rest, $\text{Na}^+$ is at higher concentration outside the cell, and $\text{K}^+$ is at higher concentration inside the cell. These gradients are not accidental — they are actively maintained.

These concentration gradients represent stored potential energy — chemical gradients waiting to drive ion movement the moment a channel opens. The electrical gradient adds a second driving force: the negative interior attracts positive ions inward. Together, these two forces make up the electrochemical gradient for each ion.

4. Differential Membrane Permeability and the Resting Potential

The resting potential is not a consequence of ion gradients existing — it depends critically on the membrane being selectively permeable to different ions at rest. This is where the concept of differential membrane permeability becomes central.

At rest, potassium leak channels are open, allowing $\text{K}^+$ to diffuse down its concentration gradient out of the cell. Sodium channels are largely closed, so $\text{Na}^+$ cannot easily enter despite its strong electrochemical gradient driving it inward. The net result of this asymmetry is a build-up of negative charge on the inside of the membrane.

Here is the crucial detail: as $\text{K}^+$ leaves the cell, the inside becomes more negative. That growing negative charge starts to attract $\text{K}^+$ back in — an electrical force opposing the outward chemical gradient. An equilibrium is reached when the electrical pull inward exactly balances the chemical push outward for potassium. At this point, the membrane potential stabilises at approximately $-70$ mV.

The small but steady leak of $\text{Na}^+$ inward (through the few sodium channels that are open at rest) would gradually depolarise the membrane if left unchecked. This is where the sodium-potassium pump becomes essential for maintaining the resting potential over time.

5. The Sodium-Potassium Pump: Maintaining the Gradients

The ion gradients that drive the resting potential would eventually run down through diffusion if nothing actively restored them. The sodium-potassium pump (Na$^+$/K$^+$-ATPase) is the mechanism that continuously resets the gradients, using energy from ATP hydrolysis.

For every molecule of ATP hydrolysed, the pump moves 3 $\text{Na}^+$ out of the cell and 2 $\text{K}^+$ into the cell. This 3:2 ratio is not arbitrary — it means more positive charge leaves than enters, making a direct electrogenic contribution to the negative interior. The pump therefore both maintains the concentration gradients and contributes a small direct negative charge to the resting potential.

Think of the pump as a rechargeable battery. The ion gradients are the stored charge; diffusion through leak channels slowly discharges it; the pump continuously recharges it. Without the pump running constantly, the resting potential would gradually collapse as $\text{Na}^+$ leaked in and $\text{K}^+$ leaked out until concentrations equalised on both sides.

6. Bringing It Together: How the Resting Potential Is Established

The resting potential emerges from three interacting mechanisms working simultaneously. It is worth tracing through the logic as a sequence, because exam questions often ask you to explain the establishment of the resting potential in a specific order.

Worked example: Establishing the Resting Potential: Step-by-Step Reasoning

Explain, in sequence, how a resting potential of approximately -70 mV is established and maintained across the axon membrane.

1. The Na$^+$/K$^+$-ATPase pump uses ATP to move 3 Na$^+$ out and 2 K$^+$ in continuously. — This creates a high [Na+] outside and high [K+] inside. These concentration gradients are the chemical component of the electrochemical gradient for each ion.
2. At rest: potassium leak channels are open; sodium channels are largely closed. K$^+$ diffuses out down its concentration gradient. Na$^+$ cannot easily enter. — The membrane is far more permeable to K+ than Na+ at rest. This asymmetry is the key to generating the charge difference.
3. As K$^+$ leaves, large negatively charged organic anions (e.g. proteins) remain inside — they cannot cross the membrane. The inside accumulates net negative charge. — The negative interior is not caused by negative ions entering; it is caused by positive ions (K+) leaving while negative charges stay behind.
4. The growing negative interior exerts an electrical attraction on K$^+$, opposing further outward diffusion. Equilibrium is reached when the electrical gradient inward equals the chemical gradient outward. — At this equilibrium, the net movement of K+ is zero. The membrane potential at this point is approximately -70 mV.
5. Slow Na$^+$ leak inward is continuously corrected by the pump removing Na$^+$. The pump also directly contributes to the negative interior via its 3:2 electrogenic ratio. — The resting potential is a dynamic steady state, not a static one. The pump must keep running (requiring ATP) to maintain it.

Answer: The resting potential of approximately -70 mV is established by: (1) the Na+/K+ pump creating ion concentration gradients; (2) differential membrane permeability allowing K+ to diffuse out while Na+ is largely excluded; (3) the resulting accumulation of negative charge inside as K+ leaves; (4) electrochemical equilibrium for K+ stabilising the potential; and (5) continuous pump activity maintaining the gradients against slow ion leaks.

Notice that the resting potential is described as approximately $-70$ mV — the minus sign indicates that the inside is negative relative to the outside. This sign convention is used consistently in exam questions, so always include it when stating a value for resting potential.

7. Exam Technique: Getting Full Marks on Resting Potential Questions

Questions on resting potential are among the most frequently mishandled in AQA A Level Biology. The marks are there for precise mechanistic language — and they are lost when students give vague or incomplete answers. Here is how to approach the most common question types.

When asked to explain the resting potential, examiners expect a causal chain: ion gradients are established by the pump, differential permeability allows selective diffusion, K$^+$ moves out, negative charge accumulates inside, electrochemical equilibrium is reached. Each link in that chain can carry a mark. Stopping at "potassium moves out" without explaining why the inside becomes negative will cost you marks.

When asked to describe the role of the sodium-potassium pump, state both the ratio (3 Na$^+$ out, 2 K$^+$ in) and the energy source (ATP hydrolysis). Stating that the pump "moves ions" without specifying direction and ratio is a common source of lost marks.

For extended-response questions that ask you to evaluate or explain the resting potential in full, structure your answer as: pump action establishes gradients → differential permeability allows selective diffusion → K$^+$ efflux creates negative interior → electrochemical equilibrium stabilises potential → pump maintains steady state. This sequence maps directly onto the mark scheme logic used by AQA examiners.

Mastering neurone structure and resting potential gives you the conceptual foundation for every subsequent topic in nervous coordination: action potentials, synaptic transmission, and the integration of signals all depend on the ion gradients and membrane properties established here.