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IB DP Biology — C: Interaction and Interdependence | SL | C2.2 Neural Signalling
📋 What You Will Learn
- Identify the resting membrane potential of a neuron and explain how it is established by ion distribution and the sodium-potassium pump
- Explain the sequence of ionic events during an action potential — depolarisation, repolarisation, and hyperpolarisation — and the role of voltage-gated ion channels
- Describe the propagation of an action potential along a myelinated axon, including the role of the myelin sheath and nodes of Ranvier
- Explain synaptic transmission from the arrival of an action potential to the generation of a postsynaptic response, including the roles of neurotransmitters and receptors
- Analyse data from oscilloscope traces and graphs of membrane potential against time to interpret action potential events
- Apply understanding of neural signalling to evaluate the effects of drugs, toxins, and disease on nerve function
🔍 Guiding Question How do neurons transmit electrical signals rapidly and precisely, and how is information passed from one neuron to the next at a synapse?
🌍 Why This Matters When a surgeon injects a local anaesthetic before a procedure, the patient feels no pain — not because the tissue is removed, but because sodium channels in nearby sensory neurons are blocked, preventing action potentials from firing. Understanding exactly how an action potential is generated and transmitted is the foundation for understanding anaesthesia, epilepsy, multiple sclerosis, and the action of every psychoactive drug ever developed.
The Problem the Neuron Had to Solve
Imagine being a multicellular organism. Your skin detects a flame. Your muscles need to contract — instantly. But the distance between skin and muscle might be a metre or more, and chemical diffusion across that distance would take minutes. The solution evolution arrived at is elegant: use electricity. Not the electricity of wires, but the electricity of ions moving across a membrane, creating a travelling wave of charge that can cross a metre in milliseconds.
That travelling wave is the action potential. To understand it, you first need to understand the state the neuron is in before it fires — the resting state. This is not passive inactivity. It is a carefully maintained, energy-expensive electrochemical condition that makes rapid signalling possible.
📖 Key Definition — Resting Membrane Potential The resting membrane potential is the electrical potential difference across the plasma membrane of a neuron when it is not transmitting a signal, typically around −70 mV (inside negative relative to outside). It is maintained by the unequal distribution of ions across the membrane and the activity of the sodium-potassium pump.
Establishing the Resting Potential
The resting potential exists because of two things working together: selective permeability of the membrane and active transport. At rest, the membrane is much more permeable to potassium ions (K⁺) than to sodium ions (Na⁺). Potassium ions leak outward through open potassium channels, carrying positive charge out of the cell. This makes the inside of the neuron negative relative to the outside.
The Sodium-Potassium Pump
The sodium-potassium pump (Na⁺/K⁺-ATPase) actively transports 3 Na⁺ out of the cell for every 2 K⁺ pumped in, using one ATP molecule per cycle. This creates and maintains concentration gradients: high Na⁺