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IB DP Biology — C: Interaction and Interdependence | SL | C2.2 Neural Signalling
📋 What You Will Learn
- Identify the structural components of a neuron — cell body, dendrites, axon, myelin sheath, nodes of Ranvier, and synaptic terminals — and relate each to its function.
- Explain how the resting membrane potential of approximately −70 mV is established and maintained through selective permeability and the sodium-potassium pump.
- Analyse the roles of ion concentration gradients and electrochemical gradients in determining the resting potential.
- Apply understanding of neuron structure and membrane potential to interpret experimental data from electrophysiology recordings.
- Distinguish between the three functional types of neuron — sensory, relay (interneuron), and motor — and their roles in reflex arcs.
🔍 Guiding Question How does the structure of a neuron and the properties of its membrane allow it to receive, process, and transmit information across the nervous system?
🌍 Why This Matters Every thought you have right now — the act of reading these words, parsing their meaning, deciding whether to take notes — depends on electrical signals racing along neurons at speeds of up to 120 metres per second. Local anaesthetics like lidocaine work by blocking sodium channels in sensory neuron membranes, preventing the resting potential from ever being disturbed. Understanding the resting membrane potential is not abstract physics — it is the molecular basis of consciousness, sensation, and movement.
The Problem the Nervous System Had to Solve
Multicellular animals face a coordination problem that single cells never encounter. When a predator appears, a muscle in your leg needs to know about it within milliseconds — not minutes. Chemical diffusion is far too slow across distances greater than a few micrometres. The nervous system evolved a radically different solution: cells that are electrically polarised at rest, primed like a compressed spring, ready to fire a signal the instant they are triggered.
That compressed spring is the resting membrane potential. To understand how it works, you first need to understand the cell that maintains it.
📖 Key Definition — Neuron A neuron is a specialised cell of the nervous system that is adapted to receive, process, and transmit electrical signals called nerve impulses. Neurons are the fundamental structural and functional units of the nervous system.
Neuron Structure: A Cell Built for Speed
Imagine a cell that has been stretched, branched, and insulated — every modification serving a single purpose: to carry a signal as fast and as far as possible. That is a neuron. Its architecture is unlike any other cell in the body, and every structural feature is a direct answer to a functional demand.
The Cell Body (Soma)
The cell body contains the nucleus and most of the cytoplasm, including the rough endoplasmic reticulum and ribosomes needed to synthesise proteins. It is the metabolic centre of the neuron — the place where the cell's housekeeping happens. Radiating outward from the cell body are two types of extensions: dendrites and an axon.
Dendrites
Dendrites are short, highly branched extensions of the cell body. Their branching increases the surface area available to receive signals from other neurons. Each branch tip forms a connection point — a synapse — with the terminal of