The Nervous System

Revise The Nervous System for Biology 5 (IAL) WBI15 (A2 Level) — revision notes and instant AI marking.

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Edexcel IAL Biology · Revision Guide

The Nervous System

Big Idea: Your body detects changes with receptors, sends tiny electrical signals along neurones to your brain and spinal cord, and then sends new signals out to muscles or glands so you can react — all in a fraction of a second.

📋 Summary — What This Chapter Covers

Neurones — the three types (sensory, relay, motor), their structure, and myelination.
The reflex pathway — how stimulus → receptor → CNS → effector → response actually works, including spinal and cranial reflexes.
The nerve impulse — resting potential, action potential, depolarisation, repolarisation, and the all-or-nothing principle.
Myelination & saltatory conduction — why myelinated neurones transmit impulses so much faster.
Synapses & neurotransmitters — how impulses cross the gap between neurones, and why synapses are more than just "relay points" (unidirectionality, divergence, summation).
Drugs and nervous transmission — how nicotine, lidocaine, cobra venom, L-dopa, and MDMA each interfere with synapses in different ways.
Detection of stimuli — the eye as a sense organ, rods vs cones, and how photoreceptors actually generate impulses (this one's a bit backwards — worth extra attention!).
Habituation — why animals "switch off" their response to a repeated, harmless stimulus, and how to investigate it experimentally.
CNS vs PNS — the big-picture map of the whole nervous system.

1️⃣ Neurones: Types & Structure

A neurone is a specialised cell built for one job: carrying electrical impulses quickly over long distances. Think of a neurone a bit like an electrical cable with a control box at one end — the "cable" part (the axon) carries the signal, while the "control box" (the cell body) contains the nucleus and keeps the cell alive.

Every neurone — no matter its type — shares three basic features:

A long fibre called an axon, which carries the impulse.
A cell body containing the nucleus and other organelles.
An axon terminal at the end, covered in nerve endings that connect to other neurones — this is what lets neurones form networks.

Many neurones are also myelinated — wrapped in a fatty insulating layer made of Schwann cells. Between each Schwann cell there's a tiny gap called a node of Ranvier. This matters hugely for speed — more on that in Topic 4.

DENDRITES CELL BODY AXON (wrapped in AXON (input from → (nucleus + → myelin, with nodes → TERMINAL other cells) organelles) of Ranvier in gaps) (output)

The Three Types of Neurone

Type	Job	Structural quirk
Sensory neurone	Carries impulses from a receptor to the CNS	Cell body branches off in the middle of the axon; NO dendrites. Has a long dendron connecting the receptor end to the cell body.
Relay neurone	Connects sensory neurones to motor neurones, entirely within the CNS	Short, with highly branched dendrites for making many connections.
Motor neurone	Carries impulses from the CNS to an effector (muscle or gland)	Large cell body sits at ONE end (in the spinal cord/brain); many branched dendrites collecting input from other neurones.