IB Biology HLNerves & SignallingPaper 1 & 2~10 min read
Neurones: Structure & Function
Your body has to get a message from your fingertip to your brain and back again before you have even noticed you touched something hot. The cells that carry those messages are shaped for exactly one job — carrying a signal a long way, fast.
📚 What you need to know
The nervous system is split into the central nervous system (CNS) — brain and spinal cord — and the peripheral nervous system (PNS), which is all the nerves.
Messages travel as electrical impulses along nerve cells called neurones. A nerve is a bundle of neurones.
A neurone has a cell body containing the nucleus, many dendrites, and one long fibre called the axon.
Schwann cells wrap around the axon to form the myelin sheath, which insulates it and stops the impulse leaking away.
There are three types: sensory (receptor → CNS), relay (inside the CNS), and motor (CNS → effector).
Each type has a slightly different shape, and the shape tells you the job.
An axon always carries impulses away from the cell body. A dendron or dendrite carries them towards it.
How the nervous system is put together
The human nervous system has two parts:
Central nervous system (CNS) — the brain and the spinal cord. This is the control centre. Impulses come in here from every part of the body, get sorted out, and go back out again.
Peripheral nervous system (PNS) — every nerve outside the CNS. These spread out to all your organs, muscles and, importantly, your sense organs.
Together they let you make sense of what is going on around you, react to it, and keep the inside of your body running properly (things like heart rate and breathing).
A common wobble in exams is the difference between a neurone and a nerve. A neurone is one cell. A nerve is a bundle of many neurones wrapped together, a bit like a cable made of many separate wires. You can see a nerve without a microscope; you cannot see a single neurone.
Watch the wording in the question. If it says "describe the structure of a nerve" and you describe an axon and a cell body, you have answered a different question and you get nothing.
What a neurone looks like
Neurones are long and thin, and that is not an accident. A signal has to get from your toe to your spinal cord without being handed over hundreds of times, so the cell itself stretches almost the whole way.
Cell body — contains the nucleus and the usual cell machinery (ribosomes, mitochondria). This is where proteins and neurotransmitters get made.
Dendrites — short branched extensions coming off the cell body and the axon terminals. They connect to other neurones, so one neurone can receive signals from many others. More branches means more connections.
Axon — the one long fibre that carries the impulse away from the cell body.
Myelin sheath — made by Schwann cells that wrap themselves round and round the axon. The sheath is mostly lipid, so it does not let charged ions through. It acts as an electrical insulator and stops the impulse leaking out sideways.
Nodes of Ranvier — small bare gaps between neighbouring Schwann cells where the axon membrane is exposed.
Axon terminals — the branched ends that pass the signal on to the next cell.
The myelin blocks are individual Schwann cells wrapped around the axon. The bare gaps between them matter enormously later on — they are what makes a myelinated neurone fast.
🧠
Which way does it go?
Axon = Away. Anything called a dendron or dendrite carries the impulse towards the cell body. Get this right and the three neurone types stop being confusing.
The three types of neurone
All three do the same basic thing — carry an impulse — but they sit in different places in the pathway, so they are built differently.
Type
Carries impulses
Shape you should describe
Sensory
From receptors to the CNS
Cell body branches off in the middle of the cell. One long dendron carrying impulses towards the cell body, and one long axon carrying them away
Relay (intermediate)
Entirely within the CNS, connecting sensory to motor
Short but highly branched axon and dendrites — it only has to reach the next cell in the same place
Motor
From the CNS to effectors (muscles or glands)
Large cell body at one end, sitting in the brain or spinal cord, with the nucleus in it. Many highly branched dendrites giving a big surface area for other neurones to connect to
The red block at the end of the motor neurone is a muscle — the effector. Notice only the sensory and motor neurones need to be long; the relay neurone never leaves the CNS.
Why so many dendrites? A motor neurone in your spinal cord may receive input from thousands of other neurones at once. All those branches give a huge surface area for other axon terminals to land on. No branches would mean one input and no decision making.
Worked examples
WE 1
Identify a neurone from its description
A neurone has a cell body that branches off the middle of the cell, a long fibre carrying impulses towards the cell body, and a second long fibre carrying impulses away from it. State the type of neurone and explain your answer. (3 marks)
Step 1: pick out the clue
The cell body sits in the middle, not at one end.
Step 2: name the two fibres
The fibre carrying impulses towards the cell body is a dendron; the one carrying them away is the axon.
Step 3: match to the job
That shape lets one cell run all the way from a receptor into the CNS without a break.
A sensory neuronesay which feature gave it away — naming the type alone is usually only one of the three marks
WE 2
Explain the role of the myelin sheath
Explain how the structure of the myelin sheath is related to its function. (3 marks)
Step 1: what it is made of
Schwann cells wrap round the axon in layers, so the sheath is built mainly from the phospholipids of their membranes.
Step 2: what lipid does
Lipid has a high electrical resistance, so charged ions cannot cross it.
Step 3: the consequence
The impulse cannot leak out through the sheath, so it is not lost along the way and it travels faster.
Layers of lipid = insulation = no loss of impulselink structure to function in every sentence — that is what "explain how structure is related to function" is asking for
💡 Exam tips
Learn the CNS and PNS split in one line each. It is an easy opening mark.
If you are asked to label a neurone diagram, use the exact words: cell body, nucleus, dendrites, axon, myelin sheath, node of Ranvier, axon terminal.
Say Schwann cells form the myelin sheath, not "myelin cells".
When comparing the three types, compare the same feature in each — position of cell body, length, branching.
Effector means a muscle or a gland. Write both.
⚠ Common mistakes
Using "nerve" and "neurone" as if they mean the same thing. A nerve is a bundle of neurones.
Saying the impulse travels along the axon towards the cell body. Axon = away.
Saying myelin makes the impulse jump straight to the brain. It jumps between nodes of Ranvier, node to node.
Describing relay neurones as long. They are short and highly branched, and they stay inside the CNS.
Forgetting the nucleus is in the cell body. Examiners do ask where it is.
Calling the myelin sheath a cell. It is a structure made by cells.
Up next: Nerve Impulses — how a neurone sitting still builds up an electrical charge across its membrane, and why it has to do that before it can send anything at all.
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