Your body needs to get a message from your fingertip to your brain and back before you have finished saying “ouch”. The cells that do it are shaped almost nothing like a typical cell — and every odd feature is there for a reason.
📚 What you need to know
The nervous system splits into the central nervous system (CNS) — brain and spinal cord — and the peripheral nervous system (PNS), all the other nerves.
Information travels as electrical impulses along nerve cells called neurones. A bundle of neurones is a nerve.
The CNS is the coordinating centre: impulses come in from the body and instructions go back out.
A neurone has a cell body containing the nucleus, many dendrites, and one long fibre, the axon.
Schwann cells wrap around the axon to form the myelin sheath, which insulates it.
An axon always carries impulses away from the cell body; a dendron carries them towards it.
Three types: sensory (receptors to CNS), relay (within the CNS), and motor (CNS to effectors).
How the nervous system is organised
The division is anatomical, and it is simpler than the names suggest. Brain and spinal cord: central. Everything else: peripheral.
Keep “nerve” and “neurone” apart in your head. A neurone is one cell; a nerve is a whole bundle of them, visible to the naked eye.
An impulse is not electricity flowing along a wire. It is a brief reversal of the electrical charge across the neurone’s membrane, travelling along like a wave. The next page unpacks exactly how.
The structure of a neurone
A neurone is built for one job: carrying a signal a long way, fast, without losing it.
The myelin blocks are separate Schwann cells, each wrapped around the axon. The small gaps between them turn out to matter enormously for speed.
Part by part
Cell body — holds the nucleus and the usual cell organelles. This is where the cell is maintained.
Dendrites — many short branches that receive impulses from other neurones. Lots of branches means a large surface area and connections to many cells, forming a network.
Axon — the one long fibre carrying the impulse away from the cell body.
Myelin sheath — formed by Schwann cells wrapping around the axon. It insulates the axon and stops the impulse leaking away.
Nodes of Ranvier — the small uninsulated gaps between Schwann cells.
Axon terminals — the branched ending that passes the signal on to the next cell.
There is one naming rule that clears up most of the confusion: axon means “away from the cell body”, dendron and dendrite mean “towards it”. Direction, not size, is what the word tells you.
The three types of neurone
The sensory neurone is the odd one out. Its cell body branches off partway along, so the impulse runs past it rather than through the middle of it.
Type
Carries impulses
Distinctive structure
Sensory
From receptors to the CNS
Cell body branches off in the middle; one long dendron in, one long axon out
Relay
Between neurones, entirely within the CNS
Short but highly branched axon and dendrites
Motor
From the CNS to effectors
Large cell body at one end, inside the CNS, with many branched dendrites
Worked examples
WORKED EXAMPLE
A neurone has a large cell body at one end with many branched dendrites, and a long myelinated axon leaving the spinal cord. Identify the type and justify your answer. [2]
Step 1: use the cell body positioncell body at one end, inside the CNS = motorStep 2: check the direction it runs
The axon leaves the spinal cord, so the impulse travels from CNS outwards.
A motor neurone, carrying impulses from the CNS to an effectora sensory neurone would have its cell body branching off partway along instead
WORKED EXAMPLE
Explain how the structure of a motor neurone suits its function. [3]
Step 1: the length
A very long axon carries the impulse a long distance in one cell, with no gaps to cross on the way.
Step 2: the insulation
The myelin sheath prevents the impulse leaking away and speeds transmission.
Step 3: the dendrites
Many branched dendrites give a large surface area for connections with other neurones.
Long axon for distance, myelin for speed, dendrites for connectionsstructure-and-function questions want a feature and its benefit each time
WORKED EXAMPLE
Put these in the order an impulse would pass through them when you touch something hot and pull away: motor neurone, receptor, relay neurone, sensory neurone, effector.
Step 1: start where the stimulus is detectedreceptor → sensory neuroneStep 2: into and across the CNSrelay neurone (inside the CNS)Step 3: back out to do something about itmotor neurone → effectorreceptor → sensory → relay → motor → effectorlearn this sequence as one string — it appears again and again
💡 Exam tip
Spell out the abbreviations at least once: central nervous system and peripheral nervous system.
Identify a neurone type by where the cell body sits, not by how long it looks in the diagram.
Say effector and then name one — a muscle or a gland.
Give the Schwann cells credit: the myelin sheath is formed by Schwann cells wrapping around the axon.
Use impulse rather than “message” or “signal” in a formal answer.
Remember relay neurones are found entirely within the CNS. That single fact answers several questions.
⚠ Common mix-up
Using “nerve” and “neurone” interchangeably. A neurone is one cell; a nerve is a bundle of them.
Thinking the impulse is a current flowing like electricity in a wire. It is a travelling change in membrane charge.
Saying the axon carries impulses towards the cell body. It always carries them away.
Placing relay neurones in the PNS. They are entirely inside the CNS.
Confusing receptors with effectors. Receptors detect; effectors respond.
Describing myelin as a single continuous coating. It is a row of separate Schwann cells with nodes between them.
Up next: Nerve Impulses — what the resting potential is, how the sodium–potassium pump creates it, and what happens in the two milliseconds when it flips.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
