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3.4.1Theme B — Form and function

Neurones: Structure and Function — IB Biology SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 13 of 13 Unit quiz

📚 IB Biology SL Hub / Topical Revision
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Part 1 of 5Structure of a Neurone

3.4.1 Neurones: Structure and Function
Cell body
  • Contains the nucleus and most organelles, and makes proteins and neurotransmitters.
Dendrites
  • Short branches that receive signals from other neurones or from receptors.
Axon
  • A long extension that carries the nerve impulse away from the cell body.
Myelin sheath
  • An insulating layer made by Schwann cells that wraps around the axon in many neurones.
Nodes of Ranvier
  • Gaps in the myelin where the impulse jumps from node to node, which speeds conduction.

Part 2 of 5Types of Neurone

Types of Neurone
Sensory neurone
  • Carries impulses from a receptor to the central nervous system.
Relay neurone
  • Connects sensory and motor neurones inside the brain or spinal cord.
Motor neurone
  • Carries impulses from the central nervous system to an effector such as a muscle or gland.
Direction
  • The impulse travels in one direction, from the dendrites through the cell body to the axon terminals.
NeuroneFromTo
SensoryReceptorCentral nervous system
RelaySensory neuroneMotor neurone
MotorCentral nervous systemEffector

Part 3 of 5A Reflex Pathway

1
A stimulus, such as heat, is detected by a receptor in the skin.
2
A sensory neurone carries the impulse to the spinal cord.
3
A relay neurone passes the signal to a motor neurone across synapses.
4
The motor neurone carries the impulse to an effector, such as a muscle.
5
The muscle contracts and withdraws the hand without waiting for conscious thought.

Part 4 of 5Structure and Function

Structure and Function
Long axons
  • Motor neurones can be over a metre in length in humans, linking the spinal cord to distant muscles.
Speed
  • Myelinated neurones conduct impulses faster than unmyelinated ones of the same diameter.
Protection
  • The brain and spinal cord form the central nervous system, which is protected by bone.

Part 5 of 5Exam Checklist & Tips

3.4.1 – Key Points for the Exam

  • Neurone A nerve cell that transmits electrical impulses.
  • Axon The long fibre that conducts an impulse away from the cell body.
  • Myelin sheath An insulating layer that speeds up impulse conduction.
  • Effector A muscle or gland that responds to a nerve impulse.

Common Mistakes

  • Saying that the dendrites carry impulses away from the cell body, when the axon does.
  • Confusing sensory neurones with motor neurones and their direction of travel.
  • Writing that myelin is part of the nerve cell; it is formed by Schwann cells around the axon.
  • Thinking that the brain and spinal cord are the peripheral nervous system.

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