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3.5.1Theme B — Form and function

Neurones- Function and Structure — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 14 of 14 Unit quiz

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Part 1 of 5Parts of a Neurone

3.5.1 Neurones- Function and Structure
Cell body
  • Contains the nucleus and most organelles, and makes the proteins and neurotransmitters the neurone needs.
Dendrites
  • Short branching extensions that receive signals from other cells, providing a large surface area for many connections.
Axon
  • A long fibre that carries the nerve impulse away from the cell body to other neurones, muscles or glands.
Myelin sheath
  • A fatty insulating layer formed by Schwann cells that wraps around the axon and greatly speeds up impulse conduction.
Nodes of Ranvier
  • Gaps between sections of myelin where the axon membrane is exposed and the impulse is regenerated.
Axon terminals
  • Swollen endings that contain vesicles of neurotransmitter and form synapses with the next cell.

Part 2 of 5Types of Neurone

Types of Neurone
Sensory neurone
  • Carries impulses from receptors in the skin, eyes and other organs towards the central nervous system.
Relay neurone
  • Lies within the brain or spinal cord and connects sensory and motor neurones, helping to process information.
Motor neurone
  • Carries impulses from the central nervous system to effectors such as muscles and glands, causing a response.
Nervous system
  • The central nervous system is the brain and spinal cord, while the peripheral nervous system is the nerves linking it to the body.
TypeDirectionCell body position
SensoryReceptor to CNSBranches off the axon, near the spinal cord
RelayWithin CNSWithin the CNS
MotorCNS to effectorAt the CNS end of the neurone

Part 3 of 5Pathway of a Response

1
A receptor detects a stimulus such as heat and a sensory neurone sends an impulse towards the spinal cord.
2
The impulse crosses a synapse to a relay neurone in the central nervous system.
3
The relay neurone passes the message across another synapse to a motor neurone.
4
The motor neurone carries the impulse to an effector, such as a muscle, which produces the response.

Part 4 of 5Worked Example

Question

  • A neurone carries information from a pressure receptor in a fingertip to the spinal cord.
  • Name its type and describe the direction of travel of the impulse, giving the structures involved.

  1. The neurone brings information from a receptor into the central nervous system, so it is a sensory neurone.
  2. The impulse is received at its dendrite ending, passes along the long axon, and reaches the axon terminals in the spinal cord.
  3. There it crosses a synapse to a relay neurone.

Answer It is a sensory neurone, conducting from the fingertip along its axon into the spinal cord.

Part 5 of 5Exam Checklist & Tips

3.5.1 – Key Points for the Exam

  • Neurone A nerve cell specialised to transmit electrical impulses.
  • Myelin sheath Insulating fatty layer around an axon that speeds impulses.
  • Dendrite Branching extension that receives signals from other cells.
  • Axon Long extension that carries impulses away from the cell body.
  • Effector A muscle or gland that carries out a response.

Common Mistakes

  • Saying the myelin sheath is part of the neurone itself; it is formed by separate Schwann cells.
  • Reversing sensory and motor neurones; sensory go to the CNS, motor go away.
  • Confusing the CNS (brain and spinal cord) with the PNS (the nerves).

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