# Neurone structure

> CIE A-Level Biology · Control and Coordination
> Source: https://www.owlsprep.com/study/cie-9700-u16-neurone-structure/

This subtopic covers the structure of mammalian neurones, the specialised cells that carry electrical impulses in the nervous system. You will learn how different neurone types are adapted for their specific roles in coordinating responses.

**Prerequisites:** [Basic eukaryotic cell structure](https://www.owlsprep.com/study/cie-9700-u1-eukaryotic-cell-structure/)

## Learning objectives

- Identify key structures of a mammalian neurone
- Compare the structure of sensory, relay and motor neurones
- Relate neurone structure to its function in impulse transmission
- Explain the role of the myelin sheath and Schwann cells

## General Structure of a Mammalian Neurone

**Neurone** — Specialised excitable cell that transmits electrical nerve impulses around the mammalian nervous system

*Example:* A motor neurone carries impulses from the central nervous system to an effector muscle

All neurones share core common structures, with specialised adaptations for rapid, efficient impulse transmission. The cell body (also called the soma) contains the nucleus and most of the cell's organelles, responsible for protein synthesis and metabolic maintenance.

- **Dendrites / Dendrons**: Cytoplasmic extensions that receive impulses from other neurones or receptors, and carry them towards the cell body
- **Axon**: A single long cytoplasmic extension that carries impulses away from the cell body towards the next neurone or effector
- **Myelin sheath**: A fatty insulating layer surrounding many axons, formed by Schwann cells in mammals
- **Nodes of Ranvier**: Gaps between adjacent Schwann cells along the axon

**Worked example:** State the function of three common structures found in all mammalian neurones

1. 1. Cell body (soma): Contains the nucleus and produces all proteins and neurotransmitters required for impulse transmission and cell maintenance
2. 2. Dendrons: These extensions receive incoming impulses from receptor cells or other neurones, transmitting the signal towards the cell body
3. 3. Axon: This long single extension carries outgoing electrical impulses away from the cell body, towards a target effector or synapse with another neurone

> **Exam tip:** CIE examiners almost always award marks for linking structure to function, so always explicitly connect a feature to its role in your answer

## Myelin Sheath and Schwann Cell Function

Most mammalian neurones that carry impulses over long distances are myelinated, meaning their axon is surrounded by a fatty myelin sheath. This structure is a key adaptation that dramatically speeds up impulse transmission.

**Myelin Sheath** — A fatty insulating layer surrounding the axon of many neurones, produced by Schwann cells in the peripheral nervous system. Gaps between Schwann cells are called nodes of Ranvier

*Notation:* Myelinated neurone

*Example:* The myelin sheath prevents leakage of electrical charge from the axon during impulse transmission

**Worked example:** Explain how the structure of a myelinated axon is adapted for fast impulse transmission

1. 1. The myelin sheath is made of multiple layers of fatty cell membrane from wrapped Schwann cells, so it acts as an electrical insulator
2. 2. This insulation prevents ion movement (and depolarisation of the membrane) across the axon membrane except at the gaps between Schwann cells, called nodes of Ranvier
3. 3. Depolarisation can only occur at the nodes, so impulses jump from node to node in a process called saltatory conduction, which is much faster than continuous conduction along an unmyelinated axon of the same length
4. 4. Ion channels are only concentrated at the nodes, which makes maintaining the resting potential more energy efficient for the neurone

> **Exam tip:** Do not confuse Schwann cells (the cells that make the myelin sheath) with the myelin sheath itself — CIE multiple choice often tests this common confusion

## Comparison of Sensory, Relay and Motor Neurones

Neurones are classified into three main functional types, each with a distinct structure adapted to their role in the nervous system:

| Neurone Type | Cell Body Position | Key Structural Features | Function |
| --- | --- | --- | --- |
| Sensory | In a ganglion outside CNS | Long dendron, short axon, cell body as side branch | Carry impulses from receptors to CNS |
| Relay (Intermediate) | Entirely within CNS | Many short dendrites, short axon | Connect sensory and motor neurones within CNS |
| Motor | Within CNS | Many short dendrites, long axon | Carry impulses from CNS to effectors |

**Worked example:** Compare and contrast the structure of a sensory neurone and a motor neurone

1. 1. Similarities: Both have a cell body containing a nucleus, extensions to receive impulses and an axon to transmit impulses away from the cell body
2. 2. Cell body position: In a sensory neurone, the cell body is located in a ganglion outside the central nervous system (CNS), while in a motor neurone the cell body is located inside the CNS
3. 3. Extension length: A sensory neurone has a very long dendron extending from the receptor to the cell body, and a short axon. A motor neurone has many short dendrites extending from the cell body, and a very long axon extending to the effector
4. 4. Overall structure: The sensory neurone cell body branches off the main axon/dendron, while the motor neurone cell body is at the start of the axon with dendrites extending directly from it

> **Exam tip:** When asked to compare, always make direct comparisons rather than just listing features separately. Use linking terms like 'while' or 'whereas' to show contrast

## Common pitfalls

- **Wrong:** Stating that dendrites carry impulses away from the cell body
  - Why it fails: Direction of impulse travel is the most common point of confusion for this topic
  - Correct: Remember: dendrites always carry impulses towards the cell body, axons carry impulses away
- **Wrong:** Claiming Schwann cells make myelin for all neurones in the nervous system
  - Why it fails: Schwann cells only produce myelin in the peripheral nervous system, not the central nervous system
  - Correct: For CIE 9700, always state Schwann cells produce the myelin sheath in peripheral neurones
- **Wrong:** Drawing the cell body of a sensory neurone at the end of the dendron near the receptor
  - Why it fails: The sensory neurone cell body branches off the main dendron mid-way, it is not located at the receptor end
  - Correct: Draw the cell body as a side branch off the main dendron, outside the CNS
- **Wrong:** Stating nodes of Ranvier slow down impulse transmission
  - Why it fails: Students often assume gaps would slow conduction, but this is incorrect
  - Correct: Nodes of Ranvier enable saltatory conduction, which dramatically increases the speed of impulse transmission

## Cheatsheet

| Structure | Function | Location |
| --- | --- | --- |
| Cell body (soma) | Stores DNA, synthesises proteins | Central region of the neurone |
| Dendron/Dendrites | Carry impulses towards cell body | Input end of the neurone |
| Axon | Carry impulses away from cell body | Output end of the neurone |
| Myelin sheath | Insulates axon, speeds conduction | Surrounds axon of myelinated neurones |
| Node of Ranvier | Site of depolarisation | Gaps between Schwann cells |
| Schwann cell | Produces myelin sheath | Surrounds peripheral axons |

## What's next

Neurone structure is the foundational concept for all topics in nervous coordination. Understanding how neurones are structured allows you to explain how resting and action potentials are generated, how impulses are transmitted along axons, and how signals pass between neurones at synapses. This topic also underpins understanding of reflex arcs and the organisation of the mammalian nervous system. Mastery of neurone structure and adaptations is required for almost all longer answer questions on control and coordination in CIE A-Level Biology, so confirm you can label all structures before moving on.

- [Nerve impulse propagation](https://www.owlsprep.com/study/cie-9700-u16-nerve-impulse-propagation/)
- [Synaptic transmission](https://www.owlsprep.com/study/cie-9700-u16-synaptic-transmission/)

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