Syllabus Edition

First teaching 2023

First exams 2025

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Neurones: Function & Structure (HL IB Biology)

Revision Note

Ruth

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Ruth

Expertise

Biology

Neurones: Function & Structure

The nervous system

  • The human nervous system consists of:
    • Central nervous system (CNS) – the brain and spinal cord
    • Peripheral nervous system (PNS) – all of the nerves in the body
  • It allows us to make sense of our surroundings and respond to them, and to coordinate and regulate body functions
  • Information is sent through the nervous system in the form of electrical impulses – these are electrical signals that pass along nerve cells known as neurones
    • A bundle of neurones is known as a nerve
  • The nerves spread out from the central nervous system to all other regions of the body and importantly, to all of the sense organs
    • The CNS acts as a central coordinating centre for the impulses that come in from, and are sent out to, any part of the body

Central Nervous System Diagram

The human nervous system, IGCSE & GCSE Biology revision notes

The human nervous system is comprised of the CNS and the PNS

Neurones

  • The following features are found in neurones:
    • Neurones have a main, long, fibre known as an axon
    • The axon is often insulated by Schwann cells which form the myelin sheath which prevents loss of nerve impulses along the axon
    • They have a cell body that contains the nucleus and other cellular structures
    • Their cell bodies and axon terminals contain many extensions called dendrites
    • These dendrites allow them to connect to many other neurones and receive impulses from them, forming a network for easy communication

Structure of a Neurone Diagram

Nerve Cell, IGCSE & GCSE Biology revision notes

Neurones have a characteristically elongated structure which allows them to transfer information between the central nervous system and the rest of the body

Different types of neurone

  • There are three main types of neurone: sensory, relay and motor
    • Sensory neurones carry impulses from receptors to the CNS (brain or spinal cord)
    • Relay (intermediate) neurones are found entirely within the CNS and connect sensory and motor neurones
    • Motor neurones carry impulses from the CNS to effectors (muscles or glands)
  • Each type of neurone has a slightly different structure
  • Motor neurones have:
    • A large cell body at one end that lies within the spinal cord or brain
    • A nucleus that is always in its cell body
    • Many highly-branched dendrites extending from the cell body, providing a large surface area for the axon terminals of other neurones
    Relay neurones have:
    • Short, but highly branched, axons and dendrites
  • Sensory neurones have:
    • A cell body that branches off in the middle of the cell
    • A single long dendron that carries impulses to the cell body and a single long axon that carries impulses away from the cell body

Three Types of Neurone Diagram

types-of-neurone-correct

The three types of neurone – the red line shows the direction of impulses. Note that the axon always carried impulses away from the cell body.

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