The Structure & Function of Neurons (AQA A Level Psychology)

Sensory, relay & motor neurons

• Neurons are cells which exist within the nervous system

• Neurons are the building blocks of communication within the body and are essential to everyday survival

• Neurons send electrical and chemical messages around the body to sense organs, glands and other organs

  • This information is then directed to the central nervous system (CNS)

• A neuron is composed of:

  • The cell body contains a nucleus (where the DNA of the neurone is stored)

  • Dendrites (like tree branches) which carry the electrical charges from one neuron to the next

  • The axon which carries the electrical charge down the length of the neurone and is covered in a protective fatty myelin sheath

  • The myelin sheath has gaps in it called Nodes of Ranvier which speed up the message’s transmission

  • Terminal knobs are found at the end of the neuron; they communicate across the synaptic cleft to the next neuron

• There are three main types of neurons, each with a distinct function:

  • Sensory neurons

  • Motor neurons

  • Relay neurons

Sensory Neurons

• The role of sensory neurons is to send information from the peripheral nervous system (PNS) towards the central nervous system (CNS)

• Sensory neurons keep the brain informed about the external and internal environment information coming from the sense organs

• Sensory neurons can only transmit messages one way and cannot receive messages i.e. like a one-way street

• Sensory neurons have long dendrites and short axons

• The cell body is usually to the side of the cell

Motor Neurons

• The role of motor neurons is to carry signals from the CNS toward organs, muscles and glands (PNS)

• They can both transmit and receive messages i.e like a two-way street

• Motor neurons control physical movements such as contraction/relaxation of muscles

• They have short dendrites and long axons

Relay Neurons

• Relay neurons connect sensory neurons to motor neurons

• Relay neurons are located in the CNS and carry signals/messages across this part of the nervous system

• Messages can be both transmitted and received i.e. a two-way street

• They have short axons and short dendrites