Sensory, Relay & Motor Neurons (AQA GCSE Psychology)

Neurons & their functions

• Neurons are cells within the nervous system

• There are thought to be as many as 100 billion neurons in the brain and over 1 billion in the spinal cord i.e. about as many neurons in the brain as there are stars in the Milky Way

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

• Neurons send electrical and chemical messages around the body to the sense organs, glands and to each other (neurotransmission) and then relay information back to the central nervous system

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

  • Sensory neurons

  • Motor neurons

  • Relay neurons

The three types of neuron cells

• A neuron is composed of:

  • The cell body containing a nucleus (the DNA of the neuron)

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

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

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

  • The terminal knobs which communicate across the synaptic cleft to the next neuron

• Sensory Neurons:

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

  • They 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

  • They have long dendrites and short axons

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

• Motor Neurons

  • Their role 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

  • They 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