6.1.2 The Structure & Function of Sensory, Relay & Motor Neurons

• 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
• To put this in context; it is believed that there are about as neurons in your brain as there are stars in our galaxy
• They are the building blocks of communication within the body and are essential to our everyday survival
• They send messages around the body to our sense organs, glands and organs and then relay information back from these to the central nervous system
• There are three main types of neuron, each with its own distinct function:
  • Sensory
  • Motor 
  • Relay

All 3 types of neuron have dendrites, cell bodies and an axon

Sensory Neurons

• These are cells that send information to the brain regarding the senses; these are also known as afferent neurons as they carry information from the Peripheral Nervous System (PNS) towards the Central Nervous System (CNS)
• They keep the brain informed about the external and internal environment via processing information coming from the eyes, ears, nose, skin and tongue
• They can only transmit messages i.e. like a one-way street
• They have long dendrites and, relatively, short axons
• The cell body is usually to the side of the cell

Motor Neurons

• These carry signals from the Central Nervous System (CNS) toward the organs, muscles and glands in the body
• In this respect, they are efferent neurons as they carry information away from the CNS toward the PNS
• As they can both transmit and receive messages i.e. like a two-way street
• They have short dendrites and, relatively long axons
• If it is connected to muscle, then it is known as a motor end plate

Relay Neurons

• These are also known as Interneurons as they can take on the role of sensory and motor neurons in the nervous system
• They connect the sensory neurons to the motor neurons
• They are based in the CNS and carry signals/messages across this part of the nervous system
• As they can both transmit and receive messages i.e. like a two-way street
• Characteristically, they have short axons and short dendrites

Diagram of the Central Nervous System and Neurons