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
Exam Tip
You may be asked to label a diagram of a neuron in the exam so make sure that you know each type of neuron well.
Worked example
Here is an example of a question you might be asked on this topic - for AO1.
AO1: You need to demonstrate knowledge and understanding of key concepts, ideas, theories and research.
After each featured question there is a ‘model’ answer i.e. one which would achieve top marks in the exam.
Question: Briefly explain the function of a motor neuron. [2]
Model answer:
- The motor neuron carries an electrical signal to a muscle which will cause that muscle to either contract or relax (move).
