The Simple Reflex Arc
- The reflex arc is an involuntary response so it does not involve the conscious part of the brain as the coordinator of the reaction
- Awareness of a response having happened occurs after the response has been carried out
- Responses are therefore automatic and rapid - this helps to protect the body from harm
The neurones of the reflex arc
- There are three main types of neurone in a reflex arc: sensory, relay and motor
- Sensory neurones carry impulses from sense organs to the CNS (brain or spinal cord)
- Relay neurones are found inside the CNS and connect sensory and motor neurones
- Motor neurones carry impulses from the CNS to effectors (muscles or glands)
![The three types of neurone](https://cdn.savemyexams.com/cdn-cgi/image/?f=auto,width=1920/uploads/2020/01/The-three-types-of-neurone.png)
There are three different types of neurone in the reflex arc
- Sensory neurones are long and have a cell body branching off the middle of the axon
- Relay neurones are short and have a small cell body at one end with many dendrites branching off it
- Motor neurones are long and have a large cell body at one end with long dendrites branching off it
An example of a reflex arc
- A stimulus (e.g. a pin) is detected by a receptor (e.g. pain/pressure/touch receptors in the skin)
- A sensory neurone sends electrical impulses to the spinal cord (the coordinator)
- An electrical impulse is passed to a relay neurone in the spinal cord
- A relay neurone synapses with a motor neurone
- A motor neurone carries an impulse to a muscle in the effector (e.g. the leg)
- The effector will initiate a response when stimulated by the motor neurone e.g. the muscle will contract and pull the foot up and away from the sharp object
![A reflex, IGCSE & GCSE Biology revision notes](https://cdn.savemyexams.com/cdn-cgi/image/w=1920,f=auto/uploads/2020/01/A-reflex.png)
Reflex actions are automatic and rapid; they do not involve the conscious part of the brain