Reflex Arcs
- There are three main types of neurone: sensory, relay and motor
- Sensory neurones carry impulses from receptors to the Central Nervous System (CNS - the brain or spinal cord)
- Relay (intermediate) neurones are found entirely within the CNS and connect sensory and motor neurones
- Motor neurones carry impulses from the CNS to effectors (muscles or glands)
- Sensory neurones, relay (intermediate) neurones and motor neurones work together to bring about a response to a stimulus
- A reflex arc is a pathway along which impulses are transmitted from a receptor to an effector without involving ‘conscious’ regions of the brain
- As it does not involve the brain, a reflex response is quicker than any other type of nervous response
- Examples of simple reflex actions that are coordinated by these pathways are:
- Removing the hand rapidly from a sharp or hot object
- Blinking
- Focusing the eye on an object
- Controlling how much light enters the eye
How sensory neurones, intermediate (relay) neurones and motor neurones work together to carry out a reflex action
- In the example above:
- A pin (the stimulus) is detected by a pain receptor in the skin
- The sensory neurone sends electrical impulses to the spinal cord (the coordinator)
- Electrical impulses are passed on to relay neurone in the spinal cord
- The relay neurone connects to the motor neurone and passes the impulses on
- The motor neurone carries the impulses to the muscle in the leg (the effector)
- The impulses cause the muscle to contract and pull the leg up and away from the sharp object (the response)
The pathway of a reflex arc
