The Reflex Arc (Cambridge O Level Biology)

Voluntary Responses

• A voluntary response is one where you make a conscious decision to carry out a particular action therefore it starts with your brain
  • An example is reaching out to pick up a cup of coffee
• An involuntary (or reflex) response does not involve the brain as the coordinator of the reaction and you are not aware you have completed it until after you have carried it out
• Involuntary actions are usually ones which are essential to basic survival and are rapid, whereas voluntary responses often take longer as we consider what the consequences might be before doing it

Reflex Responses

• An involuntary (or reflex) response does not involve the brain as the coordinator of the reaction and you are not aware you have completed it until after you have carried it out
• This is an automatic and rapid response to a stimulus such as touching something sharp or hot
• As it does not involve the brain, a reflex response is quicker than any other type of nervous response
• This helps to minimise the damage to the body

Reflex Arc Diagram

The reflex arc consists of a specific pathway from stimulus to response through the three neurone types

Reflex Arc Example

A reflex 

Example of a reflex arc

1. The pin (the stimulus) is detected by a pain/pressure/touch receptor in the skin
2. The sensory neurone sends electrical impulses to the spinal cord (the coordinator)
3. The electrical impulse is passed on to the relay neurone in the spinal cord 
4. Relay neurones connect to motor neurones and pass the impulse on
5. The motor neurone carries impulse to a muscle in the leg (the effector)
   • Note that effectors can be either muscles or glands
6. The muscle will contract and pull the foot up and away from the sharp object (the response) 

The reflex pathway

Synapses

• Where two neurons meet, they do so at a junction called a synapse
• Synapses are junctions between neurones, so are important in the nervous system being a connected network of neurones
• Nerve impulses can transmit across synapses and be directed along the appropriate route by them e.g. to the correct part of the brain
  • Think about the analogy of railway points that guide the trains onto the appropriate tracks based on that train's destination.