3.1.2 Generating a Nervous Response

• Neurones reach all parts of the body to coordinate responses
• Each neurone is made up of the following key components:
  • The axon is the long fibre which transmits electrical impulses away from the cell body
  • Axons are adapted to ensure fast transmission of the electrical impulse: 
    • A fatty layer called the myelin sheath may cover the axon to insulate it and increase the speed of the impulse
    • Some axons are very long (more than a metre in length) this reduces the number of synapses, and speeds up transmission
  • Branched endings called dendrites, create links between neurones
    • One neurone can link with many other neurones which increases the response from a stimulus

Synapses

• A synapse is a gap between two neurones found between the terminal end of the axon of one neurone and the membrane of a dendrite of another
• Electrical impulses cannot cross synapses, instead chemicals called neurotransmitters (such as dopamine and serotonin) transmit the impulse from one neurone to the next
  
  • This is the only part of the nervous system where messages are chemical as opposed to electrical
• Neurotransmitters diffuse across the synapse which slows down the transmission of electrical impulses
• Synapses ensure that impulses only travel in one direction

• The electrical impulse travels along the first axon
• When an electrical impulse arrives at the terminal end of the axon on the presynaptic neurone, chemical messengers called neurotransmitters are released from vesicles
• The neurotransmitters diffuse across the synaptic gap and bind with receptor molecules on the membrane on the dendrite of the second neurone (known as the postsynaptic membrane)
• This stimulates the second neurone to generate an electrical impulse that travels down the second axon
• The neurotransmitters are then destroyed or recycled to prevent continued stimulation of the second neurone which would cause repeated impulses to be sent

• A voluntary response is one where you make a conscious decision to carry out a particular action therefore it starts with your brain
  • Voluntary responses often take longer than involuntary responses as we consider what the response might be before doing it
• 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
  • The passage of information in an involuntary response is called a reflex arc
  • Involuntary actions are usually ones which are rapid and are essential to basic survival
  • An example of a reflex action might be pulling your hand away from a hot object to prevent harm