Fight-or-flight response, neurones and synapses

• Adrenal Glands

  The adrenal glands sit above the kidneys and have two regions – the cortex (outside) and the medulla (centre)

  The medulla produces adrenaline in response to stress.

  
• Fight or flight response

  Perception of threat leads to the “fight or flight” response, which leads to a period of heightened awareness brought about by the sympathetic nervous system and adrenaline

  • pupils dilate
  • Sweat production increases
  • Metabolic rate increases
  • Heart rate and blood pressure increase
• Neurones

  Electrical impulses travel along neurones to carry information throughout the nervous system

  • Sensory neurones – transmit impulses from sensory receptors to relay neurones in the CNS
  • Motor neurones – transmit impulses from the CNS out to effectors (muscles/glands)
  • Relay neurones (also called interconnecting neurones) have their cell bodies within the CNS and connect motor and sensory neurones

  
• Neurone structure

  Three distinct parts:

  • cell body contains the nucleus, cytoplasm and organelles
  • Dendrites are thin strands of cytoplasm that receive impulses and conduct them TOWARD the cell body
  • axon (nerve fibre) is a longer cytoplasmic extension that carries nerve impulses AWAY from the cell body

  
• Neurone structure

  In sensory neurones there is also a single dendron that carries impulses towards the cell body

  • Sensory neurones have a long dendron and a short axon

  • Motor neurones have short dendrites and a long axon

  • Relay neurones have many short dendrons and many short axons
• Myelin sheath

  In some neurones Schwann cells wrap around the axon to form a fatty (lipid)-based myelin sheath, which provides electrical insulation

  • The purpose of the myelin sheath is to allow impulses to transmit more quickly along the nerve cells – this is called saltatory conduction

  
• What is a synapse?

  A synapse is the junction between two neurones

  Where two neurones don't quite touch, there's a small gap called the synaptic cleft.

  Electrical impulses can't jump the gap so the impulse crosses via the diffusion of neurotransmitter chemicals

  These trigger an impulse in the next neurone
• What is a synapse? - Key terms

  Pre-synaptic neurone – carries the impulse to the synapse

  Post-synaptic neurone - carries the impulse away from the synaptic cleft – the gap

  Axon terminal – swellings at the end of an axon

  Vesicles – sacs that contain the neurotransmitter molecules Neurotransmitter – a chemical that diffuses across the synaptic cleft (e.g. serotonin, acetylcholine, dopamine)

  
• Synaptic transmission

  When an electrical impulse arrives at the pre-synaptic neurone, it triggers vesicles containing neurotransmitter molecules to release their contents into the synaptic cleft

  • The neurotransmitters diffuse across the gap and bind to receptors on the post-synaptic neurone’s membrane

  
• Features of synaptic transmission

  • Unidirectional - ensures information is transmitted in the correct direction

  • Neurotransmitters can be classed as excitatory or inhibitory

  Excitatory neurotransmitters (e.g. noradrenaline) increase the likelihood of the post-synaptic neurone generating an electrical impulse

  Inhibitory neurotransmitters reduce the likelihood of the post-synaptic neurone generating an electrical impulse