Synapses

• What is the synaptic cleft ?

  • The gap between the cells at the synapse
  • 20-30 nm
• Where are neurotransmitters released from ?

  Presynaptic neurone
• What is the synaptic knob ?

  The end of the axon on the presynaptic neurone
• What features of the synaptic knob are required for the production of neurotransmitters ?

  • Many mitochondria
  • Large amount of endoplasmic reticulum
• What happens once neurotransmitters are released ?

  • They diffuse across to the post synaptic neurone
  • Where neurotransmitters are received by receptor proteins
  • This leads to a new action potential in the post synaptic neurone
• What are excitatory synapses ?

  Synapses that produce new action potentials
• What are the features of synapses ?

  • Unidirectional
  • Spatial summation
  • Temporal summation
• How are synapses unidirectional ?


  They pass information in one direction
• What can low frequency action potentials lead to ?

  • Release of insufficient concentrations of neurotransmitter to trigger a new action potential in the post synaptic neurone
  • Summation combats this
• What is spatial summation ?

  • Multiple presynaptic neurones release enough neurotransmitter to exceed the threshold value of the post synaptic neurone
  • Enabling a new action potential to be triggered
• What is temporal summation ?

  • A single presynaptic neurone releases the neurotransmitter many times in a short period
  • To increase concentration
  • This means the threshold value can be reached
  • Triggering an action potential in the post synaptic neurone
• What are low frequency action potentials ?

  • Lead to neurotransmitter being down rapidly
  • Concentration is below threshold to trigger action potential in the post synaptic neurone
• What are high frequency action potentials ?

  • Lead to release of neurotransmitter in a short time
  • Concentration is above the threshold
  • So an action potential is triggered in the post synaptic neurone
• What are inhibitory synapses ?

  Synapses that make it less likely that a new action potential will be created on the post synaptic neurone
• How do inhibitory synapses operate ?

  • Hyperpolarisation
  • Makes it less likely that a new action potential will be created because a larger influx of sodium ions is needed to produce one
• How does hyperpolarisation occur ?

  • Presynaptic neurone releases a type of neurotransmitter that binds to chloride ion channels on the post synaptic neurone, causing them to open
  • Chloride ions move into the post synaptic neurone by facilitated diffusion
  • The binding of neurotransmitters causes the opening of nearby potassium ion protein channels
  • So potassium ions move out of the post synaptic neurone into the synapse
  • Inside of the post synaptic membrane becomes more negative and the outside more positive