Synaptic transmission

• What is a synapse?


  • a specialised junction where one part of the neurone contacts and communicates with another neurone or cell
• What type of synapses allows for the direct transfer of ionic current between cells?

  Electrical synapses
• How do electrical synapses facilitate communication between neurons?

  They allow for the direct transfer of ionic current from one cell to the next
• Where do electrical synapses occur?

  At gap junctions
• What is the characteristic of most gap junctions regarding ionic current?

  They allow ionic current to pass bidirectionally
• What happens when gap junctions interconnect neurons?

  They can function as an electrical synapse
• What occurs when two neurons are electrically coupled?

  An action potential in the presynaptic neuron causes ionic current to flow into the other neuron
• What is the result of the ionic current flowing into the second neuron?

  It causes an electrically mediated postsynaptic potential (PSP) in the second neuron
• What happens when the second neuron generates an action potential?

  It induces a PSP in the first neuron
• Chemical synapses 


  • most synaptic transmission is in chemical synapses
  • the pre and post synaptic membranes are separated by a synaptic cleft
  • neuromuscular junctions are chemical synapses that occur between axons of motor neurones of the spinal cord and skeletal muscles
  • neuromuscular junctions are one of the largest synapses in the body
• Stage 1 and 2 of synaptic transmission 


  stage 1:

  • Neurotransmitter is synthesised in the presynaptic neurone

  stage 2

  • neurotransmitter is stored in synaptic vesicles
• Stage 3 of synaptic transmission 


  • the neurotransmitter is released due to the arrival of an action potential
  • the depolarisation of the axon terminal causes voltage gated calcium ions to open - calcium ions flood synaptic knob
  • the influx of calcium cause vesicles to fuse with membrane
  • vesicles release via exocytosis
• stage 4 of synaptic transmission 


  • neurotransmitter binds to receptors on the postsynaptic membrane
  • this binding causes conformational changes
  • 2 types of receptors
• types of receptors on postsynaptic neurone 


  transmitter gated ion channels

  • membrane spanning proteins that comes together to from a pre between them
  • when neurotransmitter binds to specific sites, the pores open

  G protein coupled receptors

  • neurotransmitter binds to receptor proteins
  • the receptor proteins activate G proteins
  • G proteins activate 'effector' proteins
• Stage 5 of synaptic transmission 


  • diffuse out of synaptic cleft
  • reuptake - broken down by specific enzyme and taken back by presynaptic
• Synaptic integration 


  • process by which multiple synaptic potentials combine within one postsynaptic neurone
  • EPSP summation represents simplest form of synaptic integration
  • spatial - adding together EPSPs generated simultaneously at different synapses
  • temporal - adding together EPSPs generated at the same synapse if occurred in rapid succession