Synaptic Transmission

• What role do neurotransmitters play in synapses?

  They carry information across the synaptic cleft
• What are synapses?


  Junctions between neurones or between neurones and their target cells
• What is the synaptic cleft?


  The gap between neurones at a synapse
• How can neurones affect target cells?

  By releasing neurotransmitters to stimulate or inhibit them
• What is a cholinergic synapse?


  A synapse where acetylcholine is the neurotransmitter
• What is acetylcholine made up of?


  Acetyl (ethanoic acid) and choline
• Where are cholinergic synapses commonly found?

  In the CNS and at neuromuscular junctions
• What happens when an impulse arrives at a synaptic terminal?

  Calcium ion channels open, allowing calcium ions to diffuse in
• What is the role of calcium ions in neurotransmitter release?

  They cause synaptic vesicles to move towards the presynaptic membrane
• What occurs after vesicles fuse with the presynaptic membrane?

  A neurotransmitter (acetylcholine) is discharged into the synaptic cleft
• How does acetylcholine affect the postsynaptic neurone?

  It binds to receptor sites on sodium ion channels, causing them to open
• What happens when sodium ion channels open in the postsynaptic neurone?

  Sodium ions enter, causing depolarisation and potentially initiating an action potential
• What is the role of acetylcholinesterase in synaptic transmission?

  It hydrolyses acetylcholine into choline and ethanoic acid
• What happens to choline and ethanoic acid after acetylcholine is broken down?

  They diffuse back across the synaptic cleft into the presynaptic neurone
• How is acetylcholine recombined in the presynaptic neurone?

  ATP released by mitochondria is used to recombine choline and ethanoic acid
• What is the effect of depolarisation of the sarcolemma in muscle fibres?

  It leads to contraction of the muscle fibre
• What is summation in synaptic transmission?

  • Several small potential changes combine to form a larger change in potential difference
  • Types of summation:
  • Temporal: several presynaptic action potentials from one neurone
  • Spatial: presynaptic action potentials from several neurones
• How can drugs interfere with neurotransmission?

  By increasing impulses, blocking reuptake, or altering neurotransmitter release
• What are the features of synapses?

  • Unidirectional: impulses pass from presynaptic to postsynaptic neurone
  • Summation: low frequency action potentials can be insufficient
  • Spatial summation: many presynaptic neurones release neurotransmitter
  • Temporal summation: a single presynaptic neurone releases neurotransmitter multiple times
  • Inhibition: chloride ion channels can open, causing hyperpolarisation
• What is the role of excitatory presynaptic cells?

  They release neurotransmitters that decrease membrane potential, making it more excitable
• What is the effect of inhibitory presynaptic cells?

  They increase membrane potential, making target cells less excitable
• What are the major neurotransmitters in the body and their roles?

  • Acetylcholine: controls muscles and regulates memory (usually excitatory)
  • Dopamine: produces pleasure (usually inhibitory)
  • GABA: major inhibitory neurotransmitter in the brain
  • Glutamate: most common excitatory neurotransmitter in the brain
  • Glycine: inhibitory neurotransmitter in the spinal cord
  • Norepinephrine: part of fight-or-flight response (usually excitatory)
  • Serotonin: involved in mood, appetite, and sensory perception (inhibitory in pain pathways)