Chemical Synaptic Transmission

• Chemical Synaptic Transmission


  A model of fast, excitatory neurotransmission at the Neuromuscular Junction (NMJ)
• The Neuromuscular Junction (NMJ) is a model of fast, excitatory neurotransmission
• Neurotransmitters


  Chemicals released at synapses that transmit signals from one neuron to another
• Criteria that define neurotransmitters

  • Must be present at the synapse
  • Must be released in response to appropriate stimuli through a Ca2+-dependent mechanism
  • Specific post-synaptic receptors must be present
• Acetylcholine was identified as a neurotransmitter by Sir Henry Dale in 1914
• Even in the 1940s, there was debate over whether transmission was chemical or electrical
• Otto Loewi and Henry Dale awarded Nobel Prize for work on chemical neurotransmission


  1936
• Parasympathetic activation


  Causes functional effects on the heart
• Transmission at a typical chemical synapse

  1. Transmitter synthesis and vesicular storage

  2. Pre-synaptic action potential

  3. Terminal depolarisation / VGCC opening / Ca2+ influx

  4. Ca2+-induced vesicle fusion and transmitter exocytosis

  5. Transmitter binding to postsynaptic receptors

  6. Postsynaptic channel opening or closing / ion flux / current flow

  7. EPSP / IPSP generation and modulation of postsynaptic cell excitability

  8. Synaptic deactivation by transmitter removal or enzymatic degradation

  9. Vesicular recycling
• The neuromuscular junction is a model of fast, excitatory neurotransmission
• Components of a monosynaptic reflex arc

  • Skeletal muscle (effector)
  • Motorneurone (efferent)
  • Central Synapse
  • Neuromuscular Junction (motor end-plate)
  • Sensory neurone (afferent)
  • Muscle spindle (sensory receptor)
• Structure of the mammalian neuromuscular junction

  • Dark field confocal fluorescence microscopy
  • Scanning electron microscopy
• Presynaptic terminal


  Delivers transmitter
• Postsynaptic terminal


  Responds to transmitter
• Transmitter synthesis and vesicular storage of acetylcholine

  1. Choline acetyltransferase (ChAT) synthesizes acetylcholine

  2. ChAT is transported to the synapse by slow axonal transport

  3. Acetylcholine is packaged into vesicles by the vesicular acetylcholine transporter
• End plate potential (EPP)

  Depolarisation in the muscle fibre in response to motorneurone stimulation, analogous to the EPSP
• EPP depolarises the muscle fibre


  If it reaches the threshold, an action potential will be generated in the muscle fibre
• Skeletal muscle contraction


  • Action potential propagation
  • Ca2+ release from sarcoplasmic reticulum
  • Ca2+ binding to troponin
  • Muscle contraction
• Safety factor of transmission at the NMJ

  EPPs are much larger than required for reliable muscle action potential generation, leading to sustained, tetanic muscle contractions even with neurotransmitter depletion
• Release of acetylcholine at the NMJ

  1. Pre-synaptic action potential

  2. Terminal depolarisation / VGCC opening / Ca2+ influx

  3. Ca2+-induced vesicle fusion and transmitter exocytosis
• Transmitter release is absolutely dependent on Ca2+ entry via voltage-gated Ca2+ channels
• Quantal release of acetylcholine


  Spontaneous miniature end plate potentials (mEPPs) represent the release of individual quanta of acetylcholine
• Evoked release of acetylcholine at the NMJ involves Ca2+-induced vesicle fusion and exocytosis mediated by SNARE protein interactions
• Molecular machinery mediating vesicle endocytosis


  1. Clathrin attaches to the vesicle membrane

  2. Clathrin polymerisation causes membrane curvature and vesicle pinching off by dynamin

  3. Clathrin is stripped off by Hsc-70/auxilin to generate the synaptic vesicle
• Knowledge and understanding of chemical synaptic transmission develops from enquiry and experimentation
• Semester 2 Practical 3: Function and Structure of Neuromuscular Synapses

  • Rat phrenic nerve-hemidiaphragm preparation used to study neuromuscular function
• The Neuromuscular Junction (NMJ)


  A model of fast, excitatory neurotransmission
• The Neuromuscular Junction (NMJ) is a model of fast, excitatory neurotransmission
• Nicotinic acetylcholine receptor (nAChR)


  A ligand-gated ion channel
• Nicotinic acetylcholine receptor (nAChR)


  • It is a macromolecular complex made up of five subunits (2α, β, δ, γ in the foetus, 2α, β, δ, ε in the adult)
  • There are two (non-identical) binding sites for acetylcholine in the extracellular domain of the receptor, at the interfaces between the α and δ and α and γ (ε) subunits
  • Both binding sites must be occupied for maximum probability that the channel will open
• Nicotinic ACh Receptor Activation


  1. A ~15º rotation of the outer subunit structure, triggered by agonist binding, is transmitted to the M2 pore-lining region which rotates in turn to open the gate

  2. The disulphide bridge (S-S) links the moving regions to the stabilised trans-membrane region thus acting as a pivot that allows M2 rotation
• Endplate potential


  The depolarisation produced by the flow of Na+ and K+ ions through the opened nAChR channels
• Endplate potential


  Must be sufficient to open neighbouring voltage-gated Na+ channels to reach the threshold for initiating an action potential in the muscle
• Acetylcholinesterase (AChE)


  The enzyme that hydrolyses acetylcholine to choline and acetate, terminating its action
• Acetylcholine is cleared from the synaptic cleft by the action of the enzyme acetylcholinesterase
• Non-depolarizing blockers


  Competitive, reversible, receptor antagonists that block the interaction of acetylcholine with nAChR at the NMJ
• Non-depolarizing blockers


  Their actions can be overcome by increasing the acetylcholine concentration in the synaptic cleft, achieved by administering an acetylcholinesterase inhibitor drug
• Depolarizing blockers


  Agonists that activate the nAChR, producing a small amount of sustained depolarisation that ultimately causes a loss of electrical excitability at the endplate
• The block produced by a depolarizing blocker is not relieved by an anti-cholinesterase
• The duration of action of a single clinical dose of suxamethonium (a depolarizing blocker) is ~5 minutes because it is rapidly broken down by plasma cholinesterases