Synaptic Transmission

• Presynaptic neuron

  • Ends in synaptic knob
  • Has lots of mitochondria, endoplasmic reticulum & vesicles of neurotransmitter
• Synaptic cleft

  20-30 nm gap between neurons
• Postsynaptic neuron

  • Has complementary receptors to neurotransmitters (ligand-gated Na+ channels)
• Electrical impulse cannot travel over the junction between neurons
• Neurotransmitters

  Send impulses between neurons/ from neurons to effectors
• Synaptic transmission across a cholinergic synapse

  1. Wave of depolarisation travels down the presynaptic neuron

  2. Causes voltage-gated Ca2+ channels to open & CA2+ enters

  3. Vesicles move towards & fuse with presynaptic membrane & release (exocytosis) of acetylcholine (ACh) neurotransmitter

  4. Neurotransmitters cross the synaptic cleft via simple diffusion

  5. Acetylcholine attaches to receptors on the postsynaptic membrane

  6. Sodium channels open; Na+ ions enter (postsynaptic neuron) & influx leads to depolarisation / action potential generated
• Synaptic transmission is unidirectional as only the presynaptic neuron contains vesicles of neurotransmitter & only the postsynaptic membrane has complementary receptors. So impulse always travels presynaptic → postsynaptic
• Temporal summation

  1 presynaptic neuron releases neurotransmitter several times in quick succession
• Spatial summation

  Presynaptic neurons release neurotransmitters
• What happens in an inhibitory synapse

  1. Neurotransmitter binds to & opens Cl- channels on postsynaptic membrane & triggers K+ channels to open

  2. Cl- moves in & K+ moves out via facilitated diffusion; p.d. becomes more negative: hyperpolarization
• What happens to acetylcholine from the synaptic cleft

  1. Hydrolysis Into Acetyl & Choline by acetylcholinesterase (AChE)

  2. Acetyl & choline Diffuse Back Into Presynaptic membrane

  3. ATP is used to reform acetylcholine for storage in vesicles
• AChE is Important as it prevents overstimulation of skeletal muscle cells Enables acetyl & choline to be recycled
• Drugs affecting synaptic transmission

  • Increase - Inhibit AChE OR Mimic shape of neurotransmitter
  • Decrease - Inhibit release of neurotransmitters. Decrease permeability of postsynaptic membrane to ions. Hyperpolarize postsynaptic membrane
• Neuromuscular junction

  • Synaptic cleft between a presynaptic neuron & a skeletal muscle cell
  • Synaptic transmission involves (ACh) & leads to the stimulation of muscle contraction