3.6.2.2 Synaptic transmission

• Describe the role of neurotransmitters at a synapse.

  • Neurotransmitters are released from the presynaptic neuron into the synaptic cleft.
  • They bind to receptors on the postsynaptic neuron, opening ion channels.
  • This triggers an action potential in the postsynaptic neuron if the threshold is reached.
  • Neurotransmitters are broken down or reabsorbed to stop the signal​
• Explain how the structure of a synapse enables one-way transmission of nerve impulses.

  • The synapse consists of a presynaptic terminal, synaptic cleft, and postsynaptic membrane.
  • Neurotransmitters are released by the presynaptic neuron and bind to receptors on the postsynaptic neuron.
  • This ensures signals only move in one direction, as receptors are only on the postsynaptic membrane​
• How do enzymes at synapses control neurotransmitters?

  • Enzymes break down neurotransmitters in the synaptic cleft
  • This prevents continuous stimulation of the postsynaptic neuron.
• Spatial summation: Multiple presynaptic neurons release neurotransmitters simultaneously.
• Temporal summation: One presynaptic neuron releases neurotransmitters repeatedly in quick succession.
• A neuromuscular junction is a synapse between a motor neuron and a muscle fibre.
• Summation is the combined effect of multiple excitatory or inhibitory signals to generate a response in the postsynaptic neuron.
• Give one similarity and one difference between a synapse and a neuromuscular junction.

  • Similarity: Both involve neurotransmitters crossing a synaptic cleft to transmit signals.
  • Difference: Synapses connect two neurons, while neuromuscular junctions connect a neuron and a muscle cell.
• Acetylcholine is released from neuromuscular junctions.
• Neurotransmitter removal:

  • To prevent continuous stimulation, neurotransmitters must be removed from the synaptic cleft.
  • Enzymatic breakdown: E.g., acetylcholinesterase breaks down acetylcholine.
  • Reuptake: Neurotransmitters are reabsorbed by the presynaptic neuron.
  • Diffusion: Some neurotransmitters diffuse away from the cleft.
• Excitatory synapses:

  • Cause depolarisation of the postsynaptic membrane.
  • Increase the likelihood of an action potential.
• Inhibitory synapses:

  • Open channels for negative ions (e.g., Cl-) or cause K+ to leave the postsynaptic neuron.
  • Hyperpolarise the membrane, making action potentials less likely.
• A synapse is the junction between two neurons or a neuron and an effector cell.
• What are the three main components of a synapse?

  1. Presynaptic neuron (with synaptic knob and vesicles).
  2. Synaptic cleft (the small gap between neurons).
  3. Postsynaptic membrane (with receptor proteins).
• Sequence of events that occurs at a typical cholinergic synapse (uses acetylcholine as neurotransmitter):

  1. Action potential causes voltage-gated calcium ion channels in presynaptic membrane to open.
  2. Calcium ions (Ca²⁺) enter the presynaptic neuron by diffusion.
  3. Calcium ion influx causes vesicles containing acetylcholine (ACh) to fuse with the presynaptic membrane.
  4. ACh is released into the synaptic cleft by exocytosis.
  5. ACh diffuses across the synaptic cleft and binds to receptor proteins on the postsynaptic membrane.
  6. This opens sodium ion (Na⁺) channels on the postsynaptic membrane.
  7. Na⁺ enters the postsynaptic neuron, causing depolarisation.
  8. If threshold is reached, a new action potential is triggered.