3.6.2.2 Synaptic transmission

• The blink reflex involves synapses. Channel proteins on presynaptic neurones are involved in reflex responses. Explain how. (3)

  • Allows calcium ions in;
  • At end of presynaptic neurone
  • Causing release of neurotransmitter
• Serotonin diffuses across the synaptic gap and binds to a receptor on the post-synaptic membrane. Describe how this causes depolarisation of the post-synaptic membrane (2)

  • Causes sodium ion channels to open
  • Sodium ions enter cell and cause depolarisation
• It’s important that a neurotransmitter such as serotonin is transported back out of synapses. Explain why. (2)

  • If not removed keeps binding to receptors
  • Keeps causing action potentials / depolarisation in post-synaptic membrane
• Substances, called hormones, can also stimulate effectors. Humans produce a large number of different hormones but only a small number of different neurotransmitters. Explain the significance of this difference. (3)

  • Hormones reach all cells (via blood);
  • Neurotransmitters secreted directly on to target cell;
  • Different hormones specific to different target cells;
• Explain the effect of myelination on the rate of nerve impulse conduction. (2)

  • myelin insulates
  • saltatory conduction - impulse jumps from node to node
• Explain how the release of acetylcholine at an excitatory synapse reduced the membrane potential of the postsynaptic membrane (3)

  • binds to receptors
  • and opens Na+ channels
  • Na+ enter and make membrane potential less negative / depolarised
• Explain what causes transmission at a synapse to occur in only one direction (3)

  • vesicles containing neurotransmitter only in presynaptic membrane
  • receptors only in postsynaptic membrane
  • so neurotransmitters diffuse down concentration gradient
• Describe the sequence of events leading to the release of acetylcholine and its binding to the postsynaptic membrane (5)

  • action potential arrives / depolarisation occurs
  • calcium ions enter synaptic knob
  • vesicles fuse with membrane
  • acetylcholine diffuses across synaptic cleft
  • binds to receptors
• Describe the sequences of events which allow information to pass from one neurone to the next neurone across a cholinergic synapse. (6)

  • impulse causes calcium ions to enter axon
  • vesicles move to / fuse with presynaptic membrane
  • acetylcholine released
  • acetylcholine diffuses across synaptic cleft
  • binds with receptors on postsynaptic membrane
  • sodium ions enter postsynaptic neurone
  • depolarisation of postsynaptic membrane
  • if above threshold nerve impulse / action potential produced
• Give two differences between a cholinergic synapse and a neuromuscular junction (2)

  • neurone to neurone and neurone to muscle
  • action potential in neurone and no action potential in muscle / sarcolemma
  • muscle response always excitatory
  • some neuromuscular junctions have different neurotransmitters
• 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.
• Describe how calcium ions are involved in synaptic transmission (2)

  • Ca+ enter presynaptic neurone/membrane
  • causes fusion of vesicles with presynaptic membrane
• Describe how the release of acetylcholine into a neuromuscular junction causes the cell membrane of a muscle fibre to depolarise (3)

  • movement by diffusion
  • binding to receptors on postsynaptic membrane
  • causing Na channels to open/Na+ move into muscle
• What is the function of the mitochondria in synaptic transmission?

  • Aerobic respiration, providing ATP (energy)
  • For neurotransmitter re-synthesis/reabsorption
  • For active transport of ions
  • To move vesicles
• How are neurotransmitters made and stored?

  • made by endoplasmic reticulum
  • in the pre-synaptic neurone
  • stored in synaptic vesicles
• Describe what happens when neurotransmitters bind to the post-synaptic membrane

  • the membrane becomes more permeable to Na+ ions
  • Na+ ion channels in the membrane open
  • Na+ ions diffuse down the electrochemical gradient into the cell
  • the inside of the axon becomes positively charged
  • the threshold is reached inside of the axon, the membrane becomes depolarised / an action potential is established
• When a nerve impulse arrives at a synapse, it causes the release of neurotransmitters from vesicles in the presynaptic knob. Describe how. (3)

  • Nerve impulses / depolarisation of membrane causes Ca2+ channel proteins to open
  • Ca2+ enters by facilitated diffusion
  • Causes presynaptic vesicles to fuse with presynaptic membrane
• 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.
• Describe the sequence of events involved in transmission across a cholinergic synapse (5)


  • depolarisation of pre-synaptic membrane
  • calcium channels open and calcium ions enter
  • causing synaptic vesicles to move/fuse with presynaptic membrane AND release acetylcholine
  • acetylcholine diffuses across synaptic cleft
  • attaches to receptors on post-synaptic membrane
  • sodium ions enter postsynaptic neurone leading to depolarisation
• 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).