Synaptic Transmission

  • Created by: Rachelezy
  • Created on: 17-02-20 17:20

Structure of a Synapse

  • Synaptic Cleft - Gap prevents electrical impulses from passing directly from one neurone to another. Contains enzymes that break down the neurotransmitter
  • Information travels from the pre-synaptic neurone to the post-synaptic neurone
  • Post-synaptic Neurone - Membrane has neurotransmitter receptors. These are specialised ion channels that open or close when a neurotransmitter binds to a specific receptor site
  • Synaptic Knob - Filled with vesicles of neurotransmitters (e.g. acetylcholine) which are chemicals that stimulate the post-synaptic neurone. Also consists of voltage-gated calcium channels that detect when an action potential arrives at the end of the axon
  • Vesicles of Neurotransmitters - made in the smooth endoplasmic reticulum (SER)

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Neuromuscular Junctions

Point at which a motor neurone meets a skeletal muscle fibre

  • At rest, vesicles containing acetylcholine present in presynaptic motor neurone
  • Action potential reaches NJ, voltage-gated calcium ion channels open, causing calcium ions to diffuse into the neurone
  • Calcium ions cause synaptic vessel to fuse with the presynaptic membrane and release acetylcholine into the synapse
  • Diffuses across synapse and binds with receptors on muscle cell membrane, results in muscle contraction
  • When action potential stops, the acetylcholine is broken down by ethanoic acid and choline by acetylcholinesterase (enzyme)
  • Ethanoic acid and choline diffuse back into presynaptic neurone
  • Once returned, ethanoic acid and choline are recombined into acetylcholine (using energy provided by mitochondria in the presynaptic region)
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Events involved in transmission across a cholinerg

  • Action potential arrives at the pre-synaptic membrane of an axon
  • Causes calcium ion channels to open
  • Calcium ions rapidly move by facilitated diffusion into the synaptic knob (because Ca2+ is higher in cleft than in the knob)
  • When Ca2+ enters, some vesicles move to pre-synaptic membrane and fuse with it, releasing acetylcholine into the synaptic cleft
  • Diffuses across short distance to post-synaptic membrane
  • Post-synaptic membrane has receptor proteins to which acetylcholine molecules attach to. (Binding sites complementary to acetylcholine)
  • Each receptor protein located next to a sodium ion channel. When acetylcholine binds, changes shape and pushing sodium channel open
  • If enough sodium ions enter, and the threshold is exceeded, an action potential is triggered
  • Travels along the neurone as an impulse until it reaches the synapses at the other end.
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Aspects of the neurone that ensures that impulses

Unidirectional - travels in one direction

  • Hyperpolarisation - inactive ion channels
  • Neurotransmitters only present in vesicles in the pre-synaptic neurone
  • Dendrites - IN (only at one end)
  • Axon Terminal - EXIT (only at one end)
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Temporal (TIme):

  • Effects of several impulses are added together in a short space of time
  • If several impulses reach the synaptic knob in quick succession, enough acetylcholine is released for the membrane potential to reach its threshold value
  • This causes sodium ion channels to open and produces an action potential


  • Motor neurones release enough neurotransmitters to exceed the threshold value which triggers an action potential
  • Several impulses arrive simultaneously at different synaptic knobs stimulating the cell body
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  • Reduces membrane potential
  • Hyperpolarises membrane (action potential cannot be created)
  • Helps to enable specific pathways to be stimulated while prevenitng random impulses all over the body

Excitatory Post-synaptic Potential - Post-synaptic membrane is depolarised. May or may not reach its threshold (goes towards the threshold) (Excitatory synapse)

Inhibitory Post-synaptic Potential - Post-synaptic membrane is hyperpolarised away from its threshold (Inhibitory synapse) - Chlorine ions enter

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Effects of drugs on synapses


  • Blocks the effects of acetylcholine by blocking nicotinic cholinergic receptors at neuromuscular junctions, so muscle cells cannot be stimulated
  • Results in the muscle being paralysed (Post-synaptic neurone cannot generate an action potential
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Neuromuscular Junctions VS Cholinergic Synapses

Excitatory or Inhibitory?

  • NJ - Only excitatory CS - Can be both

What does this structure link?

  • NJ - Neurones to muscles CS - Neurone to neurone OR Neurone to other effector

Structure's relation to action potentials?

  • NJ - Action potential ends here CS - Another a.p. is generated along a different neurone

What type of neurone is involved?

  • NJ - Motor neurones CS - Intermediate, motor and sensory neurones

Where does acetylcholine bind?

  • NJ - Receptors on membrane of muscle fibre CS - Receptors on membrane of post-synaptic membrane
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