15 Synapses

  • Created by: lee8444
  • Created on: 20-02-20 09:36

Structure of a synapse

  • Synapses transmit information, not impulses
  • The synaptic cleft is 20-30nm wide
  • The presynaptic neurone has a bulged synaptic knob and has lots of mitochondria and a lot of endoplasmic reticulum to make the neurotransmitters
  • The neurotransmitter is stored in synaptic vesicles
  • The neurotransmitter diffuses across the cleft to the postsynaptic neurone on to specif protein receptors
  • The presynaptic neurone has multiple calcium ion channels
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Features of synapses

  • Unidirectionality - information can only be transmitted in one direction due to the receptor proteins only being on the postsynaptic neurone, acts as a valve
  • Summation - low-frequency action potentials are often unable to generate enough neurotransmitter to cause a new action potential as they cannot overcome the threshold value
  • Spatial summation - multiple presynaptic neurones release neurotransmitter together to trigger a new action potential by overcoming the threshold value
  • Temporal summation - a single presynaptic neurone releases neurotransmitter multiple times over a short time period to overcome the threshold value
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Inhibitory synapses

  • The presynaptic neurone releases a neurotransmitter that binds to chlorine ion channels on the postsynaptic neurone
  • This opens the chlorine ion channels
  • Chlorine ions move into the postsynaptic neurone
  • This opens potassium ion channels
  • Potassium ions move out of the postsynaptic neurone
  • This makes the inside of the postsynaptic neurone very negative and the outside more positive
  • This results in the new membrane potential being -80mV compared to the usual -65mV at resting potential
  • This is called hyperpolarisation and decreases the chance of an action potential being generated and a larger influx of sodium ions will be needed
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Cholinergic synapses

  • A cholinergic synapse is where the neurotransmitter is acetylcholine
  • Cholinergic synapses are very common in vertebrates where they happen in the CNS and at neuromuscular junctions
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Transmission across a synapse

  • The arrival of the action potential at the presynaptic knob opens calcium ion channels and calcium ions flow into the presynaptic neurone by facilitated diffusion
  • This influx of calcium ions causes synaptic vesicles containing neurotransmitter to fuse with the presynaptic membrane releasing acetylcholine into the synaptic cleft
  • As the acetylcholine diffuses across the cleft, it binds to receptor sites on the postsynaptic neurone. These receptor sites are sodium ion channels
  • The binding causes the sodium ion channels to open allowing sodium ions to rapidly flow into the postsynaptic neurone
  • This generates an action potential in the postsynaptic neurone
  • Acetylcholinesterase hydrolyses the acetylcholine back into acetyl and choline molecules which diffuse back across the synaptic cleft and diffuse back into the presynaptic neurone
  • This recycling prevents the acetylcholine from constantly generating an action potential in the postsynaptic neurone
  • ATP released by mitochondria recombines the acetyl and choline back into acetylcholine which is then stored in synaptic vesicles for the future
  • The sodium ion channels close when no acetylcholine is present
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