Synapses

• Synapses are the gaps between neurones
• Information is sent between neurones by chemical transmission
• Neurotransmitters pass across the synaptic cleft from the presynaptic neurone
• The synaptic cleft is a small gap 20-30nm wide between neurones
• Synaptic transmission is one way only - a presynaptic neurones will always be presynaptic and a postsynaptic neurone will always be postsynaptic
• Most synapses are cholinergic synapses that function with the neurotransmitter acetylcholine
• Synapses allow a single impulse to be transmitted to a number of different neurones creating a simultaneous response
• Synapses allow a number of impulses to be combined at a synapse allowing stimuli from different receptors to interact for a single response
• Synapse

  A) Open calcium channel

  B) Acetylcholinesterase

  C) Open ion channel

  
• 1. An action potential arrives at the presynaptic membrane
• 2. The action potential (the influx of sodium ions) causes the calcium ion channels to open and the influx of calcium ions
• 3. The calcium ions cause the synaptic vesicles to bind with the presynaptic membrane
• 4. Neurotransmitter is released via exocytosis, and it diffuses across the synaptic cleft rapidly
• 5. The neurotransmitter will bind to specific receptors in the postsynaptic neurone
• 6. The sodium ion channels open in the postsynaptic neurone, causing the influx of sodium ions. If a threshold value is reached, the action potential continues down the postsynaptic neurone
• 7. Acetylcholinesterase breaks down acetylcholine into acetyl (ethanoic acid) and choline
• 8. Sodium ion channels close and acetyl and choline diffuse back into the presynaptic neurone, which allows the neurotransmitter to be recycled
• 9. ATP released by the mitochondria is used to recombine acetyl and choline, this is then stored in the synaptic vesicles for future use
• 10. Sodium ion channels close in the absence of neurotransmitters at their receptor sites
• Synapses are unidirectional

  • there are only synaptic vesicles in the presynaptic neurone and specific receptors are only on the postsynaptic neurone
• Summation

  • Low frequency action potentials often produce insufficient amounts of neurotransmitter to trigger a new action potential in the postsynaptic neurone, they can however be made to do so in summation
• Spatial summation

  • A number of different presynaptic neurones release enough neurotransmitter to exceed the threshold value of the postsynaptic neurone
  • Together they trigger a new action potential
• Temporal summation

  • A single presynaptic neurone releases neurotransmitter many times over a short period
  • If the total amount exceeds the threshold value of the postsynaptic neurone, then a new action potential is triggered
• Synaptic summation

  A) Temporal

  B) Spatial

  
• Acetylcholine acts as an excitatory neurotransmitter at synapses connecting motor neurones with skeletal muscle
• Inhibition

  • Chloride ion channels open on the postsynaptic membrane
  • Chlorine ions are negatively charged
  • Causes hyperpolarisation (-80mv)
  • Less likely the threshold value is reached
• Excitory neurotransmitters open Na+ channels
• Inhibitory neurotransmitters open the Cl- channels, and can also move potassium ions out
• Noradrenaline

  • sympathetic nervous system
  • Increases heart rate, breathing rate, inhibits digestion
• Dopamine

  • Endorphins
• Serotonin

  • Regulates sleeps and emotions
• Acetylcholine

  • General neurotransmitter
• GABA

  • Inhibits muscle contraction
• Drug interference with neurotransmitters

  • Stimulate the nervous system
  • Inhibit the nervous system
• Drugs stimulate the nervous system by mimicking neurotransmitters or inhibiting the breakdown enzyme
• Drugs inhibit the nervous system by inhibiting the release of neurotransmitters, blocking specific receptors or blocking Na+ channels
• Morphine and codeine are similar shapes to endorphins, so they block pain pathways and prevent action potentials
• Prozac inhibits the breakdown or reuptake of serotonin, forcing it to be in the specific receptors or synapse for longer
• Valium enhances GABA which causes less involuntary muscle contractions and can be used to treat or lessen the symptoms of epilepsy, restless legs syndrome and anxiety