6.2.2 synaptic transmission

• structure of a synapse

  presynaptic neurone ->

  axon

  vesicle containing neurotransmitter

  voltage-gated calcium ion channel

  axon terminal

  synaptic cleft

  postsynaptic neurone ->

  receptor and sodium ion channel
• cholinergic synapses

  synapses that use the neurotransmitter acetylcholine (ACh)
• transmission across a cholinergic synapse - presynaptic neurone

  depolarisation of pre-synaptic membrane causes opening of voltage-gated Ca2+ channels

  Ca2+ diffuse into pre-synaptic neurone / knob

  causing vesicles containing ACh to move and fuse with pre-synaptic membrane

  releasing ACh into the synaptic cleft (by exocytosis)
• transmission across a cholinergic synapse - > postsynaptic neurone

  ACh diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane

  causing Na+ channels to open

  Na+ diffuse into post-synaptic knob causing depolarisation

  if threshold is met, an action potential is initiated
• explain what happens to acetylcholine after synaptic transmission

  it is hydrolysed by acetylcholinesterase

  products are reabsorbed by the presynaptic neurone

  to stop overstimulation - if not removed it would keep binding to receptors, causing depolarisation
• explain how synapses result in unidirectional nerve impulses

  neurotransmitter only made in / released from pre-synaptic neurone

  receptors only on post-synaptic membrane
• summation by synapses

  addition of a number of impulses converging on a single post-synaptic neurone

  causing rapid buildup of neurotransmitter (NT)

  so threshold more likely to be reached to generate an action potential

  importance - low frequency action potentials release insufficient neurotransmitter to exceed threshold
• spatial summation

  many pre-synaptic neurones share one synaptic cleft / post-synaptic neurone

  collectively release sufficient NT to reach threshold to trigger an action potential
• temporal summation

  one pre-synaptic neurone releases neurotransmitter many times over a short time

  sufficient NT to reach threshold to trigger an action potential
• inhibition by inhibitory synapses

  inhibitory neurotransmitters hyperpolarise postsynaptic membrane as

  Cl- channels open → Cl- diffuse in

  K+ channels open → K+ diffuse out

  more Na+ required for depolarisation

  reduces likelihood of threshold being met / action potential formation

  at post-synaptic membranes

  importance - both excitatory and inhibitory neurones forming synapses with the same postsynaptic membrane gives control of whether it ‘fires’ an action potential
• structure of a neuromuscular junction

  receptors are on muscle fibre instead of postsynaptic membrane and there are more

  muscle fibre forms clefts to store enzyme e.g acetylcholinesterase to break down neurotransmitter
• compare transmission across cholinergic synapses and neuromuscular junctions

  in both transmission is unidirectional

  in cholinergic synapse neurone to neurone (or effectors, glands)

  in neuromuscular junction (motor) neurone to muscle

  in cholinergic synapse neurotransmitters can be excitatory or inhibitory

  in neuromuscular junction always excitatory

  in cholinergic synapse action potential may be initiated in postsynaptic neurone

  in neuromuscular junction action potential propagates along sarcolemma down T tub
• effect of drugs on a synapse

  some drugs stimulate the nervous system, leading to more action potentials e.g

  similar shape to neurotransmitter

  stimulate release of more neurotransmitter

  inhibit enzyme that breaks down neurotransmitter → Na+ continues to enter

  some drugs inhibit the nervous system, leading to fewer action potentials e.g

  inhibit release of neurotransmitter e.g prevent opening of calcium ion channels

  block receptors by mimicking shape of neurotransmitter