6.2.2 Synaptic transmission Flashcards
(13 cards)
Describe the structure of a synapse
What are cholinergic synapses?
Synapses that use the neurotransmitter acetylcholine (ACh)
Describe transmission across a cholinergic synapse - At pre-synaptic 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)
Describe transmission across a cholinergic synapse - At post-synaptic 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
Explain 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
Describe spatial summation
● Many pre-synaptic neurones share one synaptic cleft / post-synaptic
neurone
● Collectively release sufficient neurotransmitter to reach threshold to trigger an action potential
Describe temporal summation
● One pre-synaptic neurone releases neurotransmitter many times over
a short time
● Sufficient neurotransmitter to reach threshold to trigger an action
potential
Describe inhibition by inhibitory synapses
● Inhibitory neurotransmitters hyperpolarise postsynaptic membrane as:
○ Cl- channels open → Cl- diffuse in
○ K+ channels open → K+ diffuse out
● This means inside of axon has a more negative charge relative to outside / below resting potential
● So more Na+ required to enter 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 post-synaptic membrane gives control of whether it ‘fires’ an action potential
Describe the structure of a neuromuscular junction
Very similar to a synapse except:
● Receptors are on muscle fibre sarcolemma instead of postsynaptic membrane and there are more
● Muscle fibre forms clefts to store enzyme eg. acetylcholinesterase to break down neurotransmitter
Compare transmission across cholinergic synapses and neuromuscular junctions
In both: transmission is unidirectional
Cholinergic synapse -
Neurone to neurone (or effectors, glands)
Neurotransmitters can be excitatory or inhibitory
Action potential may be initiated in postsynaptic neurone
Neuromuscular junction -
(Motor) neurone to muscle
Always excitatory
Action potential propagates along sarcolemma down T tubules
Use examples to explain the effect of drugs on a synapse
● Some drugs stimulate the nervous system, leading to more action potentials, eg.:
○ 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, eg.:
○ Inhibit release of neurotransmitter eg. prevent opening of calcium ion channels
○ Block receptors by mimicking shape of neurotransmitter
