Synaptic transmission

Question 1

Define a synapse

Answer 1

junctions between a neurone and a:

• neurone to trigger an action potential
• muscle cell to trigger a contraction
• gland cell to cause a hormone to be secreted

Question 2

Explain the sequence of events during synaptic transmission

Answer 2

1. action potential arrives at axon terminal, pre-synaptic membrane becomes more permeable to Ca2+ due to depolarisation so Ca2+ voltage gated channels open so Ca2+ enter by diffusion.
2. synaptic vesicles fuse with pre-synaptic membrane.
3. neurotransmitter ACh released by exocytosis and diffuses across synaptic cleft.
4. ACh binds to receptors on post-synaptic membrane causing Na+ channels to open.
5. Na+diffuses in through channels and depolarises post-synaptic membrane. With sufficient depolarisation above the threshold, action potential/impulse is generated.
6. ACh broken down by enzyme AChE and reabsorbed into presynaptic neurone later resynthesised (needs ATP)

Question 3

Explain what a neuromuscular junction is

Answer 3

synapse between motor neurone and muscle cell and is a special type of excitatory cholinergic synapse (receptors called nicotinic cholinergic receptors)

post-synaptic membrane has more ACh receptors so higher chance of Na+ channels opening causing depolarisation + has lots of fold which store AChE

Question 4

State similarities between neuromuscular junctions and cholinergic synapses

Answer 4

• neurotransmitters diffuse across cleft
• neurotransmitters bind to receptors on post-synaptic membrane causing Na+ to diffuse in
• neurotransmitters broken down by enzymes in cleft
• axon returns to resting potential due to Na/K pump

Question 5

State difference between neuromuscular junctions and cholinergic synapses

Answer 5

• NM excitatory, CS both inhibitory and excitatory
• NM neurones to muscles, CS neurones to other neurones or to effectors
• NM action potential ends here, CS action potential could be generated across a different neurone
• NM only motor neurones, CS intermediate, sensory or motor neurones involved
• NM ACh binds to receptors on post-synaptic membrane of muscle fibre, CS ACh binds to receptors on membrane of post-synaptic neurone

Question 6

Define unidirectionality of synapses

Answer 6

impulse travels in one direction only+one way transmission (neurotransmitter presynaptic knob but receptors on post-synaptic neurone only)

Question 7

Define spatial summation

Answer 7

several impulses from different pre-synaptic neurones arrive together to reach threshold potential.

Question 8

Define temporal summation

Answer 8

ACh breaks down in few ms so several impulses needed an quick succession to reach threshold potential

Question 9

Explain inhibition at synapses

Answer 9

• some neurotransmitters released to stop impulses e.g. K+ channels stimulated to open so K+ flows out hyperpolarising neurone
• post-synaptic neurone may have excitatory or inhibitory presynaptic neurons so end result depends on which one is stimulated