synaptic transmission

structure of synapse?

cholinergic synapses?

synapses that use the neurotransmitter acetylcholine (ACh)

what happens at the pre-synaptic neurone during transmission across a cholinergic synapse?

depolarisation causes voltage-gated Ca2+ channels to open → Ca2+ diffuses in

causing vesicles w/ Ache to move + fuse w/ pre-synaptic membrane → releasing Ash into synaptic cleft by exocytosis

what happens at the post-synaptic neurone during transmission across a cholinergic synapse?

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

causing Na+ channels to open

Na+ diffuses into post synaptic knob → depolarisation

if threshold met, action potential initiated

what happens to acetylcholine after synaptic transmission?

hydrolysed by acetylcholinesterase

products are reabsorbed by presynaptic neurone

to stop overstimulation - if not removed it would keep binding to receptors causing depolarisation

how do 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 covering on a single post-synaptic neurone

causing rapid buildup of neurotransmitter

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

why is synapse summation important?

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 neurotransmitter to reach threshold to trigger action potential

temporal summation?

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

sufficient neurotransmitter to reach threshold to trigger an action potential

inhibition by inhibitory synapses?

inhibitory neurotransmitters hyperpolarise postsynaptic membrane as:

• Cl- channels open → diffuses out

• K+ channels open → K+ diffuse out

inside of axon has more negative change than outside/below resting potential

more Na+ req. to enter for depolarisation

reduced likelihood of threshold being met/action potential formation at post-synaptic membranes

importance of inhibitory synapses?

both excitatory + inhibitory neurones forming synapses w/ the same post-synaptic membrane gives control of whether it “fires” an action potential

structure of neuromuscular junction?

• receptors are on muscle fibre sarcolemma instead of post synaptic membrane + there are more

• muscle fibre forms cleft to story enzyme eg. acetylcholinesterase to break down neurotransmitter

transmission across cholinergic synapses vs neuromuscular junctions?

C:

• neurone to neurone

• neurotrans. can be excitatory or inhibitory

• action potential may be initiated in postsynaptic neurone

N:

• neurone to muscle

• only excitatory

• action potential propagates along sarcolemma down T tubules

effect of drugs on synapse?

stimulate nervous system → more action potentials:

• similar shape to neurotransmitter

• stimulate release of more neurotransmitter

• inhibit enzyme that breaks neurotrans. down → Na+ continues to enter

inhibit nervous system → fewer action potentials:

• inhibit NT release eg. prevent opening of Ca+ channels

• block receptors by mimicking of neurotransmitter