Synapses process Flashcards
One neurotransmitter used in nervous system
acetylcholine (aCh)
What are synapses called that use ACh
cholinergic synapses
Process of synaptic transmission calcium ions (first part)
1) Incoming action potential causes synaptic knob to depolarise
2) Calcium channels open
3) Calcium ions (Ca2+) flood into synaptic knob diffusing down concentration gradient
4) Influx of Ca2+ causes synaptic vesicles to fuse with membrane releasing ACh into cleft
Process of synaptic transmission sodium ions (second part)
7) Acetylcholine diffuses across synaptic cleft
8) Acetylcholine binds to receptor site on Na+ channels
9) Sodium ions open channels
10) Allowing Na+ to flood in
11) So impulse cannot travel back (unidirectional)
Process of synaptic transmission threshold (third part)
12) If enough Na+ rush innto post-synaptic neurone, a threshold will be reached and a new AP is sent along the axon of the post synaptic neurone
Process of synaptic transmission acetylcholine (fourth part)
14) Hydrolytic enzyme breaks up acetylcholine to prevent sodium ion channels staying open
15) ATP released by mitochondria reforms acetylcholine
16) Which is stored in the synaptic vesicles for future use
17) More acetyl choline can be made at the SER
18) Na+ channels close in absence of acetylcholine at receptor sites
19) Synapse is now ready to be used again
Unidirectionality of synapse
Synapse can only travel in one direction
Summarise whole of synaptic transmission using acetylcholine (ACh)
1) Action potential arrives, depolarising presynnaptic membrane
2) Calcium ion channel proteins open. Calcium ions diffuse in.
3) Presynaptic vesicles fuse with membrane
4) ACh released
5) ACh diffuses across synaptic cleft
6) ACh binds to receptor proteins
7) Receptor proteins open, NA+ diffuse through
8) Postsynaptic membrane is depolarised
9) ACh broken down into acetate and choline by enzyme
10) Choline recycled into ACh
Why might an impulse be insufficient
Only a small amount of acetylcholine is released into synaptic cleft
Small number of the gated ion channels are opened in axon membrane
Insufficient number of Na+ pass through the membrane
Threshold potential is not reached
The small amount of acetylcholine attached to receptors is broken down rapidly by acetylcholinesterase
Temporal summation
multiple impulses arrive within quick succession the effect of the impulses can be added together to generate an action potential
How can temporal summation happen
A large amount of acetylcholine is released into the synaptic cleft
A large number of the gated ion channels open
A sufficient number of sodium ions pass through the membrane
Spatial summation
Multiple impulses arriving simultaneously at different synaptic knobs stimulating the same cell body can also generate an action potential through spatial summation
What does summation state
low frequency AP often produce insufficient amounts of neurotransmitter to trigger a new AP in the postsynaptic neurone
Need multiple presynaptic neurones to reach threshold
Describe the process of an action potential along an axon
1) Na+ gated channels open
2) Na+ flood into axon
3) Potential difference reversed
4) Na+ gates close
5) K+ gated channels open
6) K+ flood out of axon
7) Inside axon returns to negative
8) Resting potential restored
Resting potential
inside of axon is negatively charged and outside positive
-50 -> 90mV
axon is polarised
What resting potential is
difference in charge across membrane when the neurone is at rest
outside of the cell is negative and inside is positive of the cell
caused by differences in ion concentrations
How is the resting potential created?
unequal distribution of ions leading to an electrical charge difference between the inside and outside of the cell.
Why is the refractory period important
1) action potentials are unidirectional
prevents action potential spreading out in both directions
2) produces discrete impulses
action potentials separated from one another
3) limits number of action potentials
limits strength of stimulus that can be detected
Describe the process of the action potential when crossing synapse
1) Action potential comes down presynaptic neurone and triggers Ca gates
2) Calcium ions diffuse into synaptic knob down a concentration gradient causing synaptic vessels to move down synaptic knob and fuse with pre synaptic membrane
3) Acetylcholine diffuses down the concentration gradient across the synaptic cleft
4) Acetylcholine binds with receptors in the post-synaptic membrane, impulse is unidirectional
5) Sodium ions rush into the post-synaptic neurone (enough reach in threshold is reached triggering depolarisation)
6) An action potential in the post-synaptic neurone is generated
Process of the resting potential
1) Na+ actively transported out of axon by sodium-potassium pumps
2) K+ into axon
3) Active transport of Na+ is greater than K+, ratio 3:2, so more Na+ in fluid out of axon than in cytoplasm, creating electrochemical gradient
4) Na+ diffuse back in and K+ back out axon
5) K+ channels open and Na+ closed
Which is depolarised action potential or resting potential
action potential = depolarised
resting potential = polarised
How is the resting potential re-established
The active transport / pumping of ions against the concentration gradient
Na+ move out of the axon, while K+s move in
Why does the voltage decrease after an action potential
The potassium channels open
And potassium ions move out
Sodium ion channels close
Process of how the axon is affected by ion movement
1) Resting potential = Na+ outside is higher than inside. K+ high inside than outside. The axon membrane is polarised (more Na+ on outside than K+ on inside)
2) Stimulus causes influx of Na+ opening Na+ channels, depolarising membrane
3) Na+ channels then close and K+ open, leaving axon. Depolarisation moves across membrane
4) Outward movement of K+ repolarises the membrane
5) Repolarisation allows Na+ to be actively transported out returning to resting potential
