Lecture 18 - synaptic connection

Question 1

Chemical synapse

Answer 1

The junction between nerve cells where and action potential is transferred from one neuron to another or to an effector cells (namely muscle)

Question 2

Synaptic connection flow chart

Answer 2

Resting potential to graded potential to action potential to synaptic activity to information processing

Question 3

Chemically gated ion channels

Answer 3

Chemical opens ion channels that allow for the electrical depolarisation of the cell or the muscle on the other side

Chemical binds to a protein and allows for conformational change in the protein and then the channel opens up

Question 4

Acetylcholine

Answer 4

One of the chemicals that can open an ion channel. Also works in neuron to neuron transmission as well as transmission at the NMJ

When this chemical unbinds then the channel shuts

Question 5

Neuron-neuron chemical synaptic transmission

Answer 5

There is a presynaptic and a postsynaptic neuron and there are regulatory neurons attached

Question 6

Calcium used for

Answer 6

It is another cation that is not just important for bones but also for the nervous system health and function

2-2.5 mM in the ECF

Question 7

Events occurring at a cholinergic synapse

Answer 7

1- Action potential triggers the opening of voltage-gated calcium channels (action potential arrives at the terminal and it depolarises which is key for calcium channels to open and because calcium is positively charged it comes down its gradient and into the cell)
2- Calcium ions diffuse into the axon terminal, and trigger synaptic vesicles to release ACh by exocytosis (vesicles move to the terminal and fuses to the presynaptic c membrane and when that happens the ACh that was inside gets released into the synaptic cleft)
3- ACh diffuses across the synaptic cleft, binds to ACh-gated sodium ion channels, and produces a graded depolarisation (also called EPSP)
4- Depolarisation ends as ACh is broken down into acetate and choline by Acetylcholinesterase (AChE) (if this enzyme is absent it can lead to overaction of the synapse which isn’t good)
5- The axon terminal reabsorbs choline from the synaptic cleft and uses it to synthesise new molecules of ACh

Note that the mitochondria facilitates the remaking of ACh and without it the synapses would run out of ACh so no transmission of the action potential would occur

Question 8

EPSP

Answer 8

Excitatory postsynaptic potentials (EPSPs) - depolarisation caused by the opening of Na+ ion channels allowing an influx of Na+ into the cell moving the cell closer to threshold to trigger an action potential

Question 9

IPSP

Answer 9

Inhibitory postsynaptic potentials (IPSP) - chemical stimulus opens potassium ion channels causing hyper polarisation as K+ moves out of the cell

Inhibitory because the membrane is going away from threshold, going more negative and when it does this there is less likelihood that an action potential being delivered

Question 10

What determines if a postsynaptic neuron is going to fire an action potential to the next neuron or not?

Answer 10

It depends on how many IPSPs and EPSPs actually take place and whether the threshold value is reached

Question 11

Synaptic transmission at NMJ

Answer 11

Synaptic transmission at NMJ is excitatory and absolute so does not have EPSP or IPSP summation. All or nothing triggering of action.

No axon hillock. This synapse is designed to activate the muscle and it coverts neuronal action potential into a muscle action potential

Postsynaptic muscle depolarisation - always depolarised sufficiently for excitation-contraction coupling to occur

No EPSP/IPSP summation because the synaptic transmission cannot fail because if it fails the muscle will not be excited and therefore will not move

Question 12

Temporal summation

Answer 12

Temporal summation - arrival of stimulus immediately after each other resulting in addition of them together (EPSPs and IPSPs occur in quick succession and add up/summate)

Depolarisation occurs at the same point but there is just a time delay between the stimuli

When there is enough stimulation, the axon hillock is able to go above threshold and allow for action potential propagation

Question 13

Spatial summation

Answer 13

Spatial summation - addition of multiple stimulus from multiple neurons to a single postsynaptic neuron (where EPSPs and IPSPs from different areas of the input zone add up/summate)

Simultaneous action potential arrive, add together which leads to a large depolarisation that can take the axon hillock above threshold leading to action potential propagation