Lecture 5 - Synaptic transmission I - general features

Question 1

Excitability of neurons conferred by

Answer 1

expression of channels that allow depolarisation when activated

Question 2

If local potentials reach threshold for voltage-gated Na+ channels…

Answer 2

an action potential is initiated

Question 3

‘Presynaptic cell’ is postsynaptic to its

Answer 3

Afferent inputs

Presynaptic and postsynaptic are relative to the synapses you are looking at in the moment
Almost every neuron you come across will have some synapses coming in and some synapses coming out therefore every single neuron itself is both presynaptic and postsynaptic

Question 4

Electrical synapse

Answer 4

Gap junctions are essentially pores formed between the membranes of cells, they are specialised proteins in the membrane that make pores, one set of proteins in the presynaptic cell and one set of proteins in the post synaptic cell that join up to form a pore and this is not open all the time and it only opens in response to a change in the membrane voltage
Opens when the presynaptic cell becomes depolarised by becoming electrically excited which causes a conformational change in the proteins which opens the pore in the middle and allows the flow of ions which means that the depolarisation can be passed on to the post synaptic cell
Closed most of the time

Communication is very fast

Ions flow from cell to cell

The way we modify things here compared to chemical synapses is by adding more or less gap junctions which takes time since you have to synthesise more proteins in order to form these pores in the electrical synapse

Can be opened by voltage, pH, Ca2+ and receptors

Question 5

Electrical synapse is also called

Answer 5

a gap junction

Question 6

Chemical synapse

Answer 6

Slower as it is a more complicated process

Relies upon chemicals crossing the gap

Complex series of events

Neurotransmitter packaged in vesicles

Synapse strength can be modified (perhaps by modifying the phosphorylation level of proteins that are involved in that transmission)

Classical transmission (small molecule neurotransmitters diffuse across the synaptic cleft and bind to postsynaptic receptors), gaseous transmission (gaseous transmitters diffs out of the cell of origin and directly into other cell) and neuropeptide transmission (peptides diffuse in extracellular space and bind to synaptic and extra synaptic G protein-complex receptors)

Question 7

The ionic conduction causes a change in membrane potential and that

Answer 7

neurotransmitter release etc. takes longer than direct conduction of ions through gap junctions

Question 8

Neurotransmistter exocytosis - Classical-full diffusion

Answer 8

Vesicles fully fuse with the membrane and essentially flatten out and become part of the membrane and everything inside the vesicle gets spilled out into the extracellular space which in this instance would be into the synaptic cleft

Question 9

Neurotransmitter exocytosis - Kiss and run- partial diffusion

Answer 9

The vesicle does not fully fuse to the membrane surface, instead partially fuses and forms a hole but does not flatten out to become part of the membrane - it essentially stays as a vesicle and just sticks to the sides of the membrane.

It is more difficult for the contents to get out therefore this type pf fusion may deliver less neurotransmitter into the synaptic cleft than full fusion particularly if its peptide

Question 10

Clathrin

Answer 10

The little proteins called Clathrin mark the membrane as special and helps machinery inside the cell to essentially fold that back into a vesicle shape and it gets taken back up into the presynaptic terminal and filled with more neurotransmitter and goes into the reserve pool and it will later be back in the readily releasable pool which is right near the surface of the icon terminal and release the contents etc

Question 11

Sequence of events during classical chemical neurotransmission

Answer 11

Action potential propagates down the axon to the presynaptic terminal (do not say synapse here as the synapse is the presynaptic terminal AND the postsynaptic membrane

Presynaptic terminal is depolarised and voltage-gated Ca2+ channels open (and you get a calcium influx)

Ca2+ ions trigger vesicle fusion with the presynaptic membrane (the fusion may be full exocytosis or partial exocytosis)

Neurotransmitter is released into the synaptic cleft - vesicle is recycled, “kiss and run” - usually happens within one second

Neurotransmitter diffuses with the synaptic cleft

Neurotransmitter binds to its specific receptors on the postsynaptic membrane

(A) Na+ channels open - local postsynaptic cell depolarisation - the excitatory post synaptic potential (EPSP) (switches the next cell off rather than turning it on) OR (B) K+ or Cl- channels open - local postsynaptic cell hyper polarisation - the inhibitory post synaptic potential (IPSP)

Neurotransmitter degraded or taken up (by glia or presynaptic terminal) (when into presynaptic terminal it will be recycled and repackaged for rerelease)

Recycling and refilling of the vesicles - Clathrin mediated in ~20 seconds

Question 12

Synaptic integration define

Answer 12

Synaptic integration is the term used to describe how neurons ‘add up’ these inputs before the generation of a nerve impulse, or action potential

May be as many as 50,000 synapses on a single neuron
Integration of those signals from those synapses takes place within the neuron

Question 13

Temporal summation

Answer 13

Summation between EPSPs from the same input that occur close enough together in time

Same synapse

Question 14

Spatial summation

Answer 14

summation between events that occur close enough together in space from different inputs - still requires them to be close in time

Two different synapses in two different places at the same time/very close in time

Question 15

Passive local potentials =

Answer 15

IPSP and EPSPS

Question 16

IPSP

Answer 16

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 17

EPSP

Answer 17

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 18

Synaptic modulation

Answer 18

Synaptic modulation is an observed change in the synaptic function

Synaptic modulation - facilitation within a single synapse
Lasts seconds to minutes
Increased presynaptic calcium and/or neurotransmitter availability
Summation is about what happens in the post synaptic cell but facilitation is happening in the presynaptic terminal
The presynaptic cell is firing so fast that the calcium in the presynaptic terminal can’t be removed so each time the action potential fires in the presynaptic cell there is still come calcium remaining from the previous one and this builds up and up so you get a bigger response because there is more calcium
Key thing is that it modulates the strength of that synapse and the more active a synapse is/the faster it fires, the more influence it has on a postsynaptic cell

Synaptic modulation - long term potentiation (and depression)
Lasts hours to days
Multiple post synaptic mechanisms
Insertion of more receptors that respond to the more active synapse

Question 19

Shunting

Answer 19

on the same dendrite therefore when they fire close enough together the channels in that dendrite are open and so when the next signal comes you can’t open them any more and this is known as SHUNTING

Question 20

Shunting causes …

Answer 20

Shunting causes sublinear summation which is where EPSPs are intiated in DIFFERENT inputs close together on the SAME dendrite

No linear summation with shunting

Question 21

EPSPs and IPSPs are recorded in the

Answer 21

cell body

Question 22

Synapses closer to the __________ will have more of an impact than synapses ________

Answer 22

cellbody

far away on a dendrite

Question 23

Long term potentiation

Answer 23

strengthening of a synapse

Insertion of more receptors because the synapse is active which shows it is important because it is being used

Question 24

Long term depression

Answer 24

weakening of a synaptic connection

Question 25

Facilitation

Answer 25

When the presynaptic neuron is stimulated with several consecutive individual stimuli, each evokes a larger postsynaptic potential than the previous stimuli
Cause = prolonged calcium channel opening
Consequently another excitatory signal entering the neuron from another source can excite the neuron very easily

A higher concentration of calcium enables synaptic vesicles to fuse to the presynaptic membrane and release their contents (neurotransmitters) into the synaptic cleft to ultimately contact with receptors on the post synaptic membrane. The amount of neurotransmitter released is correlated with the amount of calcium influx. Therefore short term facilitation results from a build up of calcium within the presynaptic terminal when action potentials propagate close together in time

When neurons are repeatedly activated their synaptic connection may become stronger and this effect may last for milliseconds

Synapse is very active and over time there is a bigger response in the post synaptic cell through modifications in the presynaptic terminal

Question 26

the axon hillock is the

Answer 26

site of action potential initiation, gate keeper of action potential

it must be big enough to reach threshold at the axon hillock in order for an action potential to be propagated

Question 27

If the input from one synapse is further away than another

Answer 27

than the recording on the electrode will show the the response form the furthest away synapase is smaller (local potential decreases over time)