Now we are going to shift the focus to how neurotransmitter binding to receptors actually causes a change in voltage across the membrane of the postsynaptic cell.
The concentration of ions inside vs outside cells differs.
There is a lot of Potassium inside cells, a lot of Sodium outside cells and almost no Calcium inside cells. And if you look at how an action potential works these concentrations make sense as they provide the gradient. (sodium in, potassium out)
Reversal potentials (equilibrium potential) for different ions is the voltage across the membrane which precisely counterbalances the tendency of a particular ion to move down its concentration gradient i.e. it reaches equilibrium. A given ion will move in order to drive the membrane potential towards its equilibrium potential. So if you want to stop potassium ions from diffusing out of the cell, you need to make it very negative inside the cell. Hence, the reversal potential for potassium is very negative. (look at table above)
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And also it's worth remembering that if you open a channel for a particular ion. The ions will move down their electrochemical gradient and that'll cause the membrane potential to move towards the reversal potential
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The Nernst equation is for working out the equilibrium potential (reversal potential) for a particular ion. This equation uses the conc. gradient of that ion to calculate a voltage/potential that exactly matches the tendency of that ion to move down its concentration gradient.
Ohms law can be used to help explain how electrical and chemical gradients combine to determine current flow.
Ions are driven across the membrane at a rate proportional to the difference between the membrane potential and the equilibrium potential for that ion. This is known as the driving force and the bigger the difference the more ions will move.
Conductance= how easy is it for the ions to move across the membrane (inverse of resistance) i.e. the higher the conductance the smaller the resistance.
So if you have lots of receptor channels for a particular ion, and they're all open, there will be a high conductance.
*Membrane potential refers to the difference in electrical charge (voltage) between the inside and outside of a cell and it influences ion movement by creating an electrostatic force that either attracts or repels ions based on their charge.
In excitatory synapses there are post synaptic Na channels (which in reality are permeable to both sodium and potassium).
The action potential moves down the axon and triggers the opening of voltage gated calcium channels which allow an influx of calcium. This causes vesicles to fuse with the membrane releasing neurotransmitter which moves across the synaptic cleft and binds to transmitter-gated ion channels.
These channels open and sodium moves down its concentration gradient from outside the cell to inside. This means positive ions are entering the cell causing a positive shift (depolarization). This allows the membrane to get more positive and get closer to action potential threshold or make it more likely that you would start opening voltage gated sodium channels and cause an action potential. This is a excitatory postsynaptic potential.
Lets look at the math:
So if we went back to our equation, we can see here we would have the outward ionic current is equal to the conductance times the driving force.