Lecture 11

Question 1

EPSPs and IPSPs

Answer 1

EPSPs arise from a temporary rise in GNa, for example, glutamate and acetylcholine receptors/channels that conduct Na+ currents (sodium ligand gated channels)

IPSPs arise from a temporary rise in GCl, for example, GABA and Glycine receptors that conduct Cl- currents (chloride ligand gated channels)

Question 2

Why are some synaptic receptors non-selective for Na and K?

Answer 2

Some synaptic receptors such as nAChR and AMPA are non-selective for Na+ vs K+, so why do they depolarize muscle cells to generate EPSPs and not IPSPs?

Question 3

So why do they depolarize muscle cells to generate EPSPs and not IPSPs?

Answer 3

This is because at RMP ~-65 mV the driving force for Na coming out is much greater than K+ flowing out. Although the pore can conduct both K+ and Na+, the force for Na+ coming out is much greater, and so activation by acetylcholine will generate inward Na+ currents and cause depolarization.

Question 4

What happens at more depolarized voltages?

Answer 4

At more depolarized voltages nAChRs conduct hyperpolarizing, outward K+ currents. This is because the driving force for K+ is much greater than Na+. When the driving force for K+ and Na+ are equal, there will be no net current, the net current will equal zero.

Question 5

What did A and N Takeuchi do in 1959?

Answer 5

In 1959 A and N Takeuchi used a two electrode voltage clamp to study the direction of nAChR currents at different voltages in frogs. The direction of synaptic potentials changed according to holding voltage. The voltage at which the direction of currents changes from inward to outward is known as the reversal potential, Erev.

Question 6

What else did they notice?

Answer 6

They noticed that if they changed the concentration of Na+ out and K+ out, which changed Ek and ENa, that Erev would also change. For a Nav channel, which conducts Na+ ions, changing the concentration of K+ had no affect on Erev. This means that nAChRs conduct both Na+ and K+.

Question 7

What can Erev be used for?

Answer 7

Erev can be used to to determine the relative conductance of a channel to Na+ vs K+

Question 8

An important point about ion channels.

Answer 8

Nav, Kv, and Cav tend to be highly selective for their respective ions (Na+, K+, and Ca2+)
However there are various other channels that do not discriminate well, especially between Na+ and K+, these channels are:

-nAChRs (cys-loop channels)
-AMPA glutamate receptors (p-loops)
-Many TRP channels (p-loops)
-degenerin/epithelial Na+ channels
-ATP gated P2X channels
-Many others