Sensitive and Self-Sensitive Cells

Last one! Then I need to stop procrastinating and do the online test... *shudders* (Even though it's only worth 2%, I am a massive perfectionist so it still makes me nervous.)

1. Explain the ionic basis of membrane potentials.

A membrane potential is essentially the difference in charge between the two sides of the membrane. Generally, the inside of cells is more electrically negative than the outside. Membrane potentials are generated by the selective permeability of cell membranes to certain ions. For example, K+ ions can diffuse out of the cell, leaving negatively-charged electrons behind. Not too many K+ ions can diffuse out, however, as eventually the cell becomes negatively charged enough for the K+ to be drawn back in. This eventually creates an equilibrium which becomes the membrane's resting potential.

2. Understand the principle of the Nernst equation and electrochemical equilibrium.

As I mentioned in the previous paragraph, eventually an equilibrium is created where there is no net movement of charge or ions across the membrane. The value that the membrane voltage would have to be for the K+ ions to be in equilibrium can be determined by the Nernst equation:

Vm = -60log(([K+]i)/([K+]o)) millivolts

where Vm is the value of the membrane voltage (that m is meant to be a subscript, by the way), [K+]i is the concentration of K+ inside the cell, and [K+]o is the concentration of K+ outside of the cell (i and o are also meant to be subscripts).

3. Understand the principle of the Goldman equation and how it relates to the steady state membrane potential.

Electrochemical equilibrium does not happen inside of cells, as there are many different ions that can move around and create multiple ion gradients. Hence the Goldman equation, instead of the Nernst equation, can be used for cells. The Goldman equation is similar to the Nernst equation, except the ([K+]i)/([K+]o) part is different. It's going to be hard to explain, so let me give you an example.

For example, let's say that you're looking at K+, Na+ and Cl-.

The numerator would be (PK)(Kin) + (PNa)(Nain) + (PCl)(Clout)
The denominator would be (PK)(Kout) + (PNa)(Naout) + (PCl)(Clin)
(Stuff in italics is meant to be subscripted. The P__ is the relative membrane permeability of the ion in question, whereas the _in and _out is the concentration of the ion inside and outside of the membrane.)

I really hope that you can see the pattern here, because I'm not sure how to explain it. Note that the Cl is swapped around- its extracellular concentration is on top, and its intracellular concentration is on the bottom. This is because Cl- is a negative ion whereas the other two are positive. If you have other ions that you need to look at, you can add them in as well.

4. Describe & explain the ionic basis of electrical signalling in excitable (nerve and muscle) cells.

The main basis of signalling in nerve and muscle cells is the action potential. Nerve and muscle cells have voltage-gated ion channels in their cell membranes. These are channels that open when they are stimulated by a high enough voltage. At low levels of stimulation, the channels do not open, but when there is enough stimulation (enough to raise the membrane potential to -55mV), Na+ channels open, allowing Na+ to rush in and depolarise the membrane. Na+ channels begin to close as K+ channels open, allowing K+ to leave and restore the charge. (Na+/K+ pumps restore the original concentrations of both ions, but this takes a small amount of time, meaning that following an action potential there is a "refractory period" during which the cell cannot fire.) The influx of Na+ ions can also stimulate nearby channels, leading to a rapid wave of depolarisation down the axon. This causes synaptic vesicles at the end to be released into the synapse, where they can then stimulate other neurons or muscle cells.