Fundamentals of Neuroscience/Membrane potential at rest
- To learn how concentration gradients affect diffusion.
- To learn how membrane permeability is regulated by selective ion channels.
- To understand how the equilibrium potential of a single ion develops.
- To understand that the resting potential of a cell is determined by which ions can flow across its membrane, and their concentration gradients.
- To understand how electrical current flows across neural membranes and changes over time.
Diffusion and Concentration Gradients
When a hydrophilic chemical is concentrated and inserted into an aqueous solution, there is a tendency for it to diffuse throughout the solution until the solution has a uniform concentration at every point. Between the time it was first introduced and the time it has fully diffused through the solution different parts of the solution have different concentrations of chemical and so, it can be said that there is a Gradient that has the highest values at the location of entry, and the lowest points at the extremes of the container furthest away from the point of entry of the chemical. There are mathematical formulae for calculating the rate of diffusion and therefore the amount of a substance that can be expected to have arrived at a particular location at a particular time after introduction.
Membrane Permeability and Selective Ion Channels
Biologically speaking there is one primary purpose for a cellular membrane, and that is to concentrate nutrients inside the cell while encouraging the diffusion of Wastes outside the cell. It therefore acts as a barrier to diffusion of some elements and freely allows others to pass. We call this effect selective permeability. One way that the cell membrane allows permeability is a mechanism called an ion channel. There are two types of ion channels : passive ion channels, that act as pores in the surface of the membrane, and allow ions of a particular type to passively pass through the membrane, and active ion channels, that actively pump ions through the membrane either out of or into the cell.
Potential and Charge of Ions
One of the interesting aspects of Aqueous Solutions is that they tend to break down Hydrophilic chemicals into single element components called ions. Essentially an Ion in this case is the atom without its usual donation or attraction to other elements of its electrons. This structure has a residual charge that is either negative or positive depending on the number of valence electrons that are part of the ion. Thus for each ion we can predict the charge of the Ion by the structure of the electron shells, and how many electrons it would take to put the elemental atom back into a neutral state. Usually one or two electrons needs to be added or taken away to gain neutrality. Thus we can say that the Potential is the EMF generated by a single electron times the number of electrons that need to be donated, or absorbed in order to complete the valence shells. Each ion that flows into the cell, adds or subtracts it's particular potential to the state of the whole cell, and because the membrane of the cell is highly polarized, it tends to build up a charge in much the same way a capacitor does at the inner surface of the cell. This charge carries with it the emf of all the ions and creates a potential. The potential of a cell that is not actively involved in signaling is called its resting potential. Nerves adjust the flow of ions within them, in order to signal between themselves, and the signaling potential or voltage is called the Action Potential.
Ionic Currents over time
When an active ion channel actually pumps ions into or out of the cell, it creates a flow of charge past a point in space, which is the requirement for measuring current. This can radically change the voltage of the cell as a whole, as it pumps charge into or out of the cell by passing it across the cellular membrane. Thus the action potential of a cell is determined by the net effect of various ion currents on its charge.