Membrane Potential - Graded Potentials

Graded Potentials

As explained above, the potential at any point in a cell's membrane is determined by the ion concentration differences between the intracellular and extracellular areas, and by the permeability of the membrane to each type of ion. The ion concentrations do not normally change very quickly (with the exception of Ca2+, where the baseline intracellular concentration is so low that even a small influx may increase it by orders of magnitude), but the permeabilities of the ions can change in a fraction of a millisecond, as a result of activation of ligand-gated ion channels. The change in membrane potential can be either large or small, depending on how many ion channels are activated and what type they are, and can be either long or short, depending on the lengths of time that the channels remain open. Changes of this type are referred to as graded potentials, in contrast to action potentials, which have a fixed amplitude and time course.

As can be derived from the Goldman equation shown above, the effect of increasing the permeability of a membrane to a particular type of ion shifts the membrane potential toward the reversal potential for that ion. Thus, opening Na+ channels pulls the membrane potential toward the Na+ reversal potential which is, usually around +100 mV. Similarly, opening K+ channels pulls the membrane potential toward about –90 mV, and opening Cl– channels pulls it toward about –70 mV (resting potential of most membranes). Because –90 to +100 mV is the full operating range of membrane potential, the effect is that Na+ channels always pull the membrane potential up, K+ channels pull it down, and Cl– channels pull it toward the resting potential.

Graded membrane potentials are particularly important in neurons, where they are produced by synapses—a temporary change in membrane potential produced by activation of a synapse by a single graded or action potential is called a postsynaptic potential. Neurotransmitters that act to open Na+ channels cause the membrane potential to become more positive, while neurotransmitters that act on K+ channels cause it to become more negative. Because the membrane potential in a neuron must rise past the threshold value to produce an action potential, an increase in membrane potential is considered excitatory, while a decrease is considered inhibitory. Thus neurotransmitters that act to open Na+ channels produce excitatory postsynaptic potentials, or EPSPs, whereas neurotransmitters that act to open K+ channels produce inhibitory postsynaptic potentials, or IPSPs. When multiple types of channels are open within the same time period, their postsynaptic potentials summate.