The cell membrane and its potentials

 

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Figure 3:  Bilipid membrane

Typical cells are surrounded by a bilipid membrane.  This structure has many purposes, but chief among them is providing a barrier between the extracellular and intracellular fluid.  Because lipid groups are not permeable to ion flow, movement of ions is only possible through proteins embedded in the bilayer. This enables the cell to maintain varying ionic concentrations in the intracellular fluid, and restore balance to normal concentrations if disrupted.

Figure4:
    The events of an action potential and their associated         membrane potentials (Click to enlarge)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             

Figure 5:
The flow of ions in a nerve cell and the role of the Na+/K+ pump (Click to enlarge)

Action potentials

Nervous impulses alter the concentration of ions in the extracellular fluid, changing the voltage difference across the membrane of a neuron.  Once this voltage difference is great enough, a “threshold potential” is reached, which causes a series of voltage-gated channels to open.  This results in an influx of ions into the cell causing depolarization and an action potential.  Repolarization occurs through cellular mechanisms like the sodium-potassium pump, and resting potential is achieved quickly through ion movement.