Question

All mammalian cells maintain the resting membrane potential across the plasma membrane. Neurons and muscle cells are capable of generating the action potential to communicate with each other. Below is a diagram showing a temporal change of the membrane potential in axon. Explain how such membrane potential is generated and/or maintained in each stage (1-3). Make sure to identify key membrane proteins and the movement of ion(s) through these membrane proteins in each stage. Calculate the membrane potential at the stage (1) above. The concentrations of ions are shown below at 37C (write the equation and calculation process). Calculate the membrane potential at the peak (between the stages 3 and 4) using the table above.

Answer 1

The voltage across the membrane is called membrane potential. It is the difference in electric potential between the interior and exterior of the cell. When the membrane potential of a cell can go for a long period of time without changing significantly, it is called a resting potential or a resting voltage. An action potential is a short lasting event in which the electrical membrane potential of a cell rises and falls rapidly. These occur in different types of cells such as muscle cells, neurons, endocrine cells, etc. Action potentials in neurons are called nerve impulses, The temporal sequence of action potentials is called spike train.

The temporal change of the membrane potential in axon is generated by special types of voltage-gated ion channels, which are embedded in a cell's plasma membrane. These channels are usually closed when the membrane potential is near the resting potential. But, they rapidly open if the membrane potential increases to the threshold value (stage 1). These channels when open they allow inward flow of sodium ions. This brings change in electrochemical gradient, which in turn produces further rise in membrane potential (stage 2). This further causes more channels to open and produce greater electric current across the membrane (Stage 2 - 3). This process continues until all the available channels open, thus resulting in a large upswing in the membrane potential.

The rapid influx of sodium ions reverse the polarity of the plasma membrane causing rapid inactivation of the ion channels. When Sodium ions no longer enter the cell, the already entered ions will be actively transported back to the extracellular fluid. At this point, potassium channels get activated, causing outward current of potassium ions. This returns the electrochemical gradient to the resting state (beyond stage 3).