Transmembrane Potential – refers to the difference in electrical potential (voltage) across the membrane of a lining cell. There are a number of factors which contribute to this difference:

  1. Differences in ion concentrations across the membrane.
  1. Sodium ions are found in a high concentration outside cells.
  2. Potassium ions are found in a high concentration inside cells.

     2.  The cell membrane is selectively permeable.

  1. K+ channels are more permeable than Na+ channels.
  2. Proteins cannot pass through channels (too large).

     3.  As a result of #’s 1 and 2, the cytoplasm tends to be negatively charged. The extracellular area is positively charged. The        resistance of the lipid bilayer to a free flow of substances across the membrane helps to maintain this potential difference        across the membrane. In a neuron this difference measures -70 millivolts.

 

Gradients

Chemical gradients

  1. High Na+ in the extracellular fluid exerts a pressure pushing Na+ to the inside.
  2. High K+ in the intracellular fluid exerts a pressure pushing K+ to the outside.

 

Electrical gradients

  1. The intracellular negativity tends to pull Na+ into the cell.
  2. The intracellular negativity tends to hinder the movement of K+ out of the cells

 

Electrochemical gradient – refers to the sum of the electrical and chemical forces acting across the cell membrane.

  1. For Na+:
  1. The chemical gradient pushes Na+ into the cell.
  2. The electrical gradient pulls Na+ into the cell.

     2.  For K+:

  1. The chemical gradient pushes K+ out of cell.
  2. The electrical gradient retards the movement of K+ out of the cell.
    1. K+ is attracted by the intracellular protein (-).
    2. K+ is repulsed by the accumulation of positive charges extracellularly.

 

Resting Potential – As a result of all of these conditions, a resting cell tends to show a fairly stable transmembrane membrane potential across the membrane. This is called the resting potential. Each cell type has a different resting potential:

Cell Type                      Resting Potential

Adipose cell                          -40mv

Thyroid cell                           -50mv

Neuron                                 -70mv

Skeletal muscle cell               -85mv

Cardiac muscle cell               -90mv