VPB 121: Veterinary Physiology - II (2+1)

RESTING POTENTIAL

The resting potentials of nerve and muscle Fibers

• In the resting state of membrane i.e. the state under which the membrane of the nerve or muscle fibers is not transmitting signals, the fluid inside the nerve or muscle cells has greater concentration of K⁺ and the fluid outside the Na⁺.
• The potential across the membrane is a negative potential and is known as resting potential.  The resting potential of the nerve as well as the muscle fiber is -75mV i.e. the potential inside the fiber is 75mV more negative than the potential outside.

Sodium - Potassium Pump mechanism 

• The mechanism is present in all cells of the body. The negative resting membrane potential is established by this mechanism.  It transports Na⁺ from the cells to the exterior and K⁺ from the exterior to the cell.  Hence it is responsible for maintaining the normal Na⁺-K⁺ concentration difference across the cell membrane.
• The pump mechanism uses a carrier protein, which is a complex of two separate globular proteins, one of which is large with a molecular weight of about 100,000 and the other is small with a molecular weight of about 45,000. The function of the latter is not known. The large one has the following 3 specific features that confirm the pump function:
  • As the carrier protein complex exists across the cell membrane the large protein protrudes on either side of the membrane.
  • It has 2 receptor sites on the outside protrusion for K⁺ and 3 on the inside protrusion for Na^+.
  • The site adjacent or close to the Na⁺ receptors has ATPase activity.
• In the operation of the pump mechanism 3Na⁺ bind (each at a receptor) from interior of the cell and 2 K⁺ (each at a receptor) from exterior of the cell. The binding of Na⁺ at the receptors activates the ATPase.  A molecule of ATP is cleaved and reduced to ADP.  Energy is liberated from a high energy phosphate bond, cause conformational change in the carrier protein molecule and this change extrudes the Na⁺ to the exterior and K⁺ to the interior of the cell.
• Since all the while the concentration of Na⁺ at the exterior and K⁺ in the interior of the cell is high, both these ions are transported by the pump mechanism against very large concentration gradients.

Electrogenicity of the Na⁺ - K⁺ pump mechanism

• At each revolution of the pump 3 Na⁺ are transported to exterior for every 2 K⁺ to the interior of the cell. This amounts to a net of one positive change removed from the interior to the exterior.
• This obviously creates positivity outside the cell, leaving (owing to the deficit of positive ions inside the cell) negativity on the inside and thus an electrical potential is established across the cell membrane.  Hence, the pump mechanism is said to be electrogenic.