MOLECULAR BASIS OF RMP AND AP

MOLECULAR BASIS OF RMP AND AP

RESTING MEMBRANE POTENTIAL (RMP)

The resting membrane potential (RMP) is the electrical charge difference across the plasma membrane of a resting or non-excited cell. It is typically around -70 mV in neurons, meaning the inside of the cell is more negative compared to the outside.

MOLECULAR MECHANISM OF RMP

1. Sodium-Potassium (Na⁺-K⁺) Pump:

   • The Na⁺-K⁺ ATPase actively transports 3 Na⁺ ions out and 2 K⁺ ions in, creating an electrochemical gradient.

   • This makes the extracellular fluid (ECF) high in Na⁺ and the intracellular fluid (ICF) high in K⁺.

     
     

2. Selective Permeability of the Membrane:

   • The cell membrane is more permeable to K⁺ than Na⁺ due to the presence of K⁺ leak channels.

   • More K⁺ ions diffuse out of the cell than Na⁺ ions enter, making the inside of the cell more negative.

     
     

3. Negative Intracellular Proteins:

   • Large anionic proteins and phosphates inside the cell contribute to the negative charge of the cytoplasm.

     
     

4. Equilibrium Potential:

   • The RMP is maintained at around -70 mV because the outward movement of K⁺ is balanced by its inward attraction due to electrostatic forces.

ACTION POTENTIAL (AP)

An action potential is a rapid, temporary change in the membrane potential that occurs when a neuron or muscle cell receives a stimulus. It follows an "all-or-none" principle and allows nerve impulses or muscle contractions.

MOLECULAR MECHANISM OF AP

1. Depolarization (Na⁺ Influx)

   • When a stimulus reaches the neuron, voltage-gated Na⁺ channels open.

   • Na⁺ rushes into the cell, making the inside more positive.

   • The membrane potential shifts from -70 mV to around +30 mV.

     
     

2. Repolarization (K⁺ Efflux)

   • Na⁺ channels close, and voltage-gated K⁺ channels open.

   • K⁺ ions exit the cell, restoring the negative charge inside.

   • The membrane potential returns towards -70 mV.

     
     

3. Hyperpolarization and Return to Resting State

   • K⁺ channels remain open slightly longer, causing a slight overshoot (membrane potential becomes more negative than -70 mV).

   • The Na⁺-K⁺ pump restores the original ion distribution, bringing the neuron back to its resting state.

     
     

SIGNIFICANCE OF RESTING AND ACTION POTENTIALS:

• Essential for nerve impulse transmission in neurons.

• Controls muscle contractions in skeletal, cardiac, and smooth muscles.

• Regulates heart rhythm and glandular secretions.

• Disruptions can lead to neurological disorders, arrhythmias, or paralysis.

These processes form the fundamental basis of nerve conduction and muscle activity, playing a crucial role in communication between cells in the nervous and muscular systems.