How the maintenance of a polarized state in resting nerve is provided?
The resting membrane potential of a neuron, also known as the polarized state, is maintained through a combination of ion concentration gradients and the selective permeability of the neuronal membrane to different ions. This process is crucial for the proper functioning of neurons and the transmission of electrical signals. Here's how the maintenance of the polarized state is achieved:
1. Ion Concentration Gradients:
- The resting membrane potential is primarily determined by the concentration gradients of potassium (K+) and sodium (Na+) ions across the neuronal membrane.
- The intracellular concentration of K+ is higher than the extracellular concentration, while the extracellular concentration of Na+ is higher than the intracellular concentration.
- These concentration gradients are established and maintained by ion pumps, such as the sodium-potassium pump (Na+/K+-ATPase).
2. Selective Permeability of the Membrane:
- The neuronal membrane is selectively permeable to different ions, allowing some ions to pass through more easily than others.
- Potassium channels are more abundant in the resting state, allowing K+ ions to move down their concentration gradient and out of the neuron.
- This efflux of K+ ions creates a net negative charge on the inside of the membrane compared to the outside, resulting in a resting membrane potential of approximately -70 millivolts (mV).
3. Role of the Sodium-Potassium Pump:
- The sodium-potassium pump is an active transport system that utilizes energy from ATP to maintain the ion concentration gradients across the neuronal membrane.
- It pumps three Na+ ions out of the neuron and two K+ ions into the neuron for each molecule of ATP hydrolyzed.
- This continuous pumping activity helps restore the initial ion gradients and counteracts the passive leak of ions through ion channels.
4. Leak Channels and Electrogenic Pump:
- In addition to K+ channels, there is a small leakage of Na+ ions into the neuron through leak channels.
- This inward movement of Na+ ions is balanced by the electrogenic nature of the sodium-potassium pump, which creates a net movement of positive charges out of the cell.
5. Chloride (Cl-) Distribution:
- Chloride ions (Cl-) also contribute to the resting membrane potential.
- Cl- ions are more concentrated outside the neuron, and they passively move into the neuron through leak channels.
- However, the negative charge of Cl- ions is counterbalanced by the positive charges carried by K+ ions moving out of the neuron, maintaining the overall electroneutrality of the cell.
In summary, the maintenance of the polarized state in resting nerve cells is achieved through the combined effects of ion concentration gradients, selective membrane permeability, the activity of the sodium-potassium pump, and the distribution of chloride ions. These mechanisms ensure the stability of the resting membrane potential, which is essential for the proper functioning and excitability of neurons.