Problem 50 - Objective Biology for NEET

Question

The correct sequence for depolarization and repolarization is (A) Stimulus applied at a site on polarized membrane (B) Increase the permeability for \(\mathrm{Na}^{+}\) (C) Generation of A.P.(Action Potential) (D) Increase the permeability for \(\mathrm{K}^{+}\) (E) Restoration of membrane potential (a) \(\mathrm{A} \rightarrow \mathrm{B} \rightarrow \mathrm{C} \rightarrow \mathrm{D} \rightarrow \mathrm{E}\) (b) \(\mathrm{B} \rightarrow \mathrm{A} \rightarrow \mathrm{C} \rightarrow \mathrm{D} \rightarrow \mathrm{E}\) (c) \(\mathrm{A} \rightarrow \mathrm{D} \rightarrow \mathrm{C} \rightarrow \mathrm{B} \rightarrow \mathrm{E}\) (d) \(\mathrm{A} \rightarrow \mathrm{B} \rightarrow \mathrm{D} \rightarrow \mathrm{C} \rightarrow \mathrm{E}\)

Step-by-Step Solution

Answer

The correct sequence for depolarization and repolarization is option (a) \(\mathrm{A} \rightarrow \mathrm{B} \rightarrow \mathrm{C} \rightarrow\mathrm{D} \rightarrow \mathrm{E}\)

Step 1: Understanding the Concepts

Depolarization and repolarization are critical elements of the action potential in neurons. An initial stimulus is applied to a polarized membrane (A) which leads to an increase in permeability of the membrane to Sodium ions \(\mathrm{Na}^{+}\) (B) which creates an influx of Sodium ions, making the internal voltage of the neuron positive, i.e., creating an Action potential (C). This is followed by an increase in permeability of the membrane to Potassium ions \(\mathrm{K}^{+}\) (D) leading to an efflux of Potassium ions \(\mathrm{K}^{+}\) out of the neuron which makes the internal voltage of the neuron negative again. Finally, there is a restoration of the membrane potential to its resting state by the Sodium-Potassium pump (E). It ensures the neuron is ready to transmit another action potential if necessary.

Step 2: Matching the Concepts with the Options

From Step 1, the correct order of the sequence is A, B, C, D, E. Using this order, one then matches the provided options to find the sequence which corresponds to the correct order. In this case, option (a) matches the correct sequence.

Key Concepts

Depolarization

Depolarization is a crucial phase in the process of an action potential. It all begins when a stimulus is applied to a polarized neuron membrane. This stimulus makes the cell more permeable to sodium ions (\(\mathrm{Na}^{+}\)). This means that sodium channels open up, allowing \(\mathrm{Na}^{+}\) ions, which are positively charged, to flow into the cell. As these ions rush in, the inside of the neuron becomes less negative, moving towards a positive charge.
This change in voltage across the neuron's membrane is what creates the action potential. In simpler terms, the neuron switches from a resting "off" state to an "on" state, ready to send signals down the nerve.

• The process starts with a stimulus on the membrane.
• Sodium ion channels open, increasing permeability.
• Sodium ions flow into the neuron, turning the interior positive.
• This leads to the generation of an action potential.

Depolarization is essentially how neurons communicate, quickly sending information by rapidly changing their electrical states.

Repolarization

Repolarization is the next important step after depolarization in the journey of an action potential. Once the neuron has depolarized, it needs to reset to its resting state. This is where repolarization comes into play. It occurs by changing the membrane's permeability again, but this time to potassium ions (\(\mathrm{K}^{+}\)).
During repolarization, potassium channels open, and \(\mathrm{K}^{+}\) ions move out of the neuron. As these positively charged ions leave, the inside of the neuron becomes more negative once again.

• Potassium ion channels open after depolarization.
• \(\mathrm{K}^{+}\) ions flow out of the neuron.
• The internal charge returns to a more negative state.

This process ensures that the electrical potential of the neuron returns to its resting state, setting up the neuron to fire another action potential in response to future stimuli.

Sodium-Potassium Pump

The sodium-potassium pump is like the unsung hero of the neuron, often working behind the scenes. After a neuron has undergone action potential, including depolarization and repolarization, it isn't quite back to its original state yet. The sodium-potassium pump helps reset the neuron's ion balance back to its resting state.
This pump uses active transport, meaning it requires energy (from ATP) to function. It works by moving sodium ions (\(\mathrm{Na}^{+}\)) out of the neuron and potassium ions (\(\mathrm{K}^{+}\)) back into the neuron. For every three sodium ions it exports, it imports two potassium ions.

• Active transport mechanism using ATP for energy.
• Moves 3 \(\mathrm{Na}^{+}\) ions out and 2 \(\mathrm{K}^{+}\) ions in.
• Reestablishes the resting membrane potential.

This delicate balance maintained by the pump is crucial for keeping neurons ready to fire again. Without it, neurons would not be able to reliably send and receive signals, impairing nervous system functions.