Question

During the transmission of nerve impulse through a nerve fibre, the potential on the inner side of the plasma membrane has which type of electric charge? (a). First positive, then negative and continue to be negative (b) First negative, then positive and continue to be positive c) First positive, then negative and again back to positive (d) First negative, then positive and aqain back to neqative

Short answer

The potential on the inner side of the plasma membrane during the transmission of a nerve impulse is first negative, then positive, and again back to negative, which corresponds to option (d).

Step-by-step solution

Understanding Resting Potential

Understand that in an unstimulated (resting) nerve fiber, the inside of the neuron is negatively charged relative to the outside. This is due to the distribution of ions, specifically the higher concentration of potassium (K+) inside the cell and the higher concentration of sodium (Na+) outside the cell.

Depolarization

When a nerve impulse (action potential) is initiated, the membrane becomes permeable to sodium ions, leading to an influx of Na+, which causes the inside to become positively charged relative to the outside. This process is known as depolarization.

Repolarization

Shortly after depolarization, there is an increased permeability to potassium ions, leading to an efflux of K+, which helps to restore the negative charge on the inside relative to the outside. This process is called repolarization.

Return to Resting Potential

Following an action potential, the neuron returns back to the resting potential through the action of the sodium-potassium pump, which restores the original ion concentration. The neuron is then ready for another nerve impulse.

Key concepts

Understanding Resting Potential

Imagine the inside of a neuron as a tiny room that has a charge, much like a battery at rest. This charge is called the resting potential. At rest, the inside of the neuron is negatively charged compared to the outside. This difference is primarily due to the unequal distribution of ions, where potassium ions (K+) dominate inside the cell, while sodium ions (Na+) are more concentrated outside. The cell membrane's selective permeability allows K+ to move somewhat freely, while restricting Na+, which creates this negative charge inside.

This creates a stable environment, almost like a calm before the storm of nerve impulse transmission. It's set at approximately -70 millivolts (mV), and it's crucial for the ability of the neuron to fire. In short, without this negative resting potential, neurons wouldn't be ready to transmit messages.

The Spark of Depolarization

Depolarization is like the spark that ignites the neuron's activity. When the neuron receives a signal, special proteins called ion channels open up suddenly, making the membrane permeable to Na+. These ions rush into the cell due to the concentration and charge differences, like guests entering a party once the doors open. This influx of positive charges flips the membrane's charge so the inside becomes positive, reaching about +30 mV.

This rapid shift from negative to positive is the essential electrical event that triggers a nerve impulse, allowing neurons to communicate with each other. It's brief but critical, like flipping a switch to send a signal along a wire.

The Reset Button: Repolarization

After the depolarization flare, the neuron must reset itself—a process known as repolarization. Following the influx of Na+, the Na+ channels close, and K+ channels open wider, permitting these positively charged ions to exit the neuron in large numbers. This mass exit of K+ helps to restore the inside of the cell to a negative state again. It's akin to guests leaving a party, taking the warmth with them, and leaving the room back to its cool, initial state.

Repolarization restores the negative electrical charge inside the neuron and is an essential step for the neuron to be able to fire again. Imagine it as hitting the reset button so that the system is ready for the next signal.

Back to the Beginning: The Sodium-Potassium Pump

Finally, the neuron re-establishes its original ionic balance using the sodium-potassium pump. This cellular 'machine' works somewhat like a bouncer at a club, removing 3 Na+ ions from the inside and bringing in 2 K+ ions for each cycle of activity. It uses energy in the form of ATP to effectively pump sodium out and potassium in against their concentration gradients.

The pump ensures that the neuron is prepped and charged, with more K+ inside and more Na+ outside, creating the conditions for another resting potential. It's an ongoing process that's vital for neurons to continue sending messages throughout the nervous system.