Resting Potential: Nature, Measurement, and Ionic Mechanisms

Overview of Nerve Information, Nature, and Propagation

• The study of nerve information focuses on how neurons transmit signals through changes in electrical potential.
• This process involves the resting state of the neuron and the propagation of impulses across the axonal membrane.

The Resting Potential

• Definition and Value:     * A non-stimulated neuron, also referred to as a resting neuron, maintains a specific electrical state known as the resting potential.     * The value of the resting potential is approximately equal to $-70\,mV$.     * The resting potential signifies that the plasma membrane of the neuron is polarized.
• Measurement Procedure:     * To prove and measure the resting potential, the potential difference across the axial membrane of a neuron is observed.     * Equipment: The measurement requires receptor microelectrodes connected to a voltmeter or an oscilloscope.     * Results from Experimental Setup (Document 1):         * Scenario 1: When both microelectrodes ($R_1$ and $R_2$) are placed on the outer surface of the nerve fiber, the potential difference ($ddp$) is $0\,mV$.         * Scenario 2: When both microelectrodes ($R_1$ and $R_2$) are placed inside the nerve fiber, the potential difference ($ddp$) is $0\,mV$.         * Scenario 3: When one electrode ($R_1$) is placed on the surface and the second electrode ($R_2$) is inserted inside the nerve fiber, the oscilloscope recorded a potential difference ($ddp$) of $-70\,mV$.
• Interpretation of the Negative Value:     * The $-70\,mV$ value indicates a potential difference across the plasma membrane between the inside and the outside of the neuron.     * Specifically, a negative value indicates that the inside of the nerve fiber is more electronegative than the outside, while the outside is more electropositive.

Ionic Basis of the Resting Potential

• Unequal Ion Distribution:     * The charge difference between the two media (intracellular and extracellular) is caused by an unequal distribution of ions on either side of the axonal membrane.     * The primary ions involved are Sodium ($Na^+$) and Potassium ($K^+$).
• Selective Permeability and Passive Transport:     * Ion movement occurs according to concentration gradients via passive transport, which does not require the expenditure of metabolic energy.     * $Na^+$ Movement: $Na^+$ ions enter the cell (influx).     * $K^+$ Movement: $K^+$ ions exit the cell (efflux).     * Rate Comparison: The efflux of $K^+$ ions is significantly greater than the influx of $Na^+$ ions.
• Diffusion Laws:     * $Na^+$ is attracted into the cell by two factors: the $Na^+$ concentration deficit inside the cell and the intracellular electronegativity.     * $K^+$ is attracted to the outside of the cell by the $K^+$ concentration deficit, though it is simultaneously repelled by the extracellular electropositivity.

The Sodium-Potassium ($Na^+/K^+$) Pump

• Mechanism of Active Transport:     * The resting potential is maintained by the $Na^+/K^+$ pump, which uses energy to transport ions against their concentration gradients.     * The pump is an enzyme known as ATPase.
• Energy Production:     * ATPase hydrolyzes Adenosine Triphosphate (ATP) to produce chemical energy necessary for transport.
• Pump Operation:     * It ensures an influx (entry) of $K^+$ ions into the cell.     * It ensures an efflux (exit) of $Na^+$ ions out of the cell.     * This active transport mechanism is essential for preserving the unequal ion distribution and thus the resting potential.

Application and Knowledge Evaluation

• Question 1: To measure the resting potential of a neuron, where do we place the electrodes?     * Correct Answer: One electrode on the surface and the other inside (Option D).
• Question 2: How is the resting potential generated?     * Correct Answer: By the unequal distribution of sodium ($Na^+$) and potassium ($K^+$) ions on each side of the membrane (Option A).
• Question 3: What does a negative value for the resting potential indicate?     * Correct Answers:         * The inside of the nerve fiber is more electronegative than the outside (Option A).         * The outside of the nerve fiber is more electropositive than the inside (Option D).
• Question 4: According to the laws of diffusion, how do ions behave?     * Correct Answers:         * $Na^+$ is attracted into the cell by the $Na^+$ deficit and by intracellular electronegativity (Option A).         * $K^+$ is attracted to the outside of the cell by the $K^+$ deficit but is repelled by extracellular electropositivity (Option D).