Problem 2 - Biology: Concepts and Applications

Question

When a neuron is at rest, _____. a. it is at threshold potential b. gated sodium channels are open c. it holds less sodium than the interstitial fluid d. both a and

Step-by-Step Solution

Answer

The neuron holds less sodium than the interstitial fluid when at rest, so the answer is c.

Step 1: Understand the Question

We are asked to determine what happens when a neuron is at rest by selecting the correct option from the given choices.

Step 2: Recall Neuron Resting Potential

When a neuron is at rest, it is not transmitting a signal. At this stage, the neuron is at resting potential and has characteristics that are specific to this state.

Step 3: Discuss Resting Potential Characteristics

The resting potential of a neuron is about -70 mV, caused by differences in ion concentration inside and outside the cell. Sodium ions (Na+) are more concentrated outside the neuron, while potassium ions (K+) are more concentrated inside.

Step 4: Examine Option C

The statement 'it holds less sodium than the interstitial fluid' aligns with the resting state of a neuron, as the concentration of sodium outside is higher than that inside the neuron.

Step 5: Evaluate Other Options

Options a (at threshold potential) and b (gated sodium channels are open) occur during action potential initiation, not at rest. 'Both a and b' in option d are inaccurate for rest.

Step 6: Select the Correct Answer

Based on our analysis, option c accurately describes the neuron when it is at rest.

Key Concepts

Ion Concentration

One of the key features of a neuron at rest is the difference in ion concentration across its cell membrane. These differences are crucial for maintaining the resting potential, a condition where the neuron is not actively transmitting signals. Inside the neuron, certain ions are more concentrated compared to the outside and vice versa. For instance:

• Potassium ions ( K^+ ): There is a higher concentration of potassium ions inside the neuron compared to the outside environment.
• Sodium ions ( Na^+ ): Conversely, sodium ions have a higher concentration outside the neuron than inside.

This distinct distribution is due to the selective permeability of the neuron's membrane and the action of the sodium-potassium pump, both of which are critical for maintaining these ion concentrations. The pump actively transports ions against their concentration gradient (3 Na⁺ out and 2 K⁺ in), using energy in the form of ATP.
As a result, this creates an electrical difference, ensuring the cell maintains its resting membrane potential and is ready to fire when needed.

Sodium and Potassium Ions

The interplay between sodium and potassium ions is fundamental to a neuron's ability to maintain its resting state. These ions are charged particles and are crucial in developing a membrane potential.

In a resting neuron:

• High External Sodium: Outside the neuron, sodium ions ( Na^+ ) have a high concentration, contributing to a positive charge environment outside the neuron.
• High Internal Potassium: Inside the neuron, potassium ions ( K^+ ) are more concentrated, creating a contrasting environment where the inside is slightly negative compared to the outside.

This balance is not static; it is actively managed through ion channels and the sodium-potassium pump. When not transmitting signals, these channels are closed, maintaining the resting potential. However, the pump continuously works, ensuring that for every three sodium ions pumped out, two potassium ions are pumped in, helping sustain the ion gradient essential for the neuron's readiness to activate.

Membrane Potential

Membrane potential refers to the difference in electrical charge across a neuron's membrane. At rest, this is known as the resting membrane potential, typically valued at around -70 mV. This resting potential results from the differential ion concentrations controlled by the sodium-potassium pump and selective ion channels.

Key points about membrane potential include:

• Negative Resting Potential: The resting potential is maintained at a negative value, which results from the disparity in positive and negative ion distribution across the membrane.
• Dynamic Equilibrium: Although the neuron is at rest, there is an ongoing dynamic process where ions are actively transported in and out, ensuring equilibrium is preserved.
• Readiness for Action Potential: Having a negative resting potential allows the neuron to be ready for rapid response when signals need to be transmitted, especially during an action potential.

These processes equip the neuron to quickly revert to a resting state after an action potential, maintaining overall functionality and readiness for subsequent neuronal signaling needs.