Question

Which of these correctly describes the distribution of ions on either side of an axon when it is not conducting a nerve signal? a. more sodium ions (Na^') outside and more potassium ions \(\left(\mathrm{K}^{+}\right)\) inside b. more \(\mathrm{K}^{+}\) outside and less \(\mathrm{Na}^{+}\) inside c. charged protein outside; \(\mathrm{Na}^{+}\) and \(\mathrm{K}^{+}\) inside d. \(\mathrm{Na}^{+}\) and \(\mathrm{K}^{+}\) outside and water only inside e. chlorine ions \(\left(\mathrm{Cl}^{-}\right)\) on outside and \(\mathrm{K}^{+}\) and \(\mathrm{Na}^{+}\) on inside

Short answer

The correct option is (a): When an axon is not conducting a nerve signal, there are more sodium ions \(\mathrm{(Na^{+})}\) outside and more potassium ions \(\mathrm{(K^{+})}\) inside.

Step-by-step solution

Understanding Resting Membrane Potential

When a neuron is at rest (not conducting a nerve signal), there is a difference in electrical charge across the membrane maintained by a balance between various ions. This is known as the resting membrane potential, which is typically around -70mV (inside being more negative).

Identify Key Ions Involved in Neurons

The main ions that play a role in maintaining the resting membrane potential of a neuron are sodium (\(\mathrm{Na}^{+}\)) and potassium (\(\mathrm{K}^{+}\)) ions. Additionally, negatively charged proteins (proteins with anions) inside the axon and chloride ions (\(\mathrm{Cl}^{-}\)) play a minor role in maintaining resting membrane potential.

Analyze the Role of Sodium and Potassium Ions

When a neuron is at rest, there is a greater concentration of sodium ions outside the neuron compared to the inside, and there is a higher concentration of potassium ions inside the neuron compared to the outside. Sodium-potassium pumps actively maintain this ionic distribution by pumping \(3\mathrm{Na}^{+}\) ions out of the cell and \(2\mathrm{K}^{+}\) ions into the cell with the consumption of one ATP molecule.

Check the Given Options

Now, let's look at each option and see if it describes the correct ionic distribution at resting state: a. more sodium ions (Na^') outside and more potassium ions \(\left(\mathrm{K}^{+}\right)\) inside: This option correctly describes the resting membrane potential of a neuron. b. more \(\mathrm{K}^{+}\) outside and less \(\mathrm{Na}^{+}\) inside: This option is incorrect, as it contradicts the resting state ion distribution. c. charged protein outside; \(\mathrm{Na}^{+}\) and \(\mathrm{K}^{+}\) inside: This option is incorrect; charged proteins are primarily found inside the cell, not outside. d. \(\mathrm{Na}^{+}\) and \(\mathrm{K}^{+}\) outside and water only inside: This option is incorrect, as it does not address the presence of potassium ions inside the cell. e. chlorine ions \(\left(\mathrm{Cl}^{-}\right)\) on the outside and \(\mathrm{K}^{+}\) and \(\mathrm{Na}^{+}\) on the inside: This option is incorrect as it omits the presence of sodium ions outside the cell.

Conclusion

Based on our analysis, option (a) correctly describes the distribution of ions on either side of an axon when it is not conducting a nerve signal: more sodium ions \(\mathrm{(Na^{+})}\) outside and more potassium ions \(\mathrm{(K^{+})}\) inside.

Key concepts

Neuron Ion Distribution

To understand how neurons function, it's essential to start with the concept of ion distribution across the neuronal membrane. This is key not only for maintaining the resting membrane potential but also for the initiation of action potentials, which are the signals neurons use to communicate.

At rest, neurons show a characteristic distribution of ions across their membrane: they have higher concentrations of sodium ions ({Na^+}) outside and higher concentrations of potassium ions ({K^+}) inside their cell body. This asymmetrical distribution is crucial for the electrical charge difference across the membrane, known as the resting membrane potential, which is typically around -70 millivolts (mV) with the inside of the neuron being more negative than the outside.

Why does this matter? Well, the distribution of ions is the foundation upon which nerve signals are built. When a neuron 'fires', the permeability of its membrane to these ions changes, causing a rapid shift in the resting membrane potential—this is the basis of neural communication. The correct answer to our exercise, option (a), encapsulates this fundamental concept.

Sodium-Potassium Pump

The sodium-potassium pump is a spectacular molecular machine responsible for maintaining the delicate balance of sodium and potassium ions in a neuron. It's an active transport mechanism that works against the gradient to keep a higher concentration of {K^+} ions inside and {Na^+} ions outside the neuron.

For each cycle, the pump uses one ATP (adenosine triphosphate) molecule to move 3 sodium ions out and bring 2 potassium ions in. This activity contributes to the negative charge inside the cell due to the unequal exchange of positive charges.

Why Is the Sodium-Potassium Pump Important?

It's not just about balance; the pump helps in restoring the resting membrane potential after a neuron has fired an action potential, essentially 'resetting' the neuron so it can fire again. Moreover, this pump is involved in regulating cell volume and is integral to the operation of other secondary active transport processes. Without the sodium-potassium pump, neurons would not be able to maintain their internal environment, leading to a failure in signal transmission.

Neuroscience Basics

Exploring neuroscience basics can help us appreciate how the brain communicates with the body. Neurons, the building blocks of the nervous system, are specialized cells designed to carry messages through electrical and chemical signals. At the heart of this communication is the concept of resting membrane potential and the dynamic ion distribution across the neuron membrane.

Neuroscience uncovers that the resting state of a neuron is not simply a static condition but a dynamic state of readiness. When a neuron receives a signal strong enough to cause a change in its resting potential, this can trigger an action potential, which races down the neuron's axon to convey information. The resting membrane potential and action potentials are fundamentally electrical phenomena that arise from ion movement and differential permeabilities of the neuronal membrane.

In summary, understanding these basics: the resting membrane potential, the active maintenance by the sodium-potassium pump, and the ionic distribution across the membrane, gives us significant insight into how our nervous system functions. The correct ion distribution described in option (a) of our original exercise is a cornerstone of these neuroscience fundamentals.