Question

Resting axonal membrane is (a) Unpolarized (b) Unpolarized and more permeable to \(\mathrm{K}^{+}\) (c) Polarized and more permeable to \(\mathrm{Na}^{+}\) (d) Polarized and more permeable to \(\mathrm{K}^{+}\)

Short answer

The resting axonal membrane is polarized and more permeable to \(\mathrm{K}^{+}\), so the correct answer is (d).

Step-by-step solution

Understanding the Concept of Polarization

Polarization refers to the state of the membrane where there is a difference in electrical charge between inside and outside the membrane, which is maintained by the sodium-potassium pump.

Understanding the Concept of Permeability

Permeability refers to the property of the cell membrane which allows the certain molecules to pass through. Both sodium and potassium ions can pass through the axonal membrane, but the permeability differs based on the state of the membrane.

Observing the Resting State

During the resting state or the rest potential of axonal membrane, it remains polarized, meaning it has a charge difference across the membrane. Moreover, the membrane exhibits more permeability to \(\mathrm{K}^{+}\) ions than to \(\mathrm{Na}^{+}\) ions, as the potassium channels remain open while the sodium channels are mostly closed.

Key concepts

Membrane Polarization

In the context of the axonal membrane, polarization is crucial for nerve function. Polarization means that there's a difference in electrical charge across the membrane. The inside of the axon has a negative charge compared to the outside.
This is essential for signals to travel along nerves because it sets the stage for action potentials, which are the electricity-like signals that travel through neurons.
These differences in charges, known as membrane potential, are maintained by the sodium-potassium pump and various ion channels. By keeping a differential distribution of ions such as sodium (\(\text{Na}^+\)) and potassium (\(\text{K}^+\)), the membrane stays polarized.

• The inside of the membrane holds more potassium ions and proteins that carry a negative charge.
• The outside is richer in sodium ions, making it more positive.

The polarized state means the axonal membrane is ready to send an impulse at a moment's notice, which is critical for the rapid communication required within our nervous system.

Membrane Permeability

Membrane permeability refers to the ability of ions to cross the nerve cell membrane.
In the resting state, the axonal membrane shows selective permeability, which means it permits certain ions to pass through more easily than others.
This selectivity is vital for maintaining the resting potential and initiating action potentials when needed.
In particular, during the resting state:

• The membrane is more permeable to potassium ions (\(\text{K}^+\)). Potassium ions move easily across the membrane, largely through potassium channels, because it is crucial for maintaining the potential difference across the membrane.
• The membrane is less permeable to sodium ions (\(\text{Na}^+\)), as most sodium channels are closed during the resting state.

This set-up lets the axon rapidly switch from resting to active states by altering the permeability in response to signals, thereby allowing the neuron to convey information efficiently.

Sodium-Potassium Pump

The sodium-potassium pump is an essential cellular mechanism that maintains the polarization of the axonal membrane.
This pump is a type of active transport, meaning it requires energy to function, specifically ATP, the molecule cells use for energy.
The pump works by moving ions against their concentration gradients, which helps maintain a stable resting membrane potential.

• It actively transports three sodium ions (\(\text{Na}^+\)) out of the axon for every two potassium ions (\(\text{K}^+\)) it brings in. This activity ensures that the inside of the nerve cell remains negatively charged relative to the outside.
• The sodium-potassium pump is crucial because without it, the concentration gradients of these ions would disappear, and the nerve cell would be unable to generate action potentials.

By continuously working in the background, the pump stabilizes the internal environment of the nerve cell, enabling it to quickly respond to and recover from changes in membrane potential, such as those caused by nerve signals.