Question

Axoplasm have (polarized) (a) High \(\mathrm{K}^{+}\) ion (b) Low \(\mathrm{Na}^{+}\) ion (c) Negatively charged proteins (d) All of these

Short answer

The correct answer is (d) All of these. In a polarized axoplasm, the concentration of \(\mathrm{K}^{+}\) ions is high, \(\mathrm{Na}^{+}\) ions is low, and it contains negatively charged proteins.

Step-by-step solution

Understanding the Components of Axoplasm

Axoplasm, the cytoplasm of a nerve axon, contains various ions and proteins. In the polarized (resting) state, the inside of the neuron is negative compared to the outside. This state is maintained by the uneven distribution of different ions across the neuron's membrane.

Identifying High \(\mathrm{K}^{+}\) ion

In the polarized state, the axoplasm has high potassium ion (\(\mathrm{K}^{+}\)) concentration inside the neuron. There is a much higher concentration of \(\mathrm{K}^{+}\) ions inside the neuron than outside.

Identifying Low \(\mathrm{Na}^{+}\) ion

In the polarized state, the axoplasm has a low sodium ion (\(\mathrm{Na}^{+}\)) concentration inside the neuron. The concentration of \(\mathrm{Na}^{+}\) ions is lower inside the neuron than outside.

Identifying Negatively Charged Proteins

Negatively charged proteins are also present in the axoplasm. These proteins contribute to the negative charge inside the neuron in the polarized state.

Key concepts

Polarized State

In the polarized state, a neuron's axoplasm remains at rest, meaning there's an electrical difference between the inside and outside of the neuron. This difference, also known as resting membrane potential, is crucial for nerve transmission. Essentially, during polarization, the inside of the neuron is more negatively charged compared to the outside. This charge differential is essential for conducting electrical impulses along neurons.
The polarization of neurons is maintained by the movement of ions across the axon's membrane. Channels and pumps regulate the movement of these ions, ensuring the correct balance between positive and negative charges, which is vital for neuron function. For example, the sodium-potassium pump actively transports ions to preserve this polarized state.

Potassium Ions

Within the axoplasm, potassium ions ( K^{+} ) play a key role in maintaining polarization in neurons. These ions are present in high concentrations inside the neuron. This high concentration results from selective permeability, where neuron membranes are much more permeable to K^{+} ions than to others like sodium ( Na^{+} ) ions.
The high concentration of K^{+} inside the neuron is essential for maintaining the resting potential. Even though some potassium ions naturally diffuse out of the neuron due to concentration gradients, the continued action of the sodium-potassium pump ensures replenishment, sustaining the polarized state.
In essence, the movement and presence of K^{+} ions in large numbers inside the neuron is critical for preparing the neuron for activation and the transmission of signals.

Sodium Ions

Sodium ions ( Na^{+} ) are present in lower concentrations inside the axoplasm during the polarized state. The neuronal membrane is less permeable to Na^{+} ions, which helps maintain this low internal concentration. However, the concentration of Na^{+} ions outside the neuron remains high.
The sodium-potassium pump plays a significant role here by actively transporting Na^{+} ions out of the neuron, keeping their internal levels low. This differential distribution is vital for maintaining the negative internal charge within the axoplasm.
When a nerve impulse is initiated, Na^{+} channels open briefly, allowing these ions to flow into the neuron, changing the membrane potential and creating an action potential that travels along the nerve.

Negatively Charged Proteins

Negatively charged proteins are another crucial factor contributing to the neuron's polarized state. These proteins are predominantly large anions that cannot cross the neuronal membrane, keeping the interior of the neuron negatively charged.
These proteins, combined with the selective permeability to ions, contribute to the resting membrane potential. They form part of the fixed negative charge of the neurons, essential for maintaining a ready state for nerve impulses.
Without these negatively charged proteins, achieving and maintaining the correct charge balance required for nerve signal transmission would be challenging. Hence, they are an integral component of the neuron’s intricate electrochemical system, working alongside ions to maintain the neuron’s potential and functional capacity.