Question

The axonal membrane is ___ to negatively charged proteins present in the axoplasm:-
A. Selectively permeable
B. Permeable
C. Semipermeable
D. Impermeable

Answer 1

Hint: Neurons are excitable cells because their membranes are in a polarized state. Different types of ion channels are present on the nerve membrane.
These ion channels are selectively permeable to different ions.
When a neuron is not conducting any impulse, i.e., resting, the axonal membrane is comparatively more permeable to potassium ions ($K^+$) and nearly impermeable to sodium ions ($Na^+$).
Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm.
Consequently, the axoplasm inside the axon contains a high concentration of K+ and negatively charged proteins and a low concentration of $Na^+$.

Complete answer: The fluid outside the axon contains a low concentration of $K^+$, a high concentration of $Na^+$, and thus forms a concentration gradient.
These Neurons are excitable cells because their membranes are in a polarized state.
Different types of ion channels are present on the nerve membrane.
These ion channels are selectively permeable to different ions.

Additional information: When a neuron is not conducting any impulse, i.e., resting, the axonal membrane is comparatively more permeable to potassium ions (K+) and nearly impermeable to sodium ions (Na+).
1. Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm.
2. Consequently, the axoplasm inside the axon contains a high concentration of K+ and negatively charged proteins and a low concentration of $Na^+$.
3. These ionic gradients across the resting membrane are maintained by the active transport of ions by the sodium-potassium pump which transports 3 Na+ outwards for 2 K+ into the cell.
4. As a result, the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged and therefore is polarised.

So the correct answer is D. Impermeable

Note: The electrical potential difference across the resting plasma membrane is called the resting potential.
At a chemical synapse, the membranes of the pre- and postsynaptic neurons are separated by a fluid-filled space called the synaptic cleft.
The rise in the stimulus-induced permeability to Na+ is extremely short lived.
It is quickly followed by a rise in permeability to K+.
Within a fraction of a second, K+ diffuses outside the membrane and restores the resting potential of the membrane at the site of excitation and the fiber becomes once more responsive to further stimulation.