Chapter 33, Problem 4A

Summary Introduction

Introduction :

The nervous system is one of the vital systems of a body. It is a system of nerves that carry information within the body from one place to another. The neuron is the structural and functional unit of the nervous system which carries information from one place to another in the form of nerve impulses.

Answer to Problem 4A

Correct answer :

The correct answer is option (a) K+ ions are entering the neuron.

Explanation of Solution

Explanation/justification for the correct answer :

Option (a) K+ ions are entering the neuron. In the given figure, we can see that there are 3 Na+ ions for 2 K+ ions. Meaning there is more sodium ions outside the cell than inside the cell and counter to that there are more potassium ions inside the cell than outside the cell. We know that ions diffuse from higher concentration to lower concentration. So, the Na⁺-K+ pump actively transports sodium ions into the cell and potassium ions outside the cell. For every two sodium ions pumped out of the cell, two potassium ions are pumped inside the cell, thereby maintaining a net negative charge inside the neuron. So, the correct answer is option (a).

Explanation for incorrect answer :

Option (b) negatively charged proteins are leaving the neuron. The negatively charged proteins are coming towards the neuron. So, this is an incorrect option.

Option (c) Na+ ions are entering the neuron. As discussed above, the Na+ ions exit the neuron. So, this is an incorrect answer.

Option (d) The myelin sheath has broken down, allowing ions to freely cross the plasma membrane. There is no free exchange of ions in the neuron. It is only possible through transport proteins. So, this is an incorrect answer.