Question

A person after sleeping overnight with the arm under his head now experiences paresis but no numbness in the morning. Which of the following is the best explanation for it?

• A. C fibers are more sensitive to pressure than A fibers
• B. A fibers are more sensitive to hypoxia than B fibers
• C. A fibers are more susceptible to pressure changes than C fibers
• D. A fibers are more susceptible to hypoxia than C fibers

Answer 1

The Correct Option is C

The scenario describes a situation where a person experiences paresis (weakness) in the morning after sleeping with an arm under the head, yet does not feel numbness. To understand this, we need to consider the nerve fibers involved and their properties:

• A fibers: These are myelinated fibers responsible for motor functions and normal sensation. They are generally larger and conduct impulses faster than non-myelinated fibers. They are also more susceptible to alterations in pressure.
• C fibers: These fibers are unmyelinated, smaller in size, and conduct impulses more slowly. They are involved in conveying dull, aching pain and temperature sensations. Due to their structure, they are less sensitive to pressure changes compared to A fibers.

When there is pressure on a nerve (such as when sleeping with an arm under the head), the larger, myelinated A fibers are more susceptible to being compressed because their size and structure make them more vulnerable to external forces. This can temporarily disrupt motor pathways, leading to paresis without affecting the sensation of numbness.

Therefore, the correct explanation for the observed condition is: A fibers are more susceptible to pressure changes than C fibers. The pressure preferentially affects myelinated fibers involved in voluntary muscle control and proprioception rather than the C fibers, which carry pain and temperature sensations.