**Question:** 1:1 ratio of myelination is seen in

A. Motor nerves
B. Sensory nerves
C. Spinal cord
D. Brain nerves

**Core Concept:** Myelination is the process by which the myelin sheath, a fatty substance produced by oligodendrocytes, surrounds and insulates nerve fibers to enhance their conduction velocity and overall function. In the central nervous system (CNS), myelination is mainly observed in white matter, while in the peripheral nervous system (PNS), it occurs in both red and white matter.

**Why the Correct Answer is Right:** The correct answer, D. Brain nerves, refers to the myelination in the CNS, where oligodendrocytes produce myelin to wrap around axons. In the CNS, the myelin sheath is in the form of compact myelin, resulting in a 1:1 ratio of axon to myelin.

**Why Each Wrong Option is Incorrect:**

A. Motor nerves (Option A) are a part of the PNS, where myelination is not observed in the same 1:1 ratio as seen in CNS.
B. Sensory nerves (Option B) are also a part of the PNS and do not exhibit the 1:1 ratio of myelination in the CNS.
C. Spinal cord (Option C) is a part of the PNS, and while myelination occurs, it doesn't follow the exact 1:1 ratio observed in the CNS.

**Clinical Pearl:** The 1:1 ratio of myelination in brain nerves is essential to understand the structure and function of the CNS, distinguishing it from the PNS. This ratio helps in recognizing the differences between CNS and PNS myelination during examination or clinical practice.