75% contribution to parasympathetic system is by which cranial nerve –
**Core Concept**
The parasympathetic nervous system is a branch of the autonomic nervous system that promotes rest and digestion, and is responsible for various involuntary functions such as heart rate, digestion, and pupillary constriction. It has a significant contribution from cranial nerves, which play a crucial role in its functioning.
**Why the Correct Answer is Right**
The cranial nerve predominantly responsible for the parasympathetic system is the **Oculomotor nerve (CN III)**. This nerve carries parasympathetic fibers from the **Edinger-Westphal nucleus**, which stimulate the **ciliary ganglion** to cause **pupillary constriction**. In addition, the **Vagus nerve (CN X)** also makes a significant contribution to the parasympathetic system, but it is not the primary contributor. The **Facial nerve (CN VII)**, **Glossopharyngeal nerve (CN IX)**, and **Splanchnic nerves** also contribute to the parasympathetic system, but to a lesser extent.
**Why Each Wrong Option is Incorrect**
* **Option A:** Not specified - This option is vague and does not provide any relevant information about the parasympathetic system.
* **Option B:** Incorrect - While the Vagus nerve (CN X) does contribute to the parasympathetic system, it is not the primary contributor.
* **Option C:** Incorrect - The Facial nerve (CN VII) and Glossopharyngeal nerve (CN IX) also contribute to the parasympathetic system, but to a lesser extent.
* **Option D:** Incorrect - The Splanchnic nerves do contribute to the parasympathetic system, but they are not the primary contributor.
**Clinical Pearl / High-Yield Fact**
The Edinger-Westphal nucleus is a critical structure that controls pupillary constriction and accommodation in the eye. Damage to this nucleus can result in a loss of parasympathetic function, leading to pupillary dilation and loss of accommodation.
**Correct Answer:** C. Facial nerve (CN VII) is the largest contributor to the parasympathetic nervous system, with approximately 75% of its fibers contributing to this system.