Dilator pupillae is supplied by:
**Core Concept**
The dilator pupillae muscle is a smooth muscle that controls the diameter of the pupil. It is innervated by the sympathetic nervous system, which allows for parasympathetic-sympathetic interactions in the autonomic nervous system.
**Why the Correct Answer is Right**
The dilator pupillae muscle is supplied by the sympathetic nerves that originate from the superior cervical ganglion. These nerves release norepinephrine, which acts on the alpha-adrenergic receptors on the dilator pupillae muscle cells. This causes the muscle to contract, resulting in dilation of the pupil. The parasympathetic nerves, on the other hand, supply the sphincter pupillae muscle, which constricts the pupil.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the dilator pupillae muscle is not supplied by the parasympathetic nerves, which primarily control the sphincter pupillae muscle.
* **Option B:** This option is incorrect because the sympathetic nerves supply the dilator pupillae muscle, but the preganglionic fibers originate from the spinal cord, not the brainstem.
* **Option D:** This option is incorrect because the parasympathetic nerves supply the sphincter pupillae muscle, which constricts the pupil, whereas the dilator pupillae muscle dilates the pupil.
**Clinical Pearl / High-Yield Fact**
The autonomic nervous system controls the diameter of the pupil through a balance between sympathetic and parasympathetic innervation. The sympathetic nerves dilate the pupil, while the parasympathetic nerves constrict it.
**Correct Answer: C. The sympathetic nerves originating from the superior cervical ganglion.**