**Question:** Pupillary reaction to light is still normal in lesion/injury to

A. Dorsal horn
B. Edinger-Westphal nucleus
C. Hypothalamus
D. Oculomotor nucleus

**Correct Answer:** D. Oculomotor nucleus

**Core Concept:** The pupillary light reflex (PLR) is a crucial part of the autonomic nervous system's response to light, involving the following structures:

1. Light-sensitive cells in the retina (photoreceptors) convert light into electrical signals.
2. The electrical signals travel through the optic nerve to the brain.
3. The optic chiasm occurs where the axons from the retina cross over to the opposite side of the brain.
4. The axons then pass through the optic tracts and reach the brainstem, where they synapse in the pretectal area.
5. The pretectal area sends signals to the Edinger-Westphal nucleus (which controls the parasympathetic pupillary response) and the Oculomotor nucleus (which controls the sympathetic pupillary response).
6. The Oculomotor nucleus, located in the midbrain, sends signals to the iris sphincter and dilator muscles, causing constriction or dilation of the pupil as needed.

**Why the Correct Answer is Right:**

The correct answer, Oculomotor nucleus (D), is involved in the sympathetic pupillary response, which is responsible for the dilation of the pupils in response to light. A lesion/injury to the Oculomotor nucleus would not impair the pupillary light reflex, as it is involved in the dilation response.

**Why Other Options are Incorrect:**

A. Dorsal horn (A) and spinal cord (B) are part of the somatic nervous system, not involved in pupillary reflex. Lesions or injuries to these structures would disrupt motor function or sensation, not pupillary response.

C. Hypothalamus (C) plays a role in the regulation of vital body functions, not specifically the pupillary reflex. Lesions or injuries to the hypothalamus would affect the autonomic response to light indirectly, not directly impair the pupillary reflex.

**Clinical Pearl:**

The pupillary light reflex is a vital part of the autonomic nervous system's response to light. In clinical practice, understanding the anatomy and physiology of the reflex can help differentiate between various neurological disorders affecting pupillary function, such as:

1. Horner's syndrome: a condition caused by a lesion or injury to the sympathetic chain, including the hypothalamus, posterior hypothalamus, and the posterior pituitary gland. In this case, pupillary dilation would be absent due to the impaired sympathetic innervation.
2. Mydriasis: an excessive dilation of the pupils, usually caused by medications like atropine, scopolamine, or certain medications affecting the parasympathetic system. In this case, the dilation response is preserved, making Oculomotor nucleus (D) the correct answer.

3. Pupil miosis: a constricted pupil due to lesions