Which of the following is the cranial nerve responsible for papillary constriction?

JSSH Staff Nurse - 2019

Applied Physiology

Easy

Appeared in: JSSH Staff Nurse - 2019

Explanation

The Oculomotor nerve (CN III) is responsible for pupillary constriction (miosis) as it carries parasympathetic motor fibers.
These fibers originate from a specific nucleus in the midbrain called the Edinger-Westphal nucleus.
The nerve fibers travel to the ciliary ganglion in the orbit, and from there, postganglionic fibers innervate the sphincter pupillae muscle of the iris.
Contraction of the sphincter pupillae muscle causes the pupil to narrow, which is the efferent (motor) part of the pupillary light reflex.

Option A: The Olfactory nerve (CN I) is a purely sensory nerve responsible for the sense of smell (olfaction) and has no motor function related to the eye.
Option B: The Optic nerve (CN II) is the sensory nerve for vision. It forms the afferent (sensory) limb of the pupillary light reflex by detecting light, but it does not execute the motor action of constriction.
Option D: The Vagus nerve (CN X) provides parasympathetic innervation to organs in the thorax and abdomen (e.g., heart, lungs, digestive tract), not the eye.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Function of Cranial Nerves (Pupillary Reflex) as background academic context rather than a clinical decision trigger.
Assessing the pupillary light reflex is a fundamental part of a neurological examination. It helps evaluate the integrity of the brainstem, specifically the midbrain where the nuclei for CN II and CN III are located.
A non-reactive, fixed, and dilated pupil can be a sign of compression or damage to the oculomotor nerve, often caused by increased intracranial pressure (ICP), head trauma, or an aneurysm. This is considered a neurological emergency requiring immediate intervention.
What if? - If a patient has a complete lesion of the right oculomotor nerve (CN III), shining a light in either eye will cause the left pupil to constrict, but the right pupil will remain fixed and dilated because the motor pathway to the right eye's sphincter muscle is broken.

How to Approach the Question

First, identify the key action in the question: 'papillary constriction'. This is a motor action (muscle contraction).
Next, analyze the options provided, which are all cranial nerves. Recall the primary function of each nerve.
Differentiate between the sensory (afferent) and motor (efferent) components of a reflex. The question asks for the nerve responsible for the constriction, implying the motor output.
Eliminate the Olfactory nerve (CN I) as it's for smell, and the Vagus nerve (CN X) as it primarily serves the trunk.
Consider the remaining two nerves related to the eye. The Optic nerve (CN II) is for vision (sensory input), while the Oculomotor nerve (CN III) controls most eye muscles and pupillary function (motor output).
Conclude that the Oculomotor nerve (CN III) is the correct choice as it provides the motor pathway for pupillary constriction.

Concept Tested & Keywords

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E6 Anatomy Applied Ashalatha 4e pp. 425-427, 426-428

E6 Anatomy Grays 43e Vol 1 Part 3 p. 99-101