Pupillary reflex is elicited by the function of which cranial nerve?

NIMHANS Nursing Officer - 2021

Applied Anatomy

Easy

Appeared in: NIMHANS Nursing Officer - 2021

Explanation

The Oculomotor nerve (CN III) is the efferent (motor) limb of the pupillary light reflex.
It carries parasympathetic nerve fibers that innervate the sphincter pupillae muscle.
Contraction of the sphincter pupillae muscle, triggered by the Oculomotor nerve, causes the pupil to constrict (miosis).
Paralysis or damage to the Oculomotor nerve results in the absence of the pupillary light reflex on the affected side.

Option B: The Optic nerve (CN II) is the afferent (sensory) limb of the reflex. It detects the light stimulus and transmits it to the brain but does not perform the motor action of pupillary constriction.
Option C: The Ophthalmic nerve is a branch of the Trigeminal nerve (CN V). It is the sensory nerve for the corneal reflex (blinking), not the pupillary light reflex.
Option D: The Olfactory nerve (CN I) is responsible for the sense of smell and is not involved in eye function or reflexes.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Cranial nerve function and the pupillary light reflex as background academic context rather than a clinical decision trigger.
Assessing the pupillary light reflex is a fundamental part of a neurological examination. The acronym PERRLA (Pupils Equal, Round, Reactive to Light and Accommodation) is used for documentation.
An abnormal pupillary reflex can indicate serious neurological problems. For example, a fixed and dilated pupil can be a sign of compression of the Oculomotor nerve (CN III), often due to increased intracranial pressure from a head injury or brain herniation.
What if? If a patient has a complete transection of the right Optic nerve (CN II) and a light is shone into the right eye, neither pupil will constrict. However, if a light is shone into the left eye, both pupils will constrict normally because the motor pathway (both Oculomotor nerves) is intact.

How to Approach the Question

First, identify the core concept of the question, which is the 'pupillary reflex' and the 'cranial nerve' responsible for it.
Recall that a reflex arc has a sensory (afferent) component and a motor (efferent) component.
Identify the nerves involved in the pupillary reflex: the Optic nerve (CN II) for sensory input and the Oculomotor nerve (CN III) for motor output.
Analyze the question's wording: 'elicited by the function of'. This can be ambiguous, but it often refers to the nerve that performs the final action. The action of the reflex is constriction, which is a function of the Oculomotor nerve.
Evaluate the options. Both CN II and CN III are involved, but CN III is responsible for the motor response. The other options (Ophthalmic and Olfactory) are involved in different functions or reflexes.
Select the option that represents the motor (efferent) pathway, as this is the nerve that executes the reflex action.

Concept Tested & Keywords

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E6 Anatomy Applied Ashalatha 4e p. 425-427

E6 Physiology Guyton 4SAE Part 3 p. 168-170