In a 67-year-old patient with equal pupils that constrict briskly to light and demonstrate an appropriate near response, which cranial nerve carries the afferent limb of the pupillary light reflex?

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The Afferent Limb of the Pupillary Light Reflex

The correct answer is A. CN II (the optic nerve) carries the afferent limb of the pupillary light reflex 1.

Neuroanatomical Pathway

The pupillary light reflex follows a well-defined pathway where light detection and signal transmission occur through distinct cranial nerves:

Afferent Pathway (CN II)

  • Light enters the eye and stimulates photoreceptors in the retina
  • The optic nerve (CN II) transmits this sensory information from the retina through the optic chiasm and optic tract to the pretectal nuclei in the midbrain 1
  • This represents the sensory/afferent limb of the reflex arc
  • An afferent pupillary defect indicates pathology in the optic nerve or anterior visual pathway 1

Efferent Pathway (CN III)

  • From the pretectal nuclei, signals project bilaterally to the Edinger-Westphal nuclei
  • The oculomotor nerve (CN III) carries parasympathetic fibers that cause pupillary constriction
  • This represents the motor/efferent limb of the reflex

Clinical Application

When examining this 67-year-old patient with equal, briskly reactive pupils:

  • Normal bilateral pupillary constriction confirms intact afferent pathways (both optic nerves) and efferent pathways (both oculomotor nerves)
  • The swinging-light test specifically assesses for relative afferent pupillary defects by comparing the afferent input from each optic nerve 1
  • An abnormal response (pupillary dilation when light is shined on the affected eye) indicates unilateral optic nerve or anterior visual pathway pathology 1

Why Other Options Are Incorrect

  • CN III (oculomotor): Carries the efferent/motor limb, not the afferent limb
  • CN IV (trochlear): Controls the superior oblique muscle; no role in pupillary reflexes
  • CN VI (abducens): Controls the lateral rectus muscle; no role in pupillary reflexes

The distinction between afferent (sensory input via CN II) and efferent (motor output via CN III) pathways is fundamental to localizing pupillary abnormalities in clinical practice 2, 3, 4.

References

3
Research

Disorders of the pupil.

Handbook of clinical neurology, 2011

4
Research

The Pupil.

Continuum (Minneapolis, Minn.), 2019

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