Tue16November0750PM 2

A 22-year-old man is brought into the Emergency department with a severe traumatic brain injury. Prior to securing his airway, you confirm that his pupils are reactive.

Which one of the following options best describes the efferent limb of the pupillary reflex arc?

(Please select 1 option)

Optic nerve fibres from the Edinger-Westphal nuclei Incorrect answer selected

Sympathetic nerve fibres from the superior cervical ganglion

Oculomotor nerve fibres to the pretectal nucleus

Optic nerve fibres to the pretectal nucleus

Oculomotor nerve fibres from the Edinger-Westphal nuclei This is the correct answer

Explanation

The pupillary light reflex is a four neurone arc.

The first neurone is an optic nerve fibre that projects from the retina to the pretectal nucleus in the midbrain at the level of the superior colliculus. Therefore, optic nerve fibres to the pretectal nucleus represents the afferent limb of the pupillary reflex arc.

The second neurone connects the pretectal nucleus to both Edinger-Westphal nuclei, thus explaining why a unilateral light stimulus evokes bilateral pupillary constriction.

The third neurone is a parasympathetic fibre from each Edinger-Westphal nucleus that is conveyed by the oculomotor nerve to the ciliary ganglion in the orbit.

The fourth neurone leaves the ciliary ganglion and innervates the sphincter muscle.

Sympathetic nerves fibres from the superior cervical ganglion mediate pupillary dilatation and are not involved in the light reflex. Oculomotor nerve fibres to the pretectal nucleus and optic nerve fibres from the Edinger-Westphal nuclei do not exist.

Answer Statistics

1

13%

2

7%

3

7%

4

9%

5

67%

Times answered: 248