Reflex Arcs

Summary table of reflex arcs

Pupillary light reflex pathway

1. Retina detects light
2. Optic CN II
3. Bilateral pretectal nucleus in midbrain
4. Bilateral Edinger-Westphal nuclei
5. Parasympathetic fibres of Oculomotor CN III
6. Ciliary ganglion
7. Short ciliary nerves
8. Sphincter pupillae muscle
9. Direct and consensual pupillary constriction

Edinger-Westphal nucleus

• is located in the midbrain
• provides preganglionic parasympathetic fibres that travel with the oculomotor CN III
• to innervate, via the ciliary ganglion, the:
  • pupillary sphincter to enable pupillary constriction
  • ciliary muscles for lens accommodation. 

Edinger-Westphal nucleus ⏩ Oculomotor CN III ⏩ Ciliary ganglion ⏩ Short ciliary nerves ⏩ Pupillary sphincter and ciliary muscles

An Edinger-Westphal nucleus lesion causes an abnormally dilated pupil, and abolishes both direct and consensual light reflexes on the side of the lesion. 

Lacrimal system

• Lacrimation reflex pathway:

Conjunctiva irritation → Lacrimal n. → Ophthalmic n. (V₁) → Trigeminal sensory centre → Superior salivary nucleus → Facial CN VII → Greater petrosal n. → Pterygopalatine ganglion → Joins zygomatic branch of trigeminal’s maxillary CN V₂ → Joins the lacrimal branch of V₁ → Lacrimal apparatus

• Arterial supply to the lacrimal system is via a lacrimal branch of the ophthalmic artery of the internal carotid artery. 
• Venous drainage is to the superior ophthalmic vein. 

📚 Exam-style questions

• A patient presents with asymmetrical pupils and abnormal light reflex. Where is the lesion most likely?
  • [[Edinger-Westphal nucleus::Midbrain nucleus for parasympathetic output for pupil constriction]]
  • ((Hippocampus::Memory processing))
  • ((Lateral geniculate body::Visual relay))
  • ((Locus coeruleus::Noradrenergic, regulates arousal))
  • ((Posterior limb of internal capsule::Motor fibres))

• What is the afferent limb of the gag reflex?
  • [[Glossopharyngeal::In the gag reflex: Glossopharyngeal CN IX is the afferent limb; vagus CN X is the efferent limb]]
  • Vagus
  • Facial
  • Temporal

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