Reflex arcs
👩⚕️ Download the pdf below and memorise all the afferents and efferents for the above reflex arcs. Then we zoom in a bit on the direct and consensual pupillary light reflex and the associated Edinger-Westphal nucleus.
Summary table of reflex arcs
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A rhyme to memorise some of them: "S1/S2 – buckle my shoe, L3/L4 – kick the door, C5/C6 – pick up sticks, C7/C8 – shut the gate."
Pupillary light reflex pathway
- Retina detects light
- Optic CN II
- Bilateral pretectal nucleus in midbrain
- Bilateral Edinger-Westphal nuclei
- Parasympathetic fibres of Oculomotor CN III
- Ciliary ganglion
- Short ciliary nerves
- Sphincter pupillae muscle
- 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.
📚 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
- Lateral geniculate body
- Locus coeruleus
- Posterior limb of internal capsule
- 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