Iris, ciliary muscle and superior tarsal muscle
Target & receptor · α1 dilator & tarsal M3 sphincter & ciliary
Two antagonistic inputs: sympathetic α1 fibres contract dilator pupillae and the superior tarsal muscle; parasympathetic M3 fibres contract sphincter pupillae and the ciliary muscle. Pupil size in dark versus light tells you which side has failed.
Traced from the start
- Sympathetic (thoracolumbar) outflow
- Intermediolateral cell column T1–L2 via white rami communicantes
- Superior cervical ganglion
- Parasympathetic (craniosacral) outflow
- Edinger–Westphal nucleus (oculomotor, CN III)
- Ciliary ganglion
- Internal and external carotid plexuses (oculosympathetic pathway)Short ciliary nerves
- Iris, ciliary muscle and superior tarsal muscle
Detail
- Sympathetic (α1)
- Dilator pupillae → mydriasis; superior tarsal (Müller) muscle → 1–2 mm lid elevation; inferior tarsal muscle → loss gives apparent enophthalmos
- Parasympathetic (M3)
- Sphincter pupillae → miosis; ciliary muscle → accommodation for near
- Reading anisocoria
- Difference greater in the dark = sympathetic failure (Horner); greater in bright light = parasympathetic failure (III palsy, Adie, pharmacological)
When it goes wrong
Any interruption of the three-neurone oculosympathetic pathway
Horner syndrome: miosis with dilation lag, partial ptosis of 1–2 mm (and slight lower lid elevation), anhidrosis only if the lesion is central or preganglionic; congenital cases have heterochromia
Mydriasis from dim light, stress or antimuscarinic drugs in an eye with a shallow anterior chamber
Acute angle-closure glaucoma: painful red eye, fixed mid-dilated pupil, haloes and vomiting — pilocarpine (M3) and acetazolamide, then laser iridotomy
Practise this structure
1 question in the bank tagged Autonomic.