Unit 3: Pupil Abnormalities, Facial Nerve Palsy & Ptosis

Sphincter Pupillae

• The iris sphincter muscle is innervated by the parasympathetic division of the 3rd cranial nerve.
• Its function is pupil constriction (miosis)
• It synapses in the ciliary ganglion
• The neurotransmitter is acetylcholine

The pupillary light reflex is a four neurone arc:

• 1st neurone passes from the retina to the pretectal nucleus.
• 2nd neurone passes from the pretectal nucleus to the Edinger-Westphal nucleus.
• 3rd neurone passes from the Edinger-Westphal nucleus to the ciliary ganglion via the 3rd cranial nerve.
• 4th neurone passes from the ciliary ganglion to the sphincter muscle via the ciliary nerves.

Dilator Pupillae

The dilator muscle is innervated by the sympathetic nervous system and dilates the pupil (mydriasis).

Pupil mydriasis involves a three neurone arc:

• 1st neurone passes from the posterior hypothalamus to the cilio-spinal centre of Budge at the C8 to T2 spinal cord level.
• 2nd neurone (preganglionic pathway) passes from the spinal cord to the superior cervical ganglion.
• 3rd order neurone (postganglionic pathway) passes from the superior cervical ganglion along the internal carotid artery through the cavernous sinus to the short posterior ciliary nerves and to the dilator muscle.