Pupillary Disorders

ANATOMY/PHYSIOLOGY

Ciliary muscle

Pupil

Cornea

Ciliary muscle

Pupil

Cornea

The iris controls the size of the pupil. It contains two groups of smooth muscle fibre:

1. Sphincter pupillae', a circular constrictor, innervated by the parasympathetic nervous system.

Dilator pupillae; a radial dilator, innervated by the sympathetic nervous system.

Pupillary size (normal 2-6 mm) depends on the balance between sympathetic and The ciliary muscle, innervated parasympathetic tone, by the parasympathetic, controls the degree of convexity of the lens through the ciliary zonule.

Pathway of pupillary constriction and the light reflex (parasympathetic)

Light source

Short ciliary nerve

Ciliary ganglion

Light source

Short ciliary nerve

Ciliary ganglion

Lateral geniculate body

Superior colliculus

III nerve '

- Midbrain

III nerve '

Edinger-

Westphal nucleus

Lateral geniculate body

Posterior commissure -

- Midbrain

A stimulus, such as a bright light shone in the left eye, will send an afferent impulse along the optic nerve to the midbrain (superior colliculus); here a second order fibre passes to the Edinger Westphal nucleus (part of the III nerve nucleus) on the same and opposite side (through the posterior commissure). Efferent fibres leave in the oculomotor nerve, pass to the ciliary ganglion and thence, in the short ciliary nerve, to the constrictor fibres of the sphincter pupillae muscle.

If all pathways are intact, shining a light in one eye will constrict both pupils at an equal rate and to a similar degree.

Superior colliculus

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