Pupillary Constriction Pathway
Overview
Pupillary constriction involves an afferent limb taking sensory information from the eye to the midbrain, and two efferent limbs from the midbrain to each eye. It is controlled by the parasympathetic nervous system.
The afferent limb
The afferent limb takes sensory information from the eye to the midbrain. There are two different afferent limbs for each eye. The afferent limb is made up of the retina, optic nerve and pretectal nucleus in the midbrain on the same side.
The efferent limb
The efferent limb is the result of the sensory information input. There are two efferent limbs, one to each eye. Each pretectal nucleus has two motor outputs, with one output going to the Edinger-Westphal nucleus on one side, and one output going to the Edinger-Westphal nucleus on the other side.
From each Edinger-Westphal nucleus, preganglionic parasympathetic nerve fibres leave with the oculomotor nerve and synapse in the ciliary ganglia. Short ciliary nerves innervate the iris sphincter and muscles of accommodation.
Pupillary Dilation Pathway
Overview
Pupillary dilation is controlled by the sympathetic nervous system and is efferent only. The pathway starts in the cerebral cortex which controls constriction. This control is intensified during concentration and arousal and lost during sleep. During sleep, the pupils are partially constricted but still react to light.
The pathway for pupillary dilation begins in the hypothalamus which sends fibres that synapse at the ciliospinal centre at C8-T1. Postsynaptic neurones travel over the lung apices and move upwards to the superior cervical ganglia. They pass through the cavernous sinus and the superior orbital fissure to the ciliary muscle.
