The Pupillary pathway

The pupillary pathway is made of the afferent pathway (from eye to brain) and the efferent pathway (from brain to eye).

The afferent pathway begins from the Retinal photoreceptors to the Optic nerve and hemidecussate at the optic chiasm, which then leads to the optic tract. It leaves the optic tract just after the LGN (lateral geniculate nucleus) and then proceeds to the Pretectal nucleus which is located in the mid-brain, where a second decussation occurs, and then finally reach the ipsilateral and contralateral Edinger-Westphal group (EWN) of the oculomotor (CN3) nucleus.

Each optic tract carries pupillary fibres from both eyes. This is because of the hemideccusation that takes place at both the chiasm and midbrain.

When the afferent pathway is stimulated, either both eye equally dilate (sympathetic) or constrict (parasympathetic).

The efferent pathway consists of two types- sympathetic and parasympathetic.

For the efferent sympathetic pathway, stimulation is mainly from higher cortical levels, which sends impulses such as fear, pain, anxiety and emotions to the hypothalamus. These cause pupillary dilation (mydriasis).

The efferent parasympathetic pathway starts from the EWN (edinger westphal nucleus) to the Primary fibres that descend from the brain stem and leads to the synapse at the ciliospinal centre of budge ( T1-T3), and then to the secondary neurones leaving the spinal cord. It then sends the impulse through the sympathetic paraspinal chain which leads to the Superior cervical ganglion and, finally, to the postganglionic neurone travelling on the surface of the carotid artery.

At the bifurcation of the internal and external carotid arteries, it may:
1. leave to follow the external carotid artery, resulting in facial sweating
2. join CN6, enter the orbit through CN5, whereby the fibres pass through the long ciliary nerves (branch of CN5) to trigger the Dilator pupillae (pupil dilates).

In general, the efferent pathway consists of two different routes:
a) Parasympathetic- Pupil constricts
b) Sympathetic- Pupil dilates