The pupillary skin reflex describes the loop involved in dilating the pupil when the skin is painfully pinched.
The afferent part of the loop comes from peripheral, cutaneous pain receptors which are thought to be connected to efferent preganglionic neurones in the lateral grey columns of the first and second thoracic segments of the spinal cord. From here, white rami communicantes pass to the sympathetic trunk and thence to preganglionic fibres ascending to the superior cervical sympathetic plexus. The postganglionic fibres pass through the internal carotid plexus and via the long ciliary nerves to the dilator pupillae muscle of the iris.
Ref: Snell, R.S. (1987). Clinical Neuroanatomy for Medical Students. 2nd ed., Little, Brown & Co.,p.402
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