I’m kinda bored, so I decided to post a complete, wiki illustrated walkthrough of reflexive tearing.
rrrrRRRRIIPPPP. In one fell swoop, the waxing lady, who at this moment is the most evil person you know, rips some hair off the edge of your eyebrow. This stimulates the nociceptors branching from your supraorbital and supratrochlear nerves which innervate your lateral and medial eyebrow, respectively.
Dermatomes innervated by supraorbital and supratrochlear nerves (In green):
The pain stimulus is conducted by the supratrochlear and supraorbital nerves to the frontal nerve, which is located above your eye, superior to the levator palpebrae which lifts your eyelid. The frontal nerve then merges into the opthalmic branch of the trigeminal nerve, which in turn merges with the other two branches of the trigeminal nerve to form the trigeminal, or semilunar, ganglion.
Superior view of the supratrochlear and supraorbital nerves merging into the frontal and opthalmic nerves:
Pain information then travels through the sensory root of the trigeminal nerve, and enters the brainstem in the middle of the pons.
Lateral view of trigeminal nerve entering pons:
Cross section of pons showing trigeminal nerve entering:
From here, sensory information is divided based on what type of nerve fiber recorded it. Fine touch, vibration, and proprioception information are interpreted by the chief trigeminal sensory nucleus which is located in the pons. Pain, which is what we feel when our eyebrows are plucked, is interpreted by the spinal trigeminal nucleus which is a large structure which spans from the pons to the medulla.
Saggital ghost view of trigeminal nerve nuclei (in blue). Big blue oval is chief sensory nucleus. Long blue line below it is the spinal trigeminal nucleus:
Cross section of medulla showing spinal trigeminal nucleus near the right side:
Pain information from the spinal trigeminal nucleus then travels to the superior salivatory nucleus in the pons. The superior salivatory nucleus is responsible for the parasympathetic activity of the facial nerve. This is the nucleus that lets us cry, secrete nasal mucus, and is partially responsible for salivation, say when we see a nice big juicy steak. Parasympathetic fibers from the superior salivatory nucleus exit the brainstem at the border between the pons and medulla in the facial nerve.
Same image as before, but look for the superior salivatory nucleus. See how close it is to the spinal trigeminal nucleus, and how it exits the brainstem with the facial nerve:
The facial nerve then travels with the vestibulocochlear nerve (it lets you hear and balance) briefly and then branches off to enter the geniculate ganglion. Here the nerves controlling your tear reflex split from the nerves controlling the muscles of your face in the greater petrosal nerve.
Facial nerve travelling to geniculate ganglion, giving off greater petrosal nerve:
The greater petrosal nerve then carries parasympathetic information to the pterygopalatine, or sphenopalatine ganglion where it synapses.
Greater petrosal nerve leading to sphenopalatine ganglion:
Here is where the nerves leading to the nasal mucosa and the lacrimal gland split. Post synaptic parasympathetic fibers heading for the lacrimal gland hitch a ride on the maxillary nerve, which branches off to the zygomatic nerve, which gives a contribution to the lacrimal nerve, which FINALLY innervates the lacrimal gland, making you cry.
Maxillary nerve giving off zygomatic nerve which contributes to lacrimal nerve:
So the next time you get your eyebrows waxed, and the waxing lady asks you why you are crying, you can tell her that =P.