Innervation of the cavernous body of the human efferent tear ducts and function in tear outflow mechanism

The lacrimal sac and nasolacrimal duct are surrounded by a wide cavernous s ystem of veins and arteries comparable to a cavernous body. The present stu dy aimed to demonstrate the ultrastructure of the nervous tissue and the lo calisation of neuropeptides involved in the innervation of the cavernous bo dy, a topic not previously investigated. Different S-100 protein antisera, neuronal markers (neuron-specific enolase, anti-200 kDa neurofilament), neu ropeptides (substance P, neuropeptide Y, calcitonin gene-related peptide, v asoactive intestinal polypeptide) and the neuronal enzyme tyrosine hydroxyl ase were used to demonstrate the distribution pattern of the nervous tissue . The ultrastructure of the innervating nerve fibres was also examined by m eans of standard transmission electron microscopy. The cavernous body contained specialised arteries and veins known as barrie r arteries, capacitance veins, and throttle veins. Perivascularly, the tiss ue was rich in myelinated and unmyelinated nerve fibres in a plexus-like ne twork. Small seromucous glands found in the region of the fundus of the lac rimal sac were contacted by nerve fibres forming a plexus around their alve oli. Many nerve fibres were positive for S-100 protein (S 100), neuron-spec ific enolase (NSE), anti-200 kDa neurofilament (RT 97), calcitonin gene-rel ated peptide (CGRP), substance P (SP), tyrosine hydroxylase (TH), and neuro peptide Y (NPY). Vasoactive intestinal polypeptide (VIP) immunoreactivity w as only demonstrated adjacent to the seromucous glands. Both the density of nerve fibres as well as the presence of various neurope ptides emphasises the neural control of the cavernous body of the human eff erent tear ducts. By means of this innervation, the specialised blood vesse ls permit regulation of blood flow by opening and closing the lumen of the lacrimal passage as effected by the engorgement and subsidence of the caver nous body, at the same time regulating tear outflow. Related functions such as a role in the occurrence of epiphora related to emotional responses are relevant. Moreover, malfunction in the innervation of the cavernous body m ay lead to disturbances in the tear outflow cycle, ocular congestion or tot al occlusion of the lacrimal passages.