ReviewThe role of corneal afferent neurons in regulating tears under normal and dry eye conditions
Section snippets
Overview
The cornea requires constant secretion from multiple glands to provide lubrication, nourishment and a protective barrier to the external environment (Dartt, 2009). Although the secretory fluids from each gland vary with regards to composition, among the common constituents of glandular secretion are water, electrolytes, mucins, and other glycoproteins and proteins. Sensory innervation of the cornea is necessary to detect environmental stressors and, through brainstem circuits, regulate the flow
Cornea primary afferent neurons
The cornea is the most densely innervated tissue in the body and, based on their conduction velocity, is exclusively innervated by A-delta and C primary afferent fibers (Muller et al., 2003, Zander and Weddell, 1951). Afferent nerve bundles innervating the cornea enter the stroma from the periphery, branching to form a midstromal plexus (Marfurt et al., 2010). Continuing to branch, nerves penetrate Bowman's layer to form the subbasal nerve plexus. In order to maintain the transparency of the
Central processing within the spinal trigeminal nucleus
Trigeminal primary afferent neurons innervating the head and face carry sensory information to Vsp, which is divided into three primary subdivisions: subnucleus oralis (Vo), interpolaris (Vi), and caudalis (Vc), from rostral to caudal (Olszewski, 1950). The processing of sensory information in Vsp is unique from that of the spinal cord dorsal horn in that there are multiple representations of the same orofacial region in different subnuclei of Vsp (Bereiter et al., 2000). As might be expected,
Properties of corneal sensory neurons in dry eye
In vivo confocal microscopy in humans has allowed for the examination of the subbasal nerve plexus in dry eye patients (Cruzat et al., 2010). Results have not been entirely consistent with regards to differences in the overall density of the subbasal plexus, which may be the result of the varying causes and stages of dry eye in these studies. However, an increase in subbasal nerve tortuosities and bead-like formations has been consistently described (Benitez-Del-Castillo et al., 2007, Erdelyi
Future directions
At present, the neural regulation of tearing under normal conditions is relatively well characterized. Noxious mechanical, thermal, and chemical stimuli activate corneal primary afferent polymodal receptors and mechanoreceptors, whereas innocuous cooling, such as that which occurs during minute-to-minute evaporation of the tear film, increases corneal cold receptor activity. These primary afferent neurons project to the Vi/Vc transition region in the brainstem, a critical relay in the
Acknowledgments
Funding was provided by the National Eye Institute R01EY021230 to I.D.M. The authors declare no competing financial interests.
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