Article

The structure of corpuscular nerve endings in the limbal conjunctiva of the human eye. J Anat

Vision Research Centre, Department of Optometry and Visual Science, London.
Journal of Anatomy (Impact Factor: 2.23). 09/1991; 177:75-84.
Source: PubMed

ABSTRACT Corpuscular nerve endings were found to be numerous within a narrow, 1.00 mm wide, annular zone of limbal conjunctiva, located approximately 0.5 mm from the corneoscleral margin. A light and electron microscopic study was carried out on these nerve endings found within samples of human eye-bank material. Corpuscular endings were found immediately under the epithelium, often within the stromal elevations which make up the limbal palisades of Vogt. They were round to oval in shape, and varied in size, with a mean maximum diameter of 30 microns. The afferent nerve fibre lost its myelin sheath soon after entry, and subsequently branched to give rise to a variable number of axon terminal varicosities, which were characterised by an accumulation of mitochondria. Neural elements within the nerve ending were invested by the cytoplasmic lamellae of Schwann-like accessory cells. The corpuscle was demarcated from the surrounding connective tissue by a delicate fibrocyte capsule. The corpuscular nerve endings described here in the conjunctiva share features common to corpuscles found in other mucosae. The function of such complex sensory nerve endings is as yet unknown, but the possibility that they represent receptors for particular sensory modalities should be explored.

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    • "The limbus is the transitional tissue from the cornea to the conjunctiva and sclera, approximately 1.5 mm wide in adult human eyes, that has rich structural variation, with organizational alteration—from scleral irregular opacity to corneal regularity and transparency [1,2], vascular variation within the conjunctiva, sclera and peripheral cornea [3], and neural passage and termination [4,5]. The limbus is also the common site for the occurrence of early corneal neovascularization, phlyctenulosis and other limbal follicles, lupus vulgaris and corneal epithelium neoplasia [6]. "
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    ABSTRACT: The limbus is the structurally rich transitional region of tissue between the cornea on one side, and the sclera and conjunctiva on the other. This zone, among other things, contains nerves passing to the cornea, blood and lymph vasculature for oxygen and nutrient delivery and for waste, CO(2) removal and drainage of the aqueous humour. In addition, the limbus contains stem cells responsible for the existence and healing of the corneal epithelium. Here we present 3D images of the healthy human limbus, acquired in vivo with a spectral domain optical coherence tomography system operating at 1060nm. Cross-sectional and volumetric images were acquired from temporal and nasal locations in the human limbus with ~3µm x 18µm (axial x lateral) resolution in biological tissue at the rate of 92,000 A-scans/s. The imaging enabled detailed mapping of the corneo-scleral tissue morphology, and visualization of structural details such as the Vogt palisades, the blood and lymph vasculature including the Schlemm's canal and the trabecular meshwork, as well as corneal nerve fiber bundles. Non-invasive, volumetric, high resolution imaging reveals fine details of the normal human limbal structure, and promises to provide invaluable information about its changes in health and disease as well as during and after corneal surgery.
    Biomedical Optics Express 07/2011; 2(7):1794-02. DOI:10.1364/BOE.2.001794 · 3.50 Impact Factor
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    • "Cold receptors have small receptive fields in the cornea and in the perilimbal area, where they are abundant and exhibit even smaller, spot like receptive fields (for review see Belmonte and Gallar, 1996). A small number of thick, fast-conducting nerve fibres also innervate the perilimbal episclera possibly with morphological specializations around their nerve endings (Krauselike corpuscles) (Lawrenson and Ruskell, 1991). These fibres respond to gentle mechanical stimulation and presumably contribute to non-noxious touch sensations evoked by gentle mechanical stimulation of the ocular surface, as those produced by blinking (Gallar, 1991). "
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    ABSTRACT: A renewed interest in the characteristics and neural basis of corneal and conjunctival sensations is developing in recent years due to the high incidence of discomfort and altered sensitivity of the cornea following refractive surgery, use of contact lenses and dry eyes. Corneal nerves are functionally heterogeneous: about 20% respond exclusively to noxious mechanical forces (mechano-nociceptors); 70% are additionally excited by extreme temperatures, exogenous irritant chemicals and endogenous inflammatory mediators (polymodal nociceptors), and 10% are cold-sensitive and increase their discharge with moderate cooling of the cornea (cold receptors). Each of these types of sensory fibres contributes distinctly to corneal sensations. Mechano-nociceptors mediate, sharp acute pain produced by touching of the cornea. Polymodal nociceptors elicit the sustained irritation and pain that accompany corneal wounding; cold receptors evoke cooling sensations. Depending on the relative activation by the stimulus of each subpopulation of corneal sensory fibres, different subqualities of irritation and pain sensations are evoked. Corneal sensations can be explored experimentally in humans with a gas esthesiometer that applies controlled mechanical, chemical and thermal stimuli to the corneal surface. When the cornea is wounded, corneal nerves are excited and eventually severed in a variable degree and local inflammation is produced. Activated corneal nerves release neuropeptides (SP, CGRP) that contribute to the inflammatory reaction (neurogenic inflammation). They also become sensitized by local inflammatory mediators, such as prostaglandins or bradykinin and thus exhibit spontaneous activity, lowered threshold and enhanced responses to new stimuli. This leads to spontaneous pain and hyperalgesia. Nerves destroyed by injury soon start to regenerate and form microneuromas that exhibit abnormal responsiveness and spontaneous discharges, due to an altered expression of ion channel proteins in the soma and in regenerating nerve terminals. Presumably, this altered excitability is the origin of the lowered sensitivity and the spontaneous pain, dry eye sensations and other disaesthesias reported in patients following refractive surgery.
    Experimental Eye Research 04/2004; 78(3):513-25. DOI:10.1016/j.exer.2003.09.023 · 3.02 Impact Factor
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    • "Cold receptors have small receptive fields in the cornea and in the perilimbal area, where they are abundant and exhibit even smaller, spot like receptive fields (for review see Belmonte and Gallar, 1996). A small number of thick, fast-conducting nerve fibres also innervate the perilimbal episclera possibly with morphological specializations around their nerve endings (Krauselike corpuscles) (Lawrenson and Ruskell, 1991). These fibres respond to gentle mechanical stimulation and presumably contribute to non-noxious touch sensations evoked by gentle mechanical stimulation of the ocular surface, as those produced by blinking (Gallar, 1991). "
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    ABSTRACT: Ocular irritation and pain are associated with many clinical situations (e.g. accidental injury, eye diseases, surgery and contact lens wearing). Pain and related ocular sensations begin with stimulation by injurious stimuli of first-order sensory neurons of the trigeminal ganglion. Neurons responding solely to application in their receptive field of noxious mechanical forces (mechanonociceptive neurons), or of irritant chemicals and heat (polymodal nociceptive neurons), have been identified electrophysiologically in the conjunctiva, cornea, sclera, iris, ciliary body and choroid. The cornea is additionally innervated by neurons responding to low temperatures, which may account for corneal discomfort caused by cold. Also, low-threshold mechanoreceptive and cold-sensitive neurons supply the conjunctiva and sclera, possibly mediating touch and thermal sensations aroused by innocuous stimuli in the front of the eye. Ocular sensory information is transmitted from the trigeminal ganglion to specific higher-order neurons located in the trigeminal brainstem nuclear complex, the thalamus and the cerebral cortex. Local ocular inflammatory responses enhance injury-induced neural activity both in ocular nociceptive terminals and in higher order neurons. In addition to signalling acute lesions, ocular primary sensory neurons participate in post-injury processes, contributing to local inflammatory reactions (neurogenic inflammation) and to the repair of damaged tissues. These effects are mediated at least in part, by substance P and CGRP, two neuropeptides contained in ocular sensory nerve cells that are released peripherally upon tissue damage. Ocular tissues have a trophic interdependence with their sensory neurons. Ocular tissues are the source of neurotrophic factors that are critical for the early development and survival of trigeminal sensory neurons. On the other hand, the morphofunctional integrity of some ocular tissues like the cornea, appears to be dependent on the presence of an intact sensory innervation. Stimulation of ocular sensory pathways by noxious mechanical, chemical and thermal stimulation of cornea, conjunctiva or of other eye structures, evokes distinct types of ocular sensations. Differences in the quality of pain sensation presumably result from the magnitudes of activation of the various sub-populations of ocular nociceptive neurons by different stimulus modalities. In addition to conscious sensations, injurious stimuli evoke protective reflexes (blinking and lacrimation) aimed at protecting the eye and minimizing further ocular damage by noxious stimuli.
    Progress in Retinal and Eye Research 01/1997; 16(1):117–156. DOI:10.1016/S1350-9462(96)00027-4 · 9.90 Impact Factor
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