Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 77

The cornea is an exquisitely sensitive tissue, and this sensitivity provides a critical protective function. Upon stimulation of the cornea, involuntary blinking occurs via intermediate relays from the ophthalmic branch of the trigeminal nerve to orbicularis oculi innervation from the facial nerve – a fundamental reaction termed the corneal or blink reflex. Concomitant with the blink reflex is reflex tearing from parasympathetic innervation to the lacrimal gland. During extreme pain, the corneal reflex is exaggerated, and blepharospasm sometimes occurs such that the eyelids cannot be opened voluntarily. Corneal sensitivity varies by species, region of the cornea, and, in the dog and cat, skull conformation. For example, corneal sensitivity in dogs, as measured by the Cochet–Bonnet esthesiometer and histology of the corneal nerves, was highest, intermediate, and lowest in the dolichocephalic, mesaticephalic, and brachycephalic skull types, respectively. Similarly, the central cornea is less sensitive in brachycephalic cats than domestic shorthair cats. Corneal sensitivity is greatest in the central cornea and lower in the peripheral cornea.

Table 2.3 Elastic moduli of layers of the cornea as determined by atomic force microscopy in rabbits and humans.

Corneal layer	Elastic modulus (kPa)
	Rabbit (Thomasy et al., 2014)	Human (Last et al., 2009, 2012)
Epithelium	0.6 ± 0.3	Not assessed
Anterior basement membrane	4.5 ± 1.2	7.5 ± 4.2
Bowman's layer	Absent	110 ± 13
Stroma	1.1 ± 0.6 (anterior) 0.4 ± 0.2 (posterior)	33 ± 6 (anterior)
Descemet's membrane	12 ± 7.4	50 ± 18
Endothelium	4.1 ± 1.7	Not assessed

The cornea is one of the most richly innervated tissues in the body. Most corneal nerve fibers are sensory in origin and respond to mechanical, chemical, and thermal stimuli via the ophthalmic branch of the trigeminal nerve. However, a small proportion of nerves are sympathetic or parasympathetic in origin and derive from the superior cervical ganglion or ciliary ganglion, respectively. Corneal nerve organization is similar across mammalian species, with only minor interspecies differences. All mammalian corneas contain a dense limbal plexus, multiple radially directed stromal nerve bundles, a dense highly anastomotic subepithelial plexus, and a richly innervated epithelium (Figure 2.4). In the dog, corneal innervation arises from the corneal limbal plexus, which comprises a 0.8–1 mm wide, ring‐like band, surrounding the peripheral cornea.

The majority of sensory fibers that innervate the cornea are activated by a variety of exogenous mechanical, chemical, and thermal stimuli, as well as endogenous factors released by tissue injury, and are thus termed polymodal nociceptors. The remainder of the sensory fibers innervating the cornea comprise mechano‐nociceptors and cold thermal receptors, which are only activated in response to mechanical forces or changes in temperature, respectively. In addition to their contributions to corneal protection via the blink reflex and reflex tearing, corneal nerves maintain corneal epithelial health through the secretion of trophic factors and maintenance of basal tear secretions.