Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 78
Iris and Pupil
ОглавлениеPupillary functions include regulating light entering the posterior segment of the eye, increasing the depth of focus for near vision, and minimizing optical aberrations by the lens. The iris muscles consist of a constrictor (sphincter) that encircles the pupil and radial dilator muscles to expand the pupil. The sphincter muscle is an annular band of smooth muscles near the pupillary margin of the iris and is derived from neural ectoderm. The dilator muscle, also derived from neural ectoderm, consists of a series of myoepithelial cells that extend from near the pupillary margin to the base of the iris and are contiguous posteriorly with the pigmented epithelium (PE) of the ciliary body. Pupil size varies on the basis of the balance between these two muscle groups. The constrictor muscle, which is the stronger of the two, is innervated by the oculomotor nerve (CN III) and provides primarily parasympathetic control; in contrast, the dilator muscle is innervated primarily by sympathetic nerves. The constrictor muscle causes miosis, and the dilator muscle is responsible for mydriasis. Bright light decreases pupil size. The sympathetic activity in the iridal dilator muscle and ciliary body musculature (discussed later) is mediated by a combination of β‐receptors (β1 and β2) and α‐receptors (α1 and α2). Components of the pupillary light reflex are listed in Table 2.4.
Figure 2.4 Schematic of corneal innervation. The limbal plexus is a ring‐like band of predominantly myelinated fibers in the sclera adjacent to the cornea. From the limbal plexus, nerve fibers enter into the corneal stroma as nerve bundles and lose their myelin as they traverse to the central cornea. The subepithelial plexus is a dense, anastomosing network of thin axons immediately underlying the anterior basement membrane. The subepithelial plexus gives rise to the subbasal plexus, a whorl‐shaped network of axons between the anterior basement membrane and basal epithelium where nerve fibers run horizontally as long parallel nerves, termed leashes. The axons of the subbasal plexus then vertically ascend to terminate in various layers of the epithelium.
Species differences of the α‐ and β‐receptors have been demonstrated among humans, rabbits, nonhuman primates, cats, and dogs, and they are summarized in Table 2.5. These receptors alter the effects of drugs on the eye. For example, feline pupils constrict with the use of timolol, a nonselective β‐adrenergic antagonist, because the feline iris sphincter muscle has primarily β‐adrenergic nerve fiber. Because β‐adrenergic nerve fibers are inhibitory to the sphincter muscle, the miosis in response to topically applied timolol is suspected to be the result of its antagonism of inhibitory input to the sphincter muscle. Most synapses in the ciliary ganglion are involved in relaying impulses that result in accommodation; the remainder are concerned with constriction of the pupil. Endogenous prostaglandin F2α appears to be involved in maintaining muscle tone in the sphincter muscle of the iris. Prostaglandins most likely act directly on these muscles, and they appear to act to a lesser extent on the dilator muscles of the canine iris. Exogenous prostaglandin analogues cause miosis in cats, dogs, and horses, and the receptors have been detected in the iris and ciliary body of several mammals. Iris color, or the amount of melanin, influences the effects of many drugs, as melanin can bind drugs, increasing or prolonging their time to onset and duration.
Table 2.4 Components of the pupillary light reflex.
| Stimulus | Illumination of the retina |
|---|---|
| Receptors | Photoreceptors (rods and cones) |
| Afferent pathway | Optic nerve–optic tract to pretectal area (ipsi‐ and contralateral via posterior commissure) |
| Efferent pathway | Pretectal area to the parasympathetic nucleus of CN III (ipsi‐ and contralateral), and then parasympathetic fibers to ciliary ganglion (via CN III) |
| Postganglionic fibers to the iris | |
| Effector | Sphincter muscle of the iris |
| Response | Miosis (constriction of the pupil both direct and consensual reflex) |
Table 2.5 Adrenergic receptors in the iris and ciliary body.
| Species | Iris sphincter | Iris dilator | Ciliary muscles |
|---|---|---|---|
| Human | α and β equally | Mainly α, very few or no β | Mainly β, very few α |
| Rabbit | Mainly β, few α | Mainly α, few β | Mainly α, few β |
| Monkey | Mainly α, perhaps β | Mainly α, few β | Only β, no α |
| Cat | Mainly β, some α | Mainly α, some β | Mainly β, some α |
| Dog | α and β | α and β | ? |
Pupil shape also varies among species. Vertical pupils are most commonly seen in terrestrial mammals and reptiles that are ambush predators, and are diurnal. Prey species tend to have horizontally elongated pupils. These respective variations are thought to keep images on the vertical and horizontal contours sharp. In domesticated cats, the constricted pupil is a vertical slit, whereas in the larger, wild felines, it is circular. On dilation, the vertical sides of the domestic feline pupil expand to produce a circular pupil. The constrictor muscle fibers are vertically oriented, and therefore contraction leads to a vertically oriented slit pupil.
In young horses, the pupil is more circular than in adults. Under illumination, the ends of the oval pupil of mature horses do not constrict, but the dorsal and ventral borders do. In bright daylight, the superior granula iridica occludes the central papillary opening, resulting in two apertures and assisting with focusing through the creation of Scheiner's disc phenomenon. With very low illumination or administration of a mydriatic, the dorsal and ventral borders of the pupil dilate, thereby forming a circular pupil.
The avian pupil is circular and highly motile. The consensual pupillary reflex is usually absent (because of total decussation of nerve fibers at the optic chiasm), but occasionally a strong beam of light may traverse the posterior ocular layers and the thin medial orbital bones to stimulate the opposite retina. As the constrictor and dilator muscles are mainly striated with varying amounts of nonstriated fibers, the avian pupil is not affected by traditional mydriatic agents, but it can be dilated variably by various neuromuscular‐blocking drugs.
