Essentials of Veterinary Ophthalmology. Kirk N. Gelatt
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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.
Nutrition of Intraocular Tissues
While allowing light transmission through the eye, nutrients are provided and waste is removed by two systems of blood vessels (i.e., retinal vessels and uveal vessels), the formation and egress of AH, and the vitreous body. Intraocular tissues lack a typical lymphatic system, and the uveal tract (i.e., iris, ciliary body, and choroid) provides this function.
Ocular Circulation
The choroid, ciliary body, and iris are supplied primarily by the uveal vessels. The outer retina in some animals (e.g., dogs, cats, ruminants, and pigs) and almost all or the entire retina in others (e.g., horses and guinea pigs) is nourished by diffusion from the choroidal vessels. The inner retina is supplied by retinal vessels in many species. Blood vessels supplying the cornea and lens in the embryo regress before birth or shortly thereafter, leaving the AH as the primary source of nutrients for the cornea and lens.
Birds have a unique structure, the pecten oculi, which is a heavily pigmented, highly vascularized, and usually fanlike structure projecting from the surface of the optic nerve into the vitreous. A similar structure occurs in reptiles, termed the conus papillaris. The avian pecten likely functions as an important source of nutrients for the inner retina. This assumption is based on the observations that the avian retina is thicker than oxygen could perfuse from the choroid and that the pectinate artery resistive and pulsatility indices are low. Several marine mammals, the bottlenose dolphin (Tursiops truncatus), spotted seal (Phoca largha), and California sea lion (Zalophus californianus), have an ophthalmic rete from which the retinal and choroidal arteries are derived.
Ocular Blood Flow
The vascular pressure promoting flow, the resistance of blood vessels, and the viscosity of the blood all influence the blood flow through all tissues, including the eye. The pressure head for blood flow (i.e., perfusion pressure) in most tissues is the difference in pressure between the arteries and the veins. However, in the eye, the IOP approximates the venous pressure, so the perfusion pressure is the difference in pressure between the small arteries entering the eye and the IOP. Of clinical importance is that the perfusion pressure to the eye is reduced by lowering the blood pressure or raising the IOP, as occurs in glaucoma. Studies of hemodynamics in the rabbit ophthalmic artery demonstrate that autoregulation maintains normal blood velocity and resistance when the IOP is below 40 mmHg. However, at higher pressures the autoregulatory capacity is limited. As a result, an IOP of about 15–17 mmHg is related to episcleral venous pressure of about 10–12 mmHg and about 5–7 mmHg of resistance from passage through the AH pathways!
Anterior Uveal Blood Flow
In most species, the major arterial circle of the iris is formed by the nasal and temporal long posterior ciliary arteries. The iris and ciliary body receive approximately 1% and 10%, respectively, of the total ocular blood flow. In humans and rabbits, additional iridal blood flow occurs from the anterior ciliary arteries via the extraocular muscles (EOMs). Blood flow to the ciliary body in most species that have been studied is provided by the iridal major arterial circle, branches of the anterior ciliary arteries, and branches of the long posterior ciliary arteries. The cat and monkey iris and ciliary body have autoregulation of their blood flow. Carbon dioxide dilates the anterior uveal vessels, and sympathetic α‐adrenergic receptors cause vasoconstriction in the anterior uvea. Parasympathetic muscarinic receptors and prostaglandins, however, cause vasodilation. Prostaglandins E1 and F2α appear to cause a two‐ to threefold increase in blood flow to the anterior uvea when applied topically.
Choroidal Blood Flow
The outer