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Biochemical Cascade of Phototransduction

Overview Phototransduction is the series of biochemical events that lead from photon capture by a photoreceptor cell to its hyperpolarization and slowing of neurotransmitter release at the synapse. This overall process includes an activation phase and a recovery phase. While the fundamental mechanisms of phototransduction seem to be constant over a wide range of light intensities and very similar in rods and cones, the quantitative features…

Outward-Directed Transport

Introduction In ocular therapeutics, the current literature describes drug delivery limitations posed by outward directed transport mechanisms of the eye. Drug delivery to ocular tissues is restricted by various factors such as limited tissue penetration, rapid clearance rate, and dose-limiting toxicity. Though diffusion plays a role, efflux transport proteins constitute a major driving force behind selective removal of therapeutic agents from ocular tissues. Therapeutic agents must…

Sensory Innervation of the Eye

Introduction The sensory innervation of the eye is provided by the peripheral axons of primary sensory neurons located in the trigeminal ganglion. The sensory nerves enter the eyeball mainly through the ciliary nerves and reach all ocular tissues with the exception of the lens and the retina. Ocular innervation is particularly rich in the cornea but all tissues of the anterior segment of the eye have…

Formation and Function of the Tear Film

1 Tear film overview The tear film overlays the ocular surface, which is comprised of the corneal and conjunctival epithelia, and provides the interface between these epithelia and the external environment. The tear film is essential for the health and protection of the ocular surface and for clear vision as the tear film is the first refractive surface of the eye. Tears produced by the ocular…

Functions of the Orbit and Eyelids

Introduction The cranium protects the brain and provides scaffolding for facial structures. During primate evolution, the orbits were enlarged and reoriented towards the front of the face. This, along with the gradual flattening of the face, allowed for improved binocular vision due to overlapping visual fields. Along with morphologic skull changes, facial mimetic musculature evolved in primates as a means of close-proximity non-vocal communication. It is…

The Function of the Retinal Pigment Epithelium

Introduction The retinal pigment epithelium (RPE) is a monolayer of pigmented cells located between the light-sensitive photoreceptor outer segments and the fenestrated endothelium of the choriocapillaris. On both sides, specialized extracellular matrices enable a close interaction of the RPE with its adjacent tissues. On the basolateral side the multilayered Bruch's membrane combines barrier function and selective transport matrix as part of the blood–retina barrier. On the…

Metabolic Interactions between Neurons and Glial Cells

Introduction The vasculature of both the retina and brain can autoregulate, meaning that blood flow is altered in response to neuronal activity. This tight coupling between neuronal activity and blood flow, or neurovascular coupling, was first described in the brain more than a century ago by Roy & Sherrington. Retinal glia are not just passive supportive cells but rather they play an active role in directly…

Production and Flow of Aqueous Humor

In the healthy eye, flow of aqueous humor against resistance generates an intraocular pressure (IOP) of approximately 15 mmHg, necessary for the proper shape and optical properties of the globe. The circulating aqueous nourishes the cornea and lens, structures which must be transparent and therefore devoid of blood vessels as well as the trabecular meshwork (TM). The aqueous also provides a transparent and colorless medium of refractive…

Ocular Circulation

Introduction The ocular circulation is unique and complex due to the presence of two distinct vascular systems, namely the retinal and uveal systems. The part of this circulation, the one that supplies the fundus of the eye, has the useful property that it can be observed using the ophthalmoscope, an optical instrument introduced by Helmholz in the middle of the 19th century. In the early 1960s,…

Neural Control of Eye Movements

Introduction Three fundamental visual sensory-motor tasks The neural control of eye movements is organized to optimize performance of three general perceptual tasks. One task is to resolve the visual field while we move either by translation or rotation through space (self motion). Our body motion causes the image of the visual field to flow across the retina and reflexive eye movements reduce or stabilize this image…

Three-Dimensional Rotations of the Eye

Eye motility In principle the eyeball, like any rigid object, has six degrees of freedom: three for rotation, and three for translation. The adult human eyeball is about 24–25mm in diameter, and can rotate about ±50° horizontally, 42° up and 48° down, and about ±30° torsionally. In contrast, the amount of translation possible for the eye is very limited: over the whole horizontal range the eyeball…

The Extraocular Muscles

The extraocular muscles (EOM) are found within the bony orbit. They function in conjugate eye movements, maintenance of primary gaze position, and motor fusion – maintaining corresponding visual elements within the binocular field on corresponding retinal loci. In addition, the eyes must be able to follow moving objects (smooth pursuit) and accomplish rapid changes in fixation (saccades). This is accomplished by a very complex oculomotor control…

The Vitreous

Introduction The vitreous body makes up approximately 80% of the volume of the eye and thus is the largest single structure of the eye ( Fig. 6.1 ). In the anterior segment of the eye, it is delineated by and adjoins the ciliary body, the zonules, and the lens. In the posterior segment of the eye, the vitreous body is delineated by and adjoins the retina.…

The Lens

The lens is a remarkably specialized epithelial tissue that is responsible for fine-tuning the image that is projected on the retina. To perform this function it must be transparent, have a higher refractive index than the medium in which it is suspended, and have refractive surfaces with the proper curvature. Because the refractive power of the lens is variable, it permits the diopteric apparatus to focus…

Cornea and Sclera

Introduction The outermost, fibrous tunic of the human eye is the cornea and the sclera ( Fig. 4.1A,B ). Both are soft connective tissues designed to provide structural integrity of the globe and to protect the inner components of the eye from physical injury. The clear, transparent cornea ( Fig. 4.1A,C ) covers the anterior 1/6th of the total surface area of the globe, while the…

Accommodation

Introduction “There is no other portion of physiological optics where one finds so many differing and contradictory ideas as concerns the accommodation of the eye where only recently in the most recent time have we actually made observations where previously everything was left to the play of hypotheses” H Von Helmholtz (1909) It is primarily due to Helmholtz that we owe our current understanding of the…

Optical Aberrations and Wavefront Sensing

Introduction Myopia, hyperopia and cylinder are refractive errors known as second-order aberrations. These aberrations result in the inability of the eye to focus images appropriately on the retina. In myopia, light rays entering the eye focus anterior to the retina. This is most often seen in an elongated eye. In contrast, hyperopia occurs in a short eye where light rays tend to focus behind the retina.…

Optics

The young eye Primate and human infants must normally pass head first through their mother's pelvis to accommodate the limited opening determined by the bony configuration. Therefore the size of the mother's pelvis limits the head and brain size of the infant. Specifically, the brain of an infant ape is 55 percent of its full size, and the brain of a present-day human infant is only…

Gestational Trophoblastic Diseases

Clinical Keys for this Chapter ▪ The majority of patients (80-90%) with gestational trophoblastic diseases (GTDs) have a benign course, and their disease spontaneously goes into remission. The benign form of GTD is called hydatidiform mole. ▪ Most patients with hydatidiform moles present with irregular or heavy vaginal bleeding during the first or early second trimester of pregnancy, but they may also present with early-onset toxemia…

Uterine Corpus Cancer

Clinical Keys for This Chapter ▪ There are two different clinicopathologic types of endometrial cancer. Type I endometrial cancers are caused by unopposed estrogenic stimulation, are endometrioid in histologic type, and generally have a good prognosis. Type II endometrial cancers are unrelated to estrogenic stimulation, are often nonendometrioid histologically (serous or clear cell), and have a poor prognosis. ▪ About 5% of endometrial cancers occur in…