Introduction: Overview of the human eye, mammalian retina, and the retinoid visual cycle

Abstract

The human eye is a part of the sensory nervous system and is the organ responsible for conscious light perception and vision. Its intricate and complex anatomy has evolved to effectively focus incoming light from the surrounding environment and to harness its energy by efficiently utilizing the physicochemical properties of retinoids. This remarkable organ is capable of phototransduction, which involves the conversion of photon energy into an integrated neural signal that propagates through the optic nerve as an action potential to the visual cortex of the brain. There, the neural signal is processed to give rise to color differentiation, brightness perception, contrast, and depth perception. Loss of visual acuity or blindness presents a significant negative impact on quality of life. Irreversible blindness affects nearly 60 million individuals worldwide, with the leading causes including age-related macular degeneration, glaucoma, and retinal vascular disease. In this chapter, we will provide basic background information of the human eye and retina required for readers unfamiliar with the field of ophthalmic drug discovery. The purpose of this chapter is to help facilitate a thorough understanding of the subsequent chapters of this volume that focus on key ophthalmic diseases, recent developments in drug discovery efforts toward treating these diseases, and the challenges faced in the delivery of drugs to their ocular targets. Thus, this chapter will provide a general overview that begins with the gross anatomical features of the human eye followed by a description of the histology of the mammalian retina. We will next provide morphological and functional details concerning the various cell types that comprise the many layers of the retina, and we will conclude by describing the series of reactions that constitute the visual retinoid cycle and the various key proteins that facilitate it.