Histology Atlas

Histology Atlas

Introduction

Human beings - indeed, all animals - must interact favorably with the environment so that they can survive and reproduce. They must be aware of their surroundings so they can react to circumstances to better their chances for survival. Awareness has been set at a very high priority by natural selection. Consequently, during the course of many millions of years, special cells that are capable of detecting various classes of environmental stimuli have evolved.

Sensory transduction

Sensory transduction is the process by which an environmental stimulus is detected, received, and transduced into bioelectric signals that are carried to the brain in the form of nerve impulses. In the eye, the retina contains the photosensitive cells, called rods and cones, that respond to light. Plate 20-1 shows how the retina is incorporated into the structure of the eyeball; Plates 20-2 and 20-3 illustrate the cellular elements of the monkey retina that receive incoming rays of light, transduce their energy into neuronal excitation, and transmit nerve impulses to the brain along the optic nerve.

In the ear, the cochlea, a tiny spiral of bone and soft tissue, contains exquisitely sensitive mechanoreceptors, the hair cells,that respond to vibrational stimuli. Plate 20-4 presents a matched pair of light and electron micrographs that illustrate the organ of Corti - the part of the inner ear that contains the hair cells - in its normal position within the cochlea.

In the nose, the olfactory epithelium contains chemoreceptors - the olfactory receptors, a series of bipolar neurons that are excited by chemical stimulation. Plates 20-5 and 20-6 illustrate the ultrastructure of human olfactory epithelium and its sensory receptors. In the oral cavity, the tongue contains several types of projections, the papillae, that bear taste buds. Each taste bud is sensitive to sweet, sour, salty, or bitter stimuli. Plate 20-6 presents a low-magnification electron image of a single human taste bud from a circumvallate papilla. Many other types of sensory receptors exist in addition to those mentioned above. In this chapter we focus on the sensory regions of the eye, ear, nose, and tongue - areas of tissue that will be of greatest interest to students of microanatomy.

The Eye: Sclera, Choroid, And Retina

In order to understand the microanatomy of the retina and its photoreceptors, it is important to see how the retina is incorporated into the structure of the eye itself. This structure is clearly shown in the plate at right, a light micrograph of a cross section through the posterior wall of the eyeball of a large monkey, the pig-tailed macaque. The retina of the macaque is quite similar to that of man, and the specimen at right was preserved unusually well by intravascular perfusion.

In this photomicrograph, the outside of the eyeball is at the top of the figure, and the inside of the eye, filled with vitreous humor (V), is at the bottom. Working from the outside in, the outermost part of the posterior wall of the eyeball is protected by pads of ...