Organs of sense are distributed throughout the body in epithelium, connective tissue, muscle, and tendon. These specialized receptors which receive stimuli from the external environment and/or the body, are composed of nerve endings or specialized cells on which the nerve endings terminate. They are classified as to type, i.e. thermoreceptors (temperature change); mechanoreceptors (pressure of touch); and chemoreceptors (chemical changes). Morphologically, receptors are grouped as having free or naked nerve endings or encapsulated nerve endings.
Some receptors have been or will be described in laboratory's on other tissues. In this laboratory, we will examine examples of encapsulated nerve endings (Pacinian and Meissner's Corpuscles), an epithelium modified for sensory reception (Olfactory Epithelium), and an organ of special sense (the Eye).
Pacinian corpuscles are mechanoreceptors (pressure) found in the dermis or subcutaneous layer of the fingers, breasts, external genitalia, bladder and joint capsules (Pacinian 1). In longitudinal section, the corpuscle is ovid in shape and contains a central nerve fiber. The nerve fiber loses its myelin sheath as it enters the corpuscle and branches into one or more "club-like" nerve endings. The nerve is covered by concentric layers (20-60) of flattened modified schwann cells. The entire corpuscle is surrounded by a connective tissue capsule (Pacinian 2).
The Meissner's Corpuscle is much smaller and located close to the surface of the skin, within the dermal papillae (Meissner 1). These structures a predominate on the palms of hands and soles of feet and are mechanoreceptors which respond to "light" touch. Meissner's Corpuscles consist of an ovoid structure within which the nerve ending branches and orients parallel to the surface of the skin. Between nerve endings are elongated Schwann cells, in a similar orientation, with the entire structure surrounded by a thin connective tissue capsule (Meissner 2).
Two olfactory regions are found in each nasal cavity lining the roof and sides. The structure represents a "mucus membrane", consisting of a pseudostratified epithelium underlaid by connective tissue, the lamina propria (Olfactory membrane 1). Within the lamina propria a numerous serous glands, the Glands of Bowman, with ducts which penetrate the epithelium to bathe the surface in secretions to put the "smells" in solution to stimulate the olfactory nerve cells (Olfactory membrane 2). The olfactory epithelium or mucosae is a tall columnar pseudostratified epithelium, containing three different cell types: (1) basal cells (stem cells), (2) sustentacular cells (supporting); and (3) olfactory receptor cells (bipolar nerve cells). At the surface of the epithelium cilia and the large olfactory bulbs of the sensory cells can be seen (Olfactory membrane 3). Within the lamina propria, small bundles of neurons are present which penetrate the membrane to form the bipolar cell layer (Olfactory membrane 4).
The eye is a photosensitive organ composed of three layers (Eye 1): (1) an outer layer of supportive dense connective tissue; the sclera, and the front covering, the cornea; (2) a middle layer, the choroid, containing most of the vasculature and musculature of the eye, including the iris and ciliary body and processes; and (3) an inner layers of the pigmented and sensory retina.
The Cornea forms the transparent covering over the lens and iris at the front. of the eye. It consists of 5 layers different layers: (1) an outer layer of stratified squamous non-keratinzed epithelium, (2) an underlying basement membrane (Bowman's Membrane), (3) the substancia propria composed of 25 parallel rows of collagen and fibroblasts organized in a helical fashion, (4) a basement membrane (Descement's Membrane) and (5) an endothelial layer (Cornea)
The Ciliary Body and Processes are found at the end of the choroid layer at the front of the eye. The ciliary body is comprised of smooth muscles oriented to pull the lens 3 directions (Ciliary Body). Budding off the ciliary body behind lens are the ciliary processes. These are villus like projections covered with both pigmented & non-pigmented epithelium (Ciliary Process). The non-pigment epithelium functions to produce the aqueous humor which fills the anterior and posterior chambers of eye.
The Iris is a pigmented disk with variable aperture which forms the pupil of the eye. It is covered by squamous epithelium on anterior surface and 2 layers of pigmented epithelium on posterior. The size of the pupil is controlled by smooth muscle and myoepithelial cells within the iris (Iris).
The Lens is made of epithelial cells which have differentiated into lens fibers. These cells will lose all their organelles and become filled with mostly crystalized lens protein. The lens is surrounded by a lens capsule consisting of a thick basement membrane and endothelial layer (Lens).
The Retina lines the interior of the posterior region of the eye, lying on the choroid and sclera layers. It is composed of a pigmented and a sensory or neural layer (Retina 1). The pigmented retina forms the outer layer and is composed of a layer of pigmented cuboidal cells. Pigment is also taken up by cells within the choroid beneath the pigmented retina. The neural or sensory retina abuts the pigmented layer and is composed of three layers of cells: (1) immediately above the pigmented epithelium are the photoreceptors, the rods and cones, (2) many rods & cones synapse with fewer bipolar cells forming the second layer, and (3) many bipolar cells synapse with fewer ganglion cells, forming the third or innermost layer (Retina 2). The axons of the ganglion cells bundle together to form the optic nerve. The rods and cones are distinguished by the characteristic shape of their outer segments (Retina 3).
