All environmental chemicals necessary for life enter the body by the nose and mouth. The senses of smell (olfaction) and taste (gustation) monitor those chemicals, determine the flavor and palatability of foods and beverages, and warn of dangerous environmental conditions, including fire, air pollution, leaking natural gas, and bacteria-laden foodstuffs. These senses contribute significantly to quality of life and, when dysfunctional, can have untoward physical and psychological consequences. A basic understanding of these senses in health and disease is critical for the physician, since thousands of patients present to doctors' offices each year with complaints of chemosensory dysfunction. Among the more important developments in neurology has been the discovery that decreased smell function is perhaps the first sign of neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD), signifying their “presymptomatic” phase.
Odorous chemicals enter the nose during inhalation and active sniffing as well as during deglutition. After reaching the highest recesses of the nasal cavity, they dissolve in the olfactory mucus and diffuse or are actively transported to receptors on the cilia of olfactory receptor cells. The cilia, dendrites, cell bodies, and proximal axonal segments of these bipolar cells are situated within a specialized neuroepithelium that covers the cribriform plate, the superior nasal septum, the superior turbinate, and sectors of the middle turbinate (Fig. 29-1). Each of the ˜6 million bipolar receptor cells expresses only one of ˜450 receptor protein types, most of which respond to more than a single chemical. When damaged, the receptor cells can be replaced by stem cells near the basement membrane. Unfortunately, such replacement is often incomplete.
Anatomy of the olfactory neural pathways, showing the distribution of olfactory receptors in the roof of the nasal cavity. [Copyright David Klemm, Faculty and Curriculum Support (FACS), Georgetown University Medical Center; used with permission.]
After coalescing into bundles surrounded by glia-like ensheathing cells (termed fila), the receptor cell axons pass through the cribriform plate to the olfactory bulbs, where they synapse with dendrites of other cell types within the glomeruli (Fig. 29-2). These spherical structures, which make up a distinct layer of the olfactory bulb, are a site of convergence of information, since many more fibers enter than leave them. Receptor cells that express the same type of receptor project to the same glomeruli, effectively making each glomerulus a functional unit. The major projection neurons of the olfactory system—the mitral and tufted cells—send primary dendrites into the glomeruli, connecting not only with the incoming receptor cell axons but with dendrites of periglomerular cells. The activity of the mitral/tufted cells is modulated by the periglomerular cells, secondary dendrites from other mitral/tufted cells, and granule cells, the most numerous cells of the bulb. The latter cells, which are largely GABAergic, receive inputs ...