Secondary Sensory Neurons Olfactory Bulb and Projections

The olfactory bulbs are complex ovoid structures located on the ventral surface of each frontal lobe dorsal to the cribriform plate. 'id Each bulb is composed of a number of types of cells, including neurons, afferent and efferent nerve fibers, microglia, astrocytes, and blood vessels, all surrounded by a thin layer of pia-arachnoid cells.'ii] The cellular elements of the bulb are arranged in six concentric layers: the olfactory nerve layer (ONL), the glomerular layer (GL), the external plexiform layer (EPL), the mitral cell layer (MCL), the internal plexiform layer (IPL), and the granule cell layer (GCL). The GCL is the largest of these layers, making up about half the volume of the entire bulb. The anterior olfactory nucleus (AON) lies within the

Figure 7-1 (Figure Not Available) Low-power electron micrograph (*1000) of a longitudinal section through a biopsy of human olfactory mucosa taken from the nasal septum. Four cell types are indicated: ciliated olfactory receptor cells (c), microvillar cells (m), supporting cells (s), and basal cells (b). The arrows point to ciliated olfactory knobs of bipolar receptor cells; d indicates degenerating cells; bs, the base of supporting cells; lp the lamina propria; n, a nerve bundle; and bg, a BowmanfFrom Moran DT, Rowley JC III, Jafek BW et al: The fine structure of the olfactory mucosa in man. J Neurocytol 1982; 11:721-746.)

core of the olfactory bulb and extends through the olfactory tract to the rostral part of the olfactory peduncle near the anterior perforated substance. This structure, which is considered to be an element of the primary olfactory cortex, contains pyramidal cells whose dendrites receive synapses from mitral and tufted cells as well as from the contralateral AON (via the anterior commissure 'AC]) and numerous central brain structures. The axons of its pyramidal cells project to ipsilateral bulb neurons (mainly granule cells) as well as to the contralateral AON, bulb, or rostral olfactory cortex via the AC. Although termed a nucleus, the AON has a cortical structure with subdivisions based on cellular architecture and connectional patterns. y

The first synapse of a CN I bipolar neuron occurs in the olfactory bulb's glomerulus, a defining feature of both vertebrate and invertebrate olfactory systems (.Fig 7-2

). The olfactory bulb of nonelderly humans has thousands of these 50- to 200-pm structures, arranged in single or double layers within the GL. With age, the number and integrity of the glomeruli greatly decrease, so that they are nearly absent in persons over the age of 80 years. y The development and maintenance of the glomeruli depend on trophic influences exerted by the receptor cells. Synapses between axons of the olfactory receptor cells and dendrites of second-order neurons occur within these structures. A given receptor cell projects to only one glomerulus, and a given glomerulus appears to receive most of its input from a restricted region of the epithelium.

The main afferent second-order neurons are termed mitral and tufted cells (see Fig 7.-2 ). The apical dendrites of these cells are influenced not only by the olfactory nerve terminals, but also by interneurons and centrifugal fibers, most of which are GABAergic or dopaminergic. y

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