The olfactory tract, which contains both afferent and efferent fibers, is relatively flat posteriorly and becomes the olfactory trigone just rostral to the anterior perforated substance (so named because of the many small holes for blood vessels found throughout this region). At the edges of the trigone, the tract divides into the medial and lateral olfactory striae.
The axons of the mitral and tufted cells arise from the caudolateral part of the olfactory bulb and form the olfactory tract at the surface of the olfactory peduncle. According to Price,y the consensus of studies performed during the last two decades is that there is no medial olfactory tract in mammals. Thus, all mitral and tufted cell axons leave the olfactory bulb via the lateral olfactory tract to synapse on structures collectively termed the primary olfactory cortex. These structures are (1) the anterior olfactory nucleus, (2) the olfactory tubercle (poorly developed in humans), (3) the piriform cortex, (4) the anterior cortical nucleus of the amygdala, (5) the periamygdaloid complex, and (6) the rostral entorhinal cortex. y The components of the olfactory cortex have rich (and reciprocal) relations with a number of higher brain structures. For example, the entorhinal cortex supplies afferent fibers along the entire length of the hippocampus, including the dentate gyrus. Indeed, because of these connections, the olfactory system has the most direct access of all sensory systems to the hippocampus. y It is generally believed that the olfactory system is unique among sensory systems in sending fibers directly to cortical regions without synapsing in the thalamus. Extensive interactions occur between the cells comprising the superficial and deeper laminae of each component of the olfactory cortex as well as among the components themselves. y
In humans, it appears that most of the projections of the mitral and tufted cells extend to the more rostral elements of the primary olfactory cortex. It is presently believed that specific sites within the bulb do not preferentially communicate with specific sites within the primary olfactory cortex; i.e., no obvious point-to-point topography exists.y Thus, small areas of the bulb can project to large areas of the olfactory cortex and vice versa. y Among the brain regions that exhibit reciprocal connections with the olfactory cortex are the orbitofrontal cortex, the dorsomedial and submedial thalamic nuclei, the lateral hypothalamus,
Figure 7-2 Diagram of major layers and types of olfactory bulb neurons in the mammalian olfactory bulb, as based upon Golgi stained material. Main layers are indicated on left as follows: ONL, olfactory nerve layer; GL, glomerular layer; EPL, external plexiform layer; MBL, mitral body layer; IPL, internal plexiform layer; GRL, granule cell layer; ON, olfactory nerves; PGb, periglomerular cells with biglomerular dendrites; PGm, periglomerular cell with monoglomerular dendrites; SAe, short-axon cell with extraglomerular dendrites; M, mitral cell; M/Td, displaced mitral or deep tufted cell; Tm, middle tufted cell; Ts, superficial tufted cell; Gm, granule cell with cell body in mitral cell layer; Gd, granule cell with cell body in deep layers; SAc, short-axon cell of Cajal; SAg, short-axon cell of Golgi; C, centrifugal fibers; AON, fibers from the anterior olfactory nucleus; AC, fibers from anterior commissure; LOT, lateral olfactory tr(Fcom Shepherd GM: Synaptic organization of the mammalian olfactory bulb. Physiol Rev 1972; 52:864-917. Used with permission.)
the amygdala, and the hippocampus.y A major pathway from the olfactory cortex to the orbitofrontal cortex is via the mediodorsal nucleus of the thalamus. Neurochemical Olfactory Transduction Processes
Peripheral olfactory transduction occurs in several stages: first, odorants move from the air phase of the nasal cavity into the aqueous phase of the olfactory mucus; second, odorants (most of which are hydrophobic) diffuse or are transported through this aqueous phase to the proteinaceous olfactory receptors; third, odorants bind to the receptors; fourth, action potentials are generated within the receptor neurons as a result of such binding; and fifth, integration of information occurs at higher levels, such as within the glomeruli of the olfactory bulb. Although some odorants stimulate nerve endings from CN V and perhaps other cranial nerves distributed in the nasal mucosa, nasal pharynx, or oral cavity, these involve primarily somatosensory sensations of the "common chemical sense," such as warmth or coolness, pungency, and irritation.
There is evidence that small water-soluble proteins, termed odorant-binding proteins, assist the movement of some hydrophobic lipid-soluble molecules through the mucus to the receptor proteins of the olfactory cilia. y Such assistance may be selective, and at least some of these proteins may serve to inactivate odorant molecules or filter the number of such molecules reaching the receptors. For example, some odor-induced signals appear to be rapidly abolished by detoxification or biotransformation enzymes within the mucus.y
After reaching the cilia, odorants bind to receptor sites located on their surface. Different models of ligand-receptor interactions have been proposed, although little is known about the specific nature of the odorant-receptor interactions. There is evidence, however, that G-proteins play an important role in olfactory transduction. G-proteins have been identified in olfactory receptor cells, y and adenylate cyclase, an enzyme that is usually coupled to a G-protein, is highly active in olfactory cilia.y The activity of this enzyme in cilia is increased in the presence of guanosine triphosphate (GTP) by a number of odorant ligands. y Interestingly, a positive correlation exists between an odorant's ability to activate adenylate cyclase activity in a frog ciliary preparation and both its perceived odor intensity to humans and the magnitude of the electro-olfactogram (EOG), a summated neural response produced in frog epithelia. This implies that a functional relation exists between the amount of adenylate cyclase activated and the intensity of odor perception. y
Recent studies indicate that there is marked genetic diversity in olfactory receptors. Buck and Axel, y in a pioneering study, identified a large multigene family that appears to code for odorant receptor proteins with seven transmembrane domains on rat olfactory sensory neurons. Subsequent studies have identified homologous gene families in a range of vertebrate forms, including humans. y As noted in a recent review by Sullivan and colleagues, y current estimates place the size of the olfactory receptor gene family in mammals at 500 to 1000 genes. This suggests
that the basis of the ability of mammals to smell thousands of odorants derives from a wide range of ligand specificities.
It has long been known, mainly from single cell electrophysiological recordings, that individual olfactory receptor cells respond to a wide variety of odorants, leading earlier workers to label olfactory receptor cells "generalists." However, on closer scrutiny, it has become evident that such cells do not respond exactly to the same sets of stimuli, which allows for the possibility of cross-neuron coding. Recent research, incorporating gene probes from in situ hybridization studies, has provided three new pieces of information about the distribution of receptors on olfactory receptor cells and the relationship of such cells to higher-order structures. y First, most rodent olfactory receptor genes are expressed in only about 0.1 percent of the population of olfactory sensory neurons, in accordance with the hypothesis that each sensory neuron expresses only one or at most a few receptor genes. Second, neurons expressing the same gene do not clump together within the olfactory epithelium but seem to be randomly distributed within the epithelium in "spatial zones," across which different sets of olfactory receptor genes are expressed. y For example, in the mouse four such zones, formed as a series of strips extending along the anterior-posterior axis of the nasal cavity, have been identified. Neurons that recognize the same odors (i.e., express the same olfactory receptor genes) are more or less confined to the same zone. Third, axons of neurons that express the same odorant receptor tend to converge on a small number of glomeruli within the olfactory bulb, suggesting that each glomerulus may be dedicated to only one or a very small number of receptor types, receiving input from only receptor cells expressing that receptor type. As noted by Sullivan and colleagues, y "a stereotyped and highly organized map of sensory information exists in the bulb, in which information provided by different ORs (odorant receptors) is mapped into discrete sites. As a single odorant can activate many glomeruli and a single glomerulus responds to many odorants, this map may well be a map of individual structural determinants, or epitopes, each shared by many odorants and recognized by different ORs."
A number of odorants produce a dose-related increase in intracellular cyclical adenosine 3 ,5
-monophosphate (cAMP) in olfactory receptor cells, y thereby triggering the opening of cAMP-gated cation channels. y There is a suggestion that a seemingly small set of other odorants increases the formation of intraciliary inositol-1,4,5-triphosphate (IP 3 ), which in turn directly gates plasma membrane Ca2+ channels that produce cell depolarization. y Stimuli with similar odor qualities, however, do not all activate the same second messenger but induce the formation of either cAMP or IP 3 .y
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