The Olfactory Cortex

Mitral cells and tufted cells send central processes within the lateral olfactory tract to the primary olfactory cortex located on the inferior surface of the temporal lobe. The neurotransmitters utilized in this pathway appear to be excitatory neuropeptides and perhaps amino acids such as glutamate and aspartate. Mitral cells release cholecystokinin; tufted cells release corticotropin-releasing hormone. This relatively simple sensory pathway is unusual in that it is the only sensory system without a major synaptic relay in the thalamus before projecting to cortical regions.

The primary olfactory cortex occupies the superficial cortical layers of the inferior aspect of the temporal lobe (Fig. 4). It overlies the hippocampalformation, with which it is functionally associated, particularly in generating olfactory memory. More anteriorly, the periamygdaloid region of the olfactory cortex is connected to the amygdala and adjacent structures of the limbic system that provide emotional context to odor recognition. The association

FIGURE 3 Neuronal circuits of the olfactory bulb. Odorant-specific subtypes of olfactory neurons (on) are scattered randomly throughout patches of the olfactory epithelium. Four examples are illustrated in (A) through (D). As their axons project into the olfactory bulb, they become rearranged when they contact the appropriate odorant-specific glomeruli. Four glomeruli are shown, each receiving approximately 25,000 primary axons (three are shown). Two types of second-order neurons, mitral cell (mc) and tufted cell (tc), have their apical dendrites in each glomerulus. Basal dendrites of these cells are located in deeper layers of the olfactory bulb (external plexiform layer), and their axons project to the brain in the olfactory tract. As axons exit the bulb, they send collaterals back to the bulb. The two classes of interneurons, both inhibitory, are shown in black. The periglomerular cell (pgc) of the external plexiform layer forms reciprocal synapses within a given glomerulus and spreads lateral inhibition to surrounding glomeruli. The granule cells (gc) form reciprocal synapses with the basal dendrites of the secondary neurons. They also receive information from mitral and tufted cell collaterals and from the brain via centrifugal fibers.

FIGURE 3 Neuronal circuits of the olfactory bulb. Odorant-specific subtypes of olfactory neurons (on) are scattered randomly throughout patches of the olfactory epithelium. Four examples are illustrated in (A) through (D). As their axons project into the olfactory bulb, they become rearranged when they contact the appropriate odorant-specific glomeruli. Four glomeruli are shown, each receiving approximately 25,000 primary axons (three are shown). Two types of second-order neurons, mitral cell (mc) and tufted cell (tc), have their apical dendrites in each glomerulus. Basal dendrites of these cells are located in deeper layers of the olfactory bulb (external plexiform layer), and their axons project to the brain in the olfactory tract. As axons exit the bulb, they send collaterals back to the bulb. The two classes of interneurons, both inhibitory, are shown in black. The periglomerular cell (pgc) of the external plexiform layer forms reciprocal synapses within a given glomerulus and spreads lateral inhibition to surrounding glomeruli. The granule cells (gc) form reciprocal synapses with the basal dendrites of the secondary neurons. They also receive information from mitral and tufted cell collaterals and from the brain via centrifugal fibers.

of specific odors with pleasant or unpleasant sensations is an important feature of the olfaction system and allows it to function in alerting the organism of potentially harmful substances.

There is evidence for a secondary olfactory pathway that, unlike the primary pathway, does have a relay in the thalamus (medial dorsal nucleus) before projecting to a cortical region in the orbitofrontal cortex. Interestingly,

A. Distribution of olfactory information

A. Distribution of olfactory information

secondary pathway-orbitofrontal cortex tertiary pathway-vomeronasal to hypothalamus primary pathway - olfactory cortex

B. Primary and bilateral olfactory pathway, medial view enlarged

B. Primary and bilateral olfactory pathway, medial view enlarged olfactory tract

olfactory tract olfactory bulb olfactory nerve lateral olfactory stria olfactory cortex amygdala (under surface) hippocampus (under surface)

C. Secondary olfactory pathway, lateral view medial dorsal nucleus of the thalamus orbitofrontal cortex

C. Secondary olfactory pathway, lateral view

FIGURE 4 Olfactory pathways. (A) Olfactory fibers leave the olfactory bulb and distribute to a number of CNS areas for further processing. (B) The primary path projects to the temporal lobe adjacent to the hippocampus and amygdala. (C) The secondary pathway projects to the thalamus and then to the frontal cortex.

olfactory bulb olfactory tract lateral olfactory stria

FIGURE 4 Olfactory pathways. (A) Olfactory fibers leave the olfactory bulb and distribute to a number of CNS areas for further processing. (B) The primary path projects to the temporal lobe adjacent to the hippocampus and amygdala. (C) The secondary pathway projects to the thalamus and then to the frontal cortex.

it has been suggested that this pathway, although relatively small, may actually be more directly responsible for conscious perception of odors, whereas the historically designated primary pathway through the lateral olfactory tract projection to the temporal lobe represents mainly emotion- and memory-related olfactory pathways. The orbitofrontal region is adjacent to the primary taste cortex. Joint projections of taste and smell information are probably combined at the interface of these two cortical areas in order to provide the general sensation of flavor from ingested food. Pathways originating from this flavor area project back to the nucleus of the solitary tract, which controls visceral autonomic responses in the gastrointestinal tract and gustatory function.

A third olfactory pathway has recently been reported. It has long been known that lower mammals (rats and rabbits) have a vomeronasal organ (VNO) consisting of an additional area of olfactory epithelium in the nasal cavity. Until recently, it was thought that the VNO was present only during infancy in humans and that it was lost during later development. New evidence indicates that, though small in area, the VNO is present in adults and provides important olfactory information about odorants, particularly pheromones. The VNO is located in a small pit in the midline nasal septum, and its olfactory receptors project to the accessory olfactory bulb. Neurons in the accessory bulb send projections to the hypothalamus, which controls important reproductive behaviors and gonadosteroid function.

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