Olfactory Bulb and Nerve

OLFACTORY PATHWAY

Olfactory Bulb. About 100 million olfactory afferent fibers enter the olfactory bulb, a flattened, oval mass lying near the lateral margin of the cribriform plate of the ethmoid bone. The incoming olfactory fibers coalesce in the outermost layer of the olfactory bulb to form presynaptic nests, or glomeruli. Each glomerulus is composed of about 25,000 receptor cell axon terminals. The terminals synapse and excite the dendrites of mitral and tufted cells, which are the second-order neurons in the olfactory bulb. Each mitral cell sends its dendrite to only a single glomerulus, while each tufted cell sends dendrites to several glomeruli. Olfactory afferents within the glomeruli also activate periglomerular cells, which then inhibit mitral and tufted cells. Further inhibition arises at the dendrodendritic contacts between mitral and tufted cells and the processes of granule cells, which lie deeper still within the olfactory bulb. These contacts are an example of two-way synaptic feedback connections: the granule cells are excited by mitral and tufted cells and, in turn, inhibit them. Integration of olfactory information occurs when excitation is spread throughout the multiple-branched granule cell processes, and also when granule cells are excited by the centrifugal efferent fibers that reach the olfactory bulb from higher centers. Another factor in this highly complex integrative process is the recurrent collaterals of mitral cells that appear to excite mitral, tufted, and granule cells.

There is a dramatic transformation in the response to odors between the glomeruli and the mitral cells. The glomeruli respond to different substances based on their physiochemical properties, whereas mitral cells respond to groups of substances that evoke subjective sensations.

Olfactory Tract and Central Connections. The axons of mitral and tufted cells form the olfactory tract, through which they project to the olfactory trigone and into the lateral and medial olfactory striae, establishing a complex pattern of central connections. Some mitral and tufted cell axons terminate in the anterior olfactory nucleus (a continuation of the granule cell layer throughout the olfactory tract) and olfactory tubercle, the sites of origin of the efferent fibers projecting to both the ipsilateral and contralateral olfactory bulbs. Other axons from the lateral stria reach the piriform lobe of the temporal cortex and terminate in the amygdala (amygdaloid body), the septal nuclei, and the hypothalamus.

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