The Olfactory Bulb

Odorant receptors and associated second-messenger systems are primarily localized within the ciliary tufts of olfactory neurons. The tufts extend from the surface of the epithelial plate and are immersed in mucous. The cell body, located in deeper epithelial layers, has a single nonmyelinated axon that must pass through small holes in the cribriform plate before synapsing on the olfactory bulb located just above the level of the eyes (Fig. 3). These are among the smallest (therefore, slowest conducting) neurons in the body, and they are highly vulnerable to injury from head trauma. Some return of function often occurs because of the regenerative capacity of the neurons, as mentioned earlier.

One of the major structural features of the olfactory bulb is the arrangement of cell processes in bundles called glomeruli, which contain the axonal endings of olfactory receptors and the apical dendrites of roughly 100 second-order olfactory neurons. Each glomerulus receives input from 25,000 primary olfactory neurons, and each primary olfactory neuron synapses with several of the secondary neurons within the glomerulus. The end result is that each second-order neuron receives several thousand synaptic inputs from olfactory receptors.

There are two types of second-order olfactory neurons: mitral and tuf ted cells. Both receive information from olfactory receptor synapses within the glomerulus. In addition, they make reciprocal synapses with neuronal processes from two types of interneurons: (1)periglomeru-lar cell processes within the glomerulus, and (2) granule cell processes in the external plexiform layer located proximal to the glomerular layer. Periglomerular cells provide short feedback loops among the glomeruli, whereas granule cells are part of a long inhibitory feedback loop involving the olfactory cortex. In this latter pathway, olfactory signals are sent through olfactory receptors to secondary neurons in the olfactory bulb and on to tertiary cells in the olfactory cortex. These tertiary cells then project back to the bulb and activate granule cells, which in turn release gamma aminobutyric acid (GABA) to inhibit mitral and tufted cells.

The regional patterns of odorant-specific neurons seen in the olfactory epithelium are also seen in the bulb; however, there does not seem to be a one-to-one transfer of odor-specific information. The recognition of a specific odor results from the spatial pattern of olfactory receptors that are activated and that in turn activate a circumscribed group of secondary neurons in the olfactory bulb. It appears that in the olfactory system, as in most other sensory systems, the location of the neurons activated by a specific input provides a code for some aspect of the stimulus. In the visual system, a retinotopic map indicates the location of light within the visual field; in the auditory system, a tonotopic map indicates the frequency of sound (pitch); and, in the olfactory system, a odorotopic map indicates not the location of the odor but the chemical properties of the odorant. Many recognizable odors are actually mixtures, and the olfactory code for these smells seems to be generated in terms of the specific sets of neurons that are activated and the specific regions of the nasal epithelium or olfactory bulb in which they reside.

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