Neural processing in the glomerular layer

The firing rate of a primary olfactory neuron is presumably proportional to the concentration in its vicinity of the odorants to which it is sensitive. Even for an ideal transducer, however, there is a necessary minimum of noise arising from the random variation of odorant concentration in a small region (Schild, 1988). There are, however, almost certainly a large number of olfactory neurons associated with each class of odorant and the sum of the activity of all or part of these neurons would be a far more precise indicator of odorant concentration. Thus, one plausible function of the initial synaptic connection in the bulb is the derivation of an average activity of input fibers. 

In addition to summation of inputs from olfactory nerve terminals, a second function of the circuitry of the glomerular layer is regulation of transmission from the olfactory nerve to mitral and tufted cells. The anatomy of the glomerular layer, reviewed above, provides a basis for such regulation (Fig. 5). 

Terminals of the olfactory nerve synapse both with the apical dendrites of mitral and tufted cells and with juxtaglomerular (JG) cells. JG cells, which send processes primarily into a single glomerulus, form reciprocal synapses with the apical dendrites of mitral and tufted cells many of these are symmetrical and, therefore, are considered inhibitory. There is thus a feedforward pathway for inhibition of mitral/tufted cells olfactory nerve terminals excite JG cells which, in turn inhibit mitral/tufted cells. The mitral/tufted side of the reciprocal synapses are asymmetrical and are, therefore, considered excitatory. Thus there is also a pathway for feedback inhibition from mitral and tufted cells back to JG cells. Some JG neurons send axons to neighboring glomeruli but it is not known if these local projections are excitatory or inhibitory (Schneider and Macrides, 

There is only limited information on the intracellular responses of JG cells to odors (Wellis and Scott, 1990) of three cells, all responded to odor stimulation with a burst of spikes associated with a depolarization. Two of the three showed a reduction in response with successive sniffs , although there was no apparent IPSP. 

Olfactory nerve stimulation activates both glomerular (JG and mitral/tufted cells which are reciprocally connected) and infraglomerular (Mitral/granule/mitral) inhibitory systems. Therefore, it is difficult to separate the relative contribution of these two inhibitory systems to mitral cell inhibition. However, stimulation of the olfactory nerve (orthodromic stimulation) results in a longer lasting inhibition of mitral cells than stimulation 

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