Mitral granule

Fig. 5.12 (a) Synaptic types along dendritic spines of M/T and GC units uni-, and bi-directional junctions, (b) Transmitter systems at a reciprocal synapse, Mitral-Granule cell junction. [Glu, glutamate (R, receptor) GABA, y-aminobutyric acid (R, receptor) E, intracellular effector and aAR, alpha-adrenergic receptor.]. (From Hayashi et al., 1993.)... 

Doucette et al. 2000). At lower doses neonates show delayed effects on eye opening and olfactory conditioned place preference (Doucette et al. 2003). The latter supports the high sensitivity of the mitral-granule cells to domoic acid as described earlier. Evidence indicates that the sensitivity of the neonates results from insufficient renal clearance of toxin, allowing increased bioavailability in the blood (Xi et al. 1997). Domoic acid has been detected in the blood of neonates, transmitted via milk from lactating mother rats previously exposed to domoic acid but at levels appearing to be well below symptomatic doses (Maucher and Ramsdell 2005). 

Mitral-granule cell interactions Anatomical considerations... 

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... 

The responses of the mitral/granule system to single or paired orthodromic and antidromic volleys are well understood. But, the function of this reciprocal synaptic mechanism in processing odor evoked activity depends on its response to spatially and temporally patterned activity in the olfactory nerve. 

The modes of operation of the mitral/granule system are too complex to be adequately summarized as self- or lateral inhibition. An adequate description of the behavior of this system may require the development of models with appropriate simplifying assumptions. One such model is based on the ability of the mitral/granule system to develop oscillations. 

Central/Tertiary structures The fish olfactory bulb is a fourlayered structure much as in higher vertebrates. Within the 2nd layer, the first synapse for olfactory input is on the dendrites of the mitral cells (MC). About 1000 ORN axons converge on one MC, a ratio similar to mammals. The MC output, from cells at various levels, leads into several glomeruli and receives (inhibitory) input from granule cells. The latter also innervate a distinct cell type in the MC layer of teleosts — the ruffed cells (RC), with which they have reciprocal synapses [Fig. 2.18(a)] both relay cells send ascending fibres to forebrain centres (Kosaka and Hama, 1982). The RC are unlike the MC since they are not stimulated by the ORNs directly. Their interactions (Chap. 5) may contribute to the processing of pheromonal stimuli (Zippel, 2000). The main bulbar pathways project to several nuclei in the forebrain via two ipsilateral tracts, the lateral and medial [Fig. 2.18(b)], the latter mediates sexual behaviour and the former probably other behaviours (Hara,... 

AOB VN Input to mitral cells (MC) Fig. 5.4 Location of major neurotransmitters in aw qc) + Granule (GC) cells modified and processed by interaction with Periglomeru ventral sympathetic fibers <=> = reciprocal synapses efferent/centrifugal (CF) input p gs. 5.12(a) and (b)... 

Cerebral cortex (layer I-VI, selected interneurons and principal cells) hippocampus (selected interneurons and principal cells) pallidum striatum (interneurons) thalamic relay nuclei olfactory bulb (mitral cells and interneurons) cerebellum (Purkinje cells and granule cells) deep cerebellar nuclei amygdala basal forebrain substantia nigra pars reticulata inferior colliculus brainstem... 

The basic circuitry of the MOB. Axons of ORNs travel in the ONL and synapse in the GL on the dendrites of mitrai ceiis (MC), tufted ceiis (externai tufted ceii, ET middie tufted ceii, MT), and generic juxtagiomeruiar (JG) neurons, which include perigiomeruiar ceiis (PG), ET ceiis, and short axon ceiis (SA). SA ceiis interconnect different giomeruii. There are serial and reciprocal synapses between the apicai dendrites of mitral/tufted cells and the processes of JG neurons. Superficial tufted cells (ST) are located in the superficial EPL or at the GL-EPL border. The lateral dendrites of mitral/tufted cells form serial and reciprocal synapses with the apical dendrites of granule cells (GC) in the EPL. GCs are located in the GCL and the MCL. The axons of mitral/tufted cells project locally to GCs (not shown) and also to primary olfactory cortex via the lateral olfactory tract (LOT). The bulb also contains other populations of interneurons neurons, including the van Gehuchten cells (VG) within the EPL... 

Modulation of Mitral/Tufted-to-Granule Cell Dendrodendritic Interactions mGluRs may also occur at the mitral-to-granule synapses, although their role is unclear. As noted earlier, activation of Group III receptors was reported presynaptically to decrease mitral-to-GC synaptic transmission (Trombley and Westbrook,... 

Davison IG, Boyd JD, Delaney KR. 2004. Dopamine inhibits mitral/tufted-granule ceU synapses in the frog olfactory bulb. J Neurosci 24 8057-8067. 

Laaris N, Ennis M. 2002. Distinct activity patterns evoked by activation of mitral/tufted cell and centrifugal fiber inputs to main olfactory bulb (MOB) granule cells. Soc Neurosci Abstr 561 14. 

Mori K, Kishi K, Ojima H. 1983. Distribution of dendrites of mitral, displaced mitral, tufted, and granule cells in the rabbit olfactory bulb. J Comp Neurol 219 339-355. 

Trombley PQ, Shepherd GM. 1992. Noradrenergic inhibition of synaptic transmission between mitral and granule cells in mammalian olfactory bulb cultures. J Neurosci 12 3985-3991. 

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