Olfaction Flashcards
What does the mammalian olfactory bulb consist of?
thousands (1800 in the mouse) of signal-processing modules called glomeruli, within which axons of olfactory receptor neurons (ORNs) form excitatory synaptic connections on the dendrites of mitral (MC) and tufted cells (TC), the output neurons of the OB
What do glomeruli associate with and what are these combined features called?
Glomeruli together with their associated neurons, here referred to as ‘glomerular modules’
What do glomerular modules do?
function as olfactory axon convergence centers for inputs originating from one type of odorant receptor (OR), propagating odorant-specific signals to mitral and tufted cells innervating the glomerulus
How is odour processed?
The odor molecule information is processed by the local neuronal circuits that mediate synaptic interactions within the glomerular module as well as between other such modules in the OB
How do mitral/tufted cells acts as signal transducers?
Mitral/tufted cells receiving monosynaptic input from ORNs generate spike bursts if the ORN-evoked EPSP reaches threshold for spike generation, thereby amplifying suprathreshold sensory input at the first stage of synaptic transfer in the olfactory system
How do mitral/tufted cells back propagate?
signal horizontally back-propagates through the secondary dendrites into the external plexiform layer (EPL), forming reciprocal dendrodendritic synapses with GABAergic granule cells (GCs) and other inhibitory interneurons, such as superficial short-axon (sSA) cells
How is there lateral inhibition in the olfactory system?
GCs receive excitatory glutamatergic input from basal dendrites of mitral and tufted cells, and release GABA, effectively blocking the activity of other mitral/tufted cells in the EPL by a process termed ‘lateral inhibition’ (Xiong and Chen, 2002)
How does mitral cell activation pattern reflect relative glomerular activation?
lateral excitatory network of tufted and sSA cells integrates heterogeneous activation levels across the bulbar input layer and delivers a uniform layer of excitation onto a subclass of PGCs, which in turn inhibit MCs, such that MC patterning reflects relative, rather than absolute, levels of glomerular activation across the OB (Cleland et al., 2007)
How does olfactory activation lead to intensity dependent representation of stimulus?
global feedforward inhibition of the glomerular microcircuit contributes to the normalization of the intensity of sensory input, which is essential for segregating quality from concentration effects and for constructing intensity-dependent representations of stimulus quality
What are the implications of feedforward inhibition on the olfactory system?
likely improves the olfactory system’s ability to recognize the same odor at different intensities, while avoiding high rates of spiking to conserve metabolic energy
