Figure 1. Principal structures of the mammalian olfactory system. Axons of olfactory receptor neurons in the olfactory epithelium sort themselves as they project to the olfactory bulb so that all of the neurons expressing a given olfactory receptor project to the same glomerulus (dashed circle), where they make synapses on to mitral cells. The mitral cells project broadly to a variety of cortical and subcortical structures. Ant. Cort. amygdala stands for the anterior cortical nucleus of the amygdala. The olfactory peduncle includes the anterior olfactory nucleus, the indusium griseum, the anterior hippocampal continuation and the ventral tenia tecta (see Cleland and Linster, 2003).

Figure 2. Odorant-specific patterns of glomerular activation in the mouse olfactory bulb. The baseline fluorescence of the olfactory bulbs of a mouse that expresses synaptopHluorin, a fluorescent indicator of transmitter release, in its olfactory nerve terminals. Bar, 1 mm (left). Image is from an anaesthetized mouse imaging through thinned bone. See Bozza et al., 2004, In vivo imaging of neuronal activity by targeted expression of a genetically encoded probe in the mouse. Neuron 42: 9–21 for details. Pseudocolour maps showing the fluorescence increase (indicating transmitter release from olfactory receptor axons) evoked by the presentation of each of the named odorants. Increases in fluorescence in individual glomeruli are clearly visible, and the pattern of fluorescence increases is unique to each odorant (right three panels).

Figure 3. Temporal mechanisms of odour coding. (a) Pseudocolour maps showing patterns of receptor input to olfactory bulb imaged in response to the odorant ethyl butyrate during one respiratory cycle of an anaesthetized mouse. These spatial patterns (and thus the glomerular representation of the odorant) change through the time course of a single sniff. The outline of the dorsal bulb is shown in white. (b) The sequence of response times for input to different glomeruli changes with the identity of the odorant. Vertical lines indicate response times for five different glomeruli imaged during one respiratory cycle. Because the sequence is different for different odorants, the timing of inputs to different glomeruli may contribute to coding information about odorant identity. (c) Action potentials of mitral/tufted cells (middle trace) are phase-locked to the local field potential (bottom trace) during odour stimulation. Upper trace shows the time course of inhalation of the odorant enanthic acid. (d) Simultaneous recordings of action potentials from two mitral/tufted cells (trace 2 and 3) show that both cells respond to odour stimulation with a burst of spikes. Trace 1 shows the respiration cycle (dark bar indicates presentation of the odorant caproic acid). Bracket indicates spikes used in (c). (e) Action potentials from bracketed part of d shown at higher temporal resolution. The spikes of these neurons tend to synchronize (arrows) during odorant inhalation. Parts 3(c)–(e) adapted from Kashiwadani et al. 1999. Reproduced by permission of the American Physiological Society.