**2. The chemical senses**

The chemical senses are the oldest senses. The earliest living organisms monitored their environment with chemoreception in order to sense the availability of nutrients [23, 24] and thus to respond to different chemicals. Higher organisms face the challenge of reacting to various internal and external chemicals, for example, hormones, neurotransmitters, neural recognition molecules, and intra- and interspecific olfactory, and gustatory signals [24, 25].

*Neuronal Architecture and Functional Organization of Olfactory Glomeruli DOI: http://dx.doi.org/10.5772/intechopen.108728*

The olfactory pathway starts with peripheral structures. In the case of vertebrates, olfactory receptor cells are located in the nasal cavity in the olfactory epithelium. In insects, sensilla on the antennae of insects houses olfactory receptor cells [26]. Two areas in olfactory research have been under heavy investigation: (a) the transduction mechanisms taking place at the olfactory receptor cells and (b) the synaptic mechanisms acting at the first synaptic relay in the olfactory pathway, including synaptic plasticity and learning, that is, in the olfactory bulb (OB) of vertebrates and the antennal lobes (ALs) of insects [1, 27–30].
