Neuron population ( POP ; Burns, 2001 ) : An arbitrarily defined set of neurons (Waldeyer, 1891) that may or may not correspond exactly to a specific gray matter region or neuron type (Bota & Swanson, 2007). For example, a neuron population is formed by a subset of the individual neurons that together constitute a neuron type. In contrast, however, all neurons within an anterograde pathway tracer injection site that spreads to involve multiple adjacent gray matter regions, or an irregular extent within a single gray matter region, form the neuron population associated with that injection site. Another example of this type would be all neurons within multiple gray matter regions that project to a single gray matter region-as demonstrated, for example, by a retrograde pathway tracer injection site. A third example would be all neurons in the hypothalamus (Kuhlenbeck, 1927) that express the corticotropin-releasing hormone (CRH) gene following hypovolemic stress, as demonstrated by in situ hybridization. In general, a site contains a neuron population. Term thus defined was introduced by Burns (2001, p. 1201 and Fig. 10).