Innervating the Brain
The Allen Brain Atlas is an online tool that combines structure, function, and gene expression data to create a comprehensive catalogue of histological sections and three-dimensional renderings of the human and mouse brains. While it was established primarily to accelerate neuroscience and neuroanatomy research, it is available free online. The images above are taken from renderings of the mouse brain showing the innervation of the olfactory bulb (bottom and top) and the expression of the App gene (middle) implicated in amyloid fibril formation in Alzheimer’s disease and, interestingly, mental retardation in Down Syndrome patients (APP, the human analogue to mouse App, is encoded on chromosome 21).
Networks of neurons are not and cannot be wires like you would see on the side of a highway; they usually eminate from a point of origin and move to connect certain points of the brain, but they are in no way disorganised. For example, while the corpus collosum maintains extensive innervation throughout both hemispheres, other areas of the brain will not. It is this sort of macro-scale compartmentation that allows different parts of the brain to perform different functions - in vertebrates like us, for example, intense lateralisation of the hemispheres gives rise to lower-level organisation, namely the specific structures that perform fundamentally different tasks (the hippocampus, the cerebrum, and the cerebellum, for example). Before in situ hybridisation, the characteristic pattern of neuronal spread throughout the brain was probed by creating lesions in different areas and noting the resulting phenotype; this worked because without excitation, neurons die. This same reality makes it highly suboptimal for the brain to organise itself as a tangled mess of fibres if many of the pathways are likely to become redundant; it both necessitates and causes an organised structure built upon the frequency of signal transduction to particular areas.