Invertebrate organisms do not have centralized brains, but rather have nervous systems that feature simple reflex pathways composed of associations of nerve cells. In most invertebrates, these pathways are controlled by localized collections of nerve cell bodies known as ganglia. Over the course of evolutionary history, as nervous systems increased in complexity, groups of ganglia became more and more centralized, eventually leading to the vertebrate brain. The characteristics of vertebrate brains vary significantly, but all contain several basic parts, including the cerebellum, cerebrum, brain stem, hypothalamus, and pituitary gland. The brains of higher organisms include additional components as well, such as the thalamus and a much more extensive cortex.

Neurofilaments, which are found specifically in neurons and are especially prominent in the axons of the cells, were targeted in a rat brain horizontal section (shown above) with mouse anti-NF-H Non PO₄ antibodies followed by goat anti-mouse secondary antibodies conjugated to Alexa Fluor 488. The glial fibrillary acidic protein was simultaneously targeted in the specimen with mouse anti-GFAP antibodies visualized with goat anti-rabbit secondary antibodies conjugated to Alexa Fluor 568. Nuclear DNA was subsequently labeled with Hoechst 33342. Images were recorded in grayscale with a 12-bit digital camera coupled to a Nikon Eclipse 80i microscope equipped with bandpass emission fluorescence filter optical blocks. During the processing stage, individual image channels were pseudocolored with RGB values corresponding to each of the fluorophore emission spectral profiles.