The first named pathways are evident to the naked eye even in a poorly preserved brain, and were named by the great anatomists of the Renaissance using cadaver material. Examples of these include the great commissures of the brain such as the corpus callosum, anterior commissure, and posterior commissure. Further examples include the pyramidal tract, crus cerebri, and cerebellar peduncles. Note that these names describe the appearance of a structure but give no information on its function, location, etc. Later, as neuroanatomical knowledge became more sophisticated, the trend was toward naming pathways by their origin and termination. For example, the nigrostriatal pathway runs from the substantia nigra to the corpus striatum. This naming can extend to include any number of structures in a pathway, such that the cerebellorubrothalamocortical pathway originates in the cerebellum, synapses in the red nucleus, on to the thalamus, and finally terminating in the cerebral cortex. Sometimes, these two naming conventions coexist. For example, the name "pyramidal tract" has been mainly supplanted by lateral corticospinal tract in most texts. Note that the "old" name was primarily descriptive, evoking the pyramids of antiquity, from the appearance of this neural pathway in the medulla oblongata. The "new" name is based primarily on its origin and termination. In the cerebellum one of the two major pathways is that of the mossy fibers. Mossy fibers project directly to the deep nuclei, but also give rise to the following pathway: mossy fibers → granule cells → parallel fibers → Purkinje cells → deep nuclei. The other main pathway is from the climbing fibers and these project to Purkinje cells and also send collaterals directly to the deep nuclei.
Functional aspects
In general, neurons receive information either at their dendrites or cell bodies. The axon of a nerve cell is, in general, responsible for transmitting information over a relatively long distance. Therefore, most neural pathways are made up of axons. If the axons have myelin sheaths, then the pathway appears bright white because myelin is primarily lipid. If most or all of the axons lack myelin sheaths, then the pathway will appear a darker beige color, which is generally called grey. Some neurons are responsible for conveying information over long distances. For example, motor neurons, which travel from the spinal cord to the muscle, can have axons up to a meter in length in humans. The longest axon in the human bodybelongs to the Sciatic Nerve and runs from the great toe to the base of the spinal cord. These are archetypal examples of neural pathways.
Basal ganglia pathways and dopamine
Neural pathways in the basal ganglia in the cortico-basal ganglia-thalamo-cortical loop, are seen as controlling different aspects of behaviour. This regulation is enabled by the dopamine pathways. It has been proposed that the dopamine system of pathways is the overall organiser of the neural pathways that are seen to be parallels of the dopamine pathways. Dopamine is provided both tonically and phasically in response to the needs of the neural pathways.
