A neural pathway, neural tract, or neural face, connects one part of the nervous system with another via a bundle of axons, the long fibers of neurons. A neural pathway that serves to connect relatively distant areas of the brain or nervous system is usually a bundle of neurons, known collectively as white matter. A neural pathway that spans a shorter distance between structures, such as most of the pathways of the major neurotransmitter systems, is usually called grey matter.
Naming of neural pathways
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 (Latin, "hard body"; not to be confused with the Latin word "colossus" - the "huge" statue), anterior commissure, and posterior commissure. Further examples of this (by no means a complete list) include the pyramidal tract, crus cerebri (Latin, "leg of the brain"), and cerebellar peduncles (Latin, "little foot of the cerebellum"). Note that these names describe the appearance of a structure but give one no information on its function or location.
Later, as neuroanatomical knowledge became more sophisticated, the trend was toward naming pathways by their origin and termination. For example, the nigrostriatal pathway, which is degenerated in Parkinson's disease, runs from the substantia nigra (Latin, "black substance") to the corpus striatum (Latin, "striped body"). 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 ("ruber" in Latin), 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 (in the primary motor cortex, Brodmann area 4) and termination (onto the alpha motor neurons of the spinal cord).
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 (i.e., are unmyelinated), then the pathway will appear a darker beige color, which is generally called grey (British English, or gray in American English).
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 body is almost two meters long in tall individuals and runs from the great toe to the medulla oblongata of the brainstem.[medical citation needed] These are archetypical examples of neural pathways.
Major neural pathways
- Haines DE. Neuroanatomy: An Atlas of Structures, Sections, and Systems ISBN 0-7817-3736-2.