In neuroanatomy, a neural pathway is the connection formed by axons that project from neurons to make synapses onto neurons in another location, to enable neurotransmission (the sending of a signal from one region of the nervous system to another). Neurons are connected by a single axon, or by a bundle of axons known as a nerve tract, or fasciculus.[1] Shorter neural pathways are found within grey matter in the brain, whereas longer projections, made up of myelinated axons, constitute white matter.
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.[citation needed] 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.[citation needed] Further examples include the pyramidal tract, crus cerebri (Latin, "leg of the brain"), and cerebellar peduncles (Latin, "little foot of the cerebellum").[citation needed] Note that these names describe the appearance of a structure but give no information on its function, location, etc.[citation needed]
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.[6]
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.[citation needed] If the axons have myelin sheaths, then the pathway appears bright white because myelin is primarily lipid.[citation needed] 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.[citation needed]
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 belongs to the Sciatic Nerve and runs from the great toe to the base of the spinal cord. These are archetypal examples of neural pathways.[citation needed]
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.[7] Dopamine is provided both tonically and phasically in response to the needs of the neural pathways.[7]
Moore, Keith; Dalley, Arthur (2005). Clinically Oriented Anatomy (5thed.). LWW. p.47. ISBN0-7817-3639-0. A bundle of nerve fibers (axons) connecting neighboring or distant nuclei of the CNS is a tract.
Witter, Menno P.; Naber, Pieterke A.; Van Haeften, Theo; Machielsen, Willem C.M.; Rombouts, Serge A.R.B.; Barkhof, Frederik; Scheltens, Philip; Lopes Da Silva, Fernando H. (2000). "Cortico-hippocampal communication by way of parallel parahippocampal-subicular pathways". Hippocampus. 10 (4): 398–410. doi:10.1002/1098-1063(2000)10:4<398::AID-HIPO6>3.0.CO;2-K. PMID10985279. S2CID25432455.
Llinas RR, Walton KD, Lang EJ (2004). "Ch. 7 Cerebellum". In Shepherd GM (ed.). The Synaptic Organization of the Brain. New York: Oxford University Press. ISBN0-19-515955-1.
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