Descending neuron

Anatomy

Descending neurons have their somas and dendrites (primary input zones) in the brain. Their axons traverse the neck in connectives, or tracts, and output onto neurons in the spinal cord (vertebrates) or ventral nerve cord (invertebrates).

Mammals possess hundreds of thousands of descending neurons. They can be divided functionally into two major pathways: pyramidal tracts, which originate in the motor cortex, and extrapyramidal tracts, which originate in the brainstem (see schematic). An example of the former is the corticospinal tract, which is responsible for voluntary movement of the body. An example of the latter is the reticulospinal tract, which contributes to the unconscious regulation of locomotion and posture. Reticulospinal neurons originate in the medullary reticular formation, where they receive information from upstream locomotor centers, such as the mesencephalic locomotor region and the basal ganglia.

Insects possess only several hundreds of descending neurons. Work in the fruit fly Drosophila melanogaster suggests that they are organized into three broad pathways (see schematic). Two direct pathways link specific regions in the brain to motor circuits in the ventral nerve cord controlling the legs and wings, respectively. A third pathway couples a broad array of brain regions to a large integrative region in the ventral nerve cord that may control both sets of appendages.