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.
Descending neurons play an important role in initiating, maintaining, modulating, and terminating behaviors. Several descending neurons involved in controlling specific behaviors have been identified in both vertebrates and invertebrates. These include descending neurons that can initiate and terminate locomotion, modulate locomotion speed and direction, and help coordinate limbs.
While some descending neurons are sufficient to elicit specific behaviors, most behaviors are likely not controlled by single, command-like descending neurons, but instead by the combined activity of different descending neurons.
Some descending pathways form direct connections with motor neurons and premotor interneurons, including central pattern generators. But how exactly descending signals are integrated in circuits in the spinal cord (vertebrates) or ventral nerve cord (invertebrates) during behavior is not well understood.
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