A descending neuron is a

neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

that conveys signals from the brain to neural circuits in the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...

(vertebrates) or

ventral nerve cord The ventral nerve cord is a major structure of the invertebrate central nervous system. It is the functional equivalent of the vertebrate spinal cord. The ventral nerve cord coordinates neural signaling from the brain to the body and vice ve ...

(invertebrates). As the sole conduits of information between the brain and the body, descending neurons play a key role in behavior. Their activity can initiate, maintain, modulate, and terminate behaviors such as locomotion. Because the number of descending neurons is several orders of magnitude smaller than the number of neurons in either the brain or spinal cord/ventral nerve cord, this class of cells represents a critical bottleneck in the flow of information from sensory systems to motor circuits.

Anatomy

Descending neurons have their somas and

dendrite A dendrite (from Ancient Greek language, Greek δένδρον ''déndron'', "tree") or dendron is a branched cytoplasmic process that extends from a nerve cell that propagates the neurotransmission, electrochemical stimulation received from oth ...

s (primary input zones) in the brain. Their

axon An axon (from Greek ἄξων ''áxōn'', axis) or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences) is a long, slender cellular extensions, projection of a nerve cell, or neuron, ...

s traverse the neck in connectives, or tracts, and output onto neurons in the

(vertebrates) or

(invertebrates). Descending pathways in mammals

Mammals possess hundreds of thousands of descending neurons. They can be divided functionally into two major pathways:

pyramidal tracts The pyramidal tracts include both the corticobulbar tract and the corticospinal tract. These are aggregations of efferent nerve fibers from the upper motor neurons that travel from the cerebral cortex and terminate either in the brainstem (''cort ...

, 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 The corticospinal tract is a white matter motor pathway starting at the cerebral cortex that terminates on lower motor neurons and interneurons in the spinal cord, controlling movements of the limbs and trunk. There are more than one million neu ...

, 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 The reticular formation is a set of interconnected nuclei in the brainstem that spans from the lower end of the medulla oblongata to the upper end of the midbrain. The neurons of the reticular formation make up a complex set of neural networks ...

, where they receive information from upstream locomotor centers, such as the

mesencephalic locomotor region The mesencephalic locomotor region (MLR) is a functionally defined area of the midbrain that is associated with the initiation and control of locomotor movements in vertebrate species. Neuroanatomical organization The MLR was first described by S ...

and the

basal ganglia The basal ganglia (BG) or basal nuclei are a group of subcortical Nucleus (neuroanatomy), nuclei found in the brains of vertebrates. In humans and other primates, differences exist, primarily in the division of the globus pallidus into externa ...

Insects possess only several hundreds of descending neurons. Work in the fruit fly ''

Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (an insect of the Order (biology), order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly", "pomace fly" ...

'' 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.

Function

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

(vertebrates) or

(invertebrates) during behavior is not well understood.

References

{{Reflist Spinal cord

Anatomy

Function

See also

References