Corticospinal Tract

Cross references:    Medial Motor Column  

Pyramidal tracts (Wiki) 
    "The pyramidal tracts refers to both the corticospinal and corticobulbar tracts
    The corticospinal tract conducts impulses from the brain to the spinal cord. It... is made up of two separate tracts in the spinal cord: the lateral corticospinal tract and the anterior corticospinal tract. "  
The corticospinal tract is concerned specifically with discrete voluntary skilled movements, such as precise movement of the fingers and toes."  
The signals cross in the medulla oblongata, this process is also known as decussation."  
The corticobulbar tract carries information to motor neurons of the cranial nerve nuclei, rather than the spinal cord."  
The corticospinal tract originates from pyramidal cells in layer V of the cerebral cortex. About half of its fibers arise from the primary motor cortex. Other contributions come from the supplementary motor area, premotor cortex, somatosensory cortex, parietal lobe, and cingulate gyrus. The average fiber diameter is in the region of 10μm; around 3% of fibres are extra-large (20μm) and arise from Betz cells, mostly in the leg area of the primary motor cortex."  

Locomotor role of the corticoreticular-reticulospinal-spinal interneuronal system.  
In vertebrates, the descending reticulospinal pathway is the primary means of conveying locomotor command signals from higher motor centers to spinal interneuronal circuits, the latter including the central pattern generators for locomotion.  
    The pathway is morphologically heterogeneous, being composed of various types of inparallel-descending axons, which terminate with different arborization patterns in the spinal cord. Such morphology suggests that this pathway and its target spinal interneurons comprise varying types of functional subunits, which have a wide variety of functional roles, as dictated by command signals from the higher motor centers.  
    Corticoreticular fibers are one of the major output pathways from the motor cortex to the brainstem. They project widely and diffusely within the pontomedullary reticular formation. Such a diffuse projection pattern seems well suited to combining and integrating the function of the various types of reticulospinal neurons, which are widely scattered throughout the pontomedullary reticular formation. The corticoreticular-reticulospinal-spinal interneuronal connections appear to operate as a cohesive, yet flexible, control system for the elaboration of a wide variety of movements, including those that combine goal-directed locomotion with other motor actions." 
My comment:   
In vertebrates, the descending reticulospinal pathway is the primary means of conveying locomotor command signals from higher motor centers to spinal interneuronal circuits, the latter including the central pattern generators for locomotion."  This answers one of my questions.  The RS pathway is still important to we humans.  
    Unfortunately, there's no mention of neurotransmitters, so it doesn't provide any insight into the roles of glutamate vs GABA in locomotion. 
    102 Related citations  
    27 Cited by's:   

The following reference was scanned from: 

Carpenter, M.B. and Sutin, J. (1983). Human Neuroanatomy, 8th Ed. Baltimore. Williams & Wilkins. 

Page 282   

Corticospinal System.

These tracts consist of all fibers which:
    (a) originate from cells within the cerebral cortex,
    (b) pass through the medullary pyramid and
    (c) enter the spinal cord.
They constitute the largest and most important descending fiber system in the human neuraxis.  

In man each tract is composed of over 1,000,000 fibers of which some 700,000 are myelinated. Approximately 90% of these myelinated fibers have a diameter of 1 to 4mu; most of the remaining myelinated fibers range in caliber from 5 to 10 mu, but included among them are some 30,000 to 40,000 very large fibers having a thickness of 10 to 22 mu.

Fibers of the corticospinal system arise from cells in the deeper part of lamina V in the precentral area (area 4), the premotor area (area 6), the postcentral gyrus (areas 3a, 3b, 1, 2) and adjacent parietal cortex (area 5).

Cells of origin of the corticospinal system, arranged in strips or clusters, vary greatly in size in different cortical areas. The largest fibers arise mainly from the giant pyramidal cells of Betz in the precentral gyrus (area 4 of Brodmann), but some may arise from adjacent cortical areas. These corticofugal fibers converge in the corona radiata and pass downward through the internal capsule, crus cerebri, pons and medulla. As this large tract descends in the brain stem, it passes close to the emerging root fibers of cranial nerves III, VI and XII.  Corticobulbar fibers conveying impulses to motor nuclei of the brain stem are closely associated with corticospinal fibers in the internal capsule and brain stem. Cells of origin of corticobulbar fibers are found in the same cortical areas as corticospinal neurons but lie more superficially in lamina V and are quite uniform in size. The corticospinal tract comes to the surface in the medullary pyramid.

At the junction of medulla and cord, the fibers undergo an incomplete decussation giving rise to three tracts:
    (a) a large lateral corticospinal tract (crossed),
    (b) an anterior corticospinal tract (uncrossed) and
    (c) a small anterolateral corticospinal tract (uncrossed).

Pages 288-289     

It must be remembered that the corticospinal tract is a complex fiber system arising from multiple cortical areas, and only a small part of it originates from the giant pyramidal cells of Betz in the precentral gyrus. These cells have been estimated to number between 34,000 and 40,000 in one hemisphere and probably account for the larger fibers (10 to 20 mu) in the corticospinal tract.  

The more numerous finer fibers come in considerable part from other cortical regions. Thus there are at least two components in the pyramidal tract: a large and fine-fibered component from the motor area, and a fine-fibered one from all other areas that contribute fibers to the tract. 

It is probable that fibers of the Betz cells are concerned with the finer isolated movements of the distal parts of the extremities, which are primarily affected in pyramidal lesions. The more numerous finer fibers may be related to grosser movement and tonic control, and injury to them may be the cause of the increase in muscle tone and the hyperactive deep tendon reflexes.  

These fibers which form an integral part of the pyramidal tract, its largest portion, descend uninterruptedly from the cerebral cortex through the medullary pyramids to the spinal cord. 

The pyramidal cells and their axons constitute the "upper motor neurons" in contrast to the "lower motor neurons" (anterior horn cells), which directly innervate the skeletal muscle. The symptoms of a pyramidal tract lesion, loss of volitional movement, spasticity, increased deep tendon reflexes, loss of superficial reflexes and the sign of Babinski, therefore are designated as "upper  motor neuron" type paralysis (spastic or supranuclear paralysis). In "lower motor neuron" paralysis, there is loss of all movement, reflex and voluntary, as well as loss of tone and atrophy of the affected muscles. 

All descending spinal tracts, other than the cortocospinal, arise from nuclear masses in the brain stem.  Three descending tracts arise from the midbrain.  These are the tectospinal, interstitiospinal and rubospinal.