The Corticospinal Tracts in Man

Abstract

The course, location and relations of the corticospinal tracts within the spinal cord of man are demonstrated on the basis of cases with lesions above the spinal cord restricted to the corticospinal tracts, of motor neuron disease, and of anterolateral cordotomies; control cases were of normal spinal cords. The following features of the lateral corticospinal tract are emphasized in the cervical cord: (1) the large extent of the white matter of the cord covered by the tract, and the anterior extent of the tract, the border being anterior to the central canal; (2) in the lower cervical cord, the separation of fibres from the main mass of the tract, which reach the periphery of the cord in the anterolateral sector; (3) the presence in many cords of the ventral crossed bundle; and (4) the relationship of the denticulate ligament to the tracts in the cervical segments. The following features of the anterior corticospinal tracts are emphasized: (1) their location, caudal extent and asymmetry; and (2) the changes in location in relation to the median fissure as the tract descends and its relationship to other tracts of the anterior column. Three-quarters of spinal cords are asymmetric and in three-quarters of asymmetric cords the right side is the larger. The asymmetry is due to a greater number of corticospinal fibres crossing to the right side. As more fibres have crossed in the decussation, the anterior tract opposite the large lateral tract is smaller than the ipsilateral anterior tract: that accounts for the asymmetry of the two halves of the cord. The greater number of corticospinal fibres in the right side of the cord is unrelated to handedness, but correlates with the fact that in three-quarters of corticospinal decussations, the crossing from left to right occurs at a more cranial level than the opposite crossing. A group of short peripheral ascending fibres is described running along the sides of the median fissure in the thoracic cord.

Functional activation in motor cortex reflects the direction and the degree of handedness

“…However this finding is at variance with the results of a study of autopsy brains that failed to show any consistent relation between the volume of motor cortex (area 4) and hand preference (10). In addition, in the latter study, the authors found no asymmetry at other levels of the sensorimotor system such as the medulla or spinal cord although such asymmetries have been reported by others (22)(23)(24). In an effort to account for the lack of structural asymmetry at the level of the motor cortex, White et al (10) have suggested that structural changes in the primary motor cortex contralateral to the dominant hand may be balanced by structural changes associated with ''less obvious capabilities'' than handedness in the motor cortex contralateral to the nondominant hand.…”

Existence of Ipsilateral Hemiparesis in Ischemic and Hemorrhagic Stroke: Two Case Reports and Review of the Literature

“…The reason for this is felt to be the predominance of contralateral corticospinal projections, which arise from the cortical regions of the brain and decussate in the caudal medulla before traveling to the spinal cord [1]. While not all fibers decussate, 70-90% of them do, resulting in this ‘crossed' clinical finding [2]. Prior neuroanatomical studies have firmly supported the notion that the primary motor cortex predominantly innervates the contralateral half of the body [3].…”

Ipsilateral Motor Responses to Focal Transcranial Magnetic Stimulation in Healthy Subjects and Acute-Stroke Patients

“…For Davidoff, 1 the function of uncrossed pyramidal axons remains unknown, but others 4 suggest that it exerts some control on bilateral proximal limb movements. The presence of functional ipsilateral corticospinal connections appears to be a normal state in infants and children aged Ͻ10 years, 5 but their presence in adult humans is still debated.…”