Function of the Lateral Corticospinal Tracts
The lateral corticospinal tract is the principal motor pathway for voluntary movements, transmitting motor commands from the motor cortex to spinal motor neurons to control skilled, voluntary movements of the limbs, particularly for fine motor control of distal extremities. 1
Primary Motor Function
The lateral corticospinal tract (LCST) originates primarily from the primary motor cortex (approximately 50% of fibers), with additional contributions from premotor areas (27%), supplementary motor area (8%), and primary somatosensory cortex (11%). 2
Over 75% of corticospinal streamlines arise from the lateral surface of the frontal lobe, which enables the voluntary control of dexterous and flexible limb movements characteristic of human motor function. 2
Anatomical Course and Organization
The LCST fibers cross the midline at the pyramidal decussation (junction between brainstem and spinal cord), then descend in the lateral white matter of the spinal cord, with the tract covering a large extent of the lateral column and extending anteriorly beyond the central canal. 3
The tract exhibits somatotopic organization, meaning motor neurons controlling specific body parts (leg, trunk, hand, face) are arranged in an orderly fashion within the tract, though this organization remains challenging to fully reconstruct even with modern imaging techniques. 4
In the cervical cord, the LCST occupies a large portion of the lateral white matter, with some fibers separating from the main tract to reach the anterolateral periphery of the cord. 3
Clinical Significance for Motor Control
LCST integrity demonstrates a significant predictive relationship with ipsilateral lower extremity motor function, with spared tissue in the right LCST correlating with right leg motor recovery (β=0.56, R=0.43) and left LCST with left leg motor recovery (β=0.66, R=0.51). 5
The LCST is critical for lateralization of motor control, with approximately 75% of spinal cords showing asymmetry due to a greater number of corticospinal fibers crossing to the right side, independent of handedness. 3
Distinction from Sensory Pathways
The LCST is distinct from the dorsal columns, which carry sensory information (light touch, proprioception), emphasizing that the lateral corticospinal tract is purely a motor pathway. 5
In motor neuron diseases like ALS, degeneration of the LCST produces upper motor neuron signs (hypertonicity, hyperreflexia, spasticity) reflecting loss of voluntary motor control, with MRI showing abnormal T2 signal in the lateral corticospinal tracts corresponding to this degeneration. 6, 7
Functional Testing
Motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation travel through the corticospinal pathway via rapidly conducting oligosynaptic connections, with central conduction time measuring the speed of transmission from motor cortex to spinal motor neurons. 6
During voluntary muscle contraction, MEP amplitude increases and latency decreases compared to rest, reflecting enhanced excitability at both cortical and spinal levels within the corticospinal pathway. 6