Corticospinal and Descending Motor Tracts

The descending motor tracts carry commands for movement from the brain to the motor neurons of the brainstem and spinal cord. The chief and most studied of these is the corticospinal (pyramidal) tract, which originates in the cerebral cortex, descends through the internal capsule, cerebral peduncle, pons, and medullary pyramids, and largely crosses to the opposite side before reaching the spinal cord. Alongside it run several extrapyramidal pathways from the brainstem that regulate posture, tone, and reflexive movement.

Definition

The descending motor tracts are projection fibre systems conveying motor signals from the cerebral cortex (corticospinal and corticobulbar) and brainstem nuclei to lower motor neurons; the corticospinal or pyramidal tract is the principal pathway for voluntary, especially fine distal, movement.

Scope

This entry covers the origin, course, decussation, and termination of the corticospinal and corticobulbar tracts and the principal brainstem descending pathways (rubrospinal, reticulospinal, vestibulospinal, tectospinal), treated as reference anatomy. It does not provide clinical assessment of weakness or management of motor disorders.

Key concepts

• Corticospinal (pyramidal) tract
• Corticobulbar tract
• Pyramidal decussation
• Lateral and anterior corticospinal tracts
• Extrapyramidal (brainstem) descending pathways
• Upper and lower motor neuron
• Somatotopy and the motor homunculus

Mechanisms

Corticospinal fibres arise from pyramidal neurons of the primary motor cortex and adjacent premotor and somatosensory areas, converge through the corona radiata and posterior limb of the internal capsule, pass through the middle of the cerebral peduncle, the basis pontis, and the medullary pyramids. At the cervicomedullary junction most fibres cross in the pyramidal decussation to form the lateral corticospinal tract of the contralateral spinal cord, while a minority continue uncrossed as the anterior corticospinal tract. The tract synapses on interneurons and, particularly in primates, directly on motor neurons that control distal musculature, supporting fractionated finger movement. Brainstem pathways, including the rubrospinal, reticulospinal, vestibulospinal, and tectospinal tracts, run in parallel and influence axial and proximal muscles, posture, and tone; Lemon describes how these descending systems are organised and how the corticospinal contribution to direct motor-neuron control is most developed in primates.

Clinical relevance

The course of the corticospinal tract explains why a lesion on one side of the brain typically produces weakness on the opposite side of the body, and why the pattern of deficit differs above and below the pyramidal decussation. The entry describes this anatomy for reference and is not a substitute for clinical evaluation.

Evidence & guidelines

Tract anatomy is established from gross dissection, myelin histology, and experimental tracer studies in animals, and is described in reference texts. In living humans, diffusion MRI tractography reconstructs the corticospinal tract from cortex to brainstem, as demonstrated in early fibre-tracking work and tract atlases; such reconstructions are descriptive and have known limitations where fibres cross.

History

The pyramidal tract was recognised from gross anatomy of the medullary pyramids and their decussation, and twentieth-century experimental work clarified its cortical origins and its direct connections to motor neurons in primates. Diffusion tractography from the late 1990s onward allowed the tract to be reconstructed non-invasively in individual living brains.

Key figures

• Roger Lemon
• Susumu Mori

Seminal works

• lemon2008
• mori1999
• wakana2004

Frequently asked questions

Why are the corticospinal tracts also called pyramidal tracts?

They form the medullary pyramids, the paired ridges on the front of the medulla through which the fibres descend, and are named for that landmark; the MeSH descriptor for the tract is Pyramidal Tracts.

Where does the corticospinal tract cross?

Most fibres cross at the pyramidal decussation at the junction of the medulla and spinal cord, forming the lateral corticospinal tract on the opposite side; a smaller uncrossed portion forms the anterior corticospinal tract.