Pyramidal tract

Where does the pyramidal tract originate?

The corticospinal pathway originates from upper motor neurons of the cerebral cortex. The main sources of fibers:

• Primary motor cortex (area 4): located in the precentral gyrus and paracentral lobule. Layer V of the cortex contains giant pyramidal Betz cells, whose axons form the thick myelinated fibers of the tract.
• Premotor area (areas 6 and 8): regions of the frontal lobe anterior to the precentral gyrus.
• Postcentral gyrus: partly contributes fibers from the parietal lobe.

The cortex has a strict somatotopic organization (motor homunculus). Projections of the legs and trunk are represented on the medial surface and upper part of the gyrus, the arms (especially the extensive hand area) in the middle, and the muscles of the face, pharynx, and larynx in the lower part.

How does the pyramidal tract run? (Topography)

Fibers from the cortex converge into the corona radiata and descend, passing through key brain structures:

• Internal capsule (capsula interna): the tract passes through the genu (fibers to the face) and the anterior two-thirds of the posterior limb (fibers to the arm, trunk, and leg). The tract lies close to the thalamus and basal ganglia.
• Midbrain (mesencephalon): fibers occupy the middle part of the basis pedunculi (pedunculus cerebri).
• Pons (pons): transverse fibers of the pons split the pyramidal tract into separate bundles.
• Medulla oblongata (medulla oblongata): on the ventral surface (anterior to the medial lemniscus), the fibers converge again, forming two longitudinal bundles — the pyramids (pyramides).

Branches and decussation of the tract

During its descent, the pyramidal tract divides into two functional pathways:

• Corticonuclear (corticobulbar) tract: branches off at the brainstem level to the motor nuclei of the cranial nerves (III, IV, V, VI, VII, IX, X, XI, XII). Fibers decussate to the nuclei of the VII and XII nerves. However, they do not terminate directly on the motor neurons of the III, IV, and VI pairs, but connect with them via interneurons.
• Corticospinal tract: the transit pathway to the spinal cord. At the junction of the medulla and spinal cord, the pyramidal decussation (decussatio pyramidum) occurs.

After decussation, the corticospinal pathway divides into:

• Lateral corticospinal tract: consists of crossed fibers, descending in the lateral funiculi of the spinal cord.
• Anterior corticospinal tract: consists of uncrossed fibers, running in the anterior funiculi.

Function

The pyramidal tract is the origin of voluntary movements.

As part of the lateral motor system, it is responsible for activating flexor muscles, ensuring, for example, the lifting and forward swing of a limb (the swing phase of gait). The tract also exerts descending control over reflex arcs at the spinal cord level, regulating muscle fiber length.

Clinical significance: levels of lesions

Symptoms depend on the level of upper motor neuron damage:

• Cerebral cortex: due to the wide distribution of neurons, a lesion often causes isolated contralateral monoparesis (arm only or face only).
• Internal capsule: fibers are compactly bundled, so a lesion causes contralateral hemiplegia (paralysis of the face, arm, and leg on the opposite side).
• Midbrain (cerebral peduncle): Weber's alternating syndrome occurs (oculomotor nerve lesion on the side of the lesion and hemiplegia on the opposite side).
• Spinal cord: compression of the lateral funiculi (e.g., in syringomyelia or extramedullary tumors) leads to spastic (upper motor neuron) paresis below the level of the lesion.

Class and exam questions

What else to study on this topic

• Motor homunculus of the cortex
• Internal capsule of the brain
• Alternating brainstem syndromes
• Extrapyramidal system

Other topics in the "Anatomical Structures" section

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