Descending motor tracts are the upper motor neurons (UMN) that originate in the brain and descend to the spinal cord to modulate the activity of lower motor neurons (LMN), specifically the α and γ motoneurons that execute reflexes.

These tracts are functionally organized into two major systems:

1. The Pyramidal (Corticospinal) System

These tracts originate primarily in the precentral gyrus (motor cortex) and descend directly to the spinal cord without synapsing in the brainstem.

• Lateral Corticospinal Tract: Comprises about 80–90% of fibers that decussate (cross over) in the medullary pyramids. It is essential for fine, skilled voluntary movements of distal limbs (like your fingers).

• Anterior Corticospinal Tract: The remaining uncrossed fibers that typically decussate later at the spinal level where they terminate.

• Role in Reflexes: The pyramidal system normally suppresses primitive reflexes. For example, damage to this system "releases" the Babinski sign (upward toe fanning), which is normally inhibited in adults.

2. The Extrapyramidal (Brainstem) System

These tracts originate in various brainstem nuclei and are primarily responsible for postural control and modulating muscle tone.

• Reticulospinal Tracts: Originating in the reticular formation (medulla and pons), these are the major pathways for regulating the γ motoneurons that set muscle spindle sensitivity.

  • Pontine fibers are generally excitatory to extensors, while medullary fibers are primarily inhibitory.

• Vestibulospinal Tracts: Arise from vestibular nuclei and process balance information. The lateral vestibulospinal tract activates "antigravity" (extensor) muscles to maintain an upright posture.

• Rubrospinal Tract: Originates in the red nucleus of the midbrain. It excites flexor motoneurons and inhibits extensors, playing a key role in the limb flexion seen in certain types of brain injury.

Functional Organization and Reflex Control

• Medial Pathways: Include the vestibulospinal, reticulospinal, and tectospinal tracts. They terminate in the ventromedial gray matter to control axial and proximal muscles for posture.

• Lateral Pathways: Include the lateral corticospinal and rubrospinal tracts. They terminate in the dorsolateral gray matter to control distal limb muscles.

• α-γ Coactivation: During voluntary movement, UMNs typically stimulate both α and γ motoneurons simultaneously. This ensures that as a muscle shortens, the spindle "takes up the slack" and remains sensitive to unexpected stretches.

Clinical Applied Aspects: UMN Lesions

When these descending tracts are damaged (e.g., by a stroke), the inhibitory influence on spinal reflexes is lost. This leads to:

• Spasticity: High muscle tone due to hyperactive γ motoneurons making spindles overly sensitive.

• Hyperreflexia: Exaggerated deep tendon reflexes (like the knee-jerk).

• Clonus: A series of rhythmic, involuntary contractions caused by repeated triggerings of the stretch reflex.