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Descending Tracts of the Spinal Cord

I. Enumeration of Descending Tracts

Lateral Motor System (lateral column)

Medial Motor System (anterior column)

The fasciculus proprius (propriospinal tract) contains both ascending and descending fibers and connects cord segments for intersegmental reflexes.

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II. The Pyramidal Tract - Detailed Description

Definition and Nomenclature

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A. Origin (Cortical Contributions)

• ~30% - Primary motor cortex (precentral gyrus, area 4)
• ~30% - Premotor cortex and supplementary motor area (area 6)
• ~40% - Somatosensory areas posterior to the central sulcus (areas 3, 1, 2)

• ~60 µm in diameter
• ~34,000 Betz cell fibers per corticospinal tract
• Axon diameter ~16 µm; conduction velocity ~70 m/sec (fastest signals from brain to cord)
• Represent only 3% of all corticospinal fibers
• The remaining 97% are fibers <4 µm in diameter carrying tonic background signals

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B. Course Through the Brain

• The internal capsule carries corticospinal fibers in a somatotopic arrangement: face and arm anteriorly, leg posteriorly

• Occupies the middle 3/5 of the crus cerebri

• Fibers break into small bundles within the basis pontis to accommodate transverse pontocerebellar fibers

• Fibers regroup at the medulla to form the prominent ventral pyramids

• ~75-90% of fibers cross the midline → form the lateral corticospinal tract
• ~10-25% remain uncrossed → form the anterior (ventral) corticospinal tract

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C. Course in the Spinal Cord

• Located in the lateral funiculus (white matter)
• Descends the full length of the spinal cord
• Terminates predominantly at cervical and lumbosacral enlargements
• Synapses mainly on interneurons in the intermediate zone (Rexed laminae V-VII), with some direct connections to anterior horn motor neurons (lamina IX) - especially for fine finger movements

• Located in the anterior funiculus
• Extends only to cervical and upper thoracic cord
• Many fibers eventually cross at segmental level via anterior white commissure before synapsing
• Controls bilateral axial and girdle musculature

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D. Fiber Composition and Somatotopy

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E. Corticobulbar Component

• Corticospinal fibers (to spinal cord motor neurons)
• Corticobulbar fibers (to cranial nerve nuclei III-XII in the brainstem)

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III. Functions of the Pyramidal Tract

1. Voluntary skilled movements - especially fine, fractionated movements of the distal limbs (fingers, hand), which require direct corticomotoneuronal connections
2. Speed and precision of movement - fast-conducting Betz cell axons allow rapid initiation of voluntary acts
3. Contralateral limb control - lateral corticospinal tract governs contralateral extremities
4. Modulation of sensory input - fibers to the dorsal horn modulate incoming sensory signals (sensory gating)
5. Muscle tone regulation - descending cortical signals maintain appropriate background excitability of alpha and gamma motor neurons
6. Fractionated finger movements - the most evolved function, dependent on direct monosynaptic corticomotoneuronal projections

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IV. Effects of Lesion at Different Levels

Distinguishing Features: UMN vs. LMN

Note: Immediately after an acute pyramidal lesion, spinal shock occurs - transient flaccidity, areflexia, and absent Babinski. Spasticity develops over days to weeks.

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Level 1: Internal Capsule

• Contralateral hemiplegia (face, arm, and leg - the homunculus is compact here)
• Corticobulbar involvement causes contralateral lower facial weakness (forehead spared - bilateral supply) and tongue deviation toward the weak side
• Due to tight packing: a small lesion (e.g., lacunar infarct) causes dense hemiplegia - "pure motor stroke"
• Signs: contralateral spastic hemiplegia, hyperreflexia, Babinski sign, dysarthria

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Level 2: Midbrain (Crus Cerebri)

• Ipsilateral CN III palsy (ptosis, mydriasis, "down-and-out" gaze) + contralateral hemiplegia = Weber's syndrome
• This occurs because the oculomotor nerve exits medially, adjacent to the descending corticospinal fibers in the crus cerebri

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Level 3: Pons

• Ipsilateral CN VI and/or VII palsy + contralateral hemiplegia = Millard-Gubler syndrome (lesion in the basis pontis)
• Ipsilateral CN VI palsy + contralateral hemiplegia = Foville's syndrome (more posterior lesion)

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Level 4: Medulla (Pyramid)

• Lesion at or just above the decussation: contralateral hemiplegia (arm and leg) with NO cranial nerve involvement (the pyramids are below CN nuclei)
• Ipsilateral pyramid lesion BELOW the decussation: ipsilateral weakness (rare, before crossing)
• Anterior spinal artery territory infarct: bilateral pyramidal signs + loss of pain/temperature (spinothalamic involved) with dorsal column sparing = medial medullary syndrome

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Level 5: Cervical Spinal Cord (above C5)

• Quadriplegia (all 4 limbs)
• UMN signs below the lesion level
• LMN signs at the level of the lesion (destruction of anterior horn cells at that segment)
• Above C3-C5: respiratory failure (phrenic nerve involvement, diaphragm paralysis)
• Loss of all sensation below the level
• Bladder and bowel dysfunction (initially retention, later automatic/reflex bladder)

• Ipsilateral spastic weakness and loss of proprioception/vibration below lesion
• Contralateral loss of pain and temperature below lesion (one to two levels lower)
• Ipsilateral LMN signs at the level of the lesion

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Level 6: Thoracic Spinal Cord (T1-T12)

• Paraplegia (lower limbs only - arms are normal, as corticospinal fibers to arms have already exited at cervical level)
• UMN signs in lower limbs: spasticity, hyperreflexia, Babinski sign
• Sensory loss with a dermatomal sensory level
• Bladder/bowel dysfunction; autonomic dysfunction
• At T1: may have Horner syndrome (disruption of sympathetic pathway)
• Above T6: risk of autonomic dysreflexia (exaggerated sympathetic response to stimuli below the lesion)

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Level 7: Lumbar Spinal Cord (L1-L5)

• Paraplegia of lower limbs; may be UMN (conus intact) or mixed UMN+LMN depending on exact level
• Below the conus medullaris (L1-L2 in adults), lesions are technically cauda equina (LMN) - flaccid paralysis, areflexia, bladder/bowel dysfunction of LMN type (overflow incontinence, impotence)

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Level 8: Sacral Cord / Conus Medullaris

• LMN bladder and bowel (atonic bladder, fecal incontinence)
• Saddle anesthesia (S3-S5 dermatomes)
• Preserved leg strength (lumbar segments above are intact)

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Summary Table: Lesion Level vs. Effect

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• Gray's Anatomy for Students - Descending Tracts, pp. 1328-1335
• Guyton and Hall Textbook of Medical Physiology - Corticospinal (Pyramidal) Tract, p. 689
• Bradley and Daroff's Neurology in Clinical Practice - Pyramidal Tract; Anterior Horn and Pyramidal Tract Syndromes; Characteristic clinical features of lesions at different levels, pp. 467, 483-484