The Corticospinal Tract (CST) is the primary descending motor pathway that carries voluntary movement commands from the cerebral cortex to the spinal cord. This tract is essential for fine motor control, dexterity, and skilled movements[^1].
In neurodegenerative diseases, the corticospinal tract undergoes significant degeneration, contributing to the characteristic motor symptoms including weakness, spasticity, and loss of voluntary movement[^2].
| Property |
Value |
| Category |
Motor Pathway |
| Location |
Motor cortex, internal capsule, cerebral peduncle, pyramids, lateral funiculus |
| Cell Types |
Upper motor neurons (Betz cells, pyramidal neurons) |
| Primary Neurotransmitter |
Glutamate |
| Key Markers |
VGLUT1, CTIP2, FoxP1, SatB2 |
The corticospinal tract originates from multiple cortical areas:
- Primary motor cortex (M1): Betz cells in layer 5 (largest corticospinal neurons)
- Premotor cortex: Supplementary motor area
- Somatosensory cortex: Posterior parietal cortex
- Frontal eye fields: For eye movement control[^3]
Approximately 1 million corticospinal neurons project from each cerebral hemisphere.
The tract descends through:
- Corona radiata - Fan-like fibers from cortex to internal capsule
- Posterior limb of internal capsule - Between thalamus and globus pallidus
- Cerebral peduncle - Midbrain
- Pyramids - Medulla (where 85% decussate)
- Lateral funiculus - Spinal cord (after decussation)
- Anterior funiculus - (15% that do not decussate)[^4]
Fibers terminate on:
- Alpha motor neurons in ventral horn (direct corticomotor neurons)
- Interneurons in Rexed laminae VII-IX
- Reticulospinal neurons (indirect pathways)
The corticospinal tract is essential for:
- Fine motor control: Manipulation, writing, buttoning
- Dexterity: Independent finger movements
- Force gradation: Precise muscle contraction control
- Motor learning: Acquisition of skilled movements[^5]
Corticospinal fibers modulate:
- Reciprocal inhibition: Coordinating agonist/antagonist muscles
- Gain setting: Adjusting reflex sensitivity
- Postural control: Maintaining balance during movement[^6]
The corticobulbar tract is a component controlling:
- Facial expression
- Jaw movement
- Swallowing
- Speech production
ALS is characterized by:
- Upper motor neuron degeneration: Loss of corticospinal tract neurons
- Lower motor neuron degeneration: Loss of spinal motor neurons
- Spasticity: Hypertonia due to loss of cortical inhibition
- Weakness: Progressive loss of voluntary movement[^7]
Degeneration of corticospinal tract causes:
- Progressive lower limb spasticity
- Weakness
- Urinary urgency
A rare disorder affecting only upper motor neurons:
- Progressive spasticity
- Slow disease progression
- Relative preservation of other functions[^8]
Upper motor neuron lesions cause:
- Spasticity: Velocity-dependent increased tone
- Hyperreflexia: Exaggerated deep tendon reflexes
- Babinski sign: Extensor plantar response
- Clonus: Rhythmic muscle contractions
- MRI: Assess corticospinal tract integrity
- Diffusion tensor imaging: Measure fractional anisotropy
- Transcranial magnetic stimulation: Evaluate corticospinal excitability
- Nerve conduction studies: Rule out peripheral causes[^9]
- Baclofen: GABA-B agonist for spasticity
- Tizanidine: Alpha-2 adrenergic agonist
- Dantrolene: Calcium channel blocker
- Intrathecal baclofen pumps: Direct spinal drug delivery
- Deep brain stimulation: For tremor and rigidity
- Tend lengthening: For contractures
- Physical therapy: Maintain range of motion
- Occupational therapy: Adaptive strategies
- Constraint-induced movement therapy: Force affected limb use[^10]
The study of Corticospinal Tract Fibers has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.
Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.
- Lemon RN., Corticospinal neurons and motor cortex function (2008)
- Kuang R., Corticospinal tract degeneration in ALS (2015)
- Nieuwenhuys R., The human central nervous system (2013)
- Duvernoy HM., The human brain: Surface, blood supply (1999)
- Shen K., Corticospinal circuit function (2020)
- Pierrot-Deseilligny E., Control of gait and posture (2004)
- Hardiman O., Amyotrophic lateral sclerosis (2017)
- Fink JK., Hereditary spastic paraplegia (2014)
- Schubert M., MRI of corticospinal tract (2005)
- Gracies JM., Physical modalities for spasticity (2005)