Corticospinal Tract | Upper Motor Neuron Pathway Explained

This extensive section explains how basal ganglia control movement through neural circuits. The motor system involves upper motor neurons (corticospinal and cortical nuclear fibers) descending from cerebral cortex to control lower motor neurons in spinal cord and brainstem. Motor plans are stored in basal ganglia and cerebellum. When initiating movement, the idea must first consult basal ganglia for planning and refinement before descending to lower motor neurons. Basal ganglia electrical activity begins even before movement execution. A cortical-basal ganglia-thalamic loop allows consultation, refinement, and execution coordination. Movement initiation begins in prefrontal cortex, proceeds to supplementary motor area and premotor area, consults basal ganglia, receives thalamic feedback, and activates primary motor and somatosensory areas before descending to lower motor neurons.

Two critical brain structures collaborate with the motor cortex for movement planning. The cerebellum provides information from previous movement attempts, helping the brain identify useful versus incorrect strategies. It modulates planned movement information by filtering out errors and reinforcing successful approaches. The basal ganglia, located deep in the brain, regulate information flow and energy distribution. They synthesize multiple fragmented signals into fewer, more powerful commands, consolidating information to improve energy efficiency. They also route only essential information to planning areas while filtering out irrelevant signals.

This lecture introduces three major motor systems: the corticospinal tract (pyramidal system), cerebellar connections, and basal ganglia circuitry. The corticospinal tract serves as the final common pathway for voluntary movements, originating from the precentral gyrus, somatosensory cortex, and premotor areas. It travels through the internal capsule and cerebral peduncle, decussating at the spinal cord level to control the opposite body side. The cerebellum connects via three peduncles and has hemispheres and vermis regions. The basal ganglia include the caudate, putamen (striatum), globus pallidus, subthalamic nucleus, and substantia nigra, working through the thalamus to influence the corticospinal tract. These three circuits work together to produce coordinated voluntary movement.

Upper motor neurons in the cortex send three types of descending fibers: corticospinal fibers to the spinal cord, corticonuclear fibers to cranial nerve nuclei, and corticopontine fibers to the cerebellum via the middle cerebellar peduncle. The substantia nigra pars compacta sends the nigrostriatal pathway to the striatum, releasing dopamine onto neurons in the direct and indirect pathways of the basal ganglia. Dopamine enhances motor activity in the direct pathway and reduces inhibition in the indirect pathway; damage leads to Parkinson's disease. The deep cerebellar nuclei send output through cerebello-rubral fibers (crossing at the superior cerebellar peduncle decussation to the red nucleus) and cerebello-thalamic fibers (crossing to the thalamus), transmitting motor plans from the cerebellum to the cortex for coordinated movement.

The basal ganglia consist of interconnected nuclei including the caudate nucleus, putamen, globus pallidus, and subthalamic nucleus. They regulate voluntary movement through direct and indirect pathways: the direct pathway facilitates movement while the indirect pathway inhibits unwanted movements. The pyramidal tract carries motor commands from cortical neurons to spinal motor neurons. Dysfunction in these circuits causes movement disorders like Parkinson's disease. The cerebellum coordinates motor timing and precision.

The extrapyramidal motor pathways are involuntary descending tracts that do not pass through the pyramids of the medulla and include five main tracts: the rubrospinal tract (originating from the red nucleus, controlling upper limb flexion and inhibiting extension), the reticulospinal tract (from the reticular formation, controlling posture, balance, and autonomic regulation), the tectospinal tract (from the superior colliculus, coordinating involuntary head and eye movements in response to visual stimuli), the vestibulospinal tract (from vestibular nuclei, maintaining posture and equilibrium), and the olivospinal tract (from the inferior olivary nucleus, involved in motor coordination). These pathways are distributed in the lateral and anterior funiculi of the spinal cord, with the tectospinal tract only extending to the cervical region.

The descending motor pathways control voluntary and involuntary movements through two main systems: the pyramidal system (corticospinal and corticobulbar tracts) which controls voluntary movements of extremities, trunk, and head/neck, and the extrapyramidal system (rubrospinal, tectospinal, vestibulospinal, and reticulospinal tracts) which regulates involuntary movements, posture, balance, and muscle tone. The pyramidal system originates from the cerebral cortex (areas 4, 6, and sensory areas 1-3) and descends through the brainstem to synapse with lower motor neurons in the spinal cord anterior horn. The extrapyramidal system originates from the premotor cortex and brainstem nuclei, with tracts like the rubrospinal tract activating flexor muscles and inhibiting extensors, while the vestibulospinal and reticulospinal tracts facilitate extensor muscles. Lesions in upper motor neurons cause spastic paralysis with hyperreflexia and Babinski sign, while lower motor neuron lesions cause flaccid paralysis with hyporeflexia.

The descending tracts of the spinal cord are divided into pyramidal tracts (voluntary motor control) and extrapyramidal tracts (involuntary motor control). The pyramidal tracts include the corticospinal tract (controlling body musculature with 75% crossing at the medulla to form the lateral tract and 25% remaining as the anterior tract) and the corticobulbar tract (controlling facial and head muscles with bilateral innervation except for cranial nerves VII and XII). The extrapyramidal tracts include the rubrospinal tract (fine motor control, contralateral), reticulospinal tracts (medial excites extensors/increases tone, lateral inhibits voluntary movement/decreases tone), vestibulospinal tract (balance and posture, ipsilateral), and tectospinal tract (head-eye coordination, contralateral). Lesions in pyramidal tracts cause upper motor neuron signs including hypertonicity, hyperreflexia, weakness, and Babinski's sign, while extrapyramidal lesions result in parkinsonism, chorea, athetosis, and dystonia.

The extrapyramidal tracts are descending motor pathways that regulate posture, balance, and reflexive movements without passing through the medullary pyramids; they include the rubrospinal tract (originating from the red nucleus, maintaining flexor tone and fine motor control of upper limbs), reticulospinal tracts (medial from pons facilitating extensor tone and lateral from medulla inhibiting voluntary movement), vestibulospinal tracts (medial with bilateral projections and lateral with ipsilateral projections, controlling anti-gravity muscle tone based on vestibular input), and tectospinal tract (from superior colliculus enabling reflexive head, eye, and trunk movements to stimuli); damage to these pathways can result in clinical conditions such as decerebrate rigidity, characterized by continuous extensor muscle contraction.

The descending motor pathways of the central nervous system are divided into pyramidal (corticospinal and corticonuclear tracts) and extrapyramidal systems (tectospinal, reticulospinal, vestibulospinal, and rubrospinal tracts). The pyramidal system, originating from the cerebral cortex, primarily stimulates flexor muscles and inhibits extensor muscles in the extremities, neck, and trunk. The extrapyramidal system, arising from structures like the midbrain, pons, and medulla, maintains muscle tone, automatic movements, and associated movements. The extrapyramidal tracts include: tectospinal (midbrain, facilitates contralateral muscles), reticulospinal medial (pons, facilitates axial and extensor muscles), reticulospinal lateral (medulla, facilitates flexor muscles), vestibulospinal lateral (facilitates extensor muscles), vestibulospinal medial (controls head position and facilitates flexors), and rubrospinal (red nucleus, activates flexor muscles). These pathways are classified into a medial system (vestibulospinal, tectospinal, reticulospinal) that facilitates extension, and a lateral system (corticospinal, rubrospinal) that facilitates flexion.

Brown-Séquard syndrome results from unilateral spinal cord hemisection, causing ipsilateral spastic paralysis and loss of proprioception due to corticospinal tract and dorsal column damage, while contralateral loss of pain and temperature sensation occurs due to spinothalamic tract disruption; diagnosis is confirmed via MRI, and causes include trauma, tumors, ischemia, or inflammatory diseases.

Brown Sequard syndrome results from hemisection of the spinal cord, commonly caused by trauma. The syndrome involves distinct sensory pathways: the dorsal column carries fine touch, vibration, proprioception, two-point discrimination, and stereognosis, ascending ipsilaterally to gracile/cuneate nuclei before crossing to the thalamus; the spinothalamic tract carries pain, temperature, and crude touch, crossing contralaterally at entry. After hemisection, at the lesion level complete anesthesia occurs; below the lesion, ipsilateral dorsal column loss and contralateral spinothalamic loss produce characteristic sensory-motor dissociation. The spino-cerebellar system carries unconscious proprioception for movement coordination, with dorsal and ventral tracts producing ipsilateral and contralateral dysmetria respectively.

Brown-Séquard syndrome results from hemisection: ipsilateral motor/vibration/proprioception loss at lesion level, contralateral pain/temperature loss below. Amyotrophic lateral sclerosis involves both upper and lower motor neurons (hyperreflexia/spasticity plus fasciculations/atrophy), never causing sensory loss. Spinal muscular atrophy affects only anterior horn cells (lower motor neurons), causing tongue fasciculations and floppy baby presentation in children. Anterior spinal artery occlusion spares dorsal columns but destroys anterior horn and corticospinal tracts, causing paralysis and pain/temperature loss without sensory ataxia.

Amyotrophic lateral sclerosis (ALS/Lou Gehrig's disease) is a progressive neurodegenerative disorder affecting both upper and lower motor neurons, causing spasticity/hyperreflexia from upper neuron damage and flaccid paralysis/atrophy from lower neuron damage. Multiple sclerosis is an autoimmune disease destroying myelin sheaths in the CNS, decreasing conduction velocity and potentially blocking signal transmission. Spinal cord injury syndromes include central cord syndrome (greater arm than leg impairment), anterior cord syndrome (anterior spinal artery infarction damaging motor and pain/temperature tracts), and Brown-Séquard syndrome (hemisection causing ipsilateral motor loss and contralateral pain/temperature loss).

Brown-Séquard syndrome results from hemisection of the spinal cord, causing ipsilateral loss of proprioception and vibration sense (due to dorsal columns crossing at the level of injury) and contralateral loss of pain and temperature sensation (spinothalamic tract crosses at spinal level). Motor function is preserved on the ipsilateral side. This classic syndrome demonstrates the decussation patterns of sensory tracts.