Corticospinal Tract and Other Motor Pathways

by

MOTOR/SENSORY CORTEX, SOMATOTOPIC ORGANIZATION

Primary Motor Cortex

  • Precentral gyrus, Brodmann’s area 4

Primary Sensory Cortex

  • Postcentral gyrus, Brodmann’s areas 3, 1, 2

Somatotopic Organization of Primary Motor/Sensory Cortex

  • Adjacent regions on cortex correspond to adjacent areas on the body surface
  • Classically depicted by Motor and Sensory Homunculus
  • Not as clear-cut and consistent as originally believed

BASIC ANATOMY OF THE SPINAL CORD

Central Gray Matter

  • Butterfly-shaped
  • Surrounded by ascending/descending white matter columns (funiculi)
  • Dorsal (posterior) horn: primarily sensory processing; sensory neurons in the dorsal root ganglia have axons that bifurcate – one conveys sensory information from the periphery, the other carries the info through the dorsal nerve root filaments to dorsal aspects of the cord
  • Intermediate zone: contains interneurons and specialized nuclei
  • Ventral (anterior) horn: contains motor neurons that send axons out via ventral nerve root filaments
  • Can also be divided into nuclei/laminae

White Matter

  • Consists of Dorsal (posterior) columns and Ventral (anterior) columns
  • Thickest in the cervical levels where most ascending fibers have already entered the cord and most descending fibers have not yet terminated; sacral cord is mostly gray matter

Cervical and Lumbosacral Enlargements

  • Give rise to the nerve plexuses for the arms and legs
  • Has more gray matter at these levels, esp. in the ventral horns where lower motor neurons for arms and legs reside

Lateral Horn

  • Thoracic level
  • Contains intermediolateral cell column

SPINAL CORD BLOOD SUPPLY

Spinal Blood Supply

  • Arises from branches of the vertebral arteries and spinal radicular arteries
  • Vertebral arteries give rise to anterior spinal artery that runs along ventral surface supplying anterior 2/3 of the cord – anterior horns, anterior and lateral columns
  • Two posterior spinal arteries arise from vertebral/posterior inferior cerebellar arteries to supply dorsal surface supplying posterior columns and part of the posterior horns
  • The anterior and posterior arteries form a spinal arterial plexus that surrounds the cord
  • 31 segmental arterial branches, most from aorta to supply meninges, 6-10 of these radicular arteries
  • Great radicular artery of Adamkiewicz: major blood supply to lumbar/sacral cord, arising from left side T5-L3, usually between T9-T12
  • Mid-thoracic, T4-T8, between lumbar and vertebral arterial supplies, vulnerable zone of decreased perfusion, risk of infarction 2ndary thoracic surgery/other conditions of decreased aortic pressure
  • Batson’s plexus: epidural veins, don’t contain valves

GENERAL ORGANIZATION OF THE MOTOR SYSTEMS

Basic Facts

  • Elaborate network of multiple, hierarchical feedback loops
  • Cerebellum, basal ganglia, thalamic participation discussed in ch. 2, cortical regions in ch. 19
  • Upper motor neurons carry output to lower motor neurons in spinal cord and brainstem which project to muscles in the periphery
  • Descending upper motor neuron pathways divided into lateral and medial motor systems

Medial Motor Systems

  • Made up of 4 systems; control proximal axial girdle muscles involved in postural tone, balance, orienting movements of head, gait-related movements
  • Descend ipsilaterally or bilaterally
  • Unilateral lesions typically produce no obvious deficits
  • Tend to terminate on interneurons that project bilaterally – multiple spinal segments
  • Anterior corticospinal tract; vestibulospinal tracts; reticulospinal tracts; tectospinal tract

Lateral Motor System

  • Rubrospinal Tract
    • Small, uncertain clinical importance
    • May take over functions of corticospinal functions after injury
    • May play role in flexor (decorticate) posturing upper extremities
  • Lateral Corticospinal Tract
    • Most clinically important descending motor pathway; pyramidal tract
    • Controls movement of the extremities; lesions produce characteristic deficits for localization
    • Over ½ of the fibers originate in primary motor cortex (area 4) located in cortical layer 5
    • The rest from premotor and supplementary motor or parietal lobe (areas 3,1,2,5,7)
    • 3% of its neurons are Betz cells – giant pyramidal cells
    • Somatotopic representation-upper extremities medial to lower extremities
    • Lies in the posterior limb of the internal capsule
      • Internal Capsule
        • Corticospinal/corticobulbar fibers form part of it
        • Anterior limb separates head of caudate from globus pallidus and putamen
        • Posterior limb separates thalamus from globus pallidus and putamen
        • Genus transition between anterior and posterior limbs
        • Somatotopic map preserved– ace most anterior, arm and leg progressively posterior
        • Fibers projecting from cortex to the brainstem are called corticobulbar instead of corticospinal bc they go to the brainstem or “bulb”
        • Fibers compact so that lesions generally produce weakness of the entire contralateral body; occasionally more selective deficits
      • Cerebral peduncles
        • Internal capsule continues into midbrain cerebral peduncles (“Feet of the brain”)
        • Basis peduncle: white matter, ventral side
        • Middle 1/3 of basis peduncle: corticospinal/corticobulbar fibers, face/arm/leg axons go medial to lateral
      • Medullary Pyramids
        • Corticospinal fibers next descend through ventral pons forming scattered fascicles which collect on ventral surface to form medullary pyramids
        • Origin of imprecise pyramidal tract label
      • Cervicomedullary Junction
        • Transition from medulla to spinal cord
        • 85% of pyramidal fibers cross over in the pyramidal decussation to enter lateral white matter columns, forming the lateral corticospinal tract
        • Axons enter spinal cord gray matter to synapse onto anterior horn cells
        • Lesions above pyramidal decussation = contralateral weakness; below = ipsilateral weakness
        • Remaining 15% of fibers continue ipsilaterally and enter the anterior white matter columns to form the anterior corticospinal tract

AUTONOMIC NERVIOUS SYSTEM (ANS)

Basic Facts

  • Controls more automatic and visceral functions in contrast to somatic motor pathways just discussed
  • Autonomic efferents: peripheral synapse in ganglion btw CNS and effector gland/smooth muscle; in contrast to somatic efferents: anterior horn/cranial nerves project directly to skeletal muscle
  • While there are sensory inputs, the ANS itself consists of only efferent paths
  • Two main divisions: sympathetic (thoracolumbar) and parasympathetic (craniosacral)

Sympathetic

  • Arises from T1 to L2/L3
  • “Fight or flight” e.g., increasing BP, Hrt rate, bronchiodilation, pupil size
  • Preganglionic neurons: in the intromediolateral cell column in lamina VII, T1-L2/L3, travel short distance
    • Two Sets of Ganglia
      • Paired paravertebral ganglia: form sympathetic chain/trunk, bilateral, cervical to sacral
      • Paired prevertebral ganglia: in celiac plexus around aorta
  • Postganglionic neurons: travel long distances to reach effector organs; release mainly norepinephrine; one exception – sweat glands (acetylcholine)
  • Synaptic transmission mediated by acetylcholine (nicotinic receptors) released by preganglionic neurons
  • Outflow controlled directly/indirectly by higher centers
  • Also regulated by afferent sensory information including internal receptors (e.g., chemoreceptors)

Parasympathetic

  • Arises from cranial nerves and S2 to S4
  • “Rest and digest” e.g., increasing gastric secretions and peristalsis, decreasing Hrt rate and pupil size
  • Axons travel long distance to terminal ganglia within or near effector organs
  • Preganglionic fibers arise from cranial nerve parasympathetic nuclei and from sacral parasympathetic nuclei in the lateral gray matter of S2-S4 and intromedial cell column
  • Postganglion neurons release mainly acetylcholine (muscarinic cholinergic receptors)
  • Synaptic transmission mediated by acetylcholine (nicotinic receptors)
  • Outflow controlled directly/indirectly by higher centers
  • Also regulated by afferent sensory information including internal receptors (e.g., chemoreceptors)

KEY CLINICAL CONCEPTS

Upper vs. Lower Motor Neuron Lesions

  • Upper motor neurons of corticospinal tract project from cortex to lower motor neurons in the anterior horn of the spinal cord
  • Lower motor neurons in turn project via peripheral nerves to skeletal muscle
  • Signs of upper motor neuron lesions: muscle weakness and increased tone and hyperreflexia (spasticity), additional abnormal reflexes, e.g., Babinski; may initially be flaccid paralysis gradually developing into spastic paresis
  • Signs of lower motor neuron lesions: muscle weakness, atrophy, fasiculations (abnormal muscle twitches), hyporeflexia
  • Weakness can be caused by lesions at any level in the motor system

Weakness Patterns and Localization

  • Unilateral face/arm/leg: hemiparesis/plegia
    • No sensory deficits: contralateral; corticospinal/bulbar below cortex/above medulla; post. Limb internal capsule; basis pontis; mid 1/3 peduncle
    • With somatosensory/oculomotor/visual/higher cortical deficits: contralateral; primary motor cortex; corticospinal/bulbar above medulla
  • Unilateral arm/leg: contralateral above pyramidal decussation; ipsilateral below pyramidal decussation; arm/leg motor cortex; corticospinal lower medulla to C5
  • Unilateral face/arm: faciobrachial paresis/plegia; face/arm motor cortex
  • Unilateral arm: brachial monoparesis/plegia; contralateral arm motor cortex; ipsilateral peripheral nerves supplying arm
  • Unilateral leg: crural monoparesis/plegia; contralateral leg motor cortex; ipsilateral lateral corticospinal below T1, or peripheral nerves supplying the leg
  • Unilateral facial: Bell’s palsy/isolated facial weakness; common: ipsilateral facial nerve (CN VII); uncommon: contralateral face motor cortex or genu of internal capsule
  • Bilateral arm: brachial diplegia; medial fibers of corticospinal; bilateral cervical ventral horn; bilateral peripheral nerve/muscle d/o’s
  • Bilateral leg: paraparesis/plegia; bilateral leg motor cortex; lateral corticospinal below T1; cauda equina syndrome/bilateral peripheral nerve/muscle d/o
  • Bilateral arm/leg: quadraparesis/plegia; tetraparesis/plegia; bilateral arm/leg motor cortex; bilateral corticospinal lower medulla to C5; peripheral nerve motor neuron/muscle d/o’s – usually also affect the face
  • Generalized: bilateral entire motor cortex; bilateral corticospinal/bulbar anywhere from corona radiata to pons; diffuse d/o involving all lower motor neurons, peripheral axons, neuromuscular junctions, or muscles
  • Patterns not listed above: consider 2 or more lesions, unusual lesions, anatomical variants, or non-neurological