Descending Tracts

Descending Tracts Dr . Niranjan Murthy H L Asst Prof of Physiology SSMC , Tumkur

• Final common path- Motor neuron • • Under the influence of Segmental 

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Suprasegmental Brainstem Cortical

Descending tracts

• Movements- Reflexive and Voluntary  - Skilled and Postural 

• Descending Tracts- Lateral and Medial  - Pyramidal and Extrapyramidal 

• Lateral pathways- Lateral corticospinal pathway and Rubrospinal Pathway 

• Medial Pathways- Reticulospinal, Vestibulospinal, Tectospinal, Anterior

Motor Areas of Cerebral Cortex

Pyramidal Tract • Corticospinal & Corticobulbar • Origin- 30% from motor cortex, 30% from premotor cortex and 40% from parietal lobe • Course: fibers converge towards internal capsule- Corona radiata àLie in genu and anterior 2/3rd of posterior limb of internal capsule àmiddle 3/5th of crus of mid-brain

àBroken up into scattered bundles by nucleus pontis à àProduce a bulge in medullapyramids à àIn lower medulla- 80% fibers cross to opposite side and descend as lateral corticospinal tract à à20% fibers descend uncrossed as anterior corticospinal tract

• Termination: directly on motor neuron or on the interneuron • 55% end in cervical region, 20% in thoracic and 25% in lumbosacral region.

Organization through the course Motor cortex- legs medial & head lateral

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Internal capsule- head anterior & legs posterior

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Mid-brain- head medial & legs lateral

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At each level, the fibers rotate by 90⁰

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Corticobulbar tracts • Origin and course is same till pons • • Fibers terminate on cranial nerve nuclei • • Termination may be ipsilateral, contralateral, or bilateral

Functions • Execution of skilled voluntary movements • • Anterior corticospinal tract controls proximal & axial muscles • • Form part of superficial reflex pathway

Rubrospinal Tract • Originates in magnocellular portion of Red nucleus • Cross over in midbrain and descend on opposite side • Terminate on motor neurons of distal muscles • Has somatotopic map in red nucleus • Concerned with skilled movements

Vestibulospinal tract • Lateral vestibulospinal tract arise from Deiter’s nucleus- receive input from utricle & saccule • • Medial vestibulospinal tract originates from medial & descending vestibular nucleireceive signals from semicircular canals •

Reticulospinal Tract • Medullary reticulospinal tract decreases tone of antigravity muscles • • Pontine reticulospinal tract increases tone of antigravity muscle • • Under the influence of higher centers

Tectospinal tract • Originate from superior colliculus • • Cross over to opposite side • • Concerned with muscles of head and neck • • Controls visually guided head movements

Medial Longitudinal Fasciculus • Originate from medial vestibular nucleus, reticular formation, superior colliculus and interstitial nucleus of cajal • • Descend uncrossed upto cervical region • • Co-ordination of reflex ocular movement and integration of eye &

Pyramidal Vs. Extrapyramidal system Characters

Pyramidal system

Extrapyramidal system

Phylogeny

New

Old

Myelination

After birth

Before birth

Functions

Fine, skillful movements

Gross postural movements

Damage

Flaccid paralysis

Spastic paralysis

Speed of conduction

Slow

Fast

Internal capsule • Mass of axons • Laterally lenticular nucleus and medially thalamus & caudate • Ascending sensory fibers occupy posterior 1/3rd of posterior limb • Corticospinal & Corticobulbar fibers occupy genu & anterior 2/3rds of posterior limb • Frontopontine fibers & thalamocortical fibers in anterior limb • Injury at this level causes hemiplegia

Applied aspects • Hemiplegia • • UMN lesions • • Pseudobulbar palsy •