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Descending pathways

Descending pathways. Motor control. Three levels of motor control: Spinal cord Brainstem Cerebral cortex (motor <- premotor) These centers are arranged in both hierarchical and parallel manner. Descending pathways. MN organization.

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Descending pathways

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  1. Descending pathways

  2. Motor control • Three levels of motor control: • Spinal cord • Brainstem • Cerebral cortex (motor <- premotor) • These centers are arranged in both hierarchical and parallel manner.

  3. Descending pathways

  4. MN organization • Cell bodies organized into motor neuron pools – longitudal column spanning 1-4 segments • Two axis of organization • Proximal\distal rule: Proximal is medial • Flexor\extensor rule: Flexors are dorsal

  5. IN organization • Segmental INs follow MN proximal/distal organization. • Propriospinal INs have axonal distribution that reflects their location: • Medial -> long • Distal -> short

  6. Orthogonal organization of INs vs. MNs

  7. 2. Brainstem pathways (BSP) • Three descending systems: • Medial reaches ventromedial part of IZ proximal & axial musculature • Lateral dorsolateral part of IZ distal muscles. • Aminergic broad, diffuse innervation of SC.

  8. MEDIAL PATHWAYS LATERAL PATHWAYS Tectum Red nucleus Medial Reticular formation Tectospinal Vestibular nuclei Reticulospinal Vestibulospinal Rubrospinal

  9. Medial BSP • Vestibulospinal (M+L) - ipsilateral • Originates in the vestibular nuclei. • Balance and posture information • Reticulospinal (M+L) – ipsilateral • Originates in reticular formation nuclei • Excitation & inhibition of INs and MNs • Control of posture • Integrates cortical and vestibular information. • Cortico-reticulospinal pathways (M1 + PM). • Tectospinal– contralteral • Originates from the sup. Colliculus • Eye-head coordination. • Supply INs + long PNs in the ventromedial part of IZ

  10. Lateral BSP • Rubrospinal tract – contralateral • Originates from the magnocellular part of RN • Reduced importance in higher primates

  11. Medial vs. lateral BSPs • Medial BSP: • Phylogenetically oldest • Balance & posture control via proximal+distal muscles • Wide termination fields • Lateral BSP: • Wider motor spectrum • Reaching & object manipulation via distal musculature • Function overtaken by motor cortical areas.

  12. Aminergic BSPs • Control the excitability of spinal neurons. • Ceruleospinal system: • NA • Locus ceruleos + pontinemedullary reticular formation. • Descend via ventral part of lateral column • Raphe-spinal system • Serotonin (5HT) • Raphe nuclei • Descend via ventral + lateral columns.

  13. Axons reach the intermediate zone and motor nuclei Single neurons sends multiple collaterals which may span the entire cord. The raphespinal system modulate sensory transmission in the dorsal horn. Aminergic BSPs

  14. Complex and precise motor actions depends on motor cortical signals. Motor cortical areas affect motor output via Direct corticospinal pathways (1*106) Indirect cortex-brainstem-spinal pathways (19*106) – CRetS, CRnS Most of the cortical command is further processed in subcortical-supraspinal centers. 3. Pyramidal tract

  15. Structure of the PT

  16. Morphology of PT • 60% of fibers from frontal lobe (motor areas) • 40% of fibers from parietal lobe (sensory areas) • 75% contralateral – dorsolateral column – lateral CST • 25% ipsilateral – ventral column – ventral CST

  17. Ventral corticospinal Lateral corticospinal Red nucleus Medial brain stem pathways Pyramidal decussation Ventral corticospinal Lateral corticospinal

  18. Cortical origin of PT

  19. Cortical origin of PT • Pyramidal cells in layer V • In the M1 these are giant cells (Betz cells) • In PM these cells are smaller

  20. Cortical organization of PT neurons • Intracellular recording of MNs and Ia-IN while stimulating M1 • Multiple patches project to single neurons • Single cortical site project to multiple neurons • Overlap between CS and CM projecting areas. • MUSCLE COLONIES

  21. Ventral shift of CST • Corticospinal and corticobulbar pathwaty appear first in mammals • In higher mammals there is a dorsal-to-ventral shift of CST termination. • From sensory gating to motor control. • Direct CM contact facilitate dexterity.

  22. Embryogenesis of CST: reduced overlap and synaptic clustering

  23. Spinal termination of the PT • Large termination field • A single fiber probably innervates both INs and MNs

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