Spinal Cord Descending Tracts

1. Spinal CordDescending Tracts Dr. Sama-ul-Haque

2. Objectives • Define the terms upper and lower motor neurons with examples. • Describe the corticospinal (pyramidal) tract and the direct motor pathways from the cortex to the trunk and limbs. • Briefly describe the indirect motor pathways from the cortex to the trunk and limbs through extrapyramidal tracts such as rubrospinal and reticulospinal tracts.

3. Descending Tracts • Bundles of nerve fibers in the white matter of the spinal cord descending from the supraspinal nerve centers. • Also called Upper Motor Neurons. • They are concerned with Motor Activity. • They originate from cerebral cortex and brain stem. • Lower Motor Neurons (Neurons which innervate the skeletal muscles). • Mainly present in the anterior gray column of the spinal cord.

4. Descending Tracts • First-order Neuron: • Cell body lies in the cerebral cortex • Second-order Neuron: • An internuncial neuron, situated in the anterior gray column of the spinal cord. • Third-order Neuron: • Axon of 2nd order neuron is short and synapse with 3rd order neuron,(Lower motor neuron), in the anterior gray column of the spinal cord. • Axon of 3rd order neuron innervates the skeletal muscle through anterior root and spinal nerve.

5. Descending TractsFirst-orderSecond-orderandThird-order Neurons

6. Descending TractsSecond-order and Third-order Neurons

7. Descending Tracts • Corticospinal tract • Reticulospinal Tract • Tectospinal Tract • Rubrospinal Tract • Vestibulospinal Tract • Olivospinal Tract

8. Descending Tracts

9. Corticospinal Tracts or Pyramidal Tract

10. Corticospinal Tracts • Fibers of corticospinal tract arise as Axons of pyramidal cells (situated in the 5th layer of cerebral cortex). • 1/3rd of the cells arise form Primary Motor cortex (Area 4). • 1/3rd of the cells arise form Secondary Motor cortex (Area 6). • 1/3rd of the cells arise form Parietal lobe (Area 3,1 and 2). • The descending fibers converge in corona radiata. • Then pass through posterior limb of the internal capsule and reaches to midbrain.

11. Areas of Cerebral Cortex

12. Corticospinal Tracts • On entering the Pons the tract is broken into many bundles by transverse pontocerebellar fibers. • In medulla oblongata the bundle of fibers grouped together anteriorly and form Pyramid(Pyramidal Tract). • Most of the fibers cross the midline at the decussation of Pyramids (Motor Decussation) and enter the lateral white column of spinal cord as Lateral corticospinal tract. • The lateral corticospinal tract lies in the whole length of spinal cord.

13. Corticospinal Tracts • The remaining fibers do not cross in the decussation and descend in anterior white column as of spinal cord as Anterior corticospinal tract. • These remaining fibers eventually cross the midline in the cervical and upper thoracic segments. • All the fibers terminate in the anterior gray column of the spinal cord by synapsing with internuncial or motor neurons. • Function: Concerned with Voluntary, discrete and Skilled movements (especially distal parts of the limbs).

14. Corticospinal Tracts

15. Corticospinal Tracts

16. Reticulospinal Tracts

17. Reticulospinal Tracts • Reticular Formation: • Group of scattered nerve cells and nerve fibers in midbrain, Pons and Medulla. • From Pons these neurons send uncrossed axons down into the anterior white column of the spinal cord and form Pontine (Medial) Reticulospinal Tract. • From medulla these neurons send crossed and uncrossed axons down into the lateral white column of the spinal cord and form Medullary (Lateral) Reticulospinal Tract.

18. Reticulospinal Tracts

19. Reticulospinal Tracts • Both sets of fibers enter in the anterior gray columns of the spinal cord. • Facilitate or inhibit the activity of Motor neurons. • Function: Facilitate or inhibit voluntary movements or reflex activity.