Spinal cord & its tracts II (Descending tracts)

A lecture for undergraduate medical students

1. Spinal cord & its tracts – II (DESCENDING TRACTS) Dr. Abdul Qadeer MBBS;(MRCS);FCPS;FICS Assistant Prof. of Surgery LCMD, Karachi.

2. Descending tracts of spinal cord A. Pyramidal ( cortico -spinal) tracts Lateral cortico -spinal tract Anterior cortico -spinal tract B. Extra-pyramidal tracts Rubro -spinal tract Medial reticulo -spinal tract Lateral reticulo -spinal tract Olivo -spinal tract Vestibulo -spinal tract Tecto -spinal tract

3. LOWER MOTOR NEURON (LMN): LOWER MOTOR NEURON (LMN): It is the neuron situated in the anterior gray horn of the spinal cord Its axon innervates skeletal muscle through the anterior roots of the spinal nerve LMN constitutes the final common pathway to the muscles

4. UPPER MOTOR NEURON (UMN) UPPER MOTOR NEURON (UMN): These are the nerve fibers that descend in the white matter from different supra-spinal centers like medulla, pons , midbrain & cerebral cortex Collectively they form descending tracts UMNs constantly send the impulses from higher centers to the LMNs

5. Anatomical organization of descending tracts First-order neuron has the cell body in the cerebral cortex Second-order neuron (an internuncial neuron) is situated in the anterior gray column of the spinal cord. It is usually short Third-order neuron is an LMN, situated in the anterior gray horn. It innervates the skeletal muscle through anterior root & spinal nerve Sometimes, 1 st order neuron (UMN) terminates directly on 3 rd order neuron (LMN)

6. Functions of the descending tracts Corticospinal tracts: are concerned with voluntary, discrete, skilled movements, especially those of the distal parts of the limbs Reticulospinal tract: may facilitate or inhibit voluntary movement or reflex activity Tectospinal tract: is concerned with reflex postural movements in response to visual stimuli and causes pupillodilatation reflex in darkness

7. Functions of the descending tracts Rubrospinal tract: facilitates the activity of flexor muscles & inhibits the activity of extensor or antigravity muscles Vestibulospinal tract: is concerned with postural activity associated with balance Olivospinal tract: may play some role in muscular activity Descending autonomic fibers: control the visceral activity

8. Cortico -spinal tracts Theses are: Lateral cortico -spinal tract Anterior cortico -spinal tract

9. Cortico -spinal tracts Their cells arise from the cerebral cortex Descending fibers converge in the corona radiata Then pass through internal capsule Pass through midbrain To pons To medulla oblongata

10. Cortico -spinal tracts within medulla oblongata In the medulla oblongata, many fibers are grouped together to form a swelling known as the pyramid Hence these fibers are called pyramidal tract At the junction of the medulla & spinal cord, most of the fibers cross the midline at the decussation of the pyramids & enter the lateral white column of the spinal cord to form the lateral corticospinal tract

11. Cortico -spinal tracts within medulla oblongata The remaining fibers do not cross in the decussation but descend in the anterior white column of the spinal cord as the anterior corticospinal tract The fibers of anterior corticospinal tract cross the midline in the cervical and upper thoracic regions and terminate in the anterior gray horn

12. Cortico -spinal tracts Most corticospinal fibers synapse with the internuncial neuron The remaining fibers synapse directly with the motor neuron The corticospinal tracts are used in performing rapid skilled movements

13. Reticulo -spinal tracts The reticular formation is formed by groups of nerve cells scattered throughout the midbrain, pons and medulla oblongata Pontine part of reticulospinal tract remain uncrossed, and descend through the anterior white column of spinal cord Medullary part of reticulospinal tract is crossed & uncrossed, descend through the lateral white column of the spinal cord

14. Functions of reticulo -spinal tracts They facilitate or inhibit the voluntary movements and reflex activity Control the sympathetic outflow and sacral parasympathetic outflow

15. Tectospinal tract Fibers arise from the nerve cells situated within the superior colliculus of the midbrain Most of the fibers cross the midline immediately & descends through the brain stem It descends through the anterior white column of spinal cord close to the anterior median fissure Majority of fibers terminate in the anterior gray horn in the upper cervical segments of the spinal cord by synapsing with internuncial neuron

16. Functions of tectospinal tract These are concerned with reflex postural movements in response to visual stimuli

17. Rubrospinal tract Red nucleus is situated in the tegmentum of the midbrain at the level of the superior colliculus The fibers cross the midline at the level of the red nucleus , descend as the rubrospinal tract through the pons & medulla oblongata to enter the lateral white column of the spinal cord The fibers terminate on the internuncial neurons in the anterior gray horn of the cord

18. Functions of rubrospinal tract The red nucleus receives afferent impulses from the cerebral cortex & cerebellum Therefore, the cerebral cortex & cerebellum indirectly infuence the spinal cord through this tract The tract facilitates the activity of the flexor muscles and inhibits the activity of the extensor or antigravity muscles

19. Vestibulospinal tract Vestibular nuclei are situated in the pons & medulla oblongata beneath the floor of the 4 th ventricle These nuclei receive afferent fibers from the inner ear through the vestibular nerve & from the cerebellum The neurons of lateral vestibular nucleus give rise to the axons that form the vestibulospinal tract

20. Vestibulospinal tract The tract descends uncrossed through the medulla and spinal cord in the anterior white column The fibers terminate on the internuncial neurons in the anterior gray horn

21. Functions of vestibulospinal tract The inner ear & the cerebellum, by means of this tract, facilitate to maintain the balance of the body by facilitating the extensor muscles and inhibiting the flexor muscles

22. Olivospinal tract Arises from the inferior olivary nucleus Descends in the lateral white column of the spinal cord Influences the activity of motor nucleus in the anterior gray horn

23. Descending autonomic fibers Higher centers of the CNS which control the autonomic activity are situated in: Cerebral cortex Hypothalamus Amagdaloid complex Reticular formation

24. Descending autonomic fibers The fibers arise from the neurons within higher centers Cross the midline in brain stem Descend in the lateral white column of spinal cord Synapse with the autonomic motor cells in the lateral gray horn in the thoracic & upper lumbar (sympathetic) and mid-sacral (parasympathetic) levels of spinal cord

25. Intersegmental tracts Short ascending and descending tracts that originate and end within spinal cord Exist in anterior, lateral and posterior white columns These tracts interconnect the neurons of different segmental levels Important in intersegmental spinal reflexes

26. Decerebrate rigidity Spinal segmental reflexes are influenced by higher centers through descending tracts In spinal shock, due to severance of spinal cord, the segmental spinal reflexes are depressed Spinal shock disappears within few weeks, the segmental spinal reflexes return & muscle tone returns, called decerebrate rigidity

27. Renshaw cells These are the internuncial neurons which make a connection between the: Collateral branch of LMN & Axon of LMN itself These internuncial neurons ( Renshaw cells) provide feedback on the lower motor neurons, hence inhibiting their activity

28. Renshaw cell

29. THE END