1. Descending tracts
D.Nimer D.Rania Gabr
D.Safaa D.Elsherbiny

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..
Compare the signs and symptoms of the upper and lower motor neuron lesions.

3. Upper motor neurons ( UMN )
Are the descending supraspinal pathways that influence the activity of the LMN
They control :
1-voluntary motor activity
2-maintenance of posture & equilibrium
3- muscle tone and
4- reflex activity
generally exert their effect on groups of muscles ( not on one specific muscle ) reciprocally on agonist and antagonist muscle group

4. descending tracts sensory inputs
UMN
cerebral cortex – midbrain - pons - medulla oblongata
descending tracts
sensory inputs
LMN

5. Lower motor neurons ( LMN )
Are motor neurons that innervate the voluntary muscles located in
1- anterior grey column of the spinal cord , and
2- motor nuclei of brainstem
form the final common pathway
LMN

6. Motor system Pyramidal system Corticospinal tract.
Corticobulbar tract.
Extrapyramidal system
Vestibulospinal,reticulospinal,olivospinal, rubrospinal and tectospinal
Motor pathway to cerebellum: corticopontocerebellar tract

8. Pyramidal system

9. Cerebral cortex Brain stem Spinal cord
Coricobulbar tract
Corticospinal tract
Brain stem ms
Spinal cord

10. Cortico-spinal tract

11. SC (AHC) 85%LCST 10% 4,6 312 15%VCST Internal capsule midbrain pons
Corona radiata
Internal capsule
midbrain
pons
85%LCST
medulla
15%VCST
SC (AHC)
10%

12. Corona radiata

14. Course of the tract
1- Corticospinal axons leave the cerebral hemispheres : Pyramidal cells (Betz cells)
2- They pass through the the corona radiata
3-Then through post. Limb of internal capsule
4- They enter the crus cerebri of the midbrain. (Cerebral peduncles)

15. 5- They pass through the ventral portion of the pons
6- Fibers reach the medulla oblongata, where they form two prominent columns on the ventral surface.
They are called the pyramids and for this reason the term pyramidal tract is used as an alternative name for the corticospinal tract.
In the caudal medulla, the fibers of the pyramids undergo subtotal decussation.
( Motor decussation)

16. About 75-90% of fibers decussate and enter the contralateral lateral corticospinal tract, which is located in the lateral part of the spinal white matter.
10-25% of pyramidal fibres remain ipsilateral and enter the ventral corticospinal tract located lateral to the ventral median fissure.

17. Function Lesion: UMNL Above decussation: contralateral hemiplegia.
Facilitation of flexor muscle tone
Initiation of high skilled movements
Lesion: UMNL
Above decussation:
contralateral hemiplegia.
Below decussation:
ipsilateral hemiplegia.

18. Signs of UMNL hypertonia. Hyper-reflexia. Spasticity. Babinski sign.
Clonus.
Loss of superficial abdominal reflexes & cremastric reflex.

19. Clonus

20. Babinski sign

21. Hyper-reflexia

22. Spasticity

23. Corticobulbar tract

24. 4,6 Lower facial nucleus Cranial nerve.nuclei Internal capsule
midbrain
pons
medulla
Lower facial nucleus
Cranial nerve.nuclei

26. Extrapyramidal system
2 from midbrain:
Rubrospinal : from red nucleus to SC- facilitate the activity of flexor ms
Tectospinal: from superior colliculus of midbrain to SC- mediate reflex movements in response to visual stimuli

27. Extrapyramidal system
2. 2 from vestibular nuclei:
Medial and lateral nucleus.
Input: from the labyrinthine system (inner ear) and from the cerebellum.
Output: SC
Function: balance by facilitate the activity of the extensor muscles

28. Extrapyramidal system
3. 2 from reticular formation: medial & lateral reticulospinal tracts.
4. 2 others: raphe spinal & descending autonomic fibers.

29. Descending autonomic fibers
Origin: hypothalamus.
Termination: sympathetic nucleus and sacral parasympathetic nucleus
Lesion: above T1…Horner's syndrome

30. Horner's syndrome
1. Ptosis
2. Miosis
3. Flushing
4. Anhydrosis

31. The Main Descending Pathways to the Spinal Cord
Function
Origin
Site of Crossover
Destination
Branches to
Corticospinal tracts
Rapid, skilled, voluntary movements, especially distal ends of limbs
Primary motor cortex (area 4), secondary motor cortex (area 6), parietal lobe (areas 3, 1, and 2)
Most cross at decussation of pyramids and descend as lateral corticospinal tracts; some continue as anterior corticospinal tracts and cross over at level of destination
Internuncial neurons or alpha motor neurons
Cerebral cortex, basal nuclei, red nucleus, olivary nuclei, reticular formation
Reticulospinal tracts
Inhibit or facilitate voluntary movement; hypothalamus controls sympathetic, para-sympathetic outflows
Reticular formation
Some cross at various levels
Alpha & gamma motor neurons
Multiple branches as they descend
Tectospinal tract
Reflex postural movements concerning sight
Superior colliculus
Soon after origin ?
Rubrospinal tract
Facilitates activity of flexor muscles and inhibits activity of extensor muscles
Red nucleus
Immediately
Alpha and gamma motor neurons
Vestibulospinal tract
Facilitates activity of extensor inhibits flexor muscles
Vestibular nuclei
Uncrossed
Olivospinal tract ??
Inferior olivary nuclei
Cross in brainstem
? Alpha &gamma motor neurons —
Descending autonomic fibers
Control sympathetic and para-sympathetic systems
Cerebral cortex, hypothalamus, amygdaloid complex, reticular formation
Sympathetic & parasympathetic outflows
aNote that the corticospinal tracts are believed to control the prime mover muscles (especially the highly skilled movements), whereas the other descending tracts are important in controlling the simple basic movements. For simplicity, the internuncial neurons are omitted from this table.