Physiology of the descending tracts

1. Physiology of the Descending Tracts Dr. Ibrahim Abdullahi Mohamed

2. Descending motor pathways from motor cortex • The motor tract originating from the cerebral cortex traditionally has been called the pyramidal tract because it traverses the medullary pyramids on its way to the spinal cord. • This path is the corticospinal tract. • All other descending motor tracts emanating from the brainstem were generally grouped together as the extrapyramidal system.

3. Pyramidal tracts • Corticospinal tracts include 30% fibers arising from the primary cortex (area 4), 30% from premotor area (area 8) and supplementry cortex and 40% arising from the somatic sensory cortex (Brodmann areas 1, 2, and 3). • All the above fibers form the fibers of upper motor neurons of the motor pathway.

4. • After originating from the cerebral cortex, the corticospinal tract fibers descend as a part of corona radiata and then pass through the posterior limb of the internal capsule and then downwards through the brain stem forming pyramids in the medulla (hence the name pyramidal tracts). • In the lower part of medulla about 90% fibres of each pyramid decussate in the mid line to reach opposite side.

5. • The fibers of corticospinal tracts are divided into two separate tracts: 1) Lateral corticospinal tract is constituted by 80% of fibers which have crossed to opposite side. • The lateral corticospinal tract fibers descend the full length of spinal cord through the posterior part of lateral white funiculus. • Most of these fibers terminate in the internuncial neurons of the spinal grey matter. • The internuncial neurons carry the impulses to the motor neurons situated in the ventral grey horn.

6. • Some fibers of the tract terminate directly on the ventral horn cells. • The axons of the ventral motor neurons supply the skeletal muscles directly by passing through the ventral nerve root. • The neurons giving origin to the fibers of pyramidal tract along with their axons constitute the upper motor neurons. • The ventral motor neurons in the spinal cord along with their axons constitute the lower motor neurons.

7. Cont. 2) Anterior corticospinal tract is formed by 20% uncrossed pyramidal fibers. • These fibers descend down through the anterior white funiculus of the same side. • The anterior corticospinal tract fibers do not reach further than the mid-thoracic region.

8. • On reaching the appropriate level of the spinal cord, the fibers of this tract cross the midline (through the anterior white commissure) to reach grey matter on the opposite side of the cord and terminate in a manner similar to that of the fibers of the lateral corticospinal tract. • Thus, the corticospinal fibers of both the lateral as well as the anterior tracts ultimately connect the cerebral cortex of one side with ventral horn cells in opposite half of the spinal cord.

9. Functions • The cerebral cortex controls voluntary fine skilled movements of the body through the corticospinal tracts. • Main Functions: • 1. Voluntary Motor Control: • Enables precise, intentional movements of the skeletal muscles. • Particularly important for distal limb movements, such as fine motor skills (e.g., writing, buttoning). • 2. Motor Coordination: • Integrates signals from the motor cortex to ensure smooth, coordinated movement patterns.

10. Cont. • 3. Modulation of Reflexes: • Suppresses or modifies spinal reflexes to allow controlled, voluntary motion. • Prevents excessive reflexive responses (e.g., hyperreflexia). • 4. Postural Adjustments: • Works in conjunction with other descending tracts to maintain posture during voluntary movements. • 5.Bilateral Motor Control: • Although primarily responsible for contralateral control (due to decussationin the medulla), some fibers remain ipsilateral to assist with coordinated bilateral movements.

11. Corticonuclear fibers • The pyramidal tract fibers also send collaterals to other areas of the motor control systems thus communicating motor command to the basal ganglia, cerebellum and the brain stem. • In their course through the brain stem, some of the fibers (corticonuclear fibers) terminate directly on the motor nuclei of cranial neurons controlling facial muscles.

12. Cont. • Since these fibers perform the same function as pyramidal tracts, they are also considered part of the pyramidal system. • Main Functions • 1.Voluntary Control of Facial and Neck Muscles: • Regulates motor functions for muscles involved in facial expressions, mastication(chewing), swallowing, and speech. • Targets specific cranial nerve motor nuclei. • 2.Control of Eye Movements: • Influences cranial nerves (III, IV, VI) to coordinate eye movements, including gaze and tracking. • 3.Speech and Articulation: • Activates muscles of the tongue (via hypoglossal nerve) and vocal cords, essential for speech production. • 4.Swallowing and Phonation: • Innervates muscles of the pharynx and larynx through the vagus and glossopharyngeal nerves to assist in swallowingand producing sound.

13. Extrapyramidal tracts • The descending tracts of spinal cord other than the pyramidal tracts are collectively called extrapyramidal tracts. These include:  Rubrospinal tract, Vestibulospinal tract,  Reticulospinal tract, esp: P.R.S tract & M.R.S tract Tectospinal tract and Olivospinal tract

14. Rubrospinal tract: • Origin. This tract arises from the large cells (nucleus magnocellularis) or red nucleus in the mid brain. • Course. After arising from the red nucleus, the fibers of this tract cross to opposite side in the lower part of the segmental of mid brain (ventral segmental decussation).

15. Cont. • Then, the tract descends through the pons and medulla and follows a course similar to that of lateral corticospinal tract in the lateral funiculus of the spinal cord. • Termination. The fibers terminate mainly on interneurons along with the corticospinal fibers.

16. • Functions. This tract exhibits facilitatory influence on the flexor muscles and inhibitory influence on the extensor muscles of the body. • The red nucleus also receives the corticorubral fibers from the ipsilateral motor cortex. • The corticorubro-spinal tract thus formed may act as an alternate route of pyramidal system to exert influence on the lower motor neurons for motor coordination, plays a role in fine tuning movements.

17. Vestibulospinal tracts: • There are two vestibulospinal tracts: I. lateral and II. medial.

18. Cont. I. Lateral vestibulospinal tract: • Origin. Fibers of this tract arise from the lateral vestibular (Deiters’) nucleus. • These fibers are somatotopically arranged. • Fibers to cervical segments arise from the cranioventral part, those to thoracic segments from the central part and those to lumbosacral segments from the dorsocaudal part of lateral vestibular nucleus.

19. • Functions. Vestibular nucleus receives afferents from the vestibular apparatus mainly from utricles. • This pathway is principally concerned with adjustment of postural muscles to linear acceleratory displacements of the body. • Lateral vestibulospinal tract mainly facilitates activity of extensor muscles and inhibits the activity of flexor muscles in association with the maintenance of balance.

20. II. Medial vestibulospinal tract: • Origin. The fibers of this tract arise from the medial vestibular nucleus. • Location and course. This tract descends through the anterior funiculus (within the sulcomarginal fasciculus). • The fibers are mostly uncrossed but some fibers are crossed. • Termination. The fibers end in the anterior motor neurons directly or through internuncial neurons of the cervical segments of spinal cord.

21. Cont. • Functions. This part of the vestibular nucleus receives signals from the vestibular apparatus mainly from the semicircular canals. • Functionally, medial vestibulospinal tract is the donor connection of medial longitudinal fasciculus. • This tract provides a reflex pathway for movements of head, neck and eyes in response to the visual and auditory stimuli.

22. Reticulospinal tracts: • There are two reticulospinal tracts: A. The medial (pontine) reticulospinal tract and B. lateral (medullary) reticulospinal tract.

23. Cont. A. Medial (pontine) reticulospinal tract: • Origin. It arises in the medial pontine reticular formation. • Course. The tract descends, mostly uncrossed, in the anterior funiculus of spinal cord. • Termination. The fibers terminate in the spinal grey matter and through internuncial neurons influence alpha and gamma neurons.

24. • Functions of reticulospinal tracts. The reticular formation of the brain stem receives input mostly from the motor cortex through the corticoreticular fibers which accompany the corticospinal tracts. • Thus the corticoreticulospinal tracts form additional polysynaptic pathways from the motor cortex to the spinal cord. • These tracts are concerned with control of movements and maintenance of muscle tone.

25. Cont. • The reticulospinal tracts, probably, also convey autonomic information from higher centers to the intermediate region of spinal grey matter and regulate respiration, circulation and sweating. • The pontine and medullary reticular nuclei mostly function antagonistic to each other.

26. Tectospinal tract: • Origin. Fibers of this tract arise from the superior colliculi. • Course. The fibers cross the midline in the lower part of segmental of the mid brain forming dorsal segmental decussation. • Then the tract descends through the pons and medulla into the anterior white funiculus of the spinal cord.

27. Cont. • Termination. The fibers terminate in upper cervical levels by synapsing on the anterior horn cells through internuncial neurons located in laminae V and VII of the spinal grey matter. • Function. This tract forms the motor limb of the reflex pathway for turning the head and moving the arms in response to visual, hearing or other exteroceptive stimuli.

28. Olivospinal tract: • Origin. This tract originates from the inferior olivary nucleus. • Course and termination. The tract fibers descend and terminate ipsilaterally in the anterior horn cells of the spinal cord.

29. Cont. • Functions. Inferior olivary nucleus receives afferent fibers from the cerebral cortex, corpus striatum, red nucleus and spinal cord. • It influences muscle activity. • Probably, it is involved in the reflex movements arising from the proprioceptors.

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