Descending Projection Systems and Motor Functions of the Spinal Cord

1. Descending Projection Systems and Motor Functions of the Spinal Cord

2. Functional anatomy of motor systems and descending pathways. A. Components of the motor systems of the CNS. B. Organization and function of the descending pathways (i.e., lateral/medial). II. Regional anatomy of the descending systems. A. Motor regions of the cerebral cortex. 1. Premotor (higher-order) cortical regions. 2. 1° motor cortex – input, somatotopic organization and outflow. B. Descending projections through internal capsule  forming the Corticospinal Tract. C. Organization of the Corticospinal tract in the base of the midbrain (Basis Pedunculi) [+ start of rubrospinal tract]. D. Passage of corticospinal tract through Ventral Pons. E. Nuclei of dorsal pons and medulla (start of vestibulospinal and reticulospinal tracts). F. Pathways through medulla (+ lateral corticospinal tract decussation). G. Input from the descending pathways within the spinal cord + topographic organization of the grey matter and white matter. H. A note about lesions of the descending cortical pathways.

3. I. Functional anatomy of the motor systems and descending pathways • Motor systems of the CNS – 4 components of motor systems (during movements of the leg and trunk by regulating skeletal muscle). 1. Descending projections pathways. 2. motor neurons and interneurons. 3. basal ganglia. 4. cerebellum. Our ‘tours’ through motor systems will be opposite from what we experienced with sensory (order of cortex---- motor neurons). Today, we will focus on the 1st 2 of the above. Individual lectures will be presented on the basal ganglia and cerebellum at a later date.

4. 1st component: descending projection pathways (Fig. 10-1)

5. Motor Cortical Areas (Fig. 10-1)

6. 2nd component; for muscles of the limbs and trunk - motor neurons and interneurons located in: ventral horn and internal zone of the spinal cord. A parallel exists for the muscles of the head: cranial nerve motor nuclei and reticular formation in the brainstem – these are analogous to above areas. • 1 function of the brainstem is to serve as the “spinal cord for the head”. • 3rd and 4th components: basal ganglia and cerebellum do not project directly to motor neurons, but rather, synapse on descending pathways and have a very important influence.

7. Overview of function using an example • See a cup  1° visual cortex  higher-order in post parietal lobe (for id object). • Prefrontal association areas (maturation and cognition – “get the idea”). • Premotor areas  “plan of action” • 1° motor cortex • Corticospinal tract – interneurons  motor neurons (grasp the cup) Cerebellum and bg also play a role in feedback control and movement intention – we will discuss shortly.

8. B. Organization and function of descending pathways. • Motor control pathways • Paths that regulate somatic sensory processing: somatic sensory relay nuclei (brainstem) and dorsal horn neurons (spinal cord). i.e., pain suppression triggering of motor reflexes damage  spasticity). • Paths regulating ANS. Cortex , amygdala, hypothal, brain stem  preganglionic autonomic nerves of brainstem and spinal cord.

9. 7 Major descending motor control pathways from cerebral cortex or brainstem nuclei: From frontal cortex: i. lateral corticospinal tract ii. Ventral corticospinal tract iii. Corticobulbar tract ( cranial n. motor nuclei). From brainstem nuclei: i. Rubrospinal tract ii. Reticulospinal tract iii. Vestibulospinal tract iv. Tectospinal tract

10. Interneuron Connections: sometimes an intermediary before motor neurons: • Segmental interneurons – short branches within single spinal cord segment. • Propiospinal neurons – projects for multiple spinal segments before synapsing onto motor n. – long projections coordinating movements of upper and lower limbs; may transmit control signals lower, for some paths ending in cervical areas.

11. Somatotopic organization-motor (Fig. 10-2) From lateral descending paths Lateral: limbs Medial: Axial

12. LATERAL DESCENDING PATHWAYS Lateral Corticospinal Tract: (Fig. 10-3) Starts at the 1° motor cortex ------ decussation zone and lateral motor nuclei of cervical and lumbrosacral cord. Why segmented? Limbs.

13. LATERAL DESCENDING PATHWAYS Rubrospinal Tract (Fig. 10-3): Red nucleus (magnocellular)  midbrain decussation  lateral intermediate zone and ventral horn. Provides some additional (residual) motor control [more important input connection is with cerebellum].

14. MEDIAL DESCENDING PATHWAYS Medial (Ventral) Corticospinal Tract (Fig. 10-4): Axial and girdle muscles. Ipsilateral ventral column  bilateral projections to medial grey matter. Controls especially for head, shoulder, and upper trunk muscles.

15. MEDIAL DESCENDING PATHWAYS Note the bilateral projections; cf., the lateral pathways (which crossed). Reticulospinal Tract (Fig 10-4): Many terminate in cervical cord. But, 2° projections to propiospinal neurons may influence lower axial muscles also. Pontine  ventral column. Medullary reticular formation  lateral column (ventrolateral). Autonomic movements: posture and repetitive movements. Tectospinal Tract (Fig. 10-4): From sup coll (deep)  coord head and eye movements.

16. Vestibulospinal Tracts: Lateral vestibular n.  lat vestibulospinal tract (to all spinal limbs). Maintains balance. Medial nucleus  med tracts for control of head position (cervical only).

17. Organization of Tracts (Fig. 10-5) Note the descending and ascending tracts and their relative positions.

18. II. Regional Anatomy of Descending Systems • Motor regions of the Cerebral Cortex – in frontal lobe. 1. Higher-order cortical regions: - planning movement. - integration of info from diverse cortex (see earlier e.g.). a. bg  VA (thal)  supplementary motor area (bimanual coordination)  prefrontal cortex b. cerebellum  VL (thal)  premotor cortex  reticulospinal tracts (control of girdle muscles). c. Cingulate motor areas (part of limbic system) – rhythmical movements (e.g., pedaling a bike). - may be important in triggering movements initiated by emotions and drives.

19. II. Regional Anatomy of Descending Systems Premotor regions Somatic sensory Thalamic (VL) (VA) Precentral gyrus (thick layer of 5 pyramidal neurons provide major output (large Betz cells) Corticospinal tract Cerebellum bg

20. Premotor Areas (Lateral View) (Fig. 10-6)

21. Premotor Areas (Medial View) (Fig. 10-6)

22. Motor “homunculus” (Fig. 10-7) Somatotropic organization – analogous to sensory cortex

23. Origins of motor cortical tracts Note that the origins of cortical tracts are shown – somewhat segmented

24. B. Descending projections through Internal capsule and corticospinal fibers (Fig. 10-9) Posterior limb of the IC contains the cortico spinal tract; Somatotropic Organization is maintained.

25. Divisions of internal capsule (Fig. 10-10)

26. C. Pathways within midbrain (Fig. 10-11) Corticospinal tract courses in base of midbrain (basis pedunculi). Somatotopic organization is retained here. Origin of Tectospinal tract (superior colliculus) and rubrospinal tract (red nucleus). Also, these tracts cross close to their origins in midbrain.

27. D. Pathways within Pons (Fig. 10-12) Descending cortical fibres are no longer on ventral surface  they dive deeper within the pons (and also branch up into fasciculi) among pontine and pontocerebellar fibres.

28. Medullary Decussation • Nuclei of Dorsal Pons and Medulla: start of Vestibulospinal and • Reticulospinal tracts (Review pathways on earlier slides). • Functions: Reticular formation has multiple projections, which makes this • System highly integrative (e.g., between motor and sensory). • Analogous to Intermediate Zone of the spinal cord (See Fig. 10-16A).

29. F. Pathways through Medulla and Decussation of Corticospinal Tract. Fibres from cortex collect once again on ventral surface, forming pyramids. Decussations occur in medulla – specifically of lateral corticospinal tract axons.

30. G. Spinal Cord Inputs: motor pathways synapse on motor neurons of ventral horn and interneurons (segmental, propiospinal) of intermediate zone. Generalized organization of grey and white matter (see Fig. 10-16A and the last slide in this set). Ventral horn: Rexed’s laminae (VIII, IX). Intermediate zone: (laminae VII).

31. Spinal cord grey matter organization (Rexed’s laminae) (Fig. 10-16A):

32. Motor Terminations at various levels of the sc. Note the descent of various pathways in lateral and ventral columns. Medially descending and terminating pathways  Girdle and axial muscles. Cross-overs and bilateral connectivities: coord contraction during postural adjustments.

33. Lateral descending/terminating paths: Distal limb muscles Note: terminations only evident in cervical and lumbrosacral segments (sensory limbs) (Fig. 10-19) Medial input = continuous Lateral input - interrupted

34. H. Some notes about lesions of the descending cortical pathways: Lesions of posterior limb of internal capsule, ventral brainstem, and sc: on 1 side produce a sequential series of symptoms (focused primarily on limb muscles --- Why?) • Flaccid paralysis (immed)  decr strength + muscle tone due to corticospinal interruption. • Spasticity (a few weeks later)  incr muscle tone + exaggerated reflexes (Babinski sign) due to reticulospinal and sensory input damage (decr feedback).

35. White Matter: Pathway GeneralizationsReview of descending and ascending tracts