1. The Motor Systems

2. What’s the motor system?
Parts of CNS and PNS specialized for control of limb, trunk, and eye movements
Also holds us together
From simple reflexes (knee jerk) to voluntary movements (96mph fast ball)
Remarkable: Muscles only contract

3. Plan Components of the motor systems
Focus on spinal control of limbs and trunk
Same principles apply to to head control via brain stem
Basic principles of movement control
What is helpful for understanding basic motor system organization
Motor programs for voluntary movement
Descending motor pathways
Note about motor system’s bad rep…

4. Motor Systems Motor systems
Cortical motor areas
Motor Systems
Basal ganglia
Cerebellum
Descending cortical motor paths
Descending brain stem paths
Spinal cord:
Intermediate zone
Ventral horn
Muscle
Motor systems

5. Functional Hierarchy of Motor Paths
Cortical
motor
areas

6. Functional Hierarchy of Motor Paths
Cortical
motor
areas

7. Functional Hierarchy of Motor Paths
Motor execution: force & direction

8. Functional Hierarchy of Motor Paths
Motor execution: force & direction

9. Parallel Organization
Association & limbic cortex
Basal ganglia
Cerebellum

10. from
Cerebellum
Internal capsule
Motor
Cortical
areas
from
Basal ganglia

11. Hierarchical & Parallel Organization of the motor systems
Top down organization of the motor pathways--opposite that of sensory paths
Subcortical motor centers--cerebellum & basal ganglia--access cortical motor areas via the thalamus (not just sensory)
Organization of multiple subcortical and cortical motor circuits-reminiscent of parallel sensory pathways

12. Organization of Movements
Hierarchical: 3 major types
Reflexes
Postural adjustments
Voluntary movements
…from simple to complex
Diverse & adaptive
Purposeful

13. Organization of Movements
Hierarchical: 3 major types
Reflexes Spinal cord circuits
Postural adjustments Spinal & Brain stem
Voluntary movements Spinal cord, Brain stem, & cortex
Postural adjustments & voluntary movements depend more on cerebellar and basal ganglia function than reflexes
Dual purpose: 1) upcoming lectures;
2) context for motor pathways

14. Reflexes Stimulus-evoked involuntary muscle contraction
Monosynaptic (+) reflex
Knee-jerk
Jaw-jerk
Simple neural representation (circuit)

15. Knee Jerk
From muscle stretch receptors
Ventral horn
to muscle

16. Reflexes Stimulus-evoked involuntary motor muscle contraction
Monosynaptic (+) reflex
Knee-jerk
Jaw-jerk
Disynaptic reflex (+)
withdrawal

17. Why Disynaptic? Greater control (neural gate) More complex response
Very simple context
More complex response

18. Greater control: from periphery from higher centers
Intermediate zone
Response inhibited by inhibition
to muscle
Spinal Circuits

19. Greater control: from periphery from higher centers
Response blocked by inhibition
to muscle
Spinal Circuits

20. Motor I/O
Knee-jerk

21. Motor I/O Knee-jerk Automatic postural adjustments
• Balance
• Limb support
Flexible than reflexes (greater #; each w/control)
Constrained than voluntary

22. Postural adjustments Context important Feedback control-reactive
Can reorganize depending on context
Feedback control-reactive
Error correction
Response lags stimulus; sometimes too late; sometimes vicious circle
Feed-forward control-predictive
Response anticipates stimulus
More timely, but depends on practice
Depends on cerebellum, brain stem pathways & spinal cord
More complex neural representation

23. Voluntary movements Organized around purposeful acts
Flexible input-output relationships
Limitless
Price to pay: whole brain

24. Voluntary movements Organized around purposeful acts
Flexible input-output relationships
Limitless
Price to pay: whole brain
Recruits all motor systems components & much of the association cortex
Discuss:
Goal representation
Motor programs

25. The goal of voluntary movements is represented… somewhere
Motor equivalence
Individual motor actions share important characteristics even when performed in different ways
Abstract representation; effector independent
Hand writing
Soccer
Goal representation
??Association & Premotor cortex

26. Voluntary movements are organized by motor programs
Translate goal into action
Formation of a movement representation, or motor program
??Premotor cortex --> Primary motor cortex
Program
To produce the desired goal, which muscles should contract and when
2 Key movement characteristics that are programmed
Spatial (hand path; joint angles) Kinematic program
Force Dynamic program

27. Kinematic & Dynamic Programs in Reaching
Reach to target--(Sensation to Action)
Visual cortex-->Association cortex-->Premotor-->1° motor
Distinct kinematic and dynamic programs
Reach up
Against gravity
More force to achieve goal
Reach down
Gravity assists
Less force to achieve goal
Flexible control

28. Summary Motor behavior hierarchy Internal/neural representations
Reflexes
Postural adjustments
Voluntary movements
Internal/neural representations
Reflexes simple; invariant
Voluntary movements complex; flexible
Goal representation
Kinematic and dynamic programs
No wonder why voluntary movement recruit entire motor system

29. Motor Pathways Motor systems
Cortical motor areas
Motor Pathways
Basal ganglia
Cerebellum
Descending cortical motor paths
Descending brain stem paths
Spinal cord:
Intermediate zone
Ventral horn
Muscle
Motor systems

30. Motor Pathways Motor systems
Cortical motor areas
Motor Pathways
1° motor cortex
Premotor cortex
Red nucleus Reticular formation Vestibular nuclei
Superior colliculus
Descending cortical motor paths
Descending brain stem paths
Spinal cord:
Intermediate zone
Ventral horn
Muscle
Motor systems

31. Motor Pathways Hierarchy
Origins of motor paths
Premotor areas
Direct
Indirect
Motor Pathways Hierarchy
Motor Pathways Hierarchy

32. Motor pathways organized around the motor nuclei

33. Spinal Motor Columns From brain Segmental interneuron Motor neuron
Short
Long
Propriospinal--
Intersegmental--
neurons
Motor columns
(motor neurons)

34. Ventral Horn Organization: Proximal - distal rule
Lateral pathways:
limb control
Medial pathways:
trunk control
Ventral Horn Organization: Proximal - distal rule

35. Ventral Horn Organization: Proximal - distal rule
Lateral pathways:
limb control
Medial pathways:
trunk control
Ventral Horn Organization: Proximal - distal rule

36. Brain Stem Motor Paths Medial Lateral Tectum Tectospinal tract
Red nucleus
Rubrospinal tract
Reticular formation
Reticulospinal tracts
Vestibulospinal tracts
Vestibular nuclei
Bilateral
Contralateral

37. Brain Stem Pathways Lateral Medial
Rubrospinal tract: distal limb control; crude
Medial
Tectospinal tract: eye-head coordination
Reticulospinal tract: automatic postural adjustments and movements (hip; shoulder)
Vestibulospinal tract: balance (axial muscles); automatic postural adjustments

38. Brain stem nuclei Superior colliculus Tectospinal tract Red nucleus
Rubrospinal tract
Vestibular nuclei
Vestibulospinal tracts
Reticular formation
Reticulospinal tracts

39. Cortical Motor Paths Medial Lateral Lateral Cortico- spinal tract
Vestibular &
Reticular
nuclei
Medial brain stem paths
Red nucleus
Rubro-
spinal
tract
Pyramidal X
Lateral Cortico- spinal tract
Ventral corticospinal tract
Bilateral
Contralateral

40. Cortical motor paths Lateral corticospinal tract
Limb control mostly
Ventral corticospinal tract
Proximal muscle control; mostly upper body
For cranial muscle control: Corticobulbar tract
with medial and lateral components

41. Lateral Brain View Primary motor Lateral and ventral CST
Corticobulbar tract
Lateral Brain View

42. Cortical Motor Areas

43. Lateral Brain View Primary motor PMC Lateral and ventral CST
Corticobulbar tract
Lateral Brain View

44. MidSagittal Brain SMA Primary motor CMA Lateral and ventral CSTs
Corticobulbar tract
MidSagittal Brain

45. Why bother study the motor pathways?
Anatomical substrates: How it works
Multiple parallel paths & diversity of spinal connections
Damage to 1° motor cortex and pre-motor cortex projections recover some lost functions
Damage to cortex and brain stem paths recover some lost functions
With spinal cord injury. loss of monosynaptic connections and alternate paths via segmental and intersegmental interneurons can recover some lost functions