Movement & the Motor System Jun 13, 2014

1. Movement & the Motor SystemJun 13, 2014

2. SENSORIMOTOR CONTROL

3. From Sensory to Motor • Sensory info -- guide motor system • Somatosensory, vestibular, visual • Book works from the TOP to BOTTOM • Sensory “up”, motor “down” • Here -- SIMPLE to COMPLEX

4. Organization HIERARCHY • Analogy to a corporation • Workers = muscles • Supervisors = spinal cord & brainstem • Quality control = basal ganglia & cerebellum • Manager = primary cortex • President/CEO = higher order cortex

5. When do you use your motor system?

6. What is actually “moving” in the “motor system” … ?

7. MOTOR UNITS

8. Muscles • Skeletal muscle - stability to bony skeleton • Move around axes • 1000s of muscle fibers • “Fast” twitch fibers – speed • “Slow” twitch fibers – endurance

9. Muscles • Innervated by motor neurons from spinal cord • ACh (neuromuscular junction) • Motor unit – motor neuron & muscle fibers • Receptors for proprioception • Ch. in length (stretch), tension

11. Muscles • Only actively contract - more than one direction depends on antagonistic pairs • Contraction/relaxation • Ex. biceps & triceps • Flexorsbend, extensorsstraighten

12. SPINAL CIRCUITS

13. Spinal Cord • Via motor neurons • Sensory feedback from muscle & somatosensory • Can be independent of the cortex • Basic reflex circuits • Copy of information to cortex • Quick, unconscious

14. Basic Motor Reflexes Withdrawal reflex Sensory info  spinal cord  motor response “Walking” reflex In response to foot contact, ankle rotation Infants

15. Basic Motor Reflexes Stretch reflex • After sudden stretching force • Muscle stretch receptor  spinal cord muscle • Compensatory contraction • Controls external force on body position Infant reflexes • Sucking, rooting, grasping, startle

16. Basic Motor Reflexes • Requires info to muscle contracting, antagonist, and contralateral • Adjust and coordinate both sides

18. Reflexes • Some directed by brainstem • Eye movements (saccades, smooth pursuit) Complex movement under control of the cortex and descending motor pathways

19. MOTOR CORTEX

20. Primary Motor Cortex Most “movement” requires cortical control Cortex to motor neurons (spinal cord) & brainstem Directed movement, interaction with the environment

21. Primary Motor Cortex • Pre-central gyrusof frontal lobe • Adjacent to somatosensory area (post-central gyrus) • Why is this relevant? • Homunculus • Fingers, tongue, face – fine motor control

23. Descending Pathways • Several tracts • Different tracts/locations  different motor deficits • Control of posture/whole body movements vs. limb movements

24. Descending Pathways Dorsolateral corticospinal tract • Distal muscles (limbs, hands, fingers, etc.) • Primary motor cortex medulla (crossover)  spinal cord  muscles

25. Descending Pathways Three other tracts… • Indirect to spinal cord • Synapse w/ cranial nerve motor info • Proximal trunk muscles 1o cortex receives info from secondary motor cortex

28. When does the motor system need to be suppressed?

29. Basal Ganglia & Cerebellum

30. Basal Ganglia • Interconnected nuclei • Communicates w/ midbrain, rest of cortex • Distribution of tone, coordinating muscle groups • Shifting weight, posture • Cognitive roles • Compulsion, motivation, reward

32. Cerebellum • Comm. w/ cortex, basal ganglia, brainstem • Corrects movements, motor errors • Based out output • Rapid movement req. fine control, adapting to changing conditions • Also has cognitive roles

34. Higher Order Motor Control

35. Secondary Motor Cortex Premotor cortex • Frontal lobe • To primary motor • Decisions, coordination, programming of motor plans (sequence of events) • Order of movements, patterns • Imagining action

36. Secondary Motor Cortex • Complex integration of secondary areas • Input about target, body position • Mirror neurons – observation of motor action • Social learning & cooperation • Input from association cortex (in prefrontal cortex)

39. Association Cortex • Dorsolateral prefrontal cortex • Project to premotor area • Control over motor system • Directing attention, eye movements • Decision to make movements • Sensory feedback

40. Association Cortex • General programs stored, adapted to situations • Can be developed w/out practice • Can be modified w/ practice • Group behaviors together • Shift control to lower hierarchy (less cortex)

42. Motor System Dysfunction

43. Motor Dysfunction At any point in the hierarchy • Muscle weakness, atrophy, paralysis • Communication betw. spinal cord & motor unit • Descending pathways • Motor control, coordination, etc. • Cerebellar or basal ganglia disorders

44. Motor Dysfunction • Apraxia – dysfunction in voluntary movement • Not due to muscle, comprehension, motivational deficit • Reappearance of infant reflexes w/ cortical damage (withdrawal, grasping) • Don’t “outgrow” them, inhibited w/ development

45. Huntington’s Chorea • Basal ganglia & cortex • Genetic case - autosomal dominant • 30-50 years old • Protein damages neurons, over-stimulates target cells • Death of neurons • Severe cognitive, motor dysfunction • Jerks, twitches, writhing (dancing movements)

46. Parkinson’s Disease • Death of neurons proj. to basal ganglia • Substantianigra • Rigidity, tremor, slowed movement; cognitive • Starting, stopping voluntary motion • Genetic, environmental causes

47. Parkinson’s Disease Treatment – replace DA • DA can’t cross BBB • L-Dopa to replace DA • Neurons continue to die • Serious side effects

48. The Frozen Addicts Awakening the Frozen Addicts (BBC, 1993) http://youtu.be/QJIMC9d9l2o MPPP (synthetic opioid)

49. MPTP Ingestion of MPTP causes severe, immediate Parkinson’s symptoms… …What is the mechanism?

50. MPTP • MPTP metabolized to MPP+ by MAO enzyme • MPP+ brought into cells – DA reuptake transporter • Selective for neurons in substantianigra • Neurotoxic