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Bi/CNS 150 Lecture 17 Wednesday November 4, 2015 Motor Systems

Bi/CNS 150 Lecture 17 Wednesday November 4, 2015 Motor Systems Chapter 14, p 309 (ALS); chapter 34, 35, 37, 38 Henry Lester. Motor systems, “top” to “bottom”. Higher motor functions Motor cortex Basal Ganglia Corticospinal tract Motor neurons Reflexes. Clues to Organization,

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Bi/CNS 150 Lecture 17 Wednesday November 4, 2015 Motor Systems

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  1. Bi/CNS 150 Lecture 17 Wednesday November 4, 2015 Motor Systems Chapter 14, p 309 (ALS); chapter 34, 35, 37, 38 Henry Lester

  2. Motor systems, “top” to “bottom” Higher motor functions Motor cortex Basal Ganglia Corticospinal tract Motor neurons Reflexes

  3. Clues to Organization, from stimulation in human motor cortex. An array is implanted . . . to localize an epileptic focus 3

  4. Progress in Understanding Motor Systems, 2015 Work from Richard Andersen’s lab Caltech Brain-Machine Interactions, Kandel, Box 37-3 http://authors.library.caltech.edu/54866/

  5. Sensory & Motor Aspects of Behavior Account for Roughly Equal Times Stages of Processing 1. Transduction 2. Perception (early) 3. Recognition (late perception) 4. Memory (association) 5. Judgment (valuation, preference) 6. Planning (goal formation) 7. Action 5

  6. Examples of motor output • Spinal reflexes and motor units • Posture and muscle tone • Locomotion • Control of distal extremities • Breathing • Eye movements • Speech • Emotions • Autonomic Nervous System (visceromotor) 6

  7. Motor output at different levels Reflexes --spinal --central "Fixed action patterns" Emotional reactions Actions Long-term plans Stimulus-coupled Stimulus-decoupled 7

  8. Motor Cortex Has Layers 8

  9. Motor System Hierarchy Motor System Hierarchy ganglia 9

  10. Key Motor Tracts Decussation in hindbrain 10

  11. Damage to Motoneuron (Cell body or axon) Example: Amyotrophic lateral sclerosis (ALS) “Lou Gehrig’s Disease” “Upper” motoneurons also degenerate Loss of motor unit innervation leads to weakness or paralysis of muscle Fasciculations (spontaneous contractions of muscle fibers); detected with electromyography (EMG) Atrophy of muscles, due to loss of trophic factors from motoneuron Hyporeflexia or areflexia Average time from diagnosis to death ~ 3 yr 11

  12. The Basal Ganglia and ventral midbrain: Most Nuclei are GABAergic “striatum” Glutamatergic Dopaminergic. Future lecture on Parkinson’s disease 12

  13. The Basal Ganglia: Major inputs “striatum” 13

  14. The Basal Ganglia: Projections among nuclei 14

  15. Behaviors in Basal Ganglia Diseases • Three common characteristics: • tremor and other involuntary movements • changes in posture and muscle tone • slowness of movement without paralysis • Cause either excess or diminished movement • Cognitive changes (via caudate nucleus) 15

  16. Some Spinal Cord Motor Concepts • Motor unit: motoneuron and all innervated muscle fibers; variable number of fibers, depending on force required • Alpha-motoneuron: final common pathway • Motoneuron terminals, endplates, muscle action potentials, muscle contraction • When MN fires, all muscle fibers contract • Recruitment: adding muscle units to increase force of contraction 16

  17. Fewer Myelinated Fibers in Lower Spinal Cord 17

  18. The Motor Unit 18

  19. Motoneuron in Typical Spinal Cord Cross Section Dorsal Horn Sensory Myelin Motoneuron Ventral Root Motor 19 Ventral Horn Motor

  20. Electrophysiology of the Motor Neuron and Muscle Fiber Previous Lectures 20

  21. Herniated Disks Compress Nerve Roots (L5 most common) 21

  22. Motor Unit Size & Physiology • Force increased by recruiting motor units • Motoneurons of different sizes: small MNS to small, slow motor units; large MNs to large, fast motor units • Size principle: smallest motor units (and smallest force) first; then larger motor units • Muscle fibers: slow (red); fatigue resistant (intermediate); fast, fatigue (white) 22

  23. 23

  24. 97% of spinal cord neurons are interneurons. Reflexes must be coordinated; this is complex • Sensorimotor integration in absence of supraspinal input • Motoneurons get input from sensory fibers, interneurons and descending fibers • Stretch reflexes • Flexion-withdrawal reflex • Crossed extensor reflex Groups of interneurons Tracts 24

  25. Ipsilateral part of the crossed extensor reflex: Interneurons inhibit extensors when the flexors are commanded, and vice-versa Figure 35-2B 25

  26. A Feedback Loop Controls Muscle Function 26

  27. Spindles Detect Stretch, Due to Elongation of the Muscle and of the Spindles Themselves External Stretch External Stretch Modified from Figure 35-3 27

  28. Golgi Tendon Organs Detect Stretch, Due to Muscle Contraction 28 Modified from Figure 35-6

  29. Damage in the Motor System 29

  30. Anterior Cingulate Cortex Lesions in this region cause impairment in one of the hierarchically highest levels of the motor system: the will to act . Patients with lesions to ACC can exhibit "akinetic mutism": they are not paralyzed and are conscious but respond poorly to their surroundings. They sometimes respond to very automatic things, like picking up a phone that rings next to their bedside (but then say nothing). They often recover, and then explain that while in this state, they were fully conscious but just lacked motivation to do anything and so did not respond or act on their surroundings. 30

  31. Links Between Perception and Action: Why Can’t You Tickle Yourself?

  32. End of Lecture 17

  33. Motor Areas of Cortex Primary Motor Cortex BA 4 Premotor/supplementary Motor cortex BA 6 Frontal Eye Fields BA 8 Prefrontal Cortex (Frontal Association Areas) Broca’s Area (left side) BA 44, 45

  34. Sensory & Motor Aspects of Behavior Account for Roughly Equal Times 34

  35. Stages of Processing 1. Transduction 2. Perception (early) 3. Recognition (late perception) 4. Memory (association) 5. Judgment (valuation, preference) 6. Planning (goal formation) 7. Action

  36. 1. Sensory Organs in Muscle Participate in the Feedback Loop Intrafusal fibers in parallel with extrafusal muscle fibers Two types of sensory fibers – primary (Group Ia fibers) and secondary (Group II fibers) spindle afferents Group Ia – change in length (dynamic) Group II – length (static) Golgi tendon organ measures tension of muscle contraction Sensory information goes to spinal cord segment, dorsal column nuclei (proprioception), and cerebellum Extrafusal fibers 36

  37. 2. Gamma motoneurons in muscle participate in the feedback loop Small MNs that project out ventral roots to intrafusal fibers Activity in gamma-MNs contracts the intrafusal muscles and makes the spindle apparatus more sensitive In turn, the group Ia and II fibers become more active Gamma-bias impacts muscle tone Extrafusal fibers 37

  38. Mirror Neurons Links Between Perception and Action: Mirror Neurons 38

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