Motor Systems

Question 1

Motor Cortex

Components

Answer 1

• Components:
  
  • Primary motor cortex ⟾ precentral gyrus (BA 4)
  • Premotor and supplementary motor areas (BA 6)
  • Frontal eye field (BA 8)
• Output from pyramidal cells in layer V
  
  • form corticospinal tracts
  • form pathways to other motor structures

Question 2

Motor System

Regulation

Answer 2

Control involves motor cortex, premotor cortex, somatosensory cortex, and frontal eye fields.

• Motor cortex and other cortical outputs regulates basal ganglia and cerebellum
• Regulates some sensory inflow
• Activates spinal motor systems
  
  • Direct pathway
  • Indirectly via rubro, pontine, and medullary connections

Question 3

Motor System

Connections

Answer 3

• Motor control areas projections
  
  • Direct ⟾ corticospinal tracts (pyramidal)
  • Indirect ⟾ cortical extrapyramidal tracts
    
    1. project to basal ganglia or via pontine n. to cerebellum
       
       • then modify cortical activity via thalamus
    2. project to nuclei in brainstem
       
       • then project to spinal cord
  • Also projects to craniofacial and oculomotor systems
• Cells intermixed in the cortex
• Fibers exit via internal capsule and descend
  
  • Extrapyramidal fibers synapses with:
    
    • striatum
    • superior colliculus
    • red nucleus
    • pontine nuclei
    • reticular formation
  • Cortibulbar fibers (of pyramidal system) → cranial nerve nuclei

Question 4

Motor Area

Projections

Answer 4

1. Control circuits for coordination
   
   • Other cortical areas:
     
     • Supplementary motor & frontal eye field ↔ posterior parietal areas 5 & 7
   • Basal ganglia
     
     • All cortical motor regions ↔ Striatum
   • Pons/Cerebellum
     
     • All cortical areas → somatopically matching pontine n. → cerebellum
2. Sensory modulation
   
   • Thalamus
     
     • Cortical areas ↔ associated thalamic input
       
       • Primary motor ↔ VL
       • S1 ↔ VPL
       • BA 6 ↔ VA
       • BA 4 & 6 ↔ CM ↔ basal ganglia
   • Dorsal column nuclei
     
     • BA 1,2,3 → LCST → dorsal columns
3. Spinal Cord Control
   
   • Direct
     
     • BA 4 & 6 → LCST
     • BA 4 & 6 → VCST
   • Indirect
     
     • BA 4 & 6 → red nucleus & LCST
     • BA 4 (trunk) & 6 → MRST, PRST, VCST

Question 5

Extrapyramidal Tract

Somatotopy

Answer 5

Cortical fibers → internal capsule → cerebral peduncle → targets.

• Somatotopically organized projections:
  
  1. 1° motor cortex → red nucleus
  2. Premotor → pontine RF → contralateral cerebellum
  3. Cortex → lateral reticular n. & inferior olive
  4. 1° motor cortex (face area) → motor n. of V & VII
     
     • mostly contralaterally
• Not somatotopically organized projections:
  
  1. 1° motor and premotor → mesencephalic RF
  2. Motor & premotor → medial RF (mostly contralateral)
  3. Cortex → lateral reticular n. and inferior olive

Question 6

Inputs to

Motor Cortex

Answer 6

1. Cortical input from posterior parietal cortex
2. Thalamic

Question 7

Corticospinal Tract

Answer 7

Origins:

premotor cortex

primary motor

primary somatosensory cortex.

Question 8

Motor Functions

Answer 8

• Primary motor cortex
  
  • execution of movement
  • fine motor control
• Premotor cortex
  
  • reflex regulation
  • motor set within a limb
• Supplementary motor cortex
  
  • sequencing
  • planning
  • bilateral integration of limb movements

Question 9

Primary Motor Cortex

Answer 9

M1 or Brodmann’s area 4

• Contains motor maps
  
  • layer V cells have detailed somatotopy
  • can be used to localize strokes or seizure origins
    
    • strokes cause local deficits in muscles groups
    • muscles are progressively recruited in a seizure as activity sweeps across motor cortex
• Can encode force
  
  • appears to be code force precisely
  • provide precise muscle specification
  • uses inhibition to focus excitation
  • provides basis for fractionated movement
• Provides flexible “long loop” reflexes
  
  • controls force and movement
  • uses projections of S1-S3 to control pyramidal outflow

Question 10

Motor Lesions

Overview

Answer 10

• Reflexes and other activites mediated by brainstem, cerebellum, and basal ganglia controlled by cortical efferents
  
  • exaggerated responses without cortical input
  • requires coarse stimulus
• Useful movement possible without motor and premotor cortex
• Lesions that leave cortex intact can have significant symptoms
• Cooperation between cortex, basal ganglia, cerebellum, VA, and VL crucial for movement
• Lower brainstem also important
  
  • ex. brainstem RF lesion can result in significant spasticity

Question 11

Primary Motor Cortex

Lesion

Answer 11

Lesions of Brodmann’s area 4 that spare M1 trunk:

• weakness or flaccid paresis of voluntary movement in affected parts
• only trunk areas of BA 4 project to MRST/PRST system
  
  • these systems responsible for spasticity seen with UMN lesions
  • all of premotor cortex influences MRST/PRST

Question 12

Premotor Cortex

Lesion

Answer 12

• Involved in reflex set up or ‘set’ within a limb
• strongly modulates indirect pathways in brainstem
• activity preceeds activity in primary motor
• Lesion:
  
  • reflex imbalance
  • cortical spasticity
  • grasp or Babinski sign
    
    • depends on area

Question 13

Supplementary Motor Cortex

Lesion

Answer 13

Concerned with planning sequences, multi-limb integration, and bimanual coordination.

Mental rehearsal of motor acts involves SMA.

Lesions:

deficit in bimanual coordination

unadjusted, poorly planned movements

sequence apraxias

Question 14

Motor Coordination

Considerations

Answer 14

Reaching to a point in external space involves:

• extraction of location from visual processing stream
• limb and joint coordinates must be planned
• muscle activations must be chosen to generate movement
• movement has to be carried out

Question 15

Order of Activation

Cortex

Answer 15

1. Posterior parietal cortex
   
   • Used for representation of extrapersonal space represented in
2. Premotor and supplementary motor areas
   
   • Set up and plan sequence and reflex patterns for motion
   • Helps organize primary motor activity
3. Primary motor cortex
   
   • Acts via corticospinal tract to execute and regulate movement

Question 16

Population Coding

Answer 16

Force and direction coded for by populations of cells.

Each cell codes for specific direction/force of movement

or for multiple.

Sum of firing results in desired movement.

Question 17

Motor Cortex

Reorganization

Answer 17

Somatotopy within the motor cortex holds true only on a coarse scale.

i.e hand area = hand area

Practice and injury can cause reorganization.

i.e. yoyo ma has a large hand representation from practicing

Complexity and degree of fractionation of movements important in plasticity.

Question 18

Plasticity

Answer 18

1. Developmental level
   
   • brain shows greater plasticity in childhood
   • there is reorganization of the motor cortex after a neonatal spinal injury
     
     • more reliant on cortex for integration and coordination
   • after early peripheral limb amputation the cortex reorganizes
2. Plasticity occurs on a task based and day to day level
   
   • Areas can alter output temporarily in the context of a task
     
     • ex. person repeatedly extends thumb for a long time, stimulation of thumb flexion area after will result in thumb extension
3. Plasticity after adult lesions
   
   • both rapid and slower reorganization occurs
   • can occur because info in cortex is distributed
   • reorganization can convert unaffected areas