Motor Systems Overview

Question 1

Motor systems have 3 major components and levels of control:

Answer 1

1. Cerebral cortex
2. Brain stem
3. Spinal cord

Question 2

What modulates all 3 levels of motor systems?

Answer 2

1. Basal ganglia
2. Cerebellum

Question 3

What does the thalamus do?

Answer 3

• relay station for imformation
  
  • basal ganglia ⇒ cortex
  • cerebellum ⇒ cortex

Question 4

Spinal cord:

Spinal motor neurons

Answer 4

Two General Types:

1. Motor neurons
   
   • lower motor neurons
2. Interneurons

Question 5

Motor Neurons:

Answer 5

• Location: ventral horn
• project to muscles and ultimately cause muscle contraction and execute movement

Question 6

How are motor neurons somatotopically organized?

Answer 6

• Medial motor neurons:
  
  • innervate proximal muscles
  • control balance, posture, movement of trunk
• Lateral motor neurons
  
  • innervate distal muscles
  • control limbs and digits and control specific limb, digit movement

Question 7

Interneurons:

Answer 7

• Location: intermediate zone
  
  1. segmental interneurons that project within a single spinal cord level
  2. propriospinal interneurons that transmit info between multiple spinal cord levels
     
     • ​​Project to motor neurons.
• Form circuits
  
  • help connect and coordinate motor neurons

Question 8

What is the final common pathway of motor systems?

Answer 8

• Spinal motor neurons
  
  • Clinically termed “lower motor neurons”

Question 9

Brainstem:

Types of Neurons

Answer 9

• Modulates activity in the spinal cord

1. Upper Motor Neurons
2. Lower Motor Neurons

Question 10

Lower Motor Neurons:

Answer 10

• directly innervate facial muscles
  
  • Facial nucleus ⇒ facial expression
  • Hypoglossal nucleus ⇒ tongue

Question 11

Upper Motor Neurons:

Answer 11

• Many groups of neurons that project down and terminate on neurons in the spinal cord gray matter
• Named based on origin and end points
• Medial & Lateral pathways

Question 12

Medial Brainstem Pathways:

Answer 12

• Reticulospinal, Vestibulospinal, Tectospinal tracts

1. ​Descend in medial ventral white matter
2. Terminate in the ventromedial area of the ventral spinal cord

• Influence axial, proximal muscles
• Provides basic postural control system
  
  • allows cortical motor areas to organize more highly differentiated movement

Question 13

Lateral Brainstem Pathway:

Answer 13

• Rubrospinal tract: Red nucleus ⇒ spinal cord

1. Descends in dorsolateral white matter
2. Terminates in the dorsolateral area of the ventral spinal cord

• Influence motor neurons that control distal muscles of limbs
• Modulate goal-directed limb movements
  
  • reaching and manipulating

Question 14

Cerebral Cortex:

Answer 14

• Modulates action of motor neurons in the brainstem and spinal cord
• Organize complex motor acts and execute fine movements with great precision
  
  • Plans movements, coordinates their execution
  • Gives descending commands to motor neurons in spinal cord and brainstem
• Major areas of motor control:
  
  1. Primary Motor Cortex
  2. Premotor Cortex
  3. Supplementary Motor Area

Question 15

Primary motor cortex:

Answer 15

• Executes commands to motor neurons:
  
  • Controls individual finger movements
  • Coordinates the force and direction of movements
• Contains somatotopic map of body:
  
  • Electrical stimulation of discrete spots produces movement of specific body parts on opposite side
• ​Brodmann’s Area 4

Question 16

Premotor cortex:

Answer 16

• Integrates motor movements with sensory input
  
  • mainly from visual system
• Coordinates complex sequences of movement
  
  • motor learning
• Brodmann’s Area 6

Question 17

Supplementary motor area:

Answer 17

• Important in internally-driven, will-driven movements
  
  • formulate an intention to make a movement
• Brodmann’s Area 6

Question 18

1. primary somatosensory cortex:
2. posterior parietal cortex:

Answer 18

1. primary somatosensory cortex: regulates incoming sensory info in the dorsal horn
   
   • ​Brodmann’s Area 3, 1, 2
2. posterior parietal cortex: helps localize where an object is with respect to body
   
   • ​​Brodmann’s Area 5,7

Question 19

What is happening here?

Answer 19

• Changes in blood flow in cerebral cortex in response to:
  
  1. simple movements
  2. complex movements
  3. mental rehearsal of complex movements
• Supplementary motor area is important in planning and learning complex, internally-generated movements
• Blood flow increases here even if movement is mentally rehearsed but not actually performed

Question 20

The cerebral cortex acts on motor neurons via two descending pathways:

Answer 20

1. **Lateral corticospinal tract: **
   
   1. ​contralateral limb, digits
   2. goal-directed reaching
2. **Ventral corticospinal tract: **
   
   1. ​neck, trunk muscles
   2. postural control

Question 21

Motor systems are organized both __________ and in ______.

Answer 21

• hierarchically
• parallel

Question 22

How are motor systems organized hierarchically?

Answer 22

• Cortex ⇒ brainstem ⇒ spinal cord ⇒ muscle movement
  
  • Simple tasks ⇒ complex tasks

Question 23

How are motor systems organized in parallel?

Answer 23

• Parallel motor pathways exist between:
  
  • cortex and spinal cord (corticospinal tracts)
  • brainstem and spinal cord (brainstem tracts)
• When cortical, brainstem or spinal cord lesions occur:
  
  • Alternative pathways can partially compensate and carry out motor tasks
    
    • person can still have basic motor functions
  • Redundancy gives flexibility and plasticity after injury

Question 24

What type of organization is found at each level of the spinal cord?

Answer 24

Somatotopic organization

Question 25

What are the 3 general types of movements?

Answer 25

1. Reflex movements
2. Automatic postural adjustments
3. Voluntary movements

Question 26

Reflex Movements:

Answer 26

• simple, involuntary coordinated patterns of muscle contraction and relaxation evoked by peripheral stimuli
  
  • Involve spinal cord, motor neurons, sensory neurons
  • sometimes brainstem neurons

Question 27

What would invoke a reflex movement?

Answer 27

• Withdraw from a painful stimulus
• Movements that can be performed unconsciously:
  
  • Repetitive, rhythmic motor patterns:
    
    • chewing, swallowing, breathing, scratching, coughing
    • contracting extensor and flexor muscles for walking

Question 28

Why are reflexes important clinically?

Answer 28

1. Higher level motor control systems:
   
   • use simple reflex circuitry to coordinate muscles during complex, purposeful movements
2. Reflexes are tested clinically to diagnose level of lesions/damage

Question 29

Automatic postural adjustments:

Answer 29

• Slightly more complex, flexible than reflexes
• Involve brainstem, spinal cord, motor neurons
• Example:
  
  • ​compensatory shift in mass to maintain your balance

Question 30

Voluntary Movements:

Answer 30

• Organized around a purposeful act
  
  • goal-directed
• Involve cerebral cortex, brainstem, spinal cord, motor neurons
• These motor pathways are the most flexible and complex in the body
• Improve with practice
  
  • ​nervous system learns to anticipate and correct for environmental obstacles
• ​response time is much longer than for reflexes
  
  • mainly due to processing time & energy needed in higher order areas