HUF 2-43 Overview of the motor system

Question 1

Different types of movement

Answer 1

1. Voluntary
2. Breathing
3. Speech
4. Locomotion and posture
5. Eye movement: saccades; smooth pursuit

Question 2

Inborn vs. learned movement

Answer 2

Inborn
- infants walking with support

Learned

• athletics skill
• musical instruments
• sports

Question 3

Hierarchical motor system

Answer 3

Motor cortex / basal ganglia / cerebellum
=> brainstem
=> corticospinal tract
=> spinal interneurons
=> muscles

Motor equivalence: similar trajectories can be produced by different body segments
- abstract representation of purposeful movement in motor system

Neural machinery
- generate ms. activation patterns that execute the planned trajectories

Question 4

Stages of movement generation

Answer 4

Task goal
↓ (movement planning)
Trajectory
↓ (inverse kinematics)
Joint angle
↓ (inverse dynamics)
Joint torques and ms. activations (joint angle profile)
↓
Ms. pattern
↓
Motion

Question 5

Features of motor trajectories

Answer 5

1. Straight-line path and bell-shaped velocity profile

2. “Two-thirds” power law (linear relationship of angular speed against curvature to the power of 2/3)

Question 6

Motor cortexes

Answer 6

1. Premotor cortex
2. Primary motor cortex
3. Supplementary motor area
4. Cingulate cortex
5. Parietal cortex

Question 7

Complexity of motor execution

Answer 7

1. High complexity in solving inverse dynamics equations
   (e. g. 2D human arm: shoulder and elbow angles => torques)
2. Redundancy in musculature
   - multiple ms. with overlapping functions
   - different ms. patterns compatible with each net joint torque pattern
3. Indeterminacy of muscular actions
   - movement by same set of ms. in same way depends on limb’s configuration and state
   - ms. force = f(ms. length, activation)
   - moment arm of ms. = f(joint angle)
4. Complexity of coordinating many ms.
   - degree of freedom
   - adjustment difficulty

Question 8

Feedforward Motor Control

Answer 8

Desired state
=> Feedforward controller
=> Motor command
=> Actuator (ms.)

• Centrally generated commands without regard to consequence

e. g. Fast (“ballistic”) upper-limb reaching
- Single velocity peak: pre-specified motor commands
- Feedback control: first peak would be identical across targets; secondary peaks from feedback observed

Question 9

Feedback Motor Control

Answer 9

Desired state
=> Comparator
=> Feedforward controller
=> Motor command
=> Actuator (ms.)
=> Sensor (ms. spindle)
=> Input processing (filtering, amplification)
=> Sensed state
=> Comparator

• Motor commands specified by comparing desired state and actual state (according to sensory info)
  e. g. Precise object lifting

Question 10

Advantages and disadvantages of feedforward control

Answer 10

Advantages

1. Fast generation; no need to wait for sensory signals
2. Precise commands can be shaped through learning

Disadvantages

1. Movement error (from imprecise planning or execution, or unexpected perturbations) cannot be corrected
2. Requires good knowledge of ms. and limbs
3. Requires not-too-noisy neural system

Question 11

Advantages and disadvantages of feedback control

Answer 11

Advantages
- Precise movement ∵ feedback-driven correction

Disadvantages
- Delayed arrival of sensor info
=> movement instability
(sensory info at time T arrives at T+delta and compared with desired state at T+delta due to delayed arrival)

Question 12

Motor program

Answer 12

• sequence of feedforward motor commands (learned / inborn) that can be executed with minimal attention
• compensate for perturbation (instead of passive compensation by relying on neural sensory feedback)
• internal representation of dynamical environment where movement is executed
• Diff. from reflexes
  > motor program can be learned
  > motor program needs not to be triggered by specific sensory stimulus