Basic Models of Motor Control

motor control

planning, initiating, monitoring and regulating motor skills

planning

selecting what motor skill to execute and specifying how to execute motor skill

intiation

transforming motor plans to commands and transmitting motor commands to spinal cord where motor neurons initiate movements (right before movement)

monitoring

gathering and processing sensory feedback to modify ongoing movements (regulation) and refine future movements (motor learning)

regulation

modifying ongoing movements to prevent and correct for movement errors detected from incoming sensory info (when errors prevent motor skill from successfully occurring)

motor control mechanisms

engages neuromuscular, musculomechnical, sensory, and neural (CNS) mechanisms

vestibular system

senses acceleration of head

models of motor control

• conceptual or theoretical frameworks of how we perform motor skills

• used to explain and predict characteristics of motor control and learning

• explain how various neuromuscular, musculoskeletal, sensory, and neural mechanisms work together to control movement

movement models purposes

• predicting movement features of new skills or variants

• developing interventions for rehab, practice and training

• predicting changes that will result from rehab, practice and training

Movement models features

• good movement models can explain and predict many features of motor behavior

  • 2 features include motor redundancy and degrees of free problem

motor redundancy

• many ways to successfully perform motor skills

• kinematics: diff combos of limb and joint movements

• intermuscular coordination: diff combos of muscle activations

• intramuscular coordination: diff combos of recruitment and rate coding

degrees of freedom problem

• CNS must select 1 movement solution from many possible solutions (motor redundancy)

• movement models explain how CNS solves degrees of freedom problem

two classes of models

open loop and closed loop

open loop (feedforward) models

• use top-down flow of info

• do NOT use sensory feedback to initiate and regulate movements

• requires comprehensive motor plans that prevent errors from occurring

• likely used to perform short duration (<50ms) movements (finger tapping)

closed loop (feedback) models

• reciprocal flow of info

• use sensory feedback to initiate and regulate movements

• do NOT require comprehensive motor plans (feedback used to correct errors)

• likely used to perform longer-duration (>50ms) movements (reaching)

hierarchical models

• complex variants of open-loop models

• use top down flow of info (brain to spinal cord)

• brain uses comprehensive motor plans (entire motor skills are preprogrammed)

• do NOT use sensory feedback (assume regulation does not occur)

• exemplified by schema theory

schema theory

• uses generalized motor programs and scheme for motor control

generalized motor programs (GMPs)

• represent diff movement classes

• separate GMPS for walking, running, etc.

  • invariant characteristics and parameters

invariant characteirstics

features of GMPs that are constant (relative force and timing, and coordination patterns can not be modified)

parameters

inputs to GMPs used to modify movement features (overall force and duration of movements modified by parameters)

schemas

• executive controllers that make decisions

  • select which GMP to use and what parameters to use w it

Strengths of hierarchical models

• brain plays central role in motor control

criticisms of hierarchical models

• no role for sensory feedback in initiating or regulating movements (ample evidence that feedback used to initiate and regulate movement)

• require comprehensive motor programs for all movements (implausible that comprehensive GMPs exist for all possible movements

• cannot easily explain how novel movements are learned (unclear how brain would create new GMPs that do not already exist)

heterarchical models

• complex variants of closed loop models

• use reciprocal (bidirectional) flow of info (feedforward and feedback)

• sensory feedback used to initiate and regulate movements

• primarily based on spinal circuits (brain not involved in planning, initiating monitoring or regulating motor skills)

• exemplified by 3 models: equilibrium point hypothesis, uncontrolled manifold hypothesis, central pattern generators

equilibrium point hypothesis

• heterarchical model based on reflexes

• mechanisms based on spinal and brainstem circuits that mediate reflexes

• sensory feedback initiates and regulates reflexes to control movements

• distinct combos of reflexes used to peform diff motor skills

uncontrolled manifold hypothesis and synergies

• heterarchical model based on synergies

  • diff combos of synergies engaged to perform diff motor skills

synergies

• groups of muscles that act together as functional units

• use hardwired spinal circuits that decrease redundancy and simplify degrees of freedom problem

• CNS needs to control fewer synergies → individual muscles simplified into groups that act together

central pattern generators (CPGs)

• heterarchical models based on hardwired circuits in brainstem and spinal cord

• sensory feedback initiates and regulates circuits to control movements

• circuits more complex than those that mediate reflexes (do NOT require motor commands from brain)

  • paralyzed cat on treadmill experiment

human central pattern generators

• several CPGs proposed in human spinal cord and brainstem

• CPGs may be engaged during gait in patients w stroke and spinal cord injury

  • theoretical basis for body-weight supported treadmill training in rehab

strengths of heterarchical models

• use sensory feedback to initiate and regulate movements

• based on known anatomy of spinal and brainstem circuits

criticisms of heterarchical models

• do not include brain regions involved in motor control (motor cortex)

• do not consider cognitive factors that influence motor control (task goals)

• do not consider behavioral factors that influence motor control (anxiety)