Motor Control Flashcards
Motor Control
- The ability to regulate or direct the mechanisms essential to movement
- Seek basic principles of controlling voluntary movements.
- How the nervous system interacts with the rest of the body and the environment
Multi-System Control of Movement
- Integrated, distributed, and interactive system of elements and operations comprised of all sub-systems that may contribute to movement function, together with the features of the external environment within which one moves and the physical laws governing the natural world
- Thus, movement dysfunction is attributable to changes in normative multi-system control for a given health condition
3 types of constraints:
Individual, task, and environment
Individual Constraints
- What systems needed?
* What does the person bring to bear on the task? Their ability and/or health condition(s)?
Task Constraints
- How do you do it?
* Why do you do it?
Environmental Constraints
- Surface features
* Lighting
What are Some Sub-Systems in the Nervous System Needed for Movement?
• Sensory and Motor Systems
• Cognitive Systems –attention, planning, problem-solving
• Motivational/Emotional Systems –important for intent and goal-
achievement
Sensory and Motor Systems in the nervous system needed for movement
- descending systems (CST, VST, RST, etc)
- spinal elements (Ia, Ib, II, Renshaw, interneurons, the various reflexes)
- Somatosensory systems (ALS, DCML)
- Perceptual systems and processes (eg, visual system but also the processes associated with interpretation and meaning
Other systems needed for movement?
- The CNS/PNS don’t act in isolation
- Biomechanical systems
- Cardiorespiratory, endocrine, lymphatic, renal, reproductive, etc
What are all of these systems a part of?
- The individual –> individual constraints
• Components of movement and the underlying systems that make them up
• Body structures and functions
Types of Task Constraints
- Discrete / serial / continuous
- Closed / open
- Stationary / moving
Environmental Constraints
- Regulatory and non-regulatory features of the environment
- Those things that should be attended to (weight, shape, size of an object to be grasped; height, width, conformability of an obstacle).
- Those things that don’t require attention (color of cup)
Explain how constraints limit choice
They help turn a seemingly massive and uncontrollable system into a manageable one –> Think of it as if boundaries have been placed on all the elements and their interactions needed for a movement
How to think about movement-
in a complex, multi-system organism, while embedded in a physical environment with structure and energy in the form of load, light, and sound (physical laws)
Forward Internal Model
- Model of what you sent into the world
- We don’t represent the world directly, we represent a model of all of the information we’ve taken in
Reflex Theory
- Reflex was the basic unit of movement.
- Reflex: “stereotypical response to some stimulus.” Need stimulus, receptor, afferent and efferent pathway, effector (motor response)
- Sensory input drives (is required for) movement
- Closed-loop system: a feedback system
Reflex Chain Hypothesis
- notion that each of a series of movements may be linked through the sensory results of prior movements
- developed to explain the observation that successive movements could be linked together into a contiguous sequence
- Complex movements are a result of a chaining together of elemental reflexes (chain hypothesis)
Therapeutic Implications of Reflex Theory
- PT can alter patient’s movement pattern by applying specific patterns of sensory stimulation (eg., proprioceptive path) –> Use of sensory inputs to facilitate (or inhibit) movement –> access CNS via sensory system
- Emphasis on eliciting reactions (particularly postural -equilibrium/righting)
- Treat spasticity before facilitating “normal” movement –> Don’t strengthen because forceful movements will increase spasticity
Limitations of Reflex Theory
- Patient not a spinal animal (not just a bundle of reflexes to be stimulated or suppressed)
- No concept of intention in this process
- Can override reflexes (delaying dropping a hot cup of tea in order not to spill it) –> “reflex chaining” is not obligatory
- Sensory input not “required” for movement –> Deafferented animal studies
More limitations of Reflex Theory
- Only “postural reflex mechanism” –> No APA in this model
* Feedback systems require too much time to account for fast (ballistic) movements –> Triphasic EMG pattern
Hierarchical Theory
- Structural-functional relationship in CNS is hierarchical
- CNS has higher and lower “centers”
- Strict top-down control model
Explain how structural-functional relationship in CNS is hierarchical
- Identification of the “Motor Strip” –The cerebrum is involved in movement
- Movements are controlled not muscles
Explain the CNS has higher and lower “centers”
- Higher centers are associated with cortical structures and are responsible for complex volitional movement (structure-function relationship).
- Lower centers are associated with brainstem and spinal structures and are responsible for primitive reflexes and other automatic movements.
- Higher centers suppress reflexes at lower centers
If higher centers control and/or inhibit lower centers’ reflexes, then ….
there is a close observational link to developmental/ maturational process
- In development it was thought higher centers gain control over lower primitive centers (an increasing corticalization of the CNS) –> Emergence of higher-level control over lower-level reflexes
What does the assumption was brain damage to cortex mimic?
- development
- Damage leads to a persistence of primitive lower-level reflexes.
- Recovery is observed through a progressive inhibition of lower level automatic / reflex responses
what does neuro-maturational Theory of Development
Depends on?
a Hierarchical (and Reflex) Model of Control
Therapeutic Implications of the Hierarchical Theory
- Positive and negative symptoms
- Recovery is hierarchical and moves in a predictable sequence.
- Move through inhibition of abnormal patterns before facilitation of normal patterns
- Then stimulate (via sensory systems) equilibrium and righting reactions in supine and sitting before moving to higher developmental positions –> This combines reflex and hierarchical theories
Hierarchical Theory- Positive and negative symptoms
- Brain damage to higher centers results in decreased volitional control –> There is a release of reflex patterns
- Lesion (cause) = abnormal response/pattern (effect)
Hierarchical Theory- Limitations
- Movement is not governed by a strictcontrol hierarchy in the CNS
- Need subcortical circuits to assist in planning and initiating complex movements –> There are no privileged levels of the nervous system
- Spinal cord is not just a relay station between cortical control and so-called primitive reflexes at the spinal level
Spinal level “control” signals
- not formulated in terms of contractions of individual muscles or even movements in individual joints
- Otherwise, loading a segment or blocking a joint would have led to inaccurate movements
- Second, the results imply the existence of very fast corrections of movement patterns, built into the “program” for wiping
Additional Limitations of Hierarchical Theories
• Primitive reflexes do not disappear due to neural maturation alone.
Environment, task and biomechanical factors explain much in the
emergence of complex movements
• Strict adherence to a hierarchical viewpoint resulted in a rigid
application of developmental sequence in physical therapy
Reflex and Hierarchical Control
Both were used to develop the traditional neurotherapeutic
approaches of:
• Bobath –Neurodevelopmental Treatment (NDT)
• Rood –sensory stimulation
• Knot and Voss –Proprioceptive Neuromuscular Facilitation (PNF) reflexes to
pattern
• Brunnstrom –reflexes to synergies
Theoretical Advances producing a dynamic for change
• Just as traditional approaches became popular (1960s), the West was
introduced to the research of N. Bernstein –> birth of motor control
• De-emphasis on neurological factor and emergence ofbiomechanical
and behavioral factors contributing to performance
• The results of the advances in science and their addition to PT = “minds on” versus “hands on” PT
Nikolai Bernstein
- Feedback or feedforward control alone are an incomplete perspective on movement control
- Emphasis on the interactionbetween central, peripheral, mechanical, and environmental factors in the organization of movement
The problem of movement is ….
- Overcoming the many DOF of the movement system: 300+ joints, 600+ muscles, 8 x 10^10 neurons
- Therefore, “coordination of movement is the process of mastering redundant degrees of freedom of the moving organ, that is, its conversion to a controllable system.”
Bernstein Stated:
• The state of the affairs of the body are always variable –> The initial conditions are always different
• The “executive” needs no context in purely feedforward
control
• Bernstein said this could never be true –> the state of affairs of the
executive are indeterminate
Mechanical Variability or Indeterminacy:
Two identical commands to the elbow flexors will produce different movements depending on the initial conditions (position; load; fatigue)
CNS without context
- Achieving the goal requires this change in force over time
- CNS “command” achieves goal
- This is perfect for a robotic arm where all forces are specified and the conditions are entirely known
CNS + Context: Forces Given for “Free”
Goal: required change in force
“Given For Free”: gravity; reactive; frictional load forces
CNS only needs to “bend” those forces given for free
Forces Given for Free
• For Bernstein, the forces given for free were as important as
central (internal feedforward) and peripheral (external feedback)
aspects of movement
• Basically, Bernstein is saying here “biomechanical factors matter
in movement control and coordination.”
Solving the DOF Problem: Synergies
• Because of many DOF, movement can be accomplished in a potentially infinite number of ways. Solution: Ensure flexible and stable performance through synergies
Synergies
- many individual muscles or elements acting together as a functional unit to achieve some goal
• Co-variation of elements to stabilize performance (variability) –> Blacksmith experiments
Blacksmith Experiment
• Endpoint (working point) of hammer has relatively low variability
(blacksmiths are well trained at hammering)
• Variability of individual joints is higher than working point
• Individual joint motions were not reproducible from trial to trial, but the motion of the working point is reproducible.
• The task appears to be organized around stabilizing the working
point. The goal (or image of achievement) is a key factor in the construction of the synergy.
Synergy in Motor Control vs Lesions
• Synergies are not hard-wired patterns of movement but flexible patterns that emerge as a result of task, individual & environmental constraints –> Stable against perturbations
What do synergies represent
- an organizational structure of the neuromuscular system –> Synergies may be linked to specific neural processes like spinal circuitry or sensory feedback loops
• Limb and postural synergies
• Flexible and stable against perturbations –> Exploit variability (DOF) rather than limit it.
Abnormal Synergies following cortical lesions…
aren’t synergies in the context of the preceding discussion
• Abnormal Flexor; Extensor of the UE; LE
• These are relatively fixed patterns of muscle activations due to lesions in the corticospinal system –> Limited variability/flexibility
Motor Program (S. Keele):
“A set of muscle commands that are structured before a movement sequence begins, and that allow the entire sequence to be carried out uninfluenced by peripheral feedback.”
Evidence for a motor program
- Movement is possible in deafferented subjects.
- Rapid movements cannot be modified by sensory feedback while in progress –> so maybe it is executed in a “bundle” (motor program)
- RT is longer for more complex movements than for simple movements –> Suggestive of “programming”
- Central pattern generator is thought of as a type of motor program
Criticism of Motor Program
• Entirely feedforward –> a reiteration of hierarchical control –> no account for sensory feedback/regulation
• Storage problem –> how are a potentially infinite number of
MPs stored?
• Novelty problem –> if you get a movement right the first
time, how could this happen?
Solution to the Criticism is something called a Generalized Motor Program
- Still a stored pattern of movement but a more abstract representation (an algorithm or schema that defines classes of action)
- Schema is a knowledge structure that can be instantiated in different ways depending on the values of its underlying variables or parameters –> writing schema
- Less storage: Can be used to perform a task with different effectors (eg, writing name)
Novelty addressed: Through rules
- parameters of the task,
- initial conditions,
- sensory consequences,
- environmental consequences (outcome)
Therapeutic Implications: Generalized Motor Program
- Promoted a BIG shift away from treating abnormal reflexes or patterns
- Shift to treating functional movements –> albeit by understanding the movement from a computational representational perspective
- Major emphasis on active motor learning principles –> learning rules or parameters of movement through practice
Criticisms of GMP
GMP is an artifactual concept vs. a natural law. It remains a symbol, a metaphor –> How do we know it’s there? Is it stored? How? What is actually stored?
Internal Model
- A system that mimics the behavior of a natural process
- So, it’s a model of the production of a movement and pre-computation is part of the process of that production of movement
How Internal Model Works
- Brain sends a movement plan to both the target body part and internally to itself through something called efference copy
- Comparing can happen when feedback of the movement process and outcome arrives back in the brain
What does the efferent copy include?
the movement plan and a prediction of what the sensory outcome will be
Self-organization:
A process in which pattern at the global level of a system emerges solely from numerous interactions among the lower-level components of the system
The pattern is an emergent property of the system rather than a property imposed on the system by an external ordering influence (a ‘program’ or ‘executive’)
Dynamical Systems Theory
A dynamical system is simply any system changing over time –> the system’s pattern changes as time passes
Stigmergy:
Indirect coordination through the environment. A type of
self-organization common in social insects, the internet, and perhaps human movement?
Understanding a Complex Dynamical System
- Parameters acting on the system
- The interacting elements themselves
- Emerging patterns or modes of coordination (the behavior we see) to which the interacting elements give rise
Parameters acting on the system in Dynamical System
- Sometimes called boundary conditions or constraints
* Think of this as limits to the kind of actions (or dynamics) that are possible
Parameters in Dynamical Systems
• If coordinated movements are the product of self-organization, it is possible to describe their intrinsic order in a few variables
Control Parameter (Independent Variable)
- The parameter whose variation can give rise to a non-linear change in the behavior of the system (eg. Temperature when boiling water; velocity in the previous slide)
- Change the control parameter to a critical level and see a change in behavior (phase shift)
- control parameters drive the system to seek a new state
Collective Variable (Dependent Variable)
- The dimension or (order) parameter that exhibits the non-linear change
- Defines the general mode (or collective behavior) of the system’s functioning
- a low-dimensional variable that describes the pattern –> mapped as an attractor
Independent variable = Control Parameter
- Change the control parameter to acritical level and see a change or shiftin behavior.
- Control parameter drives the systemto a new state
Dependent variable = Collective Variable or Order Parameter
Describes the pattern. The stability of this pattern and its preferred state are characterized (using math) as an attractor well
Movement as emergent feature of the system’s interactions
• Rather than being imposed by some “agent” a dynamical system
possesses self-organization: spatial and temporal organization
(coordination) arising from the interaction of many individual
elements (or subsystems)
• Systems are assemblies or groups of components that together have
certain features or characteristics (typically task –specific features)
These features or characteristics IMPOSE CONSTRAINTS to
movement
Fosters ordered structure without the need for a program or prior
instruction…. But there might be a simple rule built into the system itself.
Therapeutic Implications of DST
- Certain control parameters (when identified) may be useful in treatment (eg., velocity- gait)
- Seek conditions of variability and/or instability to assist or force changes in the movement synergy (coordination pattern)
- Sensitive dependence on initial conditions could be exploited
- Importance of internal and external constraints driving behavior vs calling up a motor program from memory
Limitations of DST
- Few identified collective variables and control parameters.
- Largely described for continuous task (ongoing rhythms).
- De-emphasis on “where in the CNS”
Ecological Approach to Perception and Action
- How movers detect environmental information
- Individual as active explorer of environment
- Optic array contains invariant information about the world in the form of higher order variables
- Perception is direct
How movers detect environmental information
Relevance to how it guides, shapes, constrains action
Individual as active explorer of environment
Retinal image (passively sensed) replaced by ambient optic array (actively sampled –importance of optical flow)
Optic array contains invariant information about the world in the form of higher order variables
- Time to contact –> Catching, diving
* Dynamic touch –> Wielding and sensing sweet spot
Perception is direct
Information is “picked up” rather than processed
Affordances
- The functional utility of an object for an animal with certain action
capabilities. The combination of environmental properties that
supports some activity for a particular animal - Links the human with the environment
- Environment elicits action from the human
- The fit between the body and environment that makes action possible
Examples of Affordances
Jump-overable objects Step-down places Falling-off places Climable-places Collide-withable surfaces Travel-throughable openings
Clinical Implications of the Ecological Approach to Perception and Action
- If control of movement evolved to cope with environment
- Then use environment to shape desired movements
- Actor (patient) solves motor problems in real world rather than practices patterns of movement outside of environmental contexts
Use environment to shape desired movements
- Manipulating space, objects, textures in environment to increase or decrease task difficulty.
- Adaptability of walking is a major goal in PT and is dependent upon embedding the patient in a challenging environment. A highly functional approach to rehab –> Cluttered/uncluttered/stable/unstable environments
Limitations
• As with Dynamical Systems Theory –> De-emphasis on neural substrate underlying movement
• Notion that perception is “direct” is often confused as a rejection of
sensory nervous system functioning
Where Do We Go From Here?
Because it can all seem rather dizzying!
How much is prescribed / controlled?
How much are the segments/elements self-organizing?
How much does the environment specify action thus decreasing the need for
representing the world and planning?
And we haven’t even begun to talk about how we learn movements or relearn
movements after a change in health (disease) or injury
