6: Action

Key Questions

• How do we select, plan, and execute movements

• What cortical and subcortical computations support the production of coordinated movements?

• How is the motor system organized and how does it support coordinated movement?

• What changes lead to improved motor performance with practice?

• What are the characteristic deficits of damage to the motor system?

Information Processing Model

Anatomy of Action

Motor Hierarchy

Pyramidal and Extrapyramidal Tracts

• Pyramidal tract

  • Originate from the cortex (M1)

    • 90% contralateral

    • Some are more than 1 meter long

  • Also called corticospinal tract

  • primary control of muscles

• Extrapyramidal tracts

  • Originate from subcortical nuclei

  • terminate in both contralateral and ipsilateral regions of the spinal cord

  • control posture, tone, fine movements

How do we move?

• Muscles activated by motor neurons

  • Alpha motor neurons

    • Pyramidal tract

    • Produce muscle contracts

  • Gamma motor neurons

    • Extrapyramidal tracts

    • Muscle tone and proprioception

  • Alpha motor neurons can be activated cortex or by spinal interneurons

    • The stretch reflex

      • helps maintain balance without control from brain

• Many animals can perform complex movement possible after spinal cord resection

• Central Pattern Generators

  • Spinal cord circuit that produces movement pattern when stimulated

Neural Coding of Movement

• Population Vector

  • A representation of movement direction, calculated from activity of multiple neurons

  • Groups of neurons encode movement as a collective signal

• Brain-Machine Interface

Parietal, Premotor, and Supplementary Motor Areas

• What actions are planned?

  • multiple levels

Hierarchical Control of Action

• Actions can be planned at different levels

  • Conceptual — what is the goal

  • Response level — motor system to achieve goal

  • Implementation level — specific motor actions to achieve goal

• Motor program

  • a full set of commands to perform an action

  • commands can be executed without feedback

  • commands are abstract

  • same program can be used under different conditions

  • same program can be used for different effectors (limbs)

Goal Selection and Action Planning

• Affordance Competition Hypothesis

• Perception-Action Cycle

• Mirror Neurons

  • yawning

  • crossing arms

  • essential for comprehending and anticipating actions

  • expert dancers vs novice watching performance

The subcortical regions involved in motor control

• Many regions are involved including the brain stem

• Basal ganglia (BG)

  • “Gatekeeper”

  • Cortex prepares responses but they are not executed until allowed by BG

• Cerebellum

  • Critical for fine motor control

    • Comparing actions with consequences

    • timing

• Subcortical regions are also involved in “high-level” processes such as planning

• Movement can be learned without the cerebellum

  • Not as fluid

  • Takes very long to learn

Cerebellum

• Cerebellar Timing

  • mediates timing of sequences motor responses

• Eye-blink conditioning experiment

• Cerebellar lesions disrupt both acquisition and execution of conditioned response

  • Not a motor deficit → eye blinks normally puff to air

• Comparator

  • compares action and consequences with expected outcomes → adjusts timing (learning)

Anatomy of the Basal Ganglia

• Five Nuclei of BG

• Striatum

  • (1) Caudate

  • (2) Putamen

• (3) GP: Globus Pallidus

  • GPe — external segment

  • GPi — internal segment

• (4) SN: Substantia nigra

  • SNc pars compacta

  • SNr pars reticulata

• (5) STN: Subthalamic Nucleus

• Direct Pathway

  • Inhibits GPi/SNr

  • Decreases inhibition on Thalamus

  • Increases activity of thalamocortical circuit

• Indirect Pathway

  • Inhibits GPe

  • Decreases inhibition on GPi/SNr and STN

  • Increases GPi/SNr activity

  • Increases inhibition on Thalamus

  • Decreases activity on thalamocortical circuit

• Pathways work in opposition

• Produce tonic inhibition on thalamocortical circuit

• Direct pathway faster

  • excites thalamocortical circuit first

  • inhibits thalamocortical circuit second

Stages of Skill Acquisition

• Cognitive Stage

  • Declarative knowledge

    • facts are memorized and rehearsed

  • High working memory and attentional demands

  • Instructions and demonstrations effective

  • What type of motor programs?

• Associative Stage

  • errors in the initial understanding detected and eliminated

  • association between movement increased

    • new and efficient motor programs (composition)

  • Declarative representation converted into a procedural representations

• Autonomous Stage

  • motor programs become increasingly automatic

  • speed and efficiency improve

    • adjust parameters in procedure

  • procedural knowledge

    • knowledge less accessible

    • less verbalization

  • Conceptual goals not implementation

Skill Learning and Motor Programs

• tying your shoe

Brain Activity during Skill Acquisition

• Mirror reversed reading vs plain text (pre-training)

• Learning related increases (basal ganglia and cerebellum)

Skill Acquisition

Disorders of the Motor System

• Motor cortex and pyramidal tract

  • Hemiplegia: Paralysis of the contralateral limb(s)

  • Hemiparesis: Weakness, impaired control of contralateral limb(s)

• Secondary/Association motor areas

  • Apraxia — deficit in performing learned movements

• Subcortical motor areas

  • Cerebellum — ataxia (lack of coordination)

  • Basal Ganglia — Parkinson’s and Huntington’s disease

Ataxia

• impairment of coordination despite intact knowledge of appropriate action

• associated with damage to cerebellum

  • (other regions as well)

• Due to failure in motor timing

Cognitive Deficits with Cerebellar Lesions

• timing/predictive processes of cerebellum recruited for cognitive tasks

Apraxia

• loss of ability to generate coordinated actions

  • NOT due to loss of muscle control

• Observed with premotor, prefrontal, or parietal lesions

  • usually in left hemisphere

Parkinson’s Disease

• Degeneration of Substantia Nigra Compacta

  • reduced dopamine levels up to 90%

• Positive Symptoms

  • Resting tremor: decreases or disappears with volitional movement

  • Rigidity: stiffness due to simultaneous activity of agonist and antagonist muscles

• Negative Symptoms

  • disorders of posture and equilibrium

  • Bradykinesia — slowness of movement execution

  • Hypokinesia — absence of voluntary movement

Huntington’s Disease

• Degeneration of striatum (part of BG)

  • Cell death up to 90%

• Symptoms

  • Clumsiness, balance problems, restlessness

  • Chorea: abnormal involuntary movements, often involving multiple major muscle groups

Disorders of the Basal Ganglia: Huntington’s and Parkinson’s