Lecture 11 Motor System

The Motor System

Goals

  • Explore the hierarchical organization of motor control.
  • Understand the roles of lower and upper motor neurons.
  • Examine the clinical syndromes associated with motor dysfunction.
  • Highlight the contributions of cortical and subcortical areas to movement planning and execution.

Overview of Motor Control

  • Adaptive Movement:
    • Behavior involves planning, selecting, learning, inhibiting, correcting, and executing movements.
    • Nearly all brain areas contribute to motor processes, making the boundary between motor and non-motor regions arbitrary.
  • Hierarchical Organization:
    • Lowest Level: Reflexes, postural adjustments, and central pattern generators (CPGs) automate repetitive movements like chewing and walking.
    • Highest Level: Goal-directed actions, motor learning, and the inhibition of competing responses.

Lower Motor Neurons

Anatomy

  • Located in the ventral horn of the spinal cord.
  • Alpha Motor Neurons:
    • Directly innervate muscle fibers and are the “final common pathway” for movement.
    • Release acetylcholine at the neuromuscular junction to induce muscle contraction.
  • Gamma Motor Neurons:
    • Regulate muscle spindle sensitivity, enabling proprioceptive feedback.

Reflexes and Local Circuits

  • Stretch Reflex:
    • Maintains muscle length by responding to changes detected by muscle spindles.
    • Example: Adding weight to a cup triggers reflexive contraction to maintain position.
  • Nociceptive Reflex:
    • Multilimb coordination (e.g., stepping on a tack causes withdrawal of one leg and extension of the other for balance).
  • Renshaw Cells:
    • Provide negative feedback to prevent excessive muscle contraction.
    • Targeted by tetanus toxin, leading to hypercontraction.

Central Pattern Generators (CPGs)

  • Found in the spinal cord and brainstem.
  • Automate rhythmic activities like walking and chewing.
  • Studies in animals with spinal cord transections demonstrate the persistence of CPG activity without descending inputs.

Upper Motor Neurons

Corticospinal Tract

  • Originates in primary motor cortex (M1), premotor cortex (PMC), and supplementary motor area (SMA).
  • Decussation:
    • 90% of fibers cross at the medulla (lateral corticospinal tract).
    • 10% descend ipsilaterally (anterior corticospinal tract) but eventually cross at their target levels.
  • Corticobulbar Tract:
    • Projects to brainstem nuclei, controlling cranial nerves for facial and oral movements.

Extrapyramidal Tracts

  • Originate in brainstem nuclei and modulate posture and balance:
    • Rubrospinal Tract: Proximal muscle control (less prominent in humans).
    • Tectospinal Tract: Head and eye movements toward stimuli.
    • Vestibulospinal Tract: Balance control.
    • Reticulospinal Tract: Postural adjustments and muscle tone.

Cortical Contributions to Motor Control

Primary Motor Cortex (M1)

  • Located anterior to the central sulcus (Brodmann Area 4).
  • Organized somatotopically into a motor homunculus:
    • Disproportionate representation of fine motor areas (e.g., hands, face).
  • Controls movements, not individual muscles:
    • Same muscles can participate in different movements depending on M1 activation.
    • Population Coding:
      • Direction of movement determined by the vector sum of neuronal activity.

Premotor Cortex (PMC)

  • Role: Prepares for movement and integrates external cues.
  • Ipsilateral and Contralateral Activity:
    • Fires bilaterally, in contrast to M1’s contralateral dominance.
  • Neurophysiology:
    • Neurons activate during movement preparation (e.g., delay response tasks).

Supplementary Motor Area (SMA)

  • Role: Plans sequences of movements and internally guided actions.
  • Lesion Effects:
    • Alien hand syndrome (involuntary movements).
    • Utilization phenomena (compulsive use of objects).

Posterior Parietal Cortex (PPC)

  • Function:
    • Integrates sensory information for motor planning and execution.
    • Contributes to reaching and grasping by analyzing object affordances.
  • Damage:
    • Impaired grip adjustments (e.g., difficulty using handles).

Movement Concepts

  • Feedforward vs. Feedback Control:
    • Feedforward: Anticipates postural changes (e.g., stabilizing before lifting).
    • Feedback: Adjusts based on sensory input (e.g., unexpected weight changes).
  • Muscle Antagonism:
    • Flexors and extensors coordinate to control joints smoothly.
  • Abstract Motor Representations:
    • Movement plans are independent of specific muscles (e.g., writing with a hand vs. a foot).

Clinical Syndromes

  • ALS (Lou Gehrig’s Disease):
    • Degeneration of alpha motor neurons, leading to progressive paralysis.
  • Tetanus:
    • Loss of Renshaw cell inhibition causes sustained muscle contraction.
  • Stroke:
    • Damage to motor areas causes contralateral hemiplegia.
  • Parietal Lobe Damage:
    • Affects grasping and visuomotor coordination.

Applications and Future Directions

  • Brain-Computer Interfaces (BCIs):
    • Decode M1 activity to control robotic limbs, offering potential for restoring movement in paralysis.
  • Motor Learning and Rehabilitation:
    • Insights into CPGs and plasticity guide strategies for spinal cord injury recovery.

This lecture laid the groundwork for understanding motor control, emphasizing the interplay between cortical, subcortical, and spinal systems. Future topics will delve into modulatory systems like the basal ganglia and cerebellum.

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