26/27

Primary Motor Cortex and Upper Motor Neurons (UMN)

  • Definition: The primary motor cortex is a region of the brain responsible for the planning, initiation, and execution of voluntary motor functions. It involves upper motor neurons (UMN) that project from different areas within the brain.

Upper Motor Neurons (UMN)

  • Functionality:

    • Cell bodies located in the brainstem and cerebral cortex.

    • Project to local circuit neurons and, occasionally, lower motor neurons (LMN).

    • Essential for voluntary and complex spatiotemporal movements.

    • Functions include eye movement, speech production (e.g., Broca’s area), and emotional processing (e.g., facial expressions).

    • Brainstem influences muscle tone and integrates sensory information with motor control.

  • Anatomical Organization:

    • Different regions of the motor pathways and their projections to muscles, further categorized:

    • Upper motor neurons in the cerebral cortex.

    • Upper motor neurons in the brainstem.

    • Motor pathways distribute to proximal muscles (those closer to the body) and distal muscles (further away).

    • The ultimate goal is to coordinate the activity of LMNs, generally through local neural circuitry.

Divisions of Descending Motor Tracts

  1. Medial Division:

    • Projects primarily for posture, balance, and orienting mechanisms.

    • Key tracts include:

      • Vestibulospinal (medial and lateral)

      • Reticulospinal

      • Colliculospinal

      • Rubrospinal

  2. Lateral Division:

    • Controls voluntary and precise skilled movements primarily of distal extremities.

    • Important tracts:

      • Corticospinal

      • Corticobulbar

Motor Cortices Functionality

  • UMN's role:

    • Plan and initiate complex temporal sequences of voluntary movements.

    • Receives modulatory input from the basal ganglia and cerebellum via the ventrolateral thalamus, as well as information from sensory regions.

Primary Motor Cortex

  • Anatomical Location: Precentral gyrus, Broadmann area 4, location of pyramidal cells (Betz cells).

  • Cell Types:

    1. Betz Cells:

    • Largest somatic cells in the central nervous system (CNS).

    • Account for approximately 5% of axons projecting down to LMN; significant for distal muscle movements.

    1. Non-Betz Cells:

    • Have similar functions but are less defined in size and structure.

  • Projections:

    • Corticobulbar:

      • Axons descend through the internal capsule and brainstem, terminating in cranial nerve motor nuclei.

      • Innervate LMNs in the brainstem; termination is bilaterally on local circuit neurons.

    • Corticospinal:

    • Form medullary pyramids, with ~90% decussating at the pyramidal decussation to form the lateral corticospinal tract, synapsing onto local circuits that regulate LMN.

    • Betz cells maintain connections to alpha motor neurons for fine motor movements.

Somatotopic Organization of Movement

  • Concept: The primary motor cortex shows a somatotopically organized structure.

    • Areas requiring fine motor control exhibit greater database.

    • Organized representation of movements rather than individual muscle groups; stimulation affects multiple muscle groups simultaneously.

    • Muscle Field: The peripheral muscle groups that a UMN innervates; characterized by large overlaps.

Recruitment of UMN for Movement Patterns

  • Population Code: Integration of activity from a large population of UMNs via intracortical firing coordinating precise movements.

    • Each UMN is broadly tuned to a movement direction.

  • Population Vector: Represents the overall direction of movement based on the collective activity of UMNs firing before and during the action.

Premotor Cortex

  • Location: Rostral to primary motor cortex, encompassing Brodmann’s areas 6, 8, 44/45, and parts of 23 and 24.

  • Functionality:

    • Receives and integrates multisensory signals related to motivation and intention.

    • Influences motor activity both indirectly (through primary motor cortex connections) and directly via axonal connections.

    • Different subdivisions impact movements based on internal versus external cues.

Input and Influence of Neuronal Activity

  • Lateral Premotor Cortex:

    • About 65% of neurons respond before and during movements; crucial for conditional tasks.

    • Lesions can cause deficits in conditional motor tasks, regardless of understanding.

  • Mirror Motor Neurons:

    • Respond similarly during actual execution and observation of goal-directed movements.

    • Suppress firing during non-ideal observational stimuli; their function may include imitation suppression.

Medial Premotor Cortex

  • Functionality: Involved in selecting movements initiated by internal cues without external instruction (e.g., recalling a motor sequence).

  • Key Divisions: Includes the frontal eye field and areas associated with emotional behavior expression.

UMNs in the Brainstem

  • Functionality:

    • Balance, posture, initiation, and visual gaze orientation.

    • Essential for skilled motor movements of distal extremities.

  • Cell Bodies: Located within the vestibular complex, reticular formation, and superior colliculus; their activities balance integration and coordination.

Vestibular Nuclei Complex

  • Division: Receives information on head position and rotational movements, playing a role in posture and balance.

  • Tracts:

    1. Medial Vestibulospinal Tract:

    • Projects from the medial vestibular nucleus, terminating in the medial ventral horn.

    • Functions in head position adjustments through neck muscle reflex activation.

    1. Lateral Vestibulospinal Tract:

    • Projects from the lateral vestibular nucleus, terminating on LMNs for proximal muscle activation.

Reticular Formation

  • Composition: Diverse clusters of neurons impacted by cortical UMNs; their roles include modulation and coordination of motor tasks.

  • Reticulospinal Tract: These neurons relay initiation signals for motor movements involved in posture stabilization during actions like walking.

Mechanisms of UMNs

  • Feedforward vs. Feedback Mechanisms: Discusses how UMNs engage in shaping muscle activity based on predictions (feedforward) versus corrections (feedback).

  • Superior Colliculus: Involved in neck musculature control, while the Rubrospinal Tract mediates upper extremity musculature control, crucial for fine motor tasks.

Upper Motor Neuron Syndrome

  • Presentation:

    • Damage to motor cortices or descending pathways typically results in contralateral effects.

    • Flaccid paralysis may arise initially due to spinal shock before functionality is regained over time.

  • Later Effects: Include Babinski sign, spasticity, increased muscle tone, and the loss of fine motor abilities due to lowered LMN functions.

Lower Motor Neuron vs Upper Motor Neuron Syndrome

  • Key Differences:

    • Strength: Lower motor neuron syndrome shows severe atrophy, whereas upper lesions exhibit mild or no atrophy.

    • Reflexes: Reflex responses dramatically differ between both syndromes, highlighting the clinical implications for therapy and rehabilitation strategies.

  • Motor Impairment Distributions:

    • Lower motor neuron syndrome exhibits localized symptoms compared to the widespread effects in upper motor neuron syndromes.

Major Focus Area for Further Study

  • Basal Ganglia: It comprises regions crucial for balancing involuntary movements, habit formation, and implicit learning processes.

  • Importance for enabling smooth transitions between motor patterns and integrating movement-based decision-making processes.