Q 1.6. Extrapyramidal System: Anatomy and Physiology


1. Introduction:

    • The extrapyramidal system (EPS) was first conceptualized by Johann Prus in 1898 when he observed that disturbances in pyramidal tracts did not entirely prevent epileptic motor activity.
    • Prus postulated the existence of alternative pathways—dubbed “extrapyramidal tracts”—that conveyed motor signals from the cerebral cortex to the spinal cord.
    • Clinically, the term “extrapyramidal” distinguishes effects resulting from basal ganglia damage from those of the classic “pyramidal” pathway.
    • Despite this distinction, the two systems have intricate anatomical and functional connections.


2. Functions of the EPS:

    • The EPS serves essential roles:
      • Postural Tone Adjustment: Maintaining posture by adjusting muscle tone.
      • Predisposing Tonic Attitudes: Preparing for involuntary movements.
      • Naturalizing Voluntary Movements: Enhancing voluntary movements.
      • Automatic Tone and Movement Modifications: Controlling reflexes during affective and attentive responses.
      • Inhibition of Involuntary Movements: Crucial in extrapyramidal diseases.


3. Anatomical Components:

    • The EPS involves several brain regions:
      • Cerebral Cortex: Initiates motor commands.
      • Cerebellum: Coordinates movement.
      • Thalamus: Relays information.
      • Reticular Substance: Modulates tone and movements.
      • Basal Ganglia:
        • Includes the caudate nucleus, putamen, substantia nigra (SN), red nucleus (RN), and subthalamic nucleus.
        • These nuclei receive projections from the cortex and send projections to the brainstem and spinal cord.
        • Anatomically, the EPS comprises nuclei and fiber tracts.
        • Functionally, it acts as a complex motor-modulation system.


4. The extrapyramidal system is like the backstage crew for our body’s movements. It doesn’t steal the spotlight (that’s the pyramidal system’s job), but it ensures the show runs smoothly. Two circuits are controling the impulses:

Direct Pathway: The Movement Initiator

    • Function: This pathway kicks off the dance routine. It’s like the choreographer saying, “Action!”
    • Steps:
      • Cortex ➡️ Striatum ➡️ Globus pallidus (pars interna) ➡️ Thalamus ➡️ Motor cortex ➡️ Spinal cord/brainstem.

Result: Smooth initiation of voluntary movements.


Indirect Pathway: The Movement Terminator

      • Function: These dancers know when to exit the stage gracefully. They’re the choreographers whispering, “Curtain down!”
      • Steps:
        • Cortex ➡️ Striatum ➡️ Globus pallidus (pars externa) ➡️ Subthalamic nucleus ➡️ Globus pallidus (pars externa) ➡️ Thalamus ➡️ Motor cortex ➡️ Spinal cord/brainstem.

Result: Precise movement termination and inhibition.


5. Clinical Aspects: The Drama Unfolds

  • Hypotonic-Hyperkinetic Disorders: Picture dancers with floppy limbs—like marionettes cut loose. Disorders like Huntington’s disease fit this bill.
  • Hypertonic-Hypokinetic Disorders: Imagine rigid, slow-motion dancers. Parkinson’s disease takes center stage here


6. Pathogenesis of Extrapyramidal Motor Disorders:

    • Alterations in EPS circuits lead to various motor disorders:
      • Parkinson Disease (PD): Degeneration in the striatum.
      • Huntington Chorea (HC): Striatal degeneration.
      • Multiple Systemic Atrophy (MSA): Affects the basal ganglia.
      • Progressive Supranuclear Palsy: Impairs voluntary eye movements.
      • Tremor: somewhat rhythmic muscle contraction and relaxation involving oscillations or twitching movements of one or more body parts. It’s the most common of all involuntary movements and can affect the hands, arms, eyes, face, head, vocal folds, trunk, and legs. 
      • Athetosis: Slow, writhing movements with a snake-like quality.
      • Dystonia: Prolonged contractions of trunk and limb muscles.
      • Ballism: Severe chorea involving proximal muscles, characterized by violent flinging movements (referred to as hemiballismus when affecting only one side of the body). Intense flinging of one arm or leg (think wild salsa moves).
      • Tics: Involuntary muscle contractions.


In Summary

The extrapyramidal system orchestrates a delicate dance of motor control, involving intricate neural pathways and nuclei. 






Verified by Dr. Petya Stefanova