Central Motor System and Descending Motor Pathways

Central Motor System and Descending Motor Pathways

Introduction

  • Overview of the central motor system
    • Responsible for controlling voluntary and involuntary movements in the body
    • Comprises anatomical and functional components of the nervous system

Goals of the Lecture

  • Describe and locate cortical motor areas:
    • Primary motor cortex
    • Premotor cortex
    • Supplementary motor cortex
  • Describe the functional arrangement of neurons in the primary motor cortex
  • Describe the four main motor tracts:
    • Starting point and termination of each tract
    • Areas of decussation (crossing over)
    • Outcomes of activation of these tracts

Components of the Central Motor System

  • Cerebral Cortex
    • Key regions involved in voluntary movement:
    • Primary Motor Cortex (located in the frontal lobe, anterior to the central sulcus, in the precentral gyrus)
    • Premotor Area (located laterally in the frontal lobe; involved in preparing for movement)
    • Supplementary Motor Area (located on the superior and medial surface of the hemisphere; important for coordination)
  • Cerebellum
    • Regulates and fine-tunes motor commands; involved in coordination, precision, and timing of movements
    • Compares sensory information with intended motor commands
  • Basal Ganglia
    • Group of nuclei involved in:
    • Motor planning
    • Coordination
    • Regulation of muscle tone
    • Initiating and stopping movements
    • Refining motor actions
  • Brainstem and Spinal Cord
    • Facilitate communications between the brain and peripheral nervous system
    • Houses various motor nuclei and spinal cord motor neurons

Specific Areas of the Cerebral Cortex

  • Primary Motor Cortex
    • Function: Provides precise contralateral control of movement
    • Arrangement of cell bodies:
    • Somatotopic organization (arranged in correspondence to different body parts)
    • Visual representation: Motor Homunculus
      • Illustrates cortical area dedicated to different body parts
      • Areas needing finer control (hands, face) are larger; less fine areas (trunk, legs) are smaller
      • Easy mnemonic: "FAT legs" (Face, Arms, Trunk, Legs in this order)
  • Premotor and Supplementary Motor Areas
    • Premotor Area: prepares for muscle activity in multiple joints
    • Supplementary Motor Area: coordinates movement for both hands before it begins

Upper Motor Neurons (UMNs)

  • Function: Transmit signals from brain to spinal cord for voluntary movement
  • Location: Primarily in the primary motor cortex
  • Pathway: Nerve fibers traversing brain stem into the spinal cord, projecting onto lower motor neurons or interneurons
UMN Classification
  • Classifications based on synapses in the spinal cord:
    • Medially
    • Laterally
    • Throughout the ventral horn

Motor Tracts

Medial Motor Tracts
  • Function: Control posture and gross limb movements; often occur automatically
  • Notable Tracts:
    • Reticulospinal Tract: originates in the brainstem (reticular formation) and facilitates posture and gross movement
    • Medial Vestibulospinal Tract: regulates balance and posture; originates in vestibular nuclei, descends bilaterally
    • Lateral Vestibulospinal Tract: also regulates balance; originates in lateral vestibular nuclei, descends ipsilaterally
    • Medial Corticospinal Tract: originates from the cortex, provides voluntary muscle control, terminates in the cervical and thoracic cord
Lateral Motor Tracts
  • Lateral Corticospinal Tract
    • Function: Major pathway for voluntary movements with selective motor control (independent muscle activation)
    • Pathway: Descends from the cortex, crosses at pyramidal decussation (approx. 88% crosses to contralateral side), synapses in contralateral spinal cord
    • Remaining 10%-20% do not cross and stay ipsilateral in medial corticospinal tract
  • Corticobrainstem Tracts
    • Function: Provide voluntary control of facial and neck muscles
    • Pathway: Arise from the motor cortex to cranial nerve nuclei in brainstem

Summary of Motor Tracts

  • A synthesized view of the major motor tracts along with their functions, origins, pathways, and terminations
    • Essential for understanding distinctions between the pathways and their implications in motor control

Key Anatomical Structures

  • Pyramid: Ridge on anterior inferior medulla formed by lateral corticospinal tracts
  • Olive: Structure involved in motor control and coordination; a relay point between medulla and cerebellum
  • Pyramidal Decussation: Important anatomical feature where majority of corticospinal fibers cross over to the opposite side
  • Cerebral Peduncles: Composed of nerve fiber bundles, carrying ascending (sensory) and descending (motor) signals between brain and spinal cord

Conclusion

  • Engage with the material by asking questions and connecting concepts as you move through the course material. Refer to resources for deeper understanding and clarification of concepts discussed.