Somatic Motor Pathways and Motor Neurons

Learning Outcomes

  • Compare upper and lower motor neurons in terms of their location and synaptic connections.
  • Describe pyramidal cells in layer V of the cerebral cortex.
  • Explain the effects of disorders of the upper motor neurons.
  • Define the concept of final common pathway.
  • Explain the effects of disorders of the lower motor neurons.
  • Sketch, describe, and indicate the course and function of the corticospinal pathways (corticobulbar tracts, anterior corticospinal tract, posterior corticospinal tract).

Somatic Motor Pathways

  • Impulses from the cerebral cortex conduct motor functions to skeletal muscles via somatic motor pathways, which consist of motor neurons that carry impulses from the CNS to skeletal muscles.
  • These pathways can be complex or straightforward.

Final Common Pathway

  • The principle of the final common path states that there is one final common path:
    • Lower motor neuron located in the anterior gray horn of the spinal cord.
    • This neuron conducts impulses to skeletal muscles.

Somatic Motor Pathway Structure

  • Upper Motor Neuron (UMN):

    • Cell body located in the CNS processing center (e.g., Precentral Gyrus).
    • Axons synapse on lower motor neurons (LMN), facilitating or inhibiting their activity.

  • Lower Motor Neuron (LMN):

    • Cell bodies found in the brainstem or spinal cord.
    • Triggers contractions in the muscle it innervates.

Classification of Somatic Motor Pathways

  1. Pyramidal Tracts:

    • Known as corticospinal tracts.
    • About 75% of fibers decussate in the medulla (cross over), while 25% do not.
    • Extend to different sides of the spinal cord (crossed and uncrossed pathways).

  2. Extrapyramidal Tracts:

    • More complex, consist of all motor tracts from the brain to the spinal cord except corticospinal tracts.
    • Involves multiple relays between motor areas in the brain and spinal cord.
    • Important for automatic movements and emotional expressions.

Direct vs. Indirect Descending Spinal Pathways

  • Direct Pathways:

    • Control muscle tone and skilled movements.
    • Include: lateral corticospinal tract, anterior corticospinal tract, and corticobulbar.

  • Indirect Pathways:

    • Synapse with intermediate nuclei, not directly to LMNs.
    • Include: rubrospinal, vestibulospinal, reticulospinal, and tectospinal tracts.

Motor Homunculus

  • Represents the corresponding areas of the motor cortex responsible for specific body regions:
    • Larger areas for hands, face, and tongue due to fine motor control capabilities.
    • Proportions similar to those in the sensory homunculus.

Corticospinal Pathways

  • Voluntary control over skeletal muscles begins in pyramidal cells of the primary motor cortex.
  • Comprise three pairs of descending tracts:
    1. Corticobulbar tracts
    2. Lateral corticospinal tracts
    3. Anterior corticospinal tracts

Summary of Key Concepts

  • The somatic motor system involves the integration of upper and lower motor neuron functions, focusing on the corticospinal pathways to control voluntary movements efficiently. Understanding these pathways is essential to grasping how motor activities are executed and the impact of motor neuron disorders on movement control.