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12.9A: Organization of Motor Neuron Pathways

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    The motor system is the part of the central nervous system that is involved with movement.

     

    LEARNING OBJECTIVES

     

    Describe the organization of motor neuron pathways

     

    KEY TAKEAWAYS

    Key Points

     

    • The pyramidal tract, which includes both the corticospinal and corticobulbar tracts, serves as the motor pathway for upper motor neuronal signals coming from the cerebral cortex and from primitive brainstem motor nuclei.
    • Peripheral motor nerves carry the motor impulses from the spinal cord to the voluntary muscles.
    • The large majority (90%) of motor neurons cross (decussate) to the contralateral side of the brain at the level of the brainstem.

     

    Key Terms

     

    • extrapyramidal system: A biological neural network that is part of the motor system that causes involuntary movements.
    • corticospinal tract: The nervous system tract that conducts impulses from the brain to the spinal cord. It contains mostly motor axons and is made up of two separate tracts in the spinal cord: the lateral corticospinal tract and the anterior corticospinal tract.
    • motor system: The part of the central nervous system that is involved with movement. It consists of the pyramidal and extrapyramidal systems.
    • cerebral cortex: The gray, folded, outermost layer of the cerebrum that is responsible for higher brain processes such as sensation, voluntary muscle movement, thought, reasoning, and memory.

    This an anatomical drawing of a brainstem. The pyramidal tract is visible, and the pyramidal decussation is labeled at lower right.

     

    Decussation of the pyramids: A deep dissection, lateral view of a brainstem. The pyramidal tract is visible in red, and pyramidal decussation is labeled at lower right.

    The motor system is the part of the central nervous system that is involved with movement. It consists of the pyramidal and extrapyramidal system.

    The motor pathway, also called the pyramidal tract or the corticospinal tract, serves as the motor pathway for upper motor neuronal signals coming from the cerebral cortex and from primitive brainstem motor nuclei. There are upper and lower motor neurons in the corticospinal tract.

    The motor impulses originate in the giant pyramidal cells (Betz cells) of the motor area, i.e., the precentral gyrus of the cerebral cortex. These are the upper motor neurons of the corticospinal tract. The axons of these cells pass from the cerebral cortex to the midbrain and the medulla oblongata. Peripheral motor nerves carry the motor impulses from the anterior horn to the voluntary muscles.

    Cortical upper motor neurons originate from Brodmann areas 1, 2, 3, 4, and 6, then descend into the posterior limb of the internal capsule, through the crus cerebri, down through the pons, and to the medullary pyramids, where about 90% of the axons cross to the contralateral side at the decussation of the pyramids. They then descend as the lateral corticospinal tract.

    These axons synapse with lower motor neurons in the ventral horns of all levels of the spinal cord. The remaining 10% of axons descend on the ipsilateral side as the ventral corticospinal tract. These axons also synapse with lower motor neurons in the ventral horns. Most of them will cross to the contralateral side of the cord (via the anterior white commissure) just before synapsing.

    This drawing shows the Brodmann areas of the brain. These regions of the human cerebral cortex were delineated by Korvinian Brodmann on the basis of cytoarchitecture.

     

    Brodmann areas of the brain: This drawing shows the regions of the human cerebral cortex as delineated by Korvinian Brodmann on the basis of cytoarchitecture.

    The midbrain nuclei include four motor tracts that send upper motor neuronal axons down the spinal cord to lower motor neurons. These are the rubrospinal tract, the vestibulospinal tract, the tectospinal tract, and the reticulospinal tract.

    The function of lower motor neurons can be divided into two different groups: the lateral corticospinal tract and the anterior corticalspinal tract. The lateral tract contains upper motor neuronal axons that synapse on the dorsal lateral lower motor neurons, which are involved in distal limb control.

    The anterior corticospinal tract descends ipsilaterally in the anterior column, where the axons emerge and either synapse on ventromedial lower motor neurons in the ventral horn ipsilaterally or descussate at the anterior white commissure where they synapse on ventromedial lower motor neurons contralaterally.

    The ventromedial lower motor neurons control the large, postural muscles of the axial skeleton. These lower motor neurons, unlike those of the dorsal lateral, are located in the ventral horn throughout the spinal cord.

    This diagram of spinal cord tracts shows the motor and efferent pathways in red and the sensory and afferent pathways in blue. Included in the diagram are the following motor pathways: corticospinal tracts (pyramidal tract), and extrapyramidal tracts (tectospinal tract not delineated).

     

    Spinal cord tracts: This diagram of spinal cord tracts shows the motor and efferent pathways in red and the sensory and afferent pathways in blue. Included in the diagram are the following motor pathways: corticospinal tracts (pyramidal tract), and extrapyramidal tracts (tectospinal tract not delineated).