Motor Neuron Pathways and Control

Motor Neuron Pathways and Muscle Control

• Motor neurons in the ventral horn of the spinal cord and brainstem motor nuclei are the "final common pathway" for muscle control, synapsing directly on striated muscles.
• Alpha (α) motor neurons supply extrafusal fibers in skeletal muscles, while gamma (γ) motor neurons supply intrafusal fibers (muscle spindles).
• The motor unit consists of a motor neuron, its axon, and all the muscle fibers it innervates.
• Neuromuscular junctions are the connections between axon terminals and muscle fibers.

Somatotopic Organization

• Spinal cord anterior horn cells are organized somatotopically, with neurons controlling axial muscles (e.g., neck) in ventromedial columns, proximal muscles in the mid-region, and distal limb muscles in lateral columns.
• Corticospinal and corticobulbar tracts carry signals from the contralateral motor cortex to motor neuronal cell groups.

Corticospinal Tract

• The corticospinal tract contains approximately 1 million fibers, with only 3% originating from Betz cells in layer V of the primary motor cortex (MC).
• All corticospinal fibers are excitatory, using glutamate as a neurotransmitter.
• Upper motor neurons give rise to corticospinal and corticobulbar tracts.

Corticospinal Tract Routes

• The corticospinal pathway controls voluntary, skilled movements of the distal limbs.
• It arises from the primary motor cortex, lateral premotor cortex (PMC), and supplementary motor area (SMA), originating from both precentral and postcentral cortical areas (precentral 60%, postcentral 40%).
• Primary MC is Brodmann area 4, mainly in the postcentral gyrus.
• PMC (Brodmann area 6) is lateral in the frontal lobe.
• SMA (Brodmann area 6) is medial in the frontal lobe.
• Other input sites include primary sensory cortex (areas 3, 1, 2, particularly 3a), anterior paracentral gyri, superior parietal lobule (areas 5 and 7), and parts of the cingulate gyrus, located on the medial hemisphere surface.
• The tract descends through the corona radiata, internal capsule, cerebral peduncle, basis pontis, and medullary pyramids.

Pyramidal Decussation

• Approximately 90% of corticospinal fibers cross the ventral midline (pyramidal decussation) in the medulla forming the lateral corticospinal tract.
• Somatotopic organization exists in the internal capsule (hand fibers lateral to foot fibers) and in the pons (proximal muscle fibers dorsal to distal).
• Pontine lesions often cause pure motor hemiparesis.

Ventral/Anterior Corticospinal Tract

• Remaining fibers that do not decussate in the medulla descend ipsilaterally in the ventral funiculus as the ventral/anterior corticospinal tract (Türck bundle).
• Most fibers in this tract further decussate in the lower spinal cord.
• A small remaining portion (approximately 2%) descends ipsilaterally as the bundle of Barnes, controlling axial trunk and proximal limb musculature.

Summary of Corticospinal Pathways

• Lateral corticospinal pathway: controls distal limb movements, decussates in the medulla
• Ventral/Anterior corticospinal pathway: controls axial and proximal limb musculature, decussates at lower spinal cord levels or remains ipsilateral.