Cellular Pathways of Hereditary Spastic Paraplegia

Voluntary movement in humans relies on the pyramidal motor system, a tortuous, multisynaptic pathway in the CNS that extends from the cerebral motor cortex to neuromuscular junctions innervating skeletal muscle. This system is arranged in two main stages (Figure 1). First, axons of large pyramidal neurons originating in layer V of the cerebral motor cortex course through the medullary pyramids, where most fibers decussate in the caudal medulla before descending as lateral corticospinal tracts within the spinal cord. Although some corticospinal axons establish synapses directly with lower motor neurons in the spinal cord anterior horn, the vast majority synapse with spinal interneurons, which then establish connections with lower motor neurons. In the next stage, lower motor neurons terminate in specialized synapses at neuromuscular junctions throughout the body to regulate skeletal muscle contractility.

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