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CHAPTER 1 The corticospinal tract originates from the primary motor cortex, the premotor cortex, the supplementary motor area, and other regions including the somatosensory cortex (Bear et al., 2001). The upper motor neurons of this tract converge and travel through the internal capsule and into the medulla where they divide into lateral (~90%) and anterior (~10%) tracts (Purves, 2012). Upper motor neuron in the lateral tract decussates in the pyramids of the caudal medulla and terminates in the spinal cord, where it synapses via interneurons or directly with lower motor neurons in the ventral horn. The lower motor neuron then synapses onto muscles to deliver fine- motor control of the limb muscles (Mendoza and Foundas, 2007). The anterior tract travels straight into the spinal cord and arrives at the medial region of the ventral spinal level. The upper motor neuron then decussates and synapses via interneurons or directly with lower motor neurons in the contralateral ventral horn. The lower motor neurons then synapses onto muscles to control movement of the trunk and proximal musculature (Mendoza and Foundas, 2007). The corticobulbar tract originates from similar regions as the corticospinal tract and travels to the brainstem, where it generally synapses bilaterally (with the exception of the lower face which have no ipsilateral projections) with brainstem lower motor neuron at various levels to control muscles of the face and the neck (Young et al., 2008). The corticobulbar tracts are not involved in spinal cord injury. Upper motor neurons of the extrapyramidal tracts originate in the brainstem and mainly include the vestibulospinal, reticulospinal, rubrospinal, and tectospinal tracts (Costanzo, 2014). They are responsible for reflex/involuntary activity in skeletal muscles and subsequent movement. Most upper motor neurons of the vestibulospinal and reticulospinal tracts terminate in the ipsilateral spinal cord, where they synapse via interneurons or directly with lower motor neurons to control the posture of (mainly) the ipsilateral muscles (Fitzgerald et al., 2012). The rubrospinal tract originates in the red nucleus of the midbrain and decussates. These upper motor neurons travel into the spinal cord contralaterally where they synapse with lower motor neurons in the ventral horn. The function of rubrospinal tract remains controversial in humans (Cramer and Darby, 2014). The tectospinal tract originates from the superior colliculus of the midbrain and decussates before the upper motor neuron synapses with the lower motor neurons in the ventral horn of upper cervical spinal cord. The tectospinal tract controls the movement of the head in relation to visual stimuli (Rea, 2015). There are a number of differences in motor pathways between human and rodents. The most obvious differences are the composition and the location of corticospinal tracts. In rodents, corticospinal tracts consist of the dorsal tract, the anterior tract and the dorsolateral tract. The dorsal corticospinal tract in rodents, equivalent to the lateral corticospinal tract in human, is the main component and is situated in the ventral aspect of the dorsal column. The anterior corticospinal tract in rodents is similar to the anterior corticospinal tract in human, situated in the ventral part of the spinal cord. The dorsolateral corticospinal tract is a unique feature in rodents 8PDF Image | Effects of Red Light Treatment on Spinal Cord Injury
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