Spinal cord descending tracts

ALS is a disorder of the motor neurons and the cortical neurons that provide their input. The disorder is characterized by rapidly progressive weakness and muscle atrophy. Most affected patients die of respiratory compromise and pneumonia after 2 to 3 years. There is prominent loss of motor neurons in the spinal cord and brainstem although the oculomotor neurons are spared. Large pyramidal motor neurons in layer V of motor cortex, which are the origin of the descending corticospinal tracts, are also lost. 

Following the development of the motor program, neurons originating in the multimodal motor association areas transmit impulses by way of association tracts to neurons of the primary motor cortex. The primary motor cortex is located in the precentral gyrus, which is the most posterior region of the frontal lobe adjacent to the multimodal motor association areas (see Figure 6.3) this area initiates voluntary contractions of specific skeletal muscles. Neurons whose cell bodies reside here transmit impulses by way of descending projection tracts to the spinal cord, where they innervate the alpha motor neurons (which innervate skeletal muscles). 

Figure 7.2 Ascending and descending tracts in white matter of the spinal cord. Tracts are formed of bundles of neuronal axons that transmit similar types of information.

Table 7.1 Ascending and Descending Tracts in White Matter of the Spinal Cord...

The corticospinal tracts originate in the cerebral cortex. Neurons of the primary motor cortex are referred to as pyramidal cells. Most of these neurons axons descend directly to the alpha motor neurons in the spinal cord. In... 

If a spinal cord is cross-sectioned, the gray matter appears as a roughly H-shaped area in its middle which is, divided into dorsal (posterior), lateral, and ventral (anterior) horns. The horns are interconnected by a crossbar, the gray commissure. The rest of the spinal cord is the white matter, made up largely of tracts of myelinated nerve fibers (axons). Ascending tracts carry afferent sensory impulses towards the brain, descending tracts transmit motor impulses from the brain to the motor neurons in the ventral or lateral horns of the gray matter. 

The pathophysiology of OIH is poorly understood. Various non-opioid chemicals, neurotransmitters, and receptors have been implicated, e.g., cholecystoki-nin (CCK), N-methyl D-aspartate (NMDA) receptor, neurokinin, etc. One mechanism for opioid-induced enhanced pain sensitivity may be the neuroplastic changes that result in part from the activation of descending pain facilitation mechanisms arising from the rostral ventromedial medulla (RVM) by increasing the activity of CCK in the RVM. These activated descending neural tracts facilitate nociceptive processing in the spinal cord. 

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