<P> The major contribution to the arterial blood supply of the spinal cord below the cervical region comes from the radially arranged posterior and anterior radicular arteries, which run into the spinal cord alongside the dorsal and ventral nerve roots, but with one exception do not connect directly with any of the three longitudinal arteries . These intercostal and lumbar radicular arteries arise from the aorta, provide major anastomoses and supplement the blood flow to the spinal cord . In humans the largest of the anterior radicular arteries is known as the artery of Adamkiewicz, or anterior radicularis magna (ARM) artery, which usually arises between L1 and L2, but can arise anywhere from T9 to L5 . Impaired blood flow through these critical radicular arteries, especially during surgical procedures that involve abrupt disruption of blood flow through the aorta for example during aortic aneursym repair, can result in spinal cord infarction and paraplegia . </P> <P> Somatosensory organization is divided into the dorsal column - medial lemniscus tract (the touch / proprioception / vibration sensory pathway) and the anterolateral system, or ALS (the pain / temperature sensory pathway). Both sensory pathways use three different neurons to get information from sensory receptors at the periphery to the cerebral cortex . These neurons are designated primary, secondary and tertiary sensory neurons . In both pathways, primary sensory neuron cell bodies are found in the dorsal root ganglia, and their central axons project into the spinal cord . </P> <P> In the dorsal column - medial leminiscus tract, a primary neuron's axon enters the spinal cord and then enters the dorsal column . If the primary axon enters below spinal level T6, the axon travels in the fasciculus gracilis, the medial part of the column . If the axon enters above level T6, then it travels in the fasciculus cuneatus, which is lateral to the fasciculus gracilis . Either way, the primary axon ascends to the lower medulla, where it leaves its fasciculus and synapses with a secondary neuron in one of the dorsal column nuclei: either the nucleus gracilis or the nucleus cuneatus, depending on the pathway it took . At this point, the secondary axon leaves its nucleus and passes anteriorly and medially . The collection of secondary axons that do this are known as internal arcuate fibers . The internal arcuate fibers decussate and continue ascending as the contralateral medial lemniscus . Secondary axons from the medial lemniscus finally terminate in the ventral posterolateral nucleus (VPL) of the thalamus, where they synapse with tertiary neurons . From there, tertiary neurons ascend via the posterior limb of the internal capsule and end in the primary sensory cortex . </P> <P> The proprioception of the lower limbs differs from the upper limbs and upper trunk . There is a four - neuron pathway for lower limb proprioception . This pathway initially follows the dorsal spino - cerebellar pathway . It is arranged as follows: proprioceptive receptors of lower limb -> peripheral process -> dorsal root ganglion -> central process -> Clarke's column -> 2nd order neuron -> medulla oblogata (Caudate nucleus) -> 3rd order neuron -> VPL of thalamus -> 4th order neuron -> posterior limb of internal capsule -> corona radiata -> sensory area of cerebrum . </P>

What links neurons together in the spinal cord