FIGURE 3-3

     Fibers of this pathway are organized in a somatotopic manner (fibers from adjacent parts of the body's surface run in adjacent parts of the pathway), and the color code used to represent this is:

     Sacral - green
     Lumbar - purple
     Low Thoracic - red
     High Thoracic - light blue
     Cervical - dark blue

     The first order neurons of this pathway are large myelinated axons which enter in the medial part of the dorsal root. The region where each dorsal root enters the spinal cord is often called the "dorsal root entry zone" and these fibers enter in the medial part of the zone. The fibers then bifurcate, with one branch passing deeply to terminate in the dorsal horn and the other branch passing medially to ascend in the posterior funiculus. Because of the way in which the ascending fibers pile up in the posterior funiculus, we say that it is somatotopically organized, with the lower part of the body represented most medially.