MODULE 3
THE DORSAL COLUMN - MEDIAL LEMNISCUS (DC-ML) PATHWAY
THE FIRST ORDER NEURON
Take another look at Figure 2-31, if you want to be reminded about the general anatomy of a spinal cord segment; if not, go directly to 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 dorsal funiculus (= dorsal column). Because of the way in which the ascending fibers pile up in the dorsal column, we say that it is somatotopically organized, with the lower part of the body represented most medially.
In Figure 3-4 an entering dorsal root fiber has been drawn in to illustrate this arrangement, and in Figure 2-34 slide 1, arrow D points to fibers of this type entering the deeper layers of the dorsal horn. These branches represent the afferent side of spinal reflexes such as the stretch reflex (Blumenfeld's page 37). Fitzgerald shows this in his Figure 15,5 and Haines does so in his Figure 17.7. Or read page 340 in Kandel and see Figure 18-3.
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