MODULE 3
THE DORSAL COLUMN - MEDIAL LEMNISCUS (DC-ML) PATHWAY
THE SECOND ORDER NEURON
- - As you can see in Figure 3-2, the second order neurons of this pathway have their cell bodies in the nucleus gracilis and the nucleus cuneatus. The somatotopic organization of the pathway is preserved, both within these nuclei and within the pathway which carries the axons of these neurons rostrally through the brainstem to the thalamus. The pathway is named the medial lemniscus. The course it takes, and the relationships it establishes with adjacent structures along the way - exiting cranial nerves, other sensory and motor pathways, etc., is of enormous clinical importance. For this reason, and because this is the first of many pathways we will trace, we will describe the process in detail. This is, in fact, the most important and most time consuming part of the module.
- - The goal in tracing pathways through the brain is to try to create a three dimensional "picture" of the pathway from a series of two dimensional cross sections. All textbooks try to help with figures and it might be useful to look at their efforts before we start our own journey, so look at one of the following: Blumenfeld Fig. 7.1, Fitzgerald Fig 15.8, Haines Fig. 17-7. Doing so will also get you to the part of each text where the pathway is described.
- - In the Pixelated Brain, we provide you with two possible ways of tracing pathways. Play around with both of them and use the one that works best for you.
METHOD 1
- - Open Figure 3-5. It is similar to Figure 2, but slide levels have been drawn in and by clicking on the numbers in the left part of the figure you can call up the corresponding slide. For starters, to trace the DC-ML pathway, click on 01 (= slide 1) and look at the position of the pathway on the slide. Then click on "key" , the option at the far right, to go rostrally to the next important slide. Keep going, till you get to the thalamus. Then step caudally through the series, back to slide 1, perhaps looking at every slide of the series. At some point, play around with the options offered in the upper left part of the blue box. For example, click the "unlabeled" link and test yourself to see if you can identify structures on an unmarked slide. Or, click on "cranial nerve" to see if any cranial nerve axons passes close to the medial lemniscus as the prepare to exit from the brainstem
METHOD 2
-- Method 1 is probably the quickest way to get to any specific slide, and for most pathways it will be the only approach available to you. One disadvantage of it, however, is that it is difficult to see where each slide fits into the overall picture. For the DC-ML pathway, and perhaps for a few others, we offer a second option. To give it a try, call up Figure 3-11. Hopefully, it will look familiar to you, because you saw it first as Figure 2-23 when you did Module 2. The letters A,T,L on the figure stand for Arm, Trunk and Leg and show the somatotopic organization of the DC-ML pathway at the spinal cord and caudal brainstem levels. As in Module 2, you can click on the brainstem sections to get an enlarged view, with the option to look at an unlabeled view, an atlas view, etc. Once you have confirmed the position of the DC-ML pathway on Figure 3-11, move forward to the second slide of the group, either by clicking on "rostral" at the bottom of the slide, or by clicking here on Figure 3-12. After you finish with it, go on to Figure 3-13.
Congratulations! One way or the other,you made it to the diencephalon! Now might be a good time to take another look at Figure 2-2. The nuclei gracilis is drawn in (but not labeled) on the dorsal surface of the medulla, just caudal to the fourth ventricle. The pathway we have just traced - the medial lemniscus - passes from this nucleus (and the nucleus cuneatus, which was not drawn in) across the midline and then ascends through the medulla, the pons, and the midbrain to terminate in the diencephalon. As Figure 2-2 shows, the diencephalon is divided into 4 sub regions. By far the largest of the four is the thalamus. Almost all the ascending sensory systems (sensory from the body's surface = somatic sensory, vision, audition) have a relay within the thalamus along the way to the cerebral cortex. Many of these nuclei are named by virtue of their position within the thalamus. Hence the nucleus ventralis posterolateralis lies in the ventral, posterior lateral part of the thalamus. Read the section starting on page 271 in Blumenfeld if you want to learn more about the thalamus. Don't get overwhelmed, and focus on the relay nuclei - particularly VPL. Or, as an alternative, read the section in Fitzgerald starting on page 297, or the one in Haines starting on page 233 (but the summary, starting on page 237, may be best). Skip the details.
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