This pathway carries the modalities discriminative touch, joint position, and the somewhat artificial "sense" of vibration from the body. Receptors for these modalities are generally specialized endings in the skin and joint capsule.
The axons enter the spinal cord and turn upward, with no synapse (see Figure 32). Those fibers entering below spinal cord level T6 (sixth thoracic spinal segmental level) form the fasciculus gracilis, the gracile tract; those entering above T6, particularly those from the upper limb, form the fasciculus cuneatus, the cuneate tract, which is situated more laterally. These tracts ascend the spinal cord between the two dorsal horns, forming the dorsal column (see Figure 32, Figure 68, and Figure 69).
The first synapse in this pathway is found in two nuclei located in the lowermost part of the medulla, in the nuclei gracilis and cuneatus (see Figure 9B, Figure 40, and Figure 67C). Topographical representation, also called somatotopic organization, is maintained in these nuclei, meaning that there are distinct populations of neurons that are activated by areas of the periphery that were stimulated.
After neurophysiological processing, axons emanate from these two nuclei, which will cross the midline. This stream of fibers, called the internal arcuate fibers, can be recognized in suitably stained sections of the lower medulla, (see Figure 40 and Figure 67C). The fibers then group together to form the medial lemniscus, which ascends through the brainstem. This pathway does not give off collaterals to the reticular formation in the brainstem. This pathway changes orientation and position as it ascends through the pons and midbrain (see Figure 40 and Figure 65-Figure 67).
The medial lemniscus terminates (i.e., synapses) in the ventral posterolateral nucleus of the thalamus, the VPL (see Figure 12 and Figure 63). The fibers then enter the internal capsule, its posterior limb, and travel to the somatosensory cortex, terminating along the post-central gyrus, areas 1, 2, and 3 (see Figure 14A and Figure 63).
The representation of the body on this gyrus is not proportional to the size of the area being represented; for example, the fingers, particularly the thumb, are given a much larger area of cortical representation than the trunk; this is called the sensory "homunculus." The lower limb, represented on the medial aspect of the hemisphere (see Figure 17), has little cortical representation.
The cross-sectional levels for this pathway include the lumbar and cervical spinal cord levels, and the brainstem levels, lower medulla, mid-pons, and upper midbrain.
In the spinal cord, the pathways are found between the two dorsal horns, as a well myelinated bundle of fibers, called the dorsal column(s). The tracts have a topographical organization, with the lower body and lower limb represented in the medially placed gracile tract, and the upper body and upper limb in the laterally placed cuneate tract. After synapsing in their respective nuclei and the crossing of the fibers in the lower medulla (internal arcuate fibers), the medial lemniscus tract is formed. This heavily myelinated tract that is easily seen in myelin-stained sections of the brainstem (e.g., see Figure 67C), is located initially between the inferior olivary nuclei and is oriented in the dorsal-ventral position (see Figure 40 and Figure 67B). The tract moves more posteriorly, shifts laterally, and also changes orientation as it ascends (see Figure 40; also Figure 65A, Figure 66A, and Figure 67A). The fibers are topographically organized, with the leg represented laterally and the upper limb medially. The medial lemnis-cus is joined by the anterolateral system and trigeminal pathway in the upper pons (see Figure 36 and Figure 40).
Lesions involving this tract will result in the loss of the sensory modalities carried in this pathway. A lesion of the dorsal column in the spinal cord will cause a loss on the same side; after the crossing in the lower brainstem, any lesion of the medial lemniscus will result in the deficit occurring on the opposite side of the body. Lesions occurring in the midbrain and internal capsule will usually involve the fibers of the anterolateral pathway, as well as the modalities carried in the trigeminal pathway (to be discussed with Figure 36 and Figure 40). With cortical lesions, the part of the body affected will be determined by the area of the post-central gyrus involved.
FIGURE 33: Dorsal Column — Medial Lemniscus — Discriminative Touch, Joint Position, and Vibration
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