The Medial Lemniscus-Dorsal Column pathway is an ascending spinal tract, carrying sensory information to the brain. It is typically depicted as a chain of three neurons: first-, second-, and third-order neurons.
This pathway mediates:
- Conscious proprioception (most clinically relevant)
- Sensation of tactile discrimination
- Vibration sense
- Form recognition
First order neurons
The first-order neurons in the pathway are located in the dorsal root ganglia at all spinal levels, giving rise to the fasciculus gracilis tract in the lower extremity and the fasciculus cuneatus tract in the upper extremity. The axons comprising these funiculi ascend ipsilaterally to the medulla, where they synapse with the second-order neurons.
Second order neurons
The second-order neurons are located in the cadual medulla, and their cell bodies form the gracile and cuneate nuclei. Their axons, referred to as internal arcuate fibers, decussate to form the medial lemniscus, which ascends the contralateral brainstem to project to the ventral posterolateral (VPL) nucleus of the thalamus.
Third order neurons
The third-order VPL neurons send axons through the posterior limb of the internal capsule to the somatosensory cortex (areas 3, 1, 2)
Spinal cord lesions affecting the dorsal column (e.g., vitamin B12 neuropathy, tabes dorsalis) result in ipsilateral sensory deficits below the lesion, because the pathway does not decussate until it is at the level of the medulla.