NUCLEUS GRACILIS & CUNEATUS & THE MEDIAL LEMNISCUS
As you learned in the spinal cord module, axons in the fasciculus gracilis and fasciculus cuneatus are the uncrossed central processes of alphabeta fibers whose cell bodies lie in the dorsal root ganglia. Fibers in fasciculus gracilis end in nucleus gracilis, while fibers in fasciculus cuneatus end in nucleus cuneatus.
Nucleus gracilis and nucleus cuneatus lie in the dorsal portion of the caudal medulla. These two nuclear groups and their associated fasciculi can be seen as slight swellings on the dorsal surface of the medulla. The bump formed by nucleus and fasciculus gracilis is called the gracile tubercle (or clava; L., club) and that caused by nucleus and fasciculus cuneatus is called the cuneate tubercle. These bumps can be seen on both of the drawings shown below.
Axons of cells within nucleus gracilis and nucleus cuneatus cross as internal arcuate fibers and form the MEDIAL LEMNISCUS. The medial lemniscus is thus a large ascending bundle of heavily myelinated axons (fast conducting) whose cell bodies lie in the contralateral nucleus gracilis and nucleus cuneatus. The medial lemniscus passes rostrally through the medulla, pons and midbrain to terminate in the ventral posterolateral (VPL) nucleus of the thalamus. Cells in the VPL then send their axons to the postcentral gyrus (somatosensory cortex) of the cerebral cortex (areas 3, 1, 2).
The dorsal column-medial lemniscal system carries information from specialized touch, pressure, vibration, and joint receptors to the cerebral cortex. While lesions of the dorsal columns (fasciculi gracilis and cuneatus) in the spinal cord result in IPSILATERAL deficits, lesions of the medial lemniscus, in the brain stem, result in CONTRALATERAL deficits (since its constituent axons have crossed). This is important to understand. Also, do not confuse VPM (head, trigeminal, trigeminothalamic) with VPL (body, medial lemniscus).
Lesions of the medial lemniscus results in loss of 2 pt. discrimination, vibration and conscious proprioception from the contralateral side of the body. Also remember, there is astereognosia, agraphesthesia, and a Romberg sign.
You may remember that there is a somatotopic representation of the body in the dorsal columns (fasciculus gracilis and fasciculus cuneatus). Caudal (sacral and lumbar) body parts are represented medially, while the rostral segments (upper thoracic and cervical) are represented laterally. Nucleus gracilis receives its input from about T7 and downward, while nucleus cuneatus receives its input from spinal levels above this.
There is also a somatotopic organization within the medial lemniscus. For instance, in the medullary segment of the medial lemniscus the body is represented in an upright position, so that the legs are ventral and the arms dorsal. Within the pontine portion of the medial lemniscus there is a rotation, so that the arms are represented medially and the legs laterally. Finally, in the midbrain portion of the medial lemniscus the arms are represented ventrally while the legs are represented dorsally.