NEUROSCIENCE: SOMATIC SENSATION
Somatosensory Pathways: Dorsal Column-Medial Lemniscal System

SOMATOSENSORY PATHWAYS

C. Dorsal Column - Medial Lemniscal System

1. Subserves sensations mediated by 1st order rapidly conducting axons (larger myelinated axons)

a. fine, discriminative touch
b. flutter and vibration
c. proprioception, particularly joint receptors (also muscle spindle and tendon organ receptors)

2. Primary (1st order) afferent neurons (==>)

a. soma in spinal dorsal root
b. form the medial division of the dorsal root

3.    Ascending branch of 1st order neuron (part of 3-neuron sensory path)

Note:  minimum of 3 neurons; some branches synapse before ascending and these collateral paths involve more than 3 neurons

a. passes medially into the spinal cord white matter to form the ipsilateral dorsal columns

b. as the columns ascend, the tract becomes laminated, with the newly added axons located laterally and axons from caudal segments more medial, leading to a somatotopic lamination

c. axons entering below the mid-thoracic level form the fasciculus gracilis; those entering above the mid-thoracic level form the fasiculus cuneatus of the ipsilateral dorsal column (dividing line: T5-T6)

d. axons synapse in the ipsilateral gracilis and cuneatus nuclei of the medulla

e. in addition to the above, afferent fibers have branches that

1) synapse in the dorsal horn and send postsynaptic axons to the medulla in the dorsal columns (may first ascend or descend 1-2 segments)
2) ascend to the cerebellum (spinocerebellar, important in coordination)
3) synapse on motor neurons in the ventral horn (basis of spinal reflexes)
4) synapse on spinal interneurons

4.    Medulla  (==>)

a. axons from the gracilis fascicle and the cuneatus fascicle synapse with the ipsilateral medullary dorsal column nuclei -- the nucleus gracilis and nucleus cuneatus -- maintaining somatotopic organization

b. 2nd order axons from n. gracilis and n. cuneatus decussate in the medulla and then continues to ascend as the medial lemniscus

5.    Medial Lemniscus

a. fibers ascend through the medulla, pons, and midbrain, maintaining the somatotopic organization

b. 2nd order fibers travel to the ventrobasal complex of the thalamus, where their axons synapse with 3rd order neurons to form the specific cortical projection system

Dorsal Column -- Medial Lemniscal System:  Overall Pathway ==>