Posterior column - Medial lemniscus pathway

Dorsal column

The dorsal column is formed by two large fasciculi (bundles of nerve fibers) running through the posterior spinal cord: fasciculus gracilis and fasciculus cuneatus. These fasciculi gather sensory information from our body's periphery via skin and joint receptors, and send this information to superior cerebral structures.

The most important conscious cutaneous receptors for the DCML system are:

• Ruffini endings (bulbous corpuscles); detect tension deep in the skin and connective tissue or fascia.
• Merkel nerve endings (Merkel discs); detect sustained pressure
• Lamellar corpuscles (Pacinian corpuscles); detect rapid vibrations (200-300Hz).

When an action potential is generated by a mechanoreceptor in the tissue, the impulse travels along the peripheral axons of the first-order neuron. This first-order neuron is pseudounipolar in shape with its body in the dorsal root ganglion. The input will travel along its axon, through the dorsal root and into the posterior horn, via medial dorsal root entry zone. 

Once in the spinal cord, these axons give off small collaterals (Lissauer’s tract) in order to facilitate spinal reflexes or send part of the sensory input through the spinothalamic tracts. The majority, however, will leave the dorsal horn gray matter without synapsing and enter the dorsal funiculus in order to constitute either the fasciculus gracilis or fasciculus cuneatus. 

Together these fasciculi ascend the spinal cord to reach the lower (closed) part of the medulla oblongata. In the medulla, axons in the gracile fasciculus synapse with the gracile nucleus and axons in the cuneate fasciculus synapse with neurons in the cuneate nucleus. The neurons in these two medullary nuclei (nucleus gracilis and cuneatus) are second-order neurons. Their axons cross over to the other side of the medulla and are named internal arcuate fibers. This crossing is known as sensory decussation. The internal arcuate fibers eventually form the medial lemniscus.

Medial lemniscus

The medial lemniscus is the rostral continuation of the dorsal column. Axons from second-order neurons decussate at the level of the medulla and travel up the brainstem as the medial lemniscus on the contralateral (opposite) side. The medial lemniscus terminates and synapses with third-order neurons, located in the ventral posterolateral nucleus (VPLN) of the thalamus within each cerebral hemisphere. 

Axons of third-order neurons ascend from the VPLN, through the posterior third of the posterior limb of the internal capsule, and then through the corona radiata, to reach the primary somatosensory cortex of the postcentral gyrus in the parietal lobe.

Function

Our skin and joints contains many various receptors, each of which are specific for different types of mechanical action (vibration, slight touch, skin stretch, proprioception, temperature, changes in texture, discrimination, pressure). When these receptors are stimulated, an action potential is generated and transmitted by the DCML pathway to the primary somatosensory area of the cerebral cortex.

Discriminative sensation/touch is well developed in the fingers of humans. This capability is given to us by the DCML pathway. It provides us with the ability to detect fine textures and to determine an unknown object by its shape, using our hands without visual or audio input (stereognosis).