The medial lemniscus, also called Reil’s band or ribbon, is a channel for sensory data to the thalamus from the gracile and cuneate nuclei in the brain. Nearly all the cutaneous mechanoreceptors in the body that transmit information about light touch, vibration, and pressure signals will travel through the dorsal-column medial lemniscus pathway. It begins in the medulla, travels into the pons, and finally ends in the midbrain.
Sensory information is collected by the gracile and the cuneate nuclei and transmitted to the thalamus by means of the medial lemniscus. Gracile and cuneate nuclei are also known as the dorsal column nuclei. A gracile tract is the location of the axon fibers that transmit lower limb information. The cuneate tract is positioned lateral to the gracile tract, and is the site of the axon fibers for the chest, neck, and upper limbs. Above the gracile and cuneate nuclei in the lower closed portion of the medulla is where the medial lemniscus begins, although it does not fully form until the open medulla farther up the brainstem.
Neurons located in the dorsal column nuclei transmit axons to the thalamus that interpret somatic sensory information from the body; the axons project into the backside of the lower part of the brainstem. Once the axons branch into this area, they form the internal arcuate tract. The fibers will then cross over the midline of the brainstem in a process called decussation, and form the elongated pathway called the medial lemniscus. Information-carrying axons from the lower limbs of the body are located in the ventral portion of the medial lemniscus; axons for the upper limbs of the body are stored in the dorsal part of the structure.
When the ribbon of the medial lemniscus travels upward within the pons and the midbrain, it turns on its side at a 90-degree angle. Once it rotates, the axons are stored in different locations. The lateral section will contain the axons for the lower limbs of the body, and the medial section will hold the upper body axons. It continues to ascend in the brain until it reaches the thalamus' ventral posterior lateral (VPL) nucleus.
Damage to the dorsal-column medial lemniscus pathway may cause difficulty in a person’s ability to detect tactile stimuli. A person may also have trouble sensing the position of his arms and legs. The inability of a damaged pathway to process the speed and direction of touch may cause a person to be unable to detect which letters and numbers are being traced on his skin during diagnostic testing.