Posterior column - Medial lemniscus pathway
The dorsal column-medial lemniscus pathway (DCML) is a sensory pathway of the central nervous system. It conveys sensation of fine touch, vibration, pressure, two-point discrimination and proprioception (position) from the skin and joints.
Also known as the posterior column - medial lemniscus pathway, it consists of two parts. The dorsal (posterior) column, which runs from the spinal cord to the medulla, and the medial lemniscus which runs as a continuation of the dorsal column, from the medulla to the cortex. In the cortex the DCML pathway projects onto the primary somatosensory cortex of the postcentral gyrus. Here sensation location is 'mapped' using a somatotopic arrangement, represented as a homunculus.
This article will describe the anatomy and function of the dorsal column-medial lemniscus pathway.
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.
The gracilis fasciculus, also known as the “column of Goll”, carries tactile and proprioceptive information from the lower half of the body (T7 to the first coccygeal nerve). It lies medially to fasciculus cuneatus within the posterior half of the posterior spinal cord. The ‘lower = medial’ spatial relationship holds true, not only for the two fasciculi, but also for the individual fibers within each fasciculus. For example, the most medially placed fibers in the fasciculus gracilis arise from the lowest positioned coccygeal dorsal root, and the most laterally placed fibers arise from the more superiorly located T7 dorsal root.
The dorsal column fibers of the fasciculus gracilis ascend, without crossing, until they reach the gracilis nucleus. Here, they synapse with second-order neurons. The axons of the second-order neurons (internal arcuate fibers) run anteromedially and cross (sensory decussation) the midline of the midbrain to the contralateral side. After decussation, the internal arcuate fibers form the medial lemniscus which ascends towards the ventral posterolateral nucleus of the thalamus and then projects onwards to the primary somatosensory cortex.
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The fasciculus cuneatus, also known as the “column of Burdach”, represents the lateral portion of the dorsal column. It carries input from C1 and T6 spinal cord levels. This part of the DCML is responsible for transmitting vibration, conscious proprioception and fine (discriminative) touch sensations from the upper body.
Similar to the fasciculus gracilis, the first-order neurons in the cuneate fasciculus ascend the spinal cord without decussation. This is unlike other ascending tracts (e.g. the spinothalamic tract) which cross at some point within the spinal cord. The fibers of fasciculus gracilis reach the ipsilateral cuneatus nucleus in the caudal medulla, where they synapse with second-order neurons. These second-order neurons then cross over as internal arcuate fibers and form the medial lemniscus on the contralateral side.
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.
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).
The posterior columns transmit impulses arising from proprioceptors and cutaneous receptors. If the tracts are dysfunctional, the brain can no longer register sensory input sent from these receptors regarding touch, pressure, our surroundings, objects or the position of our body.
Damage to these pathways below the crossing point will cause symptoms on the same side of the body as the lesion. If damage occurs above the level of decussation, then the symptoms will show up on the opposite side of the body.
The clinical signs of dorsal column-medial lemniscus pathway lesion include:
- Astereognosis: the patient cannot recognize an object by its shape and weight using touch alone.
- Agraphesthesia: the patient cannot recognize, by touch, a number or letter drawn in the palm of the patient’s hand by the examiner.
- Loss of discrimination: the patient cannot differentiate a single touch from a two-point touch.
- Loss of vibration sense: the patient cannot perceive the vibration of a tuning fork placed on a bone.
- Loss of sense of position and movement: the patient cannot state the position of his body in space, without visual input.
- Positive Romberg test: the patient cannot maintain a vertical position due to the loss of the sense of the floor under the feet.
The diseases that target these pathways are usually degenerative in nature. Tabes dorsalis in tertiary syphilis or degeneration of the spinal cord seen in B12 vitamin deficiency. However, the DCML pathway can also be affected by trauma or infarction of the posterior spinal artery, causing what is known as posterior (spinal) cord syndrome.