Dorsal column-medial lemniscus (DCML) pathway

In this note, we are going to discuss the anatomy and function of the tracts of the brain and spinal cord.

The tracts within the brain and spinal cord are represented by groups of nerve fibers that are linked by a common origin, direction, and function.

They interconnect different areas of the nervous system, providing control over all neural activity.

There are three types of the tracts within the brain and spinal cord:

Ascending (or sensory)
Descending (or motor)
Propriospinal

Let’s start with the ascending pathways. These carry sensory (afferent) information from peripheral receptors to the structures of the central nervous system for further processing.

Within this category, the following tracts are distinguished:

The posterior columns (also known as the gracile and cuneate fasciculi) are responsible for conducting proprioception, fine touch, pressure, and vibratory sensations.
The spinothalamic tracts (anterior and lateral) are responsible for conducting pain and temperature sensations, as well as crude touch and sensation of superficial pressure.
The spinocerebellar tracts (anterior and posterior) are responsible exclusively for proprioception.
Additionally, this group may include the spinoreticular and spinomesencephalic tracts. They share a similar function to the spinothalamic pathways.

Let’s take a closer look at the dorsal column–medial lemniscus pathway, also known as the gracile and cuneate fasciculi.

Let’s start by defining the function.

This tract carries deep sensations from several types of receptors.

Proprioceptors. They provide information about the fine touch and proprioception, that is the sense that lets us detect the location, movement, and action of parts of the body
Exteroceptors. They provide information about pressure and vibratory sensations

Three neurons are involved in the dorsal column–medial lemniscus pathway.

Fibers from the receptors are directed to the spinal ganglion, where the first-order neuron of this pathway is located.

Then, the fibers enter into the spinal cord via the dorsal root, into the posterior funiculus, where they form two large bundles: the gracile fasciculus

and the cuneate fasciculus.

The gracile fasciculus is located medially and carries sensory information from the lower parts of the trunk (below the level of T6) and the lower limbs, while the cuneate fasciculus is situated laterally and carries sensory information from the upper parts of the trunk (above the level of T6) and the upper limbs.

Further, the fibers ascend to the medulla oblongata, where they synapse with the second-order neurons in the gracile nucleus and cuneate nucleus.

After this, the majority of fibers cross over to the contralateral side (decussate). These fibers are also known as the internal arcuate fibers.

However, some fibers, upon exiting the nuclei, do not cross over but travel along the same side to the cerebellum, forming the posterior external arcuate fibers.

The decussated fibers, ascending further, form a thick bundle, known as the medial lemniscus.

Medial lemniscus (level of the medulla oblongata)

Medial lemniscus (level of the midbrain)

As it travels upward, fibers from the nuclei of the trigeminal, facial, glossopharyngeal, and vagus cranial nerves join the lemniscus, carrying information about the same types of deep sensations.

Eventually, the medial lemniscus reaches the thalamus, where it synapses with the third-order neuron in the ventral posterolateral nucleus (VPL).

Then, the fibers pass through the posterior limb of the internal capsule,

reaching the primary somatosensory cortex of the postcentral gyrus in the parietal lobe.

The postcentral gyrus has a particular pattern of somatotopic arrangement.

Its upper and medial parts are responsible for the sensory input from the lower limbs, and the lower and lateral parts, respectively, from the head and neck. This concept is called Penfield’s sensory homunculus.

Moreover, the proportions of the homunculus differ from those of an actual human. These differences are due to the number of receptors in a specific area of the body. For example, there are significantly more tactile receptors on the lips than on the back, which is why the cortical area responsible for the lips is significantly wider.

Golosary