The DCML pathway is made up of the axons of first, second, and third-order
sensory neurons, beginning in the
dorsal root ganglia. The axons from the
first-order neurons form the ascending tracts of the
gracile fasciculus, and the
cuneate fasciculus (the
dorsal columns) which synapse on the second-order neurons in the
gracile nucleus and the
cuneate nucleus known together as the
dorsal column nuclei; axons from these neurons ascend as the
internal arcuate fibers; the fibers cross over at the
sensory decussation and form the
medial lemniscus which connects with the
thalamus; the axons synapse on neurons in the
ventral posterolateral nucleus of the thalamus, which then send axons to the
postcentral gyrus in the
parietal lobe. The gracile fasciculus carries sensory information from the lower half of the body, i.e. the nerves entering the spinal cord below
T6. The cuneate fasciculus carries sensory information from the upper half of the body (upper limbs, trunk, neck, and the
posterior third of the scalp), i.e. the nerves entering the spinal cord at or above
T6. Note that sensory information from the face and anterior 2/3 of the scalp is not carried by the dorsal column-medial lemniscus pathway but by the
trigeminal lemniscus tract. The gracile fasciculus is wedge-shaped on transverse section and lies next to the posterior median septum. Its base is at the surface of the spinal cord, and its apex directed toward the posterior gray commissure. The gracile fasciculus increases in size from inferior to superior. The cuneate fasciculus is triangular on transverse section and lies between the gracile fasciculus and the posterior column, its base corresponding with the surface of the spinal cord. Its fibers, larger than those of the gracile fasciculus, are mostly derived from the same source, viz., the posterior nerve roots. Some ascend for only a short distance in the tract, and, entering the gray matter, come into close relationship with the cells of the dorsal nucleus, while others can be traced as far as the medulla oblongata, where they end in the gracile nucleus and cuneate nucleus. The two ascending tracts meet at the level of the
sixth thoracic vertebra (T6). Ascending tracts typically have three levels of neurons, namely
first-order,
second-order, and
third-order neurons, that relay information from the physical point of reception to the actual point of interpretation in the brain.
First-order neurons Periphery and spinal cord (shown left), with a single
axon originating from the
cell body then splitting into two branches. The body is situated in the dorsal root ganglion, with one axon traveling peripherally to tissue, and one traveling into the dorsal column. On the right is a
bipolar neuron.|left When an
action potential is generated by a
mechanoreceptor in the
tissue, the action potential will travel along the peripheral axon of the first-order neuron. The first-order neuron is
pseudounipolar in shape with its
body in the
dorsal root ganglion. The action signal will continue along the central axon of the neuron through the
posterior root, into the
posterior horn, and up the
posterior column of the
spinal cord. Axons from the lower body enter the posterior column below the level of
T6 and travel in the midline section of the column called the gracile fasciculus. Axons from the upper body enter at or above T6 and travel up the posterior column on the outside of the gracile fasciculus in a more
lateral section called the cuneate fasciculus. These fasciculi are in an area of white matter, the
posterior funiculus (a
funiculus) that lies between the
posterolateral and the
posterior median sulcus. They are separated by a partition of
glial cells which places them on either side of the posterior intermediate sulcus. The column reaches the junction between the spinal cord and the
medulla oblongata, where lower body axons in the gracile fasciculus connect (
synapse) with neurons in the
gracile nucleus, and upper body axons in the cuneate fasciculus synapse with neurons in the
cuneate nucleus. Their axons cross over to the other side of the medulla and are now named as the
internal arcuate fibers, that form the
medial lemniscus on each side. This crossing over is known as the
sensory decussation. At the medulla, the medial lemniscus is orientated perpendicular to the way the fibres travelled in their tracts in the posterior column. For example, in the column,
lower limb is medial,
upper limb is more lateral. At the medial lemniscus, axons from the leg are more ventral, and axons from the arm are more dorsal. Fibres from the
trigeminal nerve (supplying the
head) come in dorsal to the arm fibres, and travel up the lemniscus too. The medial lemniscus rotates 90 degrees at the
pons. The secondary axons from neurons giving sensation to the head, stay at around the same place, while the leg axons move outwards. The axons travel up the rest of the brainstem, and synapse at the
thalamus (at the
ventral posterolateral nucleus for sensation from the neck, trunk, and extremities, and at the
ventral posteromedial nucleus for sensation from the head).
Third-order neurons Thalamus to cortex Axons from the third-order neurons in the
ventral posterior nucleus in the thalamus, ascend the posterior limb of the
internal capsule. Those originating from the head and the leg swap their relative positions. The axons synapse in the
primary somatosensory cortex, with lower body sensation most medial (e.g., the paracentral lobule) and upper body more lateral. ==Function==