Posterior column

The dorsal column–medial lemniscus pathway (DCML) (also known as the posterior column-medial lemniscus pathway (PCML)) is a sensory pathway of the central nervous system that conveys sensations of fine touch, vibration, two-point discrimination, and proprioception (position) from the skin and joints. It transmits information from the body to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe of the brain. The pathway receives information from sensory receptors throughout the body, and carries this in nerve tracts in the white matter of the dorsal columns of the spinal cord, to the medulla where it is continued in the medial lemniscus, on to the thalamus and relayed from there through the internal capsule and transmitted to the somatosensory cortex. The name dorsal-column medial lemniscus comes from the two structures that carry the sensory information: the dorsal columns of the spinal cord, and the medial lemniscus in the brainstem.2° (Spinomesencephalic tract → Superior colliculus of Midbrain tectum) The dorsal column–medial lemniscus pathway (DCML) (also known as the posterior column-medial lemniscus pathway (PCML)) is a sensory pathway of the central nervous system that conveys sensations of fine touch, vibration, two-point discrimination, and proprioception (position) from the skin and joints. It transmits information from the body to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe of the brain. The pathway receives information from sensory receptors throughout the body, and carries this in nerve tracts in the white matter of the dorsal columns of the spinal cord, to the medulla where it is continued in the medial lemniscus, on to the thalamus and relayed from there through the internal capsule and transmitted to the somatosensory cortex. The name dorsal-column medial lemniscus comes from the two structures that carry the sensory information: the dorsal columns of the spinal cord, and the medial lemniscus in the brainstem. There are three groupings of neurons that are involved in the pathway: first-order neurons, second-order neurons, and third-order neurons. The first-order neurons are sensory neurons located in the dorsal root ganglia, that send their afferent fibers through the two dorsal columns – the gracile fasciculus, or gracile tract, and the cuneate fasciculus or cuneate tract. The first-order axons make contact with second-order neurons at the gracile nucleus and the cuneate nucleus in the lower medulla. The second-order neurons send their axons to the thalamus. The third-order neurons are in the ventral nuclear group in the thalamus and fibres from these ascend to the postcentral gyrus. Sensory information from the upper half of the body is received at the cervical level of the spinal cord and carried in the cuneate tract, and information from the lower body is received at the lumbar level and carried in the gracile tract. The gracile tract is medial to the more lateral cuneate tract. The axons of second-order neurons of the gracile and cuneate nuclei are known as the internal arcuate fibers and when they cross over the midline, at the sensory decussation in the medulla, they form the medial lemniscus which connects with thalamus; the axons synapse on neurons in the ventral nuclear group which then send axons to the postcentral gyrus in the parietal lobe.. All of the axons in the DCML pathway are rapidly conducting, large, myelinated, alpha delta fibers. The DCML is made up of the axons of first, second, and third-order neurons, beginning in the dorsal root ganglia. These axons from the first-order neurons form the ascending tracts of the gracile fasciculus and the cuneate fasciculus which synapse on the dorsal column nuclei, the second-order neurons in the gracile nucleus and the cuneate nucleus; 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 thalamus; the axons synapse on neurons in the ventral nuclear group which then send axons to the postcentral gyrus in the parietal lobe. The gracile tract carries sensory information from the lower half of the body entering the spinal cord at the lumbar level. The cuneate tract carries sensory information from the upper half of the body (upper limbs, trunk, and neck) entering the spinal cord at the cervical level. 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 fasiculus in a more lateral section called the cuneate fasiculus. These fasciculi are in an area known as the posterior 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 tract connect (synapse) with neurons in the gracile nucleus, and upper body axons in the cuneate tract synapse with neurons in the cuneate nucleus. First-order neurons secrete substance P in the dorsal horn as a chemical mediator of pain signaling. The dorsal horn of the spinal cord transmits pain and non-noxious signals from the periphery to the spinal cord itself. Adenosine is another local molecule that modulates dorsal horn pain transmission