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Dorsal column–medial lemniscus pathway
The dorsal column–medial lemniscus pathway (DCML) (also known as the posterior column-medial lemniscus pathway (PCML) is the major sensory pathway of the central nervous system that conveys sensations of fine touch, vibration, two-point discrimination, and proprioception (body position) from the skin and joints. It transmits this information to the somatosensory cortex of the postcentral gyrus in the parietal lobe of the brain. The pathway receives information from sensory receptors throughout the body, and carries this in the gracile fasciculus and the cuneate fasciculus, tracts that make up the white matter dorsal columns (also known as the posterior funiculi) of the spinal cord. At the level of the medulla oblongata, the fibers of the tracts decussate and are 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 first-order axons make contact with second-order neurons of the dorsal column nuclei (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 posterolateral nucleus 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 the thalamus; the axons synapse on neurons in the ventral posterolateral nucleus which then send axons to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe. All of the axons in the DCML pathway are rapidly conducting, large, myelinated fibers.
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.
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Dorsal column–medial lemniscus pathway
The dorsal column–medial lemniscus pathway (DCML) (also known as the posterior column-medial lemniscus pathway (PCML) is the major sensory pathway of the central nervous system that conveys sensations of fine touch, vibration, two-point discrimination, and proprioception (body position) from the skin and joints. It transmits this information to the somatosensory cortex of the postcentral gyrus in the parietal lobe of the brain. The pathway receives information from sensory receptors throughout the body, and carries this in the gracile fasciculus and the cuneate fasciculus, tracts that make up the white matter dorsal columns (also known as the posterior funiculi) of the spinal cord. At the level of the medulla oblongata, the fibers of the tracts decussate and are 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 first-order axons make contact with second-order neurons of the dorsal column nuclei (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 posterolateral nucleus 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 the thalamus; the axons synapse on neurons in the ventral posterolateral nucleus which then send axons to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe. All of the axons in the DCML pathway are rapidly conducting, large, myelinated fibers.
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.