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| Dorsal column-medial lemniscus pathway | |
|---|---|
 The formation of the spinal nerve from the dorsal and ventral roots. | |
 Originating in peripheralsensory receptors, the dorsal column-medial lemniscus pathway transmits fine touch and consciousproprioceptive information to the brain. | |
| Details | |
| Precursor | Neural tube andcrest |
| System | Somatosensory system |
| Decussation | Medial lemniscus |
| To | Sensorimotor cortex |
| Function | Transmit sensation of fine touch, vibration and proprioception |
| Identifiers | |
| Latin | via columnae posterioris lemniscique medialis |
| Acronym(s) | DCML |
| Anatomical terms of neuroanatomy | |
Thedorsal column–medial lemniscus pathway (DCML) (also known as theposterior column-medial lemniscus pathway (PCML) is the majorsensory pathway of thecentral nervous system that conveyssensations offine touch,vibration,two-point discrimination, andproprioception (body position) from the skin and joints. It transmits this information to thesomatosensory cortex of thepostcentral gyrus in theparietal lobe of the brain.[1][2] The pathway receives information from sensory receptors throughout the body, and carries this in thegracile fasciculus and thecuneate fasciculus, tracts that make up the white matterdorsal columns (also known as the posteriorfuniculi) of the spinal cord. At the level of themedulla oblongata, the fibers of the tractsdecussate and are continued in themedial lemniscus, on to thethalamus and relayed from there through theinternal 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 thespinal cord, and the medial lemniscus in thebrainstem.
There are three groupings of neurons that are involved in the pathway:first-order neurons,second-order neurons, andthird-order neurons. The first-order neurons aresensory neurons located in thedorsal root ganglia, that send theirafferent fibers through the two dorsal columns.[3] The first-orderaxonsmake contact with second-order neurons of thedorsal column nuclei (thegracile nucleus and thecuneate nucleus) in the lowermedulla. The second-order neurons send their axons to thethalamus. The third-order neurons are in theventral 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 thecervical level of the spinal cord and carried in the cuneate tract, and information from the lower body is received at thelumbar 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 theinternal arcuate fibers and when they cross over the midline, at thesensory 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 postcentral gyrus in the parietal lobe. All of the axons in the DCML pathway are rapidly conducting, large,myelinated fibers.[2]
The DCML pathway is made up of the axons of first, second, and third-ordersensory neurons, beginning in thedorsal root ganglia. The axons from thefirst-order neurons form the ascending tracts of thegracile fasciculus, and thecuneate fasciculus (thedorsal columns) which synapse on the second-order neurons in thegracile nucleus and thecuneate nucleus known together as thedorsal column nuclei; axons from these neurons ascend as theinternal arcuate fibers; the fibers cross over at thesensory decussation and form themedial lemniscus which connects with thethalamus; the axons synapse on neurons in theventral nuclear group which then send axons to thepostcentral gyrus in theparietal lobe.
The gracile fasciculus carries sensory information from the lower half of the body entering the spinal cord at the lumbar level. The cuneate fasciculus carries sensory information from the upper half of the body (upper limbs, trunk, and neck) entering the spinal cord at the cervical level.[4] 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 thesixth thoracic vertebra (T6). Ascending tracts typically have three levels of neurons, namelyfirst-order,second-order, andthird-order neurons, that relay information from the physical point of reception to the actual point of interpretation in the brain.
When anaction potential is generated by amechanoreceptor in thetissue, the action potential will travel along the peripheral axon of the first-order neuron. The first-order neuron ispseudounipolar in shape with itsbody in thedorsal root ganglion. The action signal will continue along the central axon of the neuron through theposterior root, into theposterior horn, and up theposterior column of thespinal cord.
Axons from the lower body enter the posterior column below the level ofT6 and travel in the midline section of the column called the gracile fasciculus.[5] 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 morelateral section called the cuneate fasciculus. These fasciculi are in an area of white matter, theposterior funiculus (afuniculus) that lies between theposterolateral and theposterior median sulcus. They are separated by a partition ofglial cells which places them on either side of the posterior intermediate sulcus.
The column reaches the junction between the spinal cord and themedulla oblongata, where lower body axons in the gracile fasciculus connect (synapse) with neurons in thegracile nucleus, and upper body axons in the cuneate fasciculus synapse with neurons in thecuneate nucleus.[6]
First-order neurons secretesubstance 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[3]
The neurons in these two nuclei (thedorsal column nuclei) are second-order neurons.[6] Their axons cross over to the other side of the medulla and are now named as theinternal arcuate fibers, that form themedial lemniscus on each side. This crossing over is known as thesensory 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 thetrigeminal nerve (supplying thehead) come in dorsal to the arm fibres, and travel up the lemniscus too.
The medial lemniscus rotates 90 degrees at thepons. 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 thethalamus (at theventral posterolateral nucleus for sensation from the neck, trunk, and extremities, and at theventral posteromedial nucleus for sensation from the head).
Axons from the third-order neurons in theventral posterior nucleus in the thalamus, ascend the posterior limb of theinternal capsule. Those originating from the head and the leg swap their relative positions. The axons synapse in theprimary somatosensory cortex, with lower body sensation most medial (e.g., the paracentral lobule) and upper body more lateral.
Discriminative sensation is well developed in the fingers ofhumans and allows the detection of fine textures. It also allows for the ability known ashaptic perception (stereognosis), to determine what an unknown object is, using the hands without visual or audio input.Fine touch is detected bycutaneous receptors calledtactile corpuscles that lie in thedermis of theskin close to theepidermis. When these structures are stimulated by slight pressure, anaction potential is started. Alternatively, proprioceptivemuscle spindles and other skin surface touchmechanoreceptors such asMerkel cells,bulbous corpuscles,lamellar corpuscles, and hair follicle receptors (peritrichial endings) may involve the first neuron in this pathway.
The sensory neurons in this pathway arepseudounipolar, meaning that they have a single process emanating from the cell body with two distinct branches: one peripheral branch that functions somewhat like adendrite of a typical neuron by receiving input (although it shouldnot be confused with a true dendrite), and one central branch that functions like a typical axon by carrying information to other neurons (again, both branches are actually part of one axon).
Damage to the dorsal column-medial lemniscus pathway below the crossing point of its fibers results in loss of vibration and joint sense (proprioception) on thesame side of the body as the lesion. Damage above the crossing point result a loss of vibration and joint sense on theopposite side of the body to the lesion. The pathway is tested withRomberg's test.
Damage to either of the dorsal column tracts can result in the permanent loss of sensation in the limbs. SeeBrown-Séquard syndrome.
The cuneate fasciculus, fasciculus cuneatus, cuneate tract, tract of Burdach, was named forKarl Friedrich Burdach. The gracile fasciculus, the tract of Goll, was named after Swiss neuroanatomistFriedrich Goll (1829–1903).