The Dorsal Column-Medial Lemniscal System
The Dorsal Column Medial Lemniscal System (DCML; Dorsal for posterior, Column for vertical organization, Medial for midline organization, Lemniscal for the important bundle of sensory fibers in the brain stem (the medial lemniscus)) is a neural pathway for conscious awareness of well-localized, fine-discrimination (epicritic) touch, pressure and muscle and joint proprioception (kinesthetic positional sense). The pathways functional circuitry is accomplished by the distribution of cutaneous mechanoreceptors in the skin, muscle and joints which feed forward information toward the brain and spinal cord for entry into the nervous system by the dorsal roots.
The DCML system consists of a three-neuron circuit that links the periphery with the cerebral cortex. In doing so it traverses the spinal cord, brain stem, diencephalon, and cerebral hemispheres. Even though there are minimally three neurons linking the periphery with the cortex, many thousands of neurons at each level are typically engaged during normal tactile experiences. It is the DCML system that allows you to find a specific key from many on a keyring and place it in a door lock without light or visualize any part of your body without looking. A lesion (a pathologic wound from a destructive tumor, hemorrhage, scar tissue, swelling, infection, traumatic injury, etc.) of the medial lemniscus would cause cause an impairment of tactile, vibratory, touch-pressure and proprioception.
Mechanoreceptor transmission stimulus enters into the spinal cord at the dorsal root entry zone, the ascending fiber tracts within the posterior funiculi are the fasciculi gracilis and cuneatus travel ipsilaterally. The first synapse of the DCML is the termination of either fasciculi at the gracile nucleus (hindlimb representation; dorsal/medial, spinal segments T7 and lower) or cuneate nucleus (forelimb representation; dorsal/lateral to the gracile, spinal segments T6 and higher) in the lower medulla or trigeminal nucleus (facial sensory representation over three branches from the head and neck; ophthalmic (I), maxillary (II), and mandibular (III)) at the pons. These nuclei are not simple relay stations but are sites of transmission modulation that are critical for sensory discrimination. The total length of the processes of the primary neurons can be very long: the longest can be over 2 meters from receptors in the big toe to the medulla oblongata!
From the cuneate and gracilis nuclei, in the lower medulla, the axons sweep caudally and decussate in a subtle curve to the contralateral side as the internal arcuate fibers and ascend in the brain stem toward the thalamus as the medial lemniscus (ribbon). Because of this decussation, sensory information from the right side of the body is processed by the left side of the brain and vice versa. The medial lemniscus is the rostral continuation of epicritic (localized and refined touch) and conscious proprioception at the open medulla. It a large, heavily myelinated bundle of fibers that terminates in specific relay nuclei of the diencephalon at the mammillary bodies.
The maps of the dermatomes do not simply repeat the body surface but their representations are integrated into a compression of their receptive fields. It contains axons from the legs, arms and trigeminal nerve, the fibers of which lie dorsally toward the fourth ventricle. By mid-pons, the medial lemniscus has rotated, the fibers from the head are medial and fibers from the leg are lateral, an orientation that continues rostrally in the midbrain.
The axons of the secondary neurons terminate in the ventral posterior lateral nucleus of the thalamus, at the level of the mammillary bodies, where the second synapse occurs with tertiary (third-order) neurons. The axons of these neurons pass rostrally through the internal capsule, a boomerang-shaped band of fibers, and through radiations of the corona radiata (subcortical white matter) to the postcentral gyrus of the somatosensory cortex for the third synapse where many sensations of the body are processed. Sensations from the lumbar and sacral regions of the body project to the superior aspects of the cortex, and thoracic and cervical regions project to more inferior aspects of the cortex; the inverted representation of the contralateral half of the body (homunculus ("little man")). The higher-order areas receive input predominantly from the primary and other cortical areas and participate in the elaboration of sensory processing leading to perception. After somatosensory cortex processing, a fourth synapse of corticocortical projections from the somatosensory cortex to the pre-central motor cortex occurs.
*Because of this pattern, the somatotopic organization of the medial lemniscus in the medulla resembles a person standing upright. In the pons the medial lemniscus is located more dorsally than in the medulla and is oriented from medial to lateral.*
*The ascending somatic sensory pathways course in two locations in the spinal cord. The dorsal column medial lemniscal system ascends in the dorsal columns. The dorsal columns have two fascicles. The gracile fascicle carries axons from the leg and lower trunk and the cuneate fascicle carries axons from the upper trunk, arm, neck, and back of the head. The majority of the axons in the dorsal columns are central branches of dorsal root ganglion neurons.*
Dorsal column axons terminate in the dorsal column nuclei in the caudal medulla. Axons of neurons in the dorsal column nuclei decussate and ascend in the medial lemniscus and terminate in the thalamus.
The axons of the medial lemniscus synapse in the ventral posterior lateral nucleus which projects to the primary somatic sensory cortex, via the posterior limb of the internal capsule. The secondary somatic sensory cortex and posterior parietal cortex receive input from the primary somatic sensory cortex. Each of these cortical areas is somatotopically organized.
Inputs from thalamus arrive at layer IV of the cortex. Efferent projections from the somatic sensory cortical areas arise from specific cortical layers. Corticocortical association connections (with other cortical areas on the same side of the cerebral cortex) are made by neurons in layers II and III. Callosal connections (with the other side of the cerebral cortex are also made by neurons in layers II and III.) Descending projections to the striatum, brain stem, and spinal cord originate from neurons located in layer V, whereas the projection to the thalamus originates from neurons located in layer VI.
Dermatomes retain their somatotopic organization in the dorsal columns. The first synapse, the gracile nucleus (hindlimb representation) or cuneate nucleus (forelimb representation) or trigeminal nucleus (facial sensory representation). formation of internal arcuate fibers, their decussation and the origin of the medial lemniscus. Maps do not simply recapitulate the body surface but integrate input and expand or compress representations via convergence of receptive fields. The elementary structure of center-surround receptive field organization. The second synapse is the projection of medial lemniscal axons to the thalamus (ventral posterior nucleus). The third synapse is the projection of thalamic neurons to the somatic sensory cortex in the postcentral gyrus which transformations and distortions of the body map in cortex reduce some representations and expand others. The fourth synapse is corticocortical projections from the postcentral somatic sensory cortex to the pre-central motor cortex.
The dorsal column system is a neural pathway for conscious appreciation of fine touch, pressure and muscle proprioception.
The DCML system consists of a three-neuron circuit that links the periphery with the cerebral cortex. In doing so it traverses the spinal cord, brain stem, diencephalon, and cerebral hemispheres. Even though there are minimally three neurons linking the periphery with the cortex, many thousands of neurons at each level are typically engaged during normal tactile experiences. It is the DCML system that allows you to find a specific key from many on a keyring and place it in a door lock without light or visualize any part of your body without looking. A lesion (a pathologic wound from a destructive tumor, hemorrhage, scar tissue, swelling, infection, traumatic injury, etc.) of the medial lemniscus would cause cause an impairment of tactile, vibratory, touch-pressure and proprioception.
Mechanoreceptor transmission stimulus enters into the spinal cord at the dorsal root entry zone, the ascending fiber tracts within the posterior funiculi are the fasciculi gracilis and cuneatus travel ipsilaterally. The first synapse of the DCML is the termination of either fasciculi at the gracile nucleus (hindlimb representation; dorsal/medial, spinal segments T7 and lower) or cuneate nucleus (forelimb representation; dorsal/lateral to the gracile, spinal segments T6 and higher) in the lower medulla or trigeminal nucleus (facial sensory representation over three branches from the head and neck; ophthalmic (I), maxillary (II), and mandibular (III)) at the pons. These nuclei are not simple relay stations but are sites of transmission modulation that are critical for sensory discrimination. The total length of the processes of the primary neurons can be very long: the longest can be over 2 meters from receptors in the big toe to the medulla oblongata!
From the cuneate and gracilis nuclei, in the lower medulla, the axons sweep caudally and decussate in a subtle curve to the contralateral side as the internal arcuate fibers and ascend in the brain stem toward the thalamus as the medial lemniscus (ribbon). Because of this decussation, sensory information from the right side of the body is processed by the left side of the brain and vice versa. The medial lemniscus is the rostral continuation of epicritic (localized and refined touch) and conscious proprioception at the open medulla. It a large, heavily myelinated bundle of fibers that terminates in specific relay nuclei of the diencephalon at the mammillary bodies.
The maps of the dermatomes do not simply repeat the body surface but their representations are integrated into a compression of their receptive fields. It contains axons from the legs, arms and trigeminal nerve, the fibers of which lie dorsally toward the fourth ventricle. By mid-pons, the medial lemniscus has rotated, the fibers from the head are medial and fibers from the leg are lateral, an orientation that continues rostrally in the midbrain.
The axons of the secondary neurons terminate in the ventral posterior lateral nucleus of the thalamus, at the level of the mammillary bodies, where the second synapse occurs with tertiary (third-order) neurons. The axons of these neurons pass rostrally through the internal capsule, a boomerang-shaped band of fibers, and through radiations of the corona radiata (subcortical white matter) to the postcentral gyrus of the somatosensory cortex for the third synapse where many sensations of the body are processed. Sensations from the lumbar and sacral regions of the body project to the superior aspects of the cortex, and thoracic and cervical regions project to more inferior aspects of the cortex; the inverted representation of the contralateral half of the body (homunculus ("little man")). The higher-order areas receive input predominantly from the primary and other cortical areas and participate in the elaboration of sensory processing leading to perception. After somatosensory cortex processing, a fourth synapse of corticocortical projections from the somatosensory cortex to the pre-central motor cortex occurs.
*Because of this pattern, the somatotopic organization of the medial lemniscus in the medulla resembles a person standing upright. In the pons the medial lemniscus is located more dorsally than in the medulla and is oriented from medial to lateral.*
*The ascending somatic sensory pathways course in two locations in the spinal cord. The dorsal column medial lemniscal system ascends in the dorsal columns. The dorsal columns have two fascicles. The gracile fascicle carries axons from the leg and lower trunk and the cuneate fascicle carries axons from the upper trunk, arm, neck, and back of the head. The majority of the axons in the dorsal columns are central branches of dorsal root ganglion neurons.*
Dorsal column axons terminate in the dorsal column nuclei in the caudal medulla. Axons of neurons in the dorsal column nuclei decussate and ascend in the medial lemniscus and terminate in the thalamus.
The axons of the medial lemniscus synapse in the ventral posterior lateral nucleus which projects to the primary somatic sensory cortex, via the posterior limb of the internal capsule. The secondary somatic sensory cortex and posterior parietal cortex receive input from the primary somatic sensory cortex. Each of these cortical areas is somatotopically organized.
Inputs from thalamus arrive at layer IV of the cortex. Efferent projections from the somatic sensory cortical areas arise from specific cortical layers. Corticocortical association connections (with other cortical areas on the same side of the cerebral cortex) are made by neurons in layers II and III. Callosal connections (with the other side of the cerebral cortex are also made by neurons in layers II and III.) Descending projections to the striatum, brain stem, and spinal cord originate from neurons located in layer V, whereas the projection to the thalamus originates from neurons located in layer VI.
Dermatomes retain their somatotopic organization in the dorsal columns. The first synapse, the gracile nucleus (hindlimb representation) or cuneate nucleus (forelimb representation) or trigeminal nucleus (facial sensory representation). formation of internal arcuate fibers, their decussation and the origin of the medial lemniscus. Maps do not simply recapitulate the body surface but integrate input and expand or compress representations via convergence of receptive fields. The elementary structure of center-surround receptive field organization. The second synapse is the projection of medial lemniscal axons to the thalamus (ventral posterior nucleus). The third synapse is the projection of thalamic neurons to the somatic sensory cortex in the postcentral gyrus which transformations and distortions of the body map in cortex reduce some representations and expand others. The fourth synapse is corticocortical projections from the postcentral somatic sensory cortex to the pre-central motor cortex.
The dorsal column system is a neural pathway for conscious appreciation of fine touch, pressure and muscle proprioception.
DCML synapses
1. Gracile, Cuneate or Trigeminal Nucleus
2. Ventral Posterior Lateral Nucleus of the Thalamus
3. Postcentral Gyrus of Somatosensory Cortex
4. Cortical Processing between Somatosensory and Pre-Motor Cortex
1. Gracile, Cuneate or Trigeminal Nucleus
2. Ventral Posterior Lateral Nucleus of the Thalamus
3. Postcentral Gyrus of Somatosensory Cortex
4. Cortical Processing between Somatosensory and Pre-Motor Cortex
Responses of neurons along the ascending pathway reveals progressive convergence of input and a gradual increase in receptive field size. The basis for the descending control of pain and temperature perception. The brain stem systems can regulate the excitability of C fibers. The central gray in the midbrain projects to the raphe nucleus in the pons. Raphe-spinal projections descend through the spinal cord near the zone of Lissauer. The nature of nociceptive reflexes after loss of supraspinal modulating systems is withdrawal is spasmodic, poorly coordinated and involuntary. Flexor reflex afferent pathways in the spinal cord can modulate crude withdrawal from nociceptive stimulation even after severe cortical damage or spinal trauma.
The main ascending afferent input is from the spinal cord.
The main ascending afferent input is from the spinal cord.