3 - Somatosensory System: Dorsal Column/Medial Lemniscus Flashcards
(32 cards)
What types of sensations does the dorsal column/medical lemniscus system transmit?
- Light touch: 2 pt disc., stereognosis (depth), graphesthesia (writing on skin).
- Pressure
- Vibration
- Proprioception/limb position
- Kinesthesia (motion sense)
- Tactile extinction - paying attention to a stimulus on one side and not the other
What are the components of the DC/ML system? How many neurons are there?
1st neuron: in sensory ganglion (afferent)
2nd neuron: in brainstem nuclei (or spinal cord) - decussates
3rd neuron: in thalamus
Describe the location and projection of the 1st neuron of the DC/ML system? What is it’s function?
Primary afferent neuron with peripheral process that transmits info from mechanoreceptors.
Cell body in DRG, central process transmits info into dorsal root and ascends within ipsilateral dorsal columns.
Terminates in dorsal column nuclei.
Describe the location and projection of the 2nd neuron of the DC/ML system?
Cell body in nucleus gracilis or nucleus cuneatus.
Axon decussates to contralateral side and ascends as medial lemniscus.
Terminates in ventral posterior lateral (VPN) nucleus of thalamus.
Describe the location and projection of the 3rd neuron of the DC/ML system?
Cell body in the VPL of the thalamus.
Axons pass through the posterior limb of internal capsule.
Terminates in S1 cortex (postcentral gyrus, primary somatosensory cortex).
What are the two dorsal columns that the 1st neuron travels through? Where is each located?
Fasciculus gracilis: located medially; axons from sp cd levels caudal to T6. Lower limbs “graceful ballet dancer”.
Fasciculus cuneatus: laterally; axons from T6 and rostral. Upper limbs (“cunningly rubs hands”).
Where is each levels dermatome located in the 1st neuron of the spinal cord? What is the purpose of this placement?
Sacral dermatomes medial.
Lumbar, thoracic, and cervical dermatomes are located progressively more laterally.
Helps preserve info about location and nature of stimulus.
What does a unilateral spinal cord lesion of the 1st neuron result in?
Loss of light touch, pressure, vibration, and proprioception on the SAME SIDE of the lesion from dermatomes just below the level of the lesion.
Loss of other sensory and motor functions.
What does a bilateral spinal cord lesion of the 1st neuron result in?
Loss of light touch, pressure, vibration, and proprioception from dermatomes just below the level of the lesion.
Loss of other sensory and motor functions.
What is posterior cord syndrome of the first neuron?
Loss of light touch, pressure, vibration, and proprioception from dermatomes below the level of the lesion (whole body except head).
Other sensory and motor functions intact.
What is the result of a large central cord lesion of the 1st neuron?
Loss of light touch, pressure, vibration, and proprioception from dermatomes below the level of the lesion.
*Sacral region spared because those regions are pushed more laterally to the back of the cord.
Loss of other sensory and motor functions.
What is the path of the 2nd neuron of the DC/ML system?
Cell bodies in nucleus gracilis and nucleus cuneatus of the caudal medulla.
Axons decussate as internal arcuate fibers.
Axons from medial lemniscus through rostral medulla, pons, and midbrain.
Terminate in the VPL.
What is the somatotopy of the 2nd neuron at the level of the rostral medulla and the pons?
Rostral medulla level: “headless hemi-man standing on the pyramids”
Pons level: fibers from face are added via trigeminal nerve.
What does a lesion of the medial lemniscus of the 2nd neuron result in? What is this called?
Loss of light touch, pressure, vibration, and proprioception from dermatomes below the level of the lesion on the CONTRALATERAL side of the body.
Medical medullary syndrome.
Where is the cell body of the 3rd neuron in the DC/ML system located? Where do axons travel and terminate?
Cell body in the VPL of thalamus.
Axons pass through posterior limb of internal capsule.
Terminate in S1 cortex (postcentral gyrus, primary somatosensory cortex).
What are the functions of the VPL and VPM of the thalamus?
VPL: sensation from the body
VPM: sensation from the face
What is the path of the third neuron. What is the somatotopy in this region?
Third order neuron cell bodies in the thalamus send axons to form the internal capsule and make contact with the cerebral cortex.
VDL concerned with sensation from the body.
Also a few fibers from face in VDM.
What do the axons of the 3rd neuron pass through? What does this tell us about injuries to this region?
The posterior limb of the internal capsule.
More distal lesions in the radiating fibers have less of an impact because the fibers are like holding a hand of cards; lower lesions have a greater impact and impact more fibers (“cards”).
What do the axons of the 3rd neuron fan out as? Where do fibers from VPL and VPM pass?
Fan out as corona radiata.
Fibers from VPL and VPM pass to the S1 cortex.
Sensation from which body parts are located more laterally in the S1 cortex? What body parts transmit sensation more medially?
Lower limb sensation occurs medially.
Torso, upper limb, and face have a huge representation in the more lateral regions of the cortex.
What results from lesions of the thalamus or S1 cortex?
Loss of sensation from CONTRALATERAL half of the body.
The S1 cortex contains which Broadmann areas? What is in each of these areas?
3a, 3b, 1, and 2
In each there’s a complete map of the skin.
What are Brodmann area 3a and 3b of the S1 cortex involved in?
3a - limb movement
3b - basic tactile info (edges, texture)
What are Brodmann area 1 and 2 of the S1 cortex involved in?
Area 1 - motion and direction of movement of objects
Area 2- limb position and shape of objects.
