Dorsal columnmedial lemniscal system

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The dorsal column-medial lemniscal system carries sensory information for discriminative touch, joint position (kinesthetic or conscious proprioceptive) sense, vibratory, and pressure sensations from the trunk and limbs (Figures IV-4-7 and IV-4-8). The primary afferent neurons in this system have their cell bodies in the dorsal root ganglia, enter the cord via class II or A-beta dorsal root fibers, and then coalesce in the fasciculus gracilis or fasciculus cuneatus in the dorsal funiculus of the spinal cord. The fasciculus gracilis, found at all spinal cord levels, is situated closest to the midline and carries input from the lower extremities and lower trunk. The fasciculus cuneatus, found only at upper thoracic and cervical spinal cord levels, is lateral to the fasciculus gracilis and carries input from the upper extremities and upper trunk. These two fasciculi form the dorsal columns of the spinal cord that carry the central processes of dorsal root ganglion cells and ascend the length of the spinal cord to reach their second neurons in the lower part of the medulla.

In the lower part of the medulla, fibers in the fasciculus gracilis and fasciculus cuneatus synapse with the second neurons found in the nucleus gracilis and nucleus cuneatus, respecdvely. Cells in these medullary nuclei give rise to fibers that cross the midline as internal arcuate fibers and ascend through the brain stem in the medial lemniscus. Fibers of the medial lemniscus terminate on cells of the ventral posterolateral (VPL) nucleus of the thalamus. From the VPL nucleus, thalamocortical fibers project to the primary somesthetic (somatosensory) area of the postcentral gyrus, located in the most anterior portion of the parietal lobe.

Cerebral cortex

Thalamus

Ventropostero-lateral nucleus (VPL)

Medulla

Function:

Conscious proprioception, fine touch, vibration, pressure, two point discrimination

Function:

Conscious proprioception, fine touch, vibration, pressure, two point discrimination

Lesion:

Loss of above senses

Site of lesion: Affected side of body

A, B, and C: Contralateral

D: Ipsilateral

Dorsal columns

Cerebral cortex

Thalamus

Ventropostero-lateral nucleus (VPL)

Medulla

Brain stem

Spinal cord

Dorsal root ganglion cell (DRG)

-< Receptor

(Pacinian corpuscle; Meissner corpuscle)

Figure IV-4-7. Dorsal Column Pathway-Medial Lemniscal System

Clinical Correlate

Lesions of the dorsal columns result in a loss of joint position sensation, vibratory and pressure sensations, and two-point discrimination. There is loss of the ability to identify the characteristics of an object called astereognosis (e.g., size, consistency, form, shape), using only the sense of touch. Typically, dorsal column-medial lemniscal lesions are evaluated by testing vibratory sense using a 128-Hz tuning fork. Romberg sign is also used to distinguish between lesions of the dorsal columns and the midline (vermal area) of the cerebellum.

Romberg sign is tested by asking the patients to place their feet together. If there is a marked deterioration of posture (if the patient sways) with the eyes closed, this is a positive Romberg sign, suggesting that the lesion is in the dorsal columns (or dorsal roots of spinal nerves). With the eyes open, interruption of proprioceptive input carried by the dorsal columns can be compensated for by visual input to the cerebellum. Therefore, if the patient has balance problems and tends to sway with their eyes open, this is indicative of cerebellar damage.

Figure IV-4-8. Dorsal Column Pathway-Medial Lemniscal System

Upper pons

Upper medulla

Midbrain

Internal arcuate fibers

Lower medulla

Fasciculus gracilis Fasciculus cuneatus

Spinal cord

Neuron #3

Medial lemniscus

Medial lemniscus

Neuron #1

Medial lemniscus

Neuron #2

(in nucleus cuneatus)

Upper pons

Upper medulla

Neuron #3

Medial lemniscus

Medial lemniscus

Neuron #1

Receptor from upper body

Midbrain

Internal arcuate fibers

Lower medulla

Fasciculus gracilis Fasciculus cuneatus

Spinal cord

Medial lemniscus

Neuron #2

(in nucleus cuneatus)

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