General Sensory Pathways of the Trunk and Limbs

Lecture Objectives • Describe gracile and cuneate tracts and pathways for conscious proprioception, touch, pressure and vibration from the limbs and trunk. • Describe dorsal and ventral spinocerebellar tracts and pathways for unconscious proprioception from the limbs and trunk. • Describe lateral spinothalamic tract and pathways for pain and temperature from the limbs and trunk. • Describe ventral spinothalamic tract and pathways for simple touch from the limbs and trunk.

Ascending Tracts

Dorsal Column Medial Lemniscal System • Functions • Touch • Discriminative touch (calipers) • Fine touch (cotton ball) • Vibration (tuning fork) • Conscious proprioception (with eyes closed, patient reports position of limbs as they are moved by examiner)

Dorsal Column Medial Lemniscal System • Neural components • Receptors – encapsulated receptors & hair shafts • 1 st order neuron • Cell body – DRG • Central Axon • Lower body – fasciculus gracilis • Upper body (above T 6) – fasciculus cuneatus

Dorsal Column Medial Lemniscal System • Neural components • 2 nd order neuron • Cell body – Posterior column nuclei (gracilis & cuneatus) • Axons • Decussate – internal arcuate fibers • Ascend – medial lemniscus

Dorsal Column Medial Lemniscal System • 3 rd order neuron • Cell body – ventral posterolateral nucleus of the thalamus (VPL) • Axon • Internal capsule (posterior limb) • Corona radiata • Somatosensory cortex – Postcentral gyrus • Lesions • In the posterior column? • Above the decussation?

Anterolateral (spinothalamic) System • Function Free nerve ending • Pain • Aδ fibers (small myelinated) • Fast pain (sharp, will localized stabbing pain) • C fibers (unmyelinated) • Slow pain (dull aching or burning pain) • Via spinoreticular tract • Temperature • Crude touch • Poorly localized & poorly identified • DO NOT compensate damage to dorsal column

Anterolateral System • Neural components • 1 st • Cell body – DRG • Axon • 2 nd • Branches ascend & descend in the Lissauer’s tract for 1‐ 2 segments • Cell body – posterior horn (substantia gelatinosa) • Axons – cross midline at anterior white commissure • 3 rd – VPL – somatosensory cortex

Somatotopic Organization of Anterolateral System At Upper Cervical Level • Sensory modalities • Anterior – crude touch • Lateral • Medial – temperature • Lateral – pain • Area • Lower limb – most lateral • Cervical – most medial

Anterolateral System • Lesions • Segment sparing (lesion at T 1 – deficit up to T 2 or T 3 dermatomes)? • Partial lesion – effect of somatotopic organization? • Lesion at anterior white commissure?

Spinoreticular (Spinoreticulothalamic) Tract • Slow pain • Forms part of the spinothalamic tract • 1 st order – DRG (C fibers) • 2 nd order – substantia gelatinosa • Project to reticular formation Reticular formation (bilaterally) Thalamus (inralaminar nuclei) Cortex • postcentral gyrus – localization of pain • insula & anterior cingulate gyrus –affective (suffering) aspect of pain

Spinocerebellar Pathways • Function • Non‐conscious proprioception • Essential for normal motor function • Lesions lead to severe motor deficits • Ataxia (uncoordinated movements) • Origin – muscle spindles, golgi tendon organs & joint receptors • All terminate in the cerebellum at the same side

Posterior Spinocerebellar Tract From trunk and leg • 1 st – DRG • 2 nd – • Cell body ‐ Clarke’s nucleus C 8‐ L 2 • Below L 2 – ascend in the fasciculus gracilis until Clarke’s nucleus • Axons – ascend in the same side • Inferior cerebellar peduncle

The Cuneocerebellar Tract From the arm & neck • 1 st • Cell body – DRG • Axon – ascend in the fasciculus cuneatus • 2 nd • Cell body – external (lateral or accessory) cuneate nucleus • Axons – inferior cerebellar peduncle

Anterior (Ventral) Spinocerebellar Tract • 2 nd • Cell body – around the border of the ventral horn • axons – mostly cross the midline • Superior cerebellar peduncle • Then cross back through middle cerebellar peduncle Rostral Spinocerebellar Tract • Same as ventral spinocerebellar tract except • From cranial region

Other Ascending Tracts • Spinoreticular tract (data affecting consciousness) • Mostly uncrossed • To reticular formation in medulla and pons • Spinotectal tract • Crossed • To superior colliculus • Affect spinovisual reflexes • Spinoolivary tract • Cross the midline • To the inferior olivary nuclei • Then cross to the cerebellum • Inferior cerebellar peduncle

Sensory Lesions • In spinal cord • Anterior white commissure – loss of pain & temperature sensation bilaterally (ring of body) • Hemisection – contralateral loss of pain and temperature & ipsilateral loss of discriminative touch • In the medulla • Medial lesions – loss of discriminative touch for the contralateral body • Lateral lesions – loss of pain & temperature for contralateral body • In the pons and above • All sensory modalities travel together • Small lesions – hemianasthesia for the contralateral half

Sensory Lesions • Pain & the thalamus • Ventral posterior thalamus • Period (months) of analgesia followed by chronic pain (thalamic pain syndrome) • Pain & the cortex • Somatosensory cortex • Reduce ability to localize pain but does not eliminate the ability to feel pain

Phantom Limb • Results from loss of a limb • Patient feels that the limb is still present • Cortical representation stay intact for a period of time? • Limb remain associated with the mental image • Amputation could be followed by severe pain in the site of the limb • Due pressure on the nerve stumps