Ascending Tract Physiology - Complete Notes

Overview

Key point: Because touch is partially carried by both pathways, isolated lesions of either one do not fully abolish touch.

1. Sensory Neuron Fiber Types

• Large-diameter myelinated fibers (A-α, A-β) enter the medial dorsal root entry zone → posterior columns
• Small-diameter and unmyelinated fibers (A-δ, C) enter the lateral dorsal root entry zone → synapse immediately in dorsal horn gray matter

2. Posterior Column - Medial Lemniscal (PCML) Pathway

Step-by-Step Course

1. 1st-order neuron - DRG cell body; peripheral receptor (Pacinian corpuscle for vibration, muscle spindles/Golgi tendon organs for proprioception). Axon enters spinal cord via medial dorsal root entry zone.
2. Ascent in posterior (dorsal) columns - Fiber travels ipsilaterally all the way to the lower medulla without synapsing.
   • Fasciculus gracilis (medial): lower trunk and leg fibers; from below ~T6
   • Fasciculus cuneatus (lateral): upper trunk, arm, neck fibers; from above ~T6
   • Somatotopic rule: fibers add on laterally as you ascend - so legs are most medial in the posterior columns.
3. 2nd-order neuron - Synapse in the posterior column nuclei of the lower medulla:
   • Fasciculus gracilis → Nucleus gracilis
   • Fasciculus cuneatus → Nucleus cuneatus
4. Decussation - Axons of 2nd-order neurons cross as internal arcuate fibers in the lower medulla.
5. Medial lemniscus - After crossing, fibers form the medial lemniscus on the contralateral side. Somatotopic orientation shifts as it ascends:
   • Medulla: vertical orientation; legs represented ventrally ("the little person stands up")
   • Pons/midbrain: inclined position; legs move laterally, arms medially ("the little person lies down")
6. Thalamic relay - Medial lemniscus terminates in the Ventral Posterior Lateral (VPL) nucleus of the thalamus.
7. 3rd-order neuron - Thalamic radiations project to primary somatosensory cortex (postcentral gyrus, Brodmann areas 3, 1, 2).

3. Anterolateral Pathways

Step-by-Step Course

1. 1st-order neuron - Small-diameter A-δ and C fibers enter via the lateral dorsal root entry zone. Some axon collaterals ascend/descend 1-2 segments in Lissauer's tract before synapsing.
2. Synapse in dorsal horn - 1st-order neurons synapse onto 2nd-order neurons mainly in:
   • Lamina I (dorsal horn marginal zone)
   • Lamina V (deeper dorsal horn)
3. Decussation in spinal cord - 2nd-order axons cross via the anterior (ventral) commissure to the contralateral anterolateral white matter. Important: it takes 2-3 spinal segments for fibers to cross - so a lateral cord lesion causes contralateral pain/temperature loss a few segments below the lesion level.
4. Ascent in anterolateral white matter - Somatotopic organization:
   • Legs most lateral, arms more medial (fibers from anterior commissure add on medially as they ascend)
   • This lateral-to-medial arrangement is preserved throughout the brainstem
   • In the medulla: located laterally, in the groove between the inferior olives and inferior cerebellar peduncles
   • In pons/midbrain: just lateral to the medial lemniscus
5. Thalamic relay - Primary relay in VPL (separate neurons from PCML fibers). Additional projections to:
   • Intralaminar thalamic nuclei (central lateral nucleus)
   • Medial thalamic nuclei (mediodorsal nucleus)
6. 3rd-order neuron - From VPL → primary somatosensory cortex (postcentral gyrus). Medial thalamic projections → frontal and cingulate cortex (emotional aspects of pain).
7. Face pain - Carried by the analogous trigeminothalamic tract (not the spinothalamic tract).

The Three Anterolateral Tracts

• Lateral spinothalamic (neo) - VPL - discriminative pain quality
• Medial spinothalamic (paleo) - Medial thalamus - autonomic and unpleasant emotional pain perception
• Spinohypothalamic/spinotelencephalic - Hypothalamus; evoke emotional behavior
• Spinocervical tract - Lateral cervical nucleus → contralateral thalamus; major alternative pain pathway (ascends uncrossed)

4. Somatosensory Cortex

• Primary somatosensory cortex - Postcentral gyrus, Brodmann areas 3, 1, 2
• Somatotopic (homuncular) organization: face most laterally, leg most medially (same as motor cortex)
• Receives from VPL (body) and VPM (face via trigeminothalamic tract)
• Projects to secondary somatosensory cortex - in the Sylvian fissure/parietal operculum (also somatotopic)
• Further processing in superior parietal lobule association cortex (areas 5 and 7)

5. Central Modulation of Pain

Gate Control Theory

• A-β fiber (large, non-pain) activation in dorsal horn inhibits pain transmission
• Clinical application: TENS (transcutaneous electrical nerve stimulation) activates A-β fibers to reduce chronic pain; rubbing an injured area does the same

Descending Modulatory Pathways

• Periaqueductal gray (PAG) - Receives inputs from hypothalamus, amygdala, cortex → key "off switch" for pain
• PAG → Rostral Ventromedial Medulla (RVM) relay → spinal cord dorsal horn
• Two key descending neurotransmitters:
  • Serotonin (5-HT) - Raphe nuclei neurons project to dorsal horn
  • Norepinephrine (NE) - Locus ceruleus (stimulated by RVM via substance P) projects to dorsal horn

Endogenous Opioid System

• Opiate receptors distributed throughout - peripheral nerves, dorsal horn, PAG, RVM
• Enkephalin and dynorphin - concentrated in PAG, RVM, dorsal horn
• β-endorphin - concentrated in hypothalamic regions projecting to PAG
• Exogenous opioids (morphine) mimic these at all these sites

6. Key Clinical Correlations

Lesion Localization Rules

Syringomyelia

• Cavitation of the central cord damages fibers crossing in the anterior commissure → bilateral loss of pain and temperature at the segmental levels of the cavity ("cape distribution"), with preserved posterior column function ("dissociated sensory loss")

7. Quick Summary Table