In which neuron lesion there is loss of Pain and temperature but touch is not affected
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Here is a complete, textbook-backed answer:
Loss of Pain & Temperature with Preserved Touch - Spinothalamic Tract (2nd Order Neuron) Lesion
The answer is a lesion of the lateral spinothalamic tract - specifically affecting the second-order neuron of the pain/temperature pathway, while sparing the dorsal column (touch/proprioception) pathway.
Why This Happens: The Two Separate Sensory Pathways
There are two anatomically distinct ascending sensory systems, and this dissociation is possible because they travel in completely separate locations:
| Feature | Pain & Temperature | Touch, Vibration, Proprioception |
|---|---|---|
| Pathway | Lateral spinothalamic tract | Dorsal column - medial lemniscal |
| 1st order neuron | Dorsal root ganglion (DRG) | Dorsal root ganglion (DRG) |
| Fiber type | A-delta (thin myelinated) & C (unmyelinated) | A-alpha/beta (large myelinated) |
| Where it synapses | Dorsal horn (Rexed laminae I, II, V) | Nucleus gracilis / cuneatus (medulla) |
| Where it crosses | At the anterior spinal commissure, within 1-2 segments of entry | In the medulla (much higher up) |
| Tract location | Contralateral anterolateral funiculus | Ipsilateral dorsal columns |
- Localization in Clinical Neurology, 8e, Anatomy of the Sensory System
- Ganong's Review of Medical Physiology, 26th Ed, Somatosensory Pathways
The Key Lesion: Lateral Spinothalamic Tract
A selective lesion of the lateral spinothalamic tract (which carries pain and temperature ONLY after crossing) will produce:
- Loss of pain and temperature - contralateral to the lesion (below the level)
- Preserved touch - because the dorsal columns are intact
This dissociation is called "dissociated sensory loss."
Classic Clinical Scenarios
1. Syringomyelia (Most Classic Example)
- A fluid-filled cavity (syrinx) expands in the central spinal cord
- It destroys the anterior white commissure where pain/temperature fibers cross
- Result: Bilateral cape-like loss of pain & temperature at the level of the lesion, with touch preserved (dorsal columns unaffected)
- Classic: "suspended" sensory level - loss at the level, normal above and below
2. Brown-Séquard Syndrome (Hemisection of Spinal Cord)
- One side of the spinal cord is damaged
- Ipsilateral side: loss of touch, proprioception (dorsal column) + motor loss
- Contralateral side: loss of pain and temperature (spinothalamic) - touch spared
- This is the textbook "crossed" dissociated sensory loss
3. Anterior Spinal Artery Occlusion
- Infarcts the anterior 2/3 of the cord (spinothalamic + corticospinal)
- Spares the posterior columns (dorsal columns survive)
- Result: Loss of pain/temperature + motor paralysis, with preserved touch/vibration/proprioception
From Frameworks for Internal Medicine: "There is preservation of fine touch, vibration, and proprioception (carried by the dorsal columns)" in spinothalamic tract lesions.
Summary: Which Neuron?
| Level of Lesion | Effect |
|---|---|
| 1st order neuron (DRG or peripheral nerve) | All sensory modalities lost together (no dissociation) |
| 2nd order neuron (spinothalamic tract, after crossing) | Pain & temperature lost, touch SPARED - dissociated loss |
| Dorsal column lesion | Touch/vibration/proprioception lost, pain/temperature spared (opposite dissociation) |
So the answer is: A lesion of the lateral spinothalamic tract (2nd order neuron / central pathway for pain and temperature), as seen classically in syringomyelia.
Diagram: Dorsal column (tactile, vibratory, proprioceptive) pathway ascending ipsilaterally to the medulla before crossing. This is why a spinothalamic lesion can selectively knock out pain/temperature while leaving touch intact.
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