All of the following regarding spinothalamic tract characteristics are true EXCEPT:

Explanation

## Spinothalamic Tract: Anatomy and Clinical Correlation **Key Point:** The spinothalamic tract is the primary pathway for pain and temperature sensation, and its decussation pattern is crucial for understanding crossed sensory loss syndromes. ### Three-Neurone Sensory Arc | Neurone | Location | Function | | --- | --- | --- | | **1st order** | Dorsal root ganglion (pseudounipolar) | Peripheral receptor → dorsal horn | | **2nd order** | Dorsal horn (substantia gelatinosa & nucleus proprius) | Decussates within 1–2 segments; ascends contralaterally | | **3rd order** | Thalamus (VPL nucleus) | Thalamus → primary sensory cortex | ### Anatomical Features **High-Yield:** The spinothalamic tract **decussates early** (within 1–2 segments of entry), meaning: - Fibres entering at L5 cross by L4–L3 - Fibres entering at C6 cross by C5–C4 - This creates a **contralateral sensory loss pattern** ### Location in Spinal Cord ```mermaid flowchart TD A["Spinothalamic Tract"]:::outcome --> B["Ventral Spinothalamic"]:::action A --> C["Lateral Spinothalamic"]:::action B --> D["Crude touch, pressure<br/>(ventral funiculus)"]:::outcome C --> E["Pain & temperature<br/>(lateral funiculus)"]:::outcome E --> F["Contralateral loss<br/>due to early decussation"]:::action ``` **Clinical Pearl:** In **Brown-Séquard syndrome** (hemisection of spinal cord): - **Ipsilateral** loss of motor function and proprioception (dorsal column & corticospinal tract are uncrossed at that level) - **Contralateral** loss of pain and temperature (spinothalamic tract has already crossed 1–2 segments above) ### Why the Correct Answer Is Wrong **Option 3 states:** "Lesions produce **ipsilateral** loss of pain and temperature sensation" **This is INCORRECT** because: - The spinothalamic tract decussates within 1–2 segments of entry - A lesion of the spinothalamic tract produces **contralateral** loss of pain and temperature - Example: A right-sided spinal cord lesion at C5 affecting the lateral spinothalamic tract → **left-sided** pain and temperature loss below C5 ### Why Each Correct Statement Is Valid | Statement | Validity | Explanation | | --- | --- | --- | | 1st-order neurones in dorsal root ganglia | ✓ Correct | Pseudounipolar cells with peripheral and central processes | | Decussation within 1–2 segments | ✓ Correct | Occurs in dorsal horn; creates contralateral pattern | | Located in ventral & lateral funiculi | ✓ Correct | Ventral spinothalamic (crude touch); lateral spinothalamic (pain/temp) | | Ipsilateral loss of pain/temperature | ✗ **WRONG** | Loss is **contralateral** due to early decussation | **Mnemonic:** **"DCCC"** = **Dorsal** columns are **Ipsilateral** (Uncrossed); **Spinothalamic** is **Contralateral** (Crossed early). Remember: **"Dorsal = Same side; Spinothalamic = Opposite side."**