Question

After a right limb amputation, the patient is experiencing severe pain phantom limb.
What is the mechanism behind this?

• A. Projection of adjacent fibres to overlap to right sensory cortex
• B. Projection of adjacent fibres to overlap to left sensory cortex
• C. Expansion of right sensory cortex
• D. Expansion of left sensory cortex

Hint:

Phantom pain = adjacent cortical fibres overlapping the deafferented zone; and a right limb maps to the contralateral (left) sensory cortex.

Answer 1

The Correct Option is B

Step 1: Recall the cause of phantom limb pain. After amputation, the cortical area in the primary somatosensory cortex (S1) that used to receive input from the now-missing limb is deprived of sensory input. The brain undergoes cortical reorganisation (maladaptive plasticity): neighbouring cortical regions (e.g. those representing the face/adjacent body parts) send their adjacent afferent fibres to invade and overlap the deafferented limb area. Stimulation of those adjacent body parts is then mis-referred to the missing limb, producing phantom sensation/pain.

Step 2: Determine which hemisphere. The somatosensory pathways DECUSSATE (cross to the opposite side). The RIGHT limb projects to the LEFT (contralateral) primary somatosensory cortex. Therefore the reorganisation/overlap of adjacent fibres affecting the right-limb representation occurs in the LEFT sensory cortex.

Step 3: Why option B is correct. The mechanism is the projection/overlap of adjacent cortical fibres invading the deafferented zone, and because the body is wired contralaterally, the right limb maps to the LEFT sensory cortex - hence "projection of adjacent fibres to overlap to left sensory cortex."

Step 4: Why the distractors are wrong. (A) Right sensory cortex represents the LEFT limb, not the amputated right limb - wrong side. (C) and (D) describe "expansion" of the cortex: phantom limb pain is due to invasion/overlap by adjacent representations into the deprived area, not an expansion of the limb's own (now silent) cortex; furthermore (C) names the wrong (right) hemisphere. "Expansion" of the deafferented limb cortex is the opposite of what happens - that area shrinks and is taken over.

Final Answer: Option B - Projection of adjacent fibres to overlap to left sensory cortex.