From the Super Mario Wiki
The Nerve Cluster is an area of Bowser's body in Mario & Luigi: Bowser's Inside Story. This area is located in his ulnar nerve. It is accessible to Mario and Luigi when Bowser finishes eating Wiggler's giga-carrot. Mario and Luigi come in here to find a pain in Bowser caused by Durmite and her Biffidus helpers. Once Mario and Luigi defeat Durmite and the Biffidus, they enter another room with more Biffidus, and find Toadbert. After finding Toadbert Mario and Luigi hammer the nerve which drops a wall blocking the exit and allows Bowser to use the sliding punch. Toadbert and Starlow then tell Bowser how to use the sliding punch.
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