In the previous chapter of "Bleach," Pernida revealed to Mayuri the former's true identity as the Left Arm of the Soul King while still being capable of retaining its own will and conscience. A preview of Chapter 639, courtesy of Kaleo, reveals how the mad scientist Mayuri counters Pernida's powerful nerve technique. 

During the fight, Mayuri's capabilities as a scientist proves useful as he is able to cut off one of Pernida's fingers. This proves to be a small triumph, however, as Pernida reveals that he can still control the finger even when it has been sliced off. In an instant, he triggers his nerve technique. 

One issue that Kaleo points out is that the nerve technique is meant to manipulate inanimate objects so how it can alter and kill Mayuri, a living being, is in question. Despite this, Pernida's technique proves effective as it begins to invade Mayuri's body. 

However, at the last moment, Mayuri reveals that he is fine. His wounds have been stitched. The mad scientist reveals that he has the ability to "rearrange" his nerves, rendering Pernida's technique useless. However, in doing so, Mayuri also exhausts himself to the point where Pernida might be able to finish him off if the battle continues in chapter 640. 

Chapter 639 ends with Pernida checking Captain Mayuri's condition, surveying if the Shinigami is weak enough for him to defeat. He also appears hesitant, carefully checking if Mayuri still has other tricks up his sleeves. 

Given how Mayuri is able to rearrange his nerves at the last minute, as well as pull off an "explosive armor" technique earlier in the battle, it is still very possible that the mad scientist might make it out alive before Pernida is able to take advantage of the situation. 

Mayuri also shows he is waiting for his allies, particularly Hantaro Yamada who has healing capabilities. If they are able to regain their full energy, then they may be able to defeat Pernida in chapter 640.