From the Super Mario Wiki
A Napse is a neuron-like enemy inside Bowser's body in Mario & Luigi: Bowser's Inside Story. They are located in the Energy Hold, and look like round red cells ringed by moving dendrites. Their name comes from "synapse", which is the junction between nerve cells.
A Napse will attack Mario or Luigi using electrical discharges that are colored either red, or green. One of its attacks includes summoning two energy balls colored red and green. It will then spin around turning both the red and green balls into blue ones. The energy balls will then hit the respective brother, depending on its former color, so the player needs to keep an eye on the order the Napse has the energy balls. This attack is avoided by jumping. Another one of its attacks includes getting red and green energy balls from other Napses around it. Once it has obtained enough, it will shoot the energy balls at the bros. This attack is avoided by jumping. This enemy can also multiply, but not as often as the Air Napse. However, when it multiplies, the Napse loses 1/2 of its remaining health and its duplicate gets the other 1/2 of its remaining health. Napses take extra damage from fire.
The idle form of the Bowser Memory ML found in the Memory Banks bears some resemblance to a Napse, however it acts different and has a mouth as well as eyes.
Names in Other Languages
|From 「シナプス」 Shinapusu, meaning synapse.
||From molécula (molecule).
||Same as English name
||Pun on synapse