Synthesize the information on sphingomyelin's structure to explain its role in nerve cell membranes.

Sphingomyelin's structure makes it a poor insulator, hindering signal transmission.

Sphingomyelin's structure makes it a poor conductor, hindering signal transmission.

Sphingomyelin's structure allows it to form a protective sheath around nerve cells, aiding in signal transmission.

Sphingomyelin's structure allows it to increase membrane fluidity, aiding in signal transmission.