Is there anybody who maintains the overall structure of the Internet?

I mean as time goes by, somebody has to modify and improve the Internet's capabilities. The Internet is a network, right? Then where is it's main server or mainframe computer?

There are several backbone servers that work as central request servers.. There use to be seven of them, I'm not sure how many there are now.There was a kid in Canada in 2001 that attacked a couple of the servers and brought the Internet worldwide to a crawl. These servers are not responsible for anything other then relaying requests from one backbone isp to another. Most smaller ISPs then purchase a branch from the backbone ISPs lines, and it spiders out further from there. As long as the technology that connects the networks is compatible there is no central group or organization that controls the development and technology of the Internet.

the Internet is a series of inter-connected networks ..that is, there is no central server or one 'controlled' network.

Any self respecting series of tubes must have a plumber!