Who reports to the Managing Director?

It would depend on the size of the company.

Normally, in a large company its other Directors like the Sales or Finance Directors etc. who reports to the MD.

Managers (e.g. sales manager, Accounts Manager) report to their own department director.

In a smaller organisation where the only director is the MD (and usually the owner of the business) then it would be the Managers.

Also, the PA to the MD or Personal Secretary would also report directly to the MD.