extend MY_Controller

[eluser]Zawardo[/eluser]
I created a MY_Controller class, putted into core folder and it's working fine.
Now i want to extend another time the controller, let's say MY_Second_controller.
If i put this class into core folder and declare it as class MY_Second_controller. extends MY_Controller {.....
when i try to use it with an application controller (like controller.php declared as class controller extends MY_Second_controller {...) i get an error of MY_Second_controller not found.
I have inserted into config-php a piece of code to autoload class from core folder and it's working.

My question is: why i had to add this autoload function? everything prefixed with MY_ should be autoloaded by default or do i have misunderstood anything?