JMS Adapter Question

We have our scenario MQ > XI > R/3.

everything is working fine, but i need to understand how XI picks up msgs from MQ. i know we define the channel name for MQ in the JMS adapter, but physically how is the msgs getting picked up from MQ.

Example: in the File adapter, there is a polling parameter which looks at a specified file system and picks a file when it comes there...

Can anyone help on this?

thanks

tony