I have a store that sends a parameter to my controller. When I define my store

Code:
    proxy: {
                type: 'ajax',
                scope: this,
                url: 'My/GetData',
                extraParams: {
                    parametervalue: Ext.getCmp('Combo').getValue() //something similar
                   }
          }
I am not updating the store on combo selection. Or else I could have just added a listener on combo and updated the store.
I want to get the value of the combobox when the grid is loaded. It also works with paging, so when the user hits next page the store loads the second page "with" the selected value in the combobox, which it passes to my controller.