How to enable a button when an item in the ComboBox is selected

Hi

I have an ajax ComboBox

<ajax:ComboBoxID="TicketTypeComboBox"runat="server"DropDownStyle="DropDown"AutoCompleteMode="SuggestAppend"RenderMode="Block"MaxLength="2"CssClass="PONComboBox"Width="20px"TabIndex="2"/>

When the form loads the ComboBox is populated from values in a database table. There is no default selection in the Combobox list of items i.e. the SelectedIndex = -1

I want to use JQuery to enable a button whenever an item is selected in the ComboBox.

Now, I understand that when an ajax ComboBox is rendered, it is rendered as a combination of three different controls - textbox, button and optionlist.

I am able to get to the button click of ComboBox but after that not able to figure out how to detect a change in ComboBox selection. 

Please help.

Thanks

Rajat