Cannot acces the control in side the contentplaceholder

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C#

ASP.NET

Javascript

I've master page in that have contentplaceholder.

And derived the content pages from that,
on page load the div tag must be minimised and on click of the arrow symbol its div tag's contents should be visible.

this is the code i've written in content page.

<div  runat="server" class="accordion">
                <div  runat="server" class="accordionHeader">Resource Mangement 
                <a href="#"  önclick="ExpandOrCollapsePanel('ResMgmt');">
                <img id="imgNext" alt="Expand" src="Images/expand.GIF" align="right"/></a></div>
                    <div id="ResMgmt" name="ResMgmt"  runat="server" class="accordionContent" style="display:none">
                            <div class="accordionHeader"  runat="server">Add/Edit Users <a href="#"  önclick="ExpandOrCollapsePanel('addUsr');"><img id="img1" alt="Expand" src="Images/expand.GIF" align="right"/></a></div>
                                <div id="addUsr"  runat="server" class="accordionHeader" style="display:none"></div>
                            </div>
                    </div>

And on click of the image hyperlink a java script will be made run
the javascript code looks like,

function ExpandOrCollapsePanel(id, panelBar) {
    var panel = document.getElementById(id);
    if (!panelBar) {
        panelBar = event.srcElement;
    }

    if (panelBar) {
        while (typeof panelBar.onclick != "undefined"
               && String(panelBar.onclick).indexOf("ExpandOrCollapsePanel") == -1) {
            panelBar = panelBar.parentElement;
        }

        if (!panelBar.getElementsByTagName("div")[0]) {
            panelBar = panelBar.parentElement;
        }
        
        var isExpanded = panelBar.expanded ? panelBar.expanded : panelBar.Expanded;
        if (isExpanded == "True") {
            eval((panelBar.expanded ? "panelBar.expanded " : "panelBar.Expanded ") + " = \"False\"");
            if (panel) {
                panel.style.display = "none";
            }
        } else {
            eval((panelBar.expanded ? "panelBar.expanded " : "panelBar.Expanded ") + " = \"True\"");
            if (panel) {
                panel.style.display = "block";
            }
        }
    }
}

in this code i'm facing the problem that "panel" variable is showing 'null' on runtime when i try to debug the code.

since this control is inside the contentplaceholder its not accessible i think.

please suggest me how to access this div tag from javascript.

even i tried

var panel = document.getElementById('<%=ResMgmt.ClientID%>');

then also it is 'null'