Pure JavaScript AJAX Call In WordPress Custom Plugin Development

I am developing a Custom WordPress Plugin in which I have some HTML form on the main plugin page and a back page where I have some functions in PHP like to get information from the database etc. To explain in detail, here is the code...

Main Plugin File:

<?php
/*
Plugin Name: WP Testing Plugin
Plugin URI: http://www.wordpress.org/WP-Testing-Plugin
Description: A Detailed Description About This Plugin.
Author: Muhammad Hassan
Version: 0.1
Author URI: http://www.wordpress.org
*/

/*____________WP Testing Plugin Admin/Script_____________*/
function wp_testingPlugin_admin() {
    echo '<form id="searchForm" onsubmit="return searchData(this)">
            <input name="WhatToSearch" type="text" />
            <input type="submit" value="Search"/>
            <input type="reset" value="Reset"/>
            <div id="showReturnData"></div>
        </form>
    ';

    echo '<script type="text/javascript">
            function searchData(incomingForm) {
                // Confirmation To Add A Data
                var answer = confirm("Are You Sure Want To Search?");
                if (answer){
                    // If User Click Ok Then Execute The Below Code     
                    var FD = new FormData(incomingForm); // Get FORM Element Object
                    FD.append("Function", "SearchFunction"); // Adding Extra Data In FORM Element Object To Hit Only This Function In Ajax Call File
                    var ajx = new XMLHttpRequest();
                    ajx.onreadystatechange = function () {
                        if (ajx.readyState == 4 && ajx.status == 200) {
                            document.getElementById("showReturnData").innerHTML = ajx.responseText;             
                        }
                    };
                    ajx.open("POST", "'.plugin_dir_url( __FILE__ ).'my_functions.php", true);
                    ajx.send(FD);
                    document.getElementById("showReturnData").innerHTML = "<div class="error">ERROR: AJAX Did Not Respond.</div>";
                }
                return false; // For Not To Reload Page
            }
        </script>
    ';

}
/*__________________________________________________________________*/


/*____________WP Testing Plugin Option_____________*/
//Adding "WP Testing Plugin" Menu To WordPress -> Tools
function wp_testingPlugin() {
    //  add_management_page( $page_title, $menu_title, $capability, $menu_slug, $function);                  Menu Under Tools
    add_management_page("WP Testing Plugin By Hassan", "WP Testing Plugin", 'activate_plugins', "WP-Testing-Plugin", "wp_testingPlugin_admin");
}
add_action('admin_menu', 'wp_testingPlugin');
/*__________________________________________________________________*/
?>

And this is my_functions.php file.

<?php
/****************************************************************************/
//Garb The Function Parameter
/****************************************************************************/
$Function = $_POST['Function'];


/****************************************************************************/
// Run Search Function
/****************************************************************************/
if ($Function == "SearchFunction"){

    if(!isset($_POST['WhatToSearch'])){
        $WhatToSearch = "Nothing";
    } else {
        $WhatToSearch = $_POST['WhatToSearch'];
    }

    echo "<div class='success'>SUCCESS: Function Is Working Perfectly And Getting Data ".$WhatToSearch.".</div>";
}

/****************************************************************************/
// Run Another Function
/****************************************************************************/
if ($Function == "AnotherFunction"){
    echo "<div class='success'>SUCCESS: Another Function Is Working Perfectly.</div>";
}

?>

But my code is not working and not hitting my_functions.php file even. Whats the problem here? Need basic step only to work in this patteren.