highlight the menu items related to the particular content type
You can use theme_menu_link function.
Add this code to template.php of your theme

function themename_menu_link(array $variables) {
  $path_parts = array();
  $element  = $variables['element'];
  $sub_menu = '';
  $menuName = $variables['element']["#original_link"]["menu_name"];
  //Check menu name
  if($menuName == 'main-menu')
  {
    $path_parts = explode('/',$element['#href']);
    if($path_parts[0]=='node' &amp;&amp; is_numeric($path_parts[1]))
    {
      $menu_load_node = node_load($path_parts[1]);
      //Check node type (In our case 'article')
      if($menu_load_node-&gt;type == 'article')
      {
        $element['#attributes']['class'][] = 'highlight';
      }
    }
  }
  if ($element['#below']) {
    $sub_menu = drupal_render($element['#below']);
  }
  $output = l($element['#title'], $element['#href'], $element['#localized_options']);
  return '&lt;li' . drupal_attributes($element['#attributes']) . '&gt;' . $output . $sub_menu . "&lt;/li&gt;\n";
}


All links to nodes with type 'article' will have class highlight