Laravel 5.2，Eloquent＆Infinite Descendants

Question

I have a model Category which has a manyToMany relation with itself:

public function parentcategories()
{
    return $this->belongsToMany('App\Category', 'category_category', 'parent_category_id', 'category_id');
}

public function categories()
{
    return $this->belongsToMany('App\Category', 'category_category', 'category_id', 'parent_category_id');
}

What is the simplest way to get all of the descendant Categories?

For example:

Cat1
-Cat2
-Cat3
--Cat4
--Cat5
---Cat6
Cat7
-Cat8

If I click Cat1 I want to get Cat2-6, and if I click Cat3 I want to get Cat4-6.