We've already looked at the innervation of the facial muscles and in particular the consequences of upper motor neurone and lower motor neurone lesions of the facial nerve here. This article will focus on the overall functional anatomy of the muscles of facial expression, the article is not an exhaustive list of the muscles of facial expression.
In this post, I will be discussing the neuroanatomical differentiation between an upper motor neuron (UMN) and lower motor neuron (LMN) facial palsy. The two pathologies in the title could also be differentiated in other ways, for example, the speed of onset, additional symptoms and signs, etc.; however understanding the neuroanatomy in the context of the clinical picture, is the best way of avoiding making a grave error.
Yes, I know, what a catchy title! Trust me this post says what it does on the tin. I came across these rules when I was a medical student (the days when using a tablet meant chiselling into stone) from Robert Whitaker's book Instant Anatomy, a superb book which I thoroughly recommend. Essentially the innervation of the muscles of the head & neck can be broadly covered by six rules;