The facial nerve receives impulses from multiple brain areas. Lower face muscles are represented more fully in the motor cortex than the upper face, allowing for more voluntary and learned control of the lower face; this provides the fine controls of that facial region required for speech articulation. The amount of bilateral v. contralateral fibers to the facial muscles differs depending on region, with the lower face being primarily contralateral and bilateral fibers increasing in the upper face .
Note that there are large individual differences in this regard, and that involuntary expressions for the most part provide bilateral activation. Voluntary and involuntary expressions are under the control of different neural tracts, with voluntary expressions controlled by impulses from the motor strip through the pyramidal tract, and involuntary expressions controlled by impulses from subcortical areas through the extrapyramidal tract. The activation of facial movements that have become habitual, although acquired voluntarily, might resemble involuntary activation, but no research on this has been done by us yet.
Once innervated, the face is intricate and differentiated, making it one of the most complex signal systems available to humans. It includes over 40 structurally and functionally anatomically independent muscles, each of which can innervate independently of each other. The facial musculature is fairly unique. They include the only somatic muscles in the body attached on one side to bone and the other to skin; this facial movements are specialized for expression. The face is also one of the few places in the body where some muscles are not attached to any bone at all (e.g., orbicularis oculi, the muscle surrounding the eyes; orbicularis oris, the muscle in the lips).
Finally, there isn’t a one to one correspondence between structure and function in some facial muscles. The corrugator muscle group, for instance, which brings the brows down and together, is comprised of three muscles that usually act together when innervated. (Note it is possible to activate just the muscle that lowers the brows without drawing them together, although it is not common.) Although the frontalis muscle is a single muscle that spans the forehead, the inner and outer parts of this muscle can move independently of each other, allowing for just the inner or outer corners of the eyebrows to rise (or the entire brow if both portions are innervated).
Because this requires separate neural supplies to these two strands of frontalis, from a functional viewpoint this should be regarded as two not one muscle. Thus when decomposing facial behaviors, it is important to understand them from the perspective of functional, not structural anatomy (i.e., how the muscles function in a living, not dead, individual). Moreover, each of the functional muscle units of the face can be innervated with different timing, intensity, and laterality characteristics. These characteristics produce the ability to create thousands of different expressions in which our deception experts can read.