Brainstem Nuclei and Tracts Related to Facial Expression Top: Ventral view of the brainstem exterior; Center: Same view, but with tissue peeled back to show the interal positions of the cranial nuclei and nerve tracts; Bottom: Lateral view of the brainstem with tissues removed to show the interna structure of the nuclei and tracts, in particular the loop the facial nerve makes around the nucleus of the abducens nucleus (the genu of the facial nerve) before it exits the central nervous system.
The brainstem is the lowest part of the brain and it is just above the end of the spinal cord.
If you put your fingers at the back of your neck at the base of your skull, the brainstem begins
right about there and goes towards the top of your head a few inches, between your ears.
The top diagram above shows the optic nerve at the level of the eyes in the head. The more anterior (higher) parts
of your brain are folded over parts of the brainstem.
The brainstem has a number of neural centers called
nuclei that manage many automatic aspects of physiology and behavior.
Some nuclei, called "motor nuclei"
control the action of muscles; other nuclei, called "sensory nuclei," monitor the information from
receptors, such as pain, taste, and olfaction.
Bundles of nerve fibers called "neural tracts" run to and from various places in the brain and from the brainstem
to and from peripheral nerves and sensors.
The tracts that run from the brainstem to the body's periphery are called cranial nerves.
This name distinguishes them from many other neural fibers that have similar functions, except that
they run from the spinal cord to the periphery, and are called "spinal nerves."
(Cranial nerves differ from spinal nerves in other ways unimportant for this discussion.) Motor tracts connect motor nuclei to the muscles that each motor nucleus controls.
A sensory tract connects sensory nuclei to sensory receptors.
Each cranial nerve is numbered in Roman numerals, e.g., CN III.
These nuclei and tracts are bilateral.
They control or process signals from the same side of the face as their own location. Thus, damage to the right facial nucleus or nerve affects the right side of the face. Higher motor neurons coming into the facial nucleus may be crossed or uncrossed.
The most important motor nuclei for facial expression are the facial nucleus, or nucleus of CN VII, and the motor
nucleus of the trigeminal nerve, or the nucleus of CN V.
The facial nerve, or CN VII, connects the facial nucleus to most of the important muscles of facial expression. The trigeminal nerve, or CN V, is a large, complicated nerve that consists mostly of sensory fibers, but also has some motor fibers, and ends in the trigeminal root. Use the other diagrams to see where these tracts course through the head to their peripheral endings. Signals of cutaneous and many other sensory aspects of the face and head are tranported by the trigeminal fibers. Some muscles of the face and head that contribute to facial appearances are innervated by other cranial nuclei and tracts.