Cortical Motor Center Connections to Facial Subnuclei and to Facial Muscle Groups

Five cortical areas have direct connections (corticobulbar) to the four facial subnuclei in the pontine tegmentum.7 These five cortical areas with corticobulbar fibers to the facial subnuclei are primary motor cortex (M1), supplementary motor cortex (M2), rostral cingulate motor cortex (M3), caudal cingulate motor cortex (M4), and ventral lateral premotor cortex (LPMCv). The dorsal lateral premotor cortex, originally thought to have major facial nucleus projects, has not been proven to be primarily important and has been excluded after further studies.1 All five cortical motor regions connect bilaterally to all facial subnuclei. However, they connect in different ways. Each of the four facial subnuclei selectively innervates different groups of facial muscles ( Figs. 16.1 and 16.2 ).1,7 The strength of corticonuclear projections are, in order, M1 and LPMCv, M2, M3, and M4.

The facial nuclei, the origin of the two facial nerves, are in the pontine tegmentum. Each facial nucleus consists of four subnuclei that are arranged in rostral-caudal columns. Two larger subnuclei are inferior (anterior), the lateral and medial subnuclei; two smaller subnuclei are located superiorly (posterior), the dorsal and intermediate subnuclei. The lateral subnucleus innervates the perioral muscles; neurons on the superior part innervate muscles of the upper lip and neurons in the inferior part muscles of the lower lip. M1, LPMCv, and M4 project predominantly to the contralateral lateral nucleus. Extremely few fibers from M1 go to the upper face. The medial subnucleus innervates auricular muscles (frontalis and platysma in humans). M2 projects bilaterally to the medial subnuclei. The dorsal and intermediate subnuclei innervate the frontalis and orbicularis oculi muscles. M3 projects bilaterally to the dorsal and intermediate subnuclei ( Fig. 16.3 ).

Corona Radiata and Internal Capsule

Projection fibers to (corticopetal) and from (corticofugal) the cerebral cortex connect the cortices with subcortical structures. Corticofugal fibers converge from all directions to form a wide fan-shaped white matter structure known as the corona radiata, which becomes the internal capsule and continues toward the brainstem in the cerebral peduncle ( Figs. 16.4 and 16.5 ).4 It is classically thought that the genu of the internal capsule contains most of the corticobulbar fibers that terminate mostly in contralateralcranial nerve motor nuclei. However, detailed information obtained from nonhuman primates suggests facial representation in the internal capsule is more complexly organized.6 Axons to superior levels in the brainstem, from the facial motor cortices, occupy the anterior limb, genu, and anterior portion of the posterior limb. These projections are arranged from anterior to posterior in the following order: M3, M2, LPMCv, M4, and M1. Projections from the cortex to the lower brainstem are in the posterior limb of the internal capsule in the same anterior–posterior order ( Fig. 16.6 ).

Specific areas of the internal capsule receive blood supply from different striate branches of the middle cerebral artery, anterior cerebral artery, internal carotid artery, and the anterior choroidal artery.3 Morecraft et al6 suggested that the differential arrangement of corticofacial fibers in the internal capsule might allow small more anterior capsular lesions to affect small specific facial regions and allow more favorable recovery. However, if the lesion is more posterior in the capsule, the relatively more compact grouping of all these axons could produce greater deficits and be associated with poorer recovery.

Corticobulbar (Corticofacial) Fibers in the Brainstem

The corticobulbar tracts and the corticospinal tracts in the brainstem are easier to understand by briefly reviewing the brain segments in order from most superior (rostral) to inferior (caudal). The prosencephalon (forebrain consisting of the two cerebral hemispheres) is divided into telencephalon, which includes the cortex and basal ganglia and corona radiata, and the diencephalon, which principally contains the thalamus, epithamalus, and hypothalamus, and the internal capsule. Internal capsule fibers converge to flank the midbrain as cerebral peduncles ( Fig. 16.7 ). The mesencephalon (midbrain), capped by the quadrageminal plate (colliculi), the shortest segment of the brain, contains the red nucleus, substantia nigra, and connects the forebrain with the rhombencephalon (hindbrain). As the corticobulbar fibers exit the internal capsule into the cerebral peduncles, the fibers then form two protrusions (peduncles) on the ventral surface of the midbrain that includes the crus cerebri (the mesial aspect of the peduncles) ( Fig. 16.8 ). The end of the brainstem is the rhombencephalon (hindbrain), which is divided into two parts: the pons and medulla oblongata. As corticobulbar fibers exit the crus cerebri to enter the pons, they are separated into fascicles by the transverse pontocerebellar fibers.4

A more detailed analysis of the corticofacial fibers within the pons and medulla shows three pathways to the facial nucleus10:

1. 
   

   Most of the corticofacial fibers course through the ventromedial pontine base to cross midline at the level of the facial nucleus to innervate the contralateral facial nucleus.

   
   
2. 
   

   However, in some individuals, the corticofacial fibers course along the dorsal edge of the base of the pons in an “aberrant bundle” then cross midline at the level of the facial nucleus to innervate the contralateral facial nucleus.

   
   
3. 
   

   And in others, the corticofacial fibers loop down into the ventral part of the upper medulla, cross the midline, and ascend in the dorsolateral medullary region to reach the contralateral facial nucleus.11

Involuntary and Emotional Facial Expressions

Involuntary and emotional facial expressions result from a complex multisynaptic control mechanism involving the thalamus, basal ganglia, limbic system, and the motor cortices on the medial side of the cerebral hemisphere, especially the cingulate gyrus.3,4 The thalamus has reciprocal projections to and from most of the cerebral cortex and is the major route for subcortical to cortical communication. The basal ganglia (caudate nucleus, putamen, globus pallidus, and amygdala) modulate movement and motivational aspects of behavior. The principal input to the basal ganglia from the rest of the brain is via the caudate nucleus and putamen, which projects in part to the globus pallidus and substantial nigra. The globus pallidus is the main output system to the thalamus. Disorders of this system include reduced movement with hypertonia (like Parkinson disease) or abnormal involuntary movements (dyskinesias). The amygdala is integrally associated with the basal ganglia and with the limbic (ring or border) system, which forms a ring around the thalamus and basal ganglia. The limbic system integrates external and internal sensory information, and links cortical sensory association areas and subcortical autonomic and endocrine areas with the emotion expressive motor systems. It also has a role in memory, food intake, arousal, sexual behavior, and motivation.

These structures modulate facial expressions in the following ways.5,8,9,12 The amygdala integrates sensory information and generates emotional responses such as rage. The emotional information is sent to higher cortical levels for memory and learning, and to subcortical levels of the hypothalamus and brainstem for autonomic, hormonal, and behavioral responses to the emotional information. This information is then sent to the rostral cigulate motor cortex (M3), which projects bilaterally to the dorsal and intermediate facial subnuclei, which innervate the upper face, especially the frontalis and orbicularis oculi. Thus, the amygdala-cingulate cortex facial nucleus system is involved in emotional expression, emotional maturation, and social adaptation.9 Additionally, the amygdala sends emotional information to M4, which supplies the contralateral lower face. In summary, M3 and M4 are critical targets for motor expression from the limbic system in the role of emotion, attention, and cognition.7,8 Pathology in this pathway may cause emotional paralysis of the face while maintaining normal volitional control; this is the opposite of classical central facial paralysis.