Craniofacial Muscles

120 B.J. Sessle et al.
(Dubner et al. 1978 ; Sessle 2006, 2009 ) have profound effects on many of the
neurons in the VBSNC or NTS that relay to thalamus or other brain regions
(Fig. 7.2 ). Because many of these neurons also contribute as interneurons to re fl ex
and other behavioral responses evoked by stimulation of orofacial tissues, responses
can also be regulated by these modulatory in fl uences on the interneurons or in some
cases, on the motoneurons themselves that are part of the re fl ex circuits. These
descending modulatory in fl uences include those from the amygdala and other parts
of the limbic system, the lateral hypothalamus, the lateral habenular nucleus, the
basal ganglia, the anterior pretectal nucleus, the red nucleus, the cerebellum, the
sensorimotor cerebral cortex, the cortical premotor and supplementary motor areas
(SMAs), and the cortical masticatory and swallowing areas. It is through these
descending excitatory or inhibitory in fl uences that the higher brain centers can exert
control over the brainstem processes and activities of motoneurons supplying not
only the masticatory but also all the orofacial musculature, and thereby initiate,
guide or regulate orofacial motor functions. The next sections examine in more
detail these higher brain center in fl uences and processes.
7.4.4 Subcortical Processes
Several areas in the CNS exert modulatory in fl uences on motor behavior via direct or
indirect projections to cranial nerve motoneuron pools. The numerous connections
between these areas mean that the neural circuitry involved in the CNS control of
motor function is extensive and complex. Only limited study has been made on these
pathways as they apply to orofacial motor control as compared to limb motor control.
In the case of the brainstem, the descending inputs as well as the afferent inputs
from peripheral receptors access the re fl ex interneurons that project to and modulate
motoneurons in the cranial nerve motor nuclei. Several of these regions also act in
concert to form the neural circuitry of the CPGs for chewing, swallowing, and other
analogous complex motor behaviors (Lund 1991 ; Jean 2001 ; Lund et al. 2009 ) .
Most research attention has focused on the CPGs underlying swallowing and especially
mastication. For the latter, this CPG (the “chewing center”) can generate
chewing-like movements independent of orofacial sensory inputs. Nonetheless,
studies in humans and animals indicate that it can utilize these inputs, especially
those from periodontal mechanoreceptors and jaw muscle spindles, in concert with
other brain regions accessing it, to provide for modi fi cation and guidance of masticatory
movements (Fig. 7.2 ). The CPG-dependent stereotyped movements typical
of chewing can be varied, and function in an integrated manner with movements of
the cheeks and tongue to allow for repositioning of the food bolus and for alterations
in masticatory force, velocity and jaw displacement as the food is crushed and
manipulated. These features explain how several factors, for example the number of
teeth, food composition and hardness, and bite force, can in fl uence the masticatory
process and provide for the appropriate reduction of food to a size suitable for swallowing.
As part of this process, the CPG can also modulate sensory inputs, such that