Craniofacial Muscles

116 B.J. Sessle et al.
Most of the masticatory motor functions are cyclic in nature, that is, they are regularly
recurring movements. These cyclic jaw movements and other associated facial
and tongue movements that characterize chewing are generated by muscles that are
driven by a program that resides in the brainstem CPGs and which itself can be
modulated by afferent inputs acting through brainstem re fl ex circuits or higher brain
centers. During chewing, there are a wide range of directions and magnitudes of bite
forces between opposing upper and lower teeth and a similarly wide range of directions
and magnitudes of forces from the tongue, lips, and cheeks that can be exerted
on the food bolus. This complexity demands highly coordinated processes that, for
example, on the one hand allow for light forces to explore and ascertain the texture
of a foodstuff and on the other hand allow for the generation of the large forces necessary
for biting through tough foodstuffs while avoiding self-injury (Lund 1991 ;
Woda et al. 2006 ) . Thus, there is the need for sophisticated neural circuits that allow
for integration bilaterally of these diverse and complex motor functions and for coordination
with other motor functions such as respiration and swallowing. Virtually all
of these functions are not purely “motor” but indeed are sensorimotor, since they
depend upon or utilize sensory inputs or feedback to initiate or guide them.
Chewing is a motor function learned after birth, distinct from swallowing which
develops in utero. Most mammals are born as suckling and swallowing animals, but
as the infant matures, the rhythmic jaw movements become increasingly under central
control and more sophisticated, engaging to a greater degree other muscle groups that
allow for more re fi nement and the emergence of chewing behavior in the full sense of
the word. The postnatal eruption of the teeth provides important sensory inputs from
periodontal receptors that also assist in the development of masticatory control.
7.4 Neural Processes
There are several features that distinguish orofacial motor functions and their underlying
mechanisms from spinal sensorimotor processes and movements (see Sessle
2009 ) . In addition to the number of muscles that may often require bilateral muscle
activities, these distinguishing features include the arrangement of the various sensory
nuclei and motor nuclei into distinct neuronal pools in the brainstem, and unique
aspects of the peripheral and central mechanisms (see below). The orofacial region
receives its sensory and motor nerve supplies from the brainstem. The major sensory
nuclei include the trigeminal (CNV) brainstem sensory nuclear complex (VBSNC)
that receives most of the general somatosensory afferent input from the orofacial
tissues, and the solitary tract nucleus (NTS) that receives visceral afferents (e.g., those
supplying lingual taste buds, and laryngeal and pharyngeal taste buds and mechanoreceptors).
The main cranial nerve motor nuclei include the CNV motor nucleus
(Motor CNV) that provides the motor innervation of most jaw muscles, the CNVII
motor nucleus supplying the muscles of facial expression, CNXII motor nucleus
supplying the intrinsic and extrinsic tongue muscles, and the nucleus ambiguus
(CNIX and CNX) that mainly supplies muscles of the palate, larynx, and pharynx.
These brainstem sensorimotor circuits are controlled by other brainstem systems