Craniofacial Muscles

124 B.J. Sessle et al.
that each output zone of face MI controls one of the many contextual functions in
which a muscle participates. This multiple representation includes ipsilateral and/or
contralateral elemental movements, for example jaw-opening, tongue protrusion, or
facial twitch, as well as complex activities such as chewing and swallowing which
can be evoked by ICMS not only from the deep and principal parts of the classical
CMA, lateral to face MI, but also from within face MI and even within the face SI
in the monkey, rabbit, and cat. Furthermore, selective cold block or ablation of each
of these regions can disrupt chewing and swallowing to varying degrees, indicating
that each may be involved differentially in the production and patterning of chewing
and swallowing. This includes the coordination of the tongue, facial, and jaw movements
that is necessary for the proper ingestion and transport of food or liquid
(Hiraba et al. 1997, 2007 ; Murray et al. 2001 ; Lund et al. 2009 ; Sessle 2009 ) .
It is also noteworthy that ICMS-evoked orofacial movements have been observed
for the SMA, an area which, like premotor cortex, has been implicated in the preparation
for movement. However, little information is available on the role of this region
in orofacial motor control except for recent studies indicating that premotor cortex as
well as other cortical areas (e.g., parietal cortex) may be involved in the preparation
and planning for ingestion, visuomotor control, and perhaps even in cognitive functions
related to the understanding and communication of ingestive motor actions,
facial recognition, and other complex behaviors involving the orofacial region.
Ablation or cold block of the monkey’s face MI or SI also disrupts the animal’s
ability to perform a learned tongue task (Murray et al. 2001 ; Sessle et al. 2007 ; Avivi-
Arber et al. 2011b ) and lesioning of the cat’s SI, MI or masticatory cortex disrupts
masticatory movements (Hiraba and Sato 2005a, b ) . Interestingly, face MI or SI ablation
or block causes much less disruption of a biting task. This is consistent with
ICMS fi ndings and single neuron recordings showing a very limited representation
of jaw closing in face MI. Single neuron recordings in monkeys or subprimates also
have revealed that many face MI and SI neurons discharge in relation to chewing or
swallowing (e.g., Fig. 7.3 ). However, most also discharge in association with more
elemental movements, with some being active in relation to jaw movements, especially
jaw opening, and many others being active in relation to tongue movements.
These fi ndings are consistent with the above ICMS fi ndings. Although single neuron
recordings in face MI also have revealed that some neurons are active during preparation
for movement, many more neurons in premotor areas appear to show this
feature. This is consistent with other studies that have placed more emphasis on
cortical and subcortical regions in addition to MI in mechanisms involved in
motor planning and preparation (Murray et al. 2001 ; Sessle et al. 2007 ; Avivi-Arber
et al. 2011b ) .
The neuronal recording and ablation fi ndings outlined above have underscored
the importance of the somatosensory cortex as well as the motor cortex in the fi ne
motor control of orofacial movements. The face SI can in fl uence orofacial movements
by its projections to face MI and other cortical areas and to subcortical regions
such as the VBSNC, NTS, reticular formation, and the cranial nerve motor nuclei
(Murray et al. 2001 ; Sessle et al. 2007 ; Avivi-Arber et al. 2011b ) . The face SI has a
somatotopically arranged array of somatosensory inputs which are predominantly