Craniofacial Muscles

7 Motor Control of Masticatory Muscles
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from facial skin and intraoral structures but may also include some inputs from deep
tissues, such as muscles. The face MI and CMA also receive somatosensory inputs.
While limb MI neurons receive inputs primarily from deep tissues, face MI neurons
receive inputs especially from super fi cial tissues of the face, mouth, and jaws, such
as skin, mucosa, and teeth (Hatanaka et al. 2005 ; Henry and Catania 2006 ; Kaas
et al. 2006 ; Iyengar et al. 2007 ) . Although most face MI neurons receive somatosensory
inputs from the same orofacial areas within which movement is evoked by
ICMS applied to the same neuronal recording site receiving the somatosensory
inputs, a substantial number of face MI neurons receive somatosensory inputs from
distant orofacial regions that have no close spatial relation with the ICMS-evoked
movement area. In addition, a well-described feature of neurons not only in face SI,
but also face MI and CMA, is that these neurons receive bilateral inputs from the
orofacial tissues. This organization of somatosensory inputs to face MI is probably
related to the need for extensive somatosensory feedback from wide bilateral peripheral
orofacial areas for the fi ne control, coordination, and modulation of the bilateral
orofacial muscle activities during orofacial movements (Murray et al. 2001 ) . These
bilateral inputs may be used by CMA, for example, to help guide masticatoryrelated
movements. Face MI may also utilize its orofacial afferent inputs for generating
and regulating orofacial movements in order to re fi ne ongoing cortical motor
activity and shape the appropriate motor response. For example, this is seen in the
control of voluntary orofacial movements such as the manipulation of the food
bolus after it is placed in the mouth, since many face MI neurons are active during
the food preparatory phase. Sensory inputs from the orofacial regions presumably
are utilized by these MI neurons for this purpose. Pain may also in fl uence masticatory
muscle function by actions on the sensorimotor cortex as will be discussed in
the following section.
Face MI and SI rely on orofacial afferent inputs to guide, correct, and control
movement by the use of sensory cues prior to movement and by using sensory
information generated during movement. These processes may involve intracortical
processing, cortical gating, and transfer of somatosensory information, as well as
corticofugal projections to subcortical sites that modulate and select somatosensory
information ascending through subcortical relay neurons in the brainstem, such as
VBSNC, NTS, and thalamus. These inputs also play critical roles in motor learning
and in the motor adjustments or adaptations that take place after a change in the
peripheral environment. This brings us to a consideration of neuroplasticity, especially
as it applies to the cortical mechanisms of orofacial motor control.
7.5 Cortical Neuroplasticity and Control
of Masticatory Muscles
Neuroplasticity is the capacity of the nervous system in general to alter its structure
(e.g., synaptogenesis, dendritic branching) and function (e.g., excitability, longterm
depression or potentiation) throughout life (Ebner 2005 ; Barnes and Finnerty