Craniofacial Muscles

118 B.J. Sessle et al.
lie immediately adjacent to the VBSNC and the NTS. These sites provide some of
the neural circuitry and processing that form the basis for the central programs
(central pattern generators, CPGs) so crucial in the initiation and control of complex
functions such as chewing and swallowing. These regions also provide a neural
substrate allowing for the initiation or modulation of brainstem re fl exes by receiving
and integrating afferent inputs from various orofacial tissues and from other
brainstem or suprabulbar areas, such as afferent inputs, descending controls, and
conscious state. As well as these afferent inputs being capable of evoking brainstem-based
re fl ex responses, afferent signals are also relayed from the brainstem to
higher brain centers involved in sensorimotor control of the muscles (Dubner et al.
1978 ; Murray et al. 2001 ; Sessle et al. 2005 ) .
7.4.2.1 Brainstem Re fl exes
The motoneurons supplying the masticatory muscles receive re fl ex afferent inputs from
free nerve endings as well as the specialized receptors in the orofacial tissues. Thus, the
masticatory neuromuscular system can be in fl uenced re fl exively by the somatosensory
inputs into the brainstem from receptors that signal pain, touch, joint position, muscle
stretch or tension, and the like. Through their connections with the previously mentioned
interneuronal circuits, these afferent inputs can activate or inhibit the cranial
nerve motoneurons supplying the masticatory musculature. Brainstem circuits also
underlie the autonomic re fl ex changes in heart rate, blood pressure, breathing, and salivation
and in more complex behaviors that can be evoked by non-noxious or noxious
stimulation of orofacial tissues. On the basis of these various types of responses, several
human behavioral paradigms have been developed in order to study the effects of
orofacial stimuli in humans; these include changes in autonomic functions (e.g., heart
rate, salivation), muscle re fl exes, and facial expression.
Given the large number of muscles in the orofacial region, and the diversity of
receptors and afferent inputs, the number of re fl exes is also vast. This applies to the
masticatory muscles as well. For example, mechanical stimulation of the jaw-closing
muscles can result in several jaw re fl ex responses that involve brainstem circuits and
are modulated by afferent and descending in fl uences. Jaw muscle stretch evokes
myotatic stretch re fl exes through activation of jaw muscle spindle afferents (Dubner
et al. 1978 ; Lund and Olsson 1983 ) . As indicated above, the primary afferent cell
bodies of these spindle afferents are located in MesV, and impulses in their central
axons can monosynaptically activate jaw-closing motoneurons in Motor CNV.
Synapses occur on these primary afferent cell bodies, and they and their central
axons have intriguing electrophysiological and neurochemical features that have
recently been reviewed (Lund et al. 2009 ) . The jaw-opening re fl ex and the re fl ex
effects of stimulation of periodontal receptors around the root of the tooth are two
examples of other well-studied jaw re fl exes. A brief excitatory re fl ex in the jaw
muscles may be elicited under certain conditions, but an inhibitory re fl ex involving
one or more so-called “silent” periods in the jaw-closing muscles has received
particular attention over the years. This inhibitory re fl ex is usually thought to provide