Craniofacial Muscles

13 Tongue Biomechanics and Motor Control
235
fi ring patterns. It is easy to understand that vestibular input (Tsuiki et al. 2000 ) , state
of wakefulness (Bailey et al. 2007a ) , hypoxia (Hwang et al. 1983 ) , or laryngeal
mechanoreceptor stimulation (Withington-Wray et al. 1988 ) will alter recruitment
and the respiratory pattern, considering that these are very different functions. The
vestibular system would provide information that the head is vertical or horizontal
in a gravity controlled environment before there is feedback of hypoxia. If the head
is horizontal, the tongue might collapse into the oro-pharyngeal space. As a result,
a greater number of GG motor units are recruited during sleep because of the recumbent
position (Tsuiki et al. 2000 ) . One might think that the recruitment of the GG
might be a simple train of potentials to pull the tongue forward during inspiration
when sleeping in the supine position. Instead, Bailey et al. ( 2007a ) found at least six
different fi ring patterns in non-REM sleep, which persist after arousal from sleep
(Wilkinson et al. 2010 ) and in quiet wakefulness (Saboisky et al. 2007 ) . Recognition
of insuf fi cient oxygen is a primitive re fl ex that attempts to maintain an open airway
at all times. Hypoxia is recognized in the blood by the chemoreceptors located in
the carotid sinuses and sensed by cranial nerve IX. The sensation is directly connected
to the breathing centers of the brainstem, which recruits the respiratory
muscles including the GG to open the airway and increase the rate of inspiration
(Hwang et al. 1983 ) . If there is food or liquid aspiration into the larynx, the recruitment
of motoneurons may shift from inspiratory to expiratory to expel the problem
(Withington-Wray et al. 1988 ) .
Recruitment of motor units and rate coding vary during respiration, depending
on how much is needed to meet the need for suf fi cient oxygen. It is suggested that
the modulation of force in the hypoglossal motoneuron pool is biased in favor of
recruitment rather than rate coding (Bailey 2011 ) . There seem to be some localized
areas of the posterior GG that are active during quiet breathing (Bailey et al. 2007b ) .
Magnetic resonance imaging (Cheng et al. 2008, 2011 ) and intramuscular stimulation
(Oliven et al. 2007 ) reveal the mechanical action of deeper transversely oriented
GG fi bers that particularly contribute to opening the pharyngeal airway. The intrinsic
motor units may be more involved in respiration than previously considered.
The documentation of respiratory-related co-activation of protrudor and retrusor
tongue muscles reinforced the hypothesis of a respiratory central pattern generator
(CPG) (Bailey and Fregosi 2004 ; Peever et al. 2002 ) . The motor units of the tongue
and chest wall fi re in synchrony at frequencies between ~1.5 and 8 Hz (Rice et al.
2011 ) . As the breathing becomes more rapid, the chest wall and diaphragm work
ef fi ciently together while leaving the tongue to other complex tasks.
13.5.3 Suckling, Acquiring and Manipulating Food,
and Swallowing
Fortunately, at birth, most animals are ready to voluntarily protrude the tongue on
the nipple using the GG, apply the upward pressure or stroking on the nipple, using
the SG and vertical intrinsic tongue muscle fi bers which causes the milk to be
expressed. The tongue intrinsic muscles and HG propel the bolus toward the back