Craniofacial Muscles

13 Tongue Biomechanics and Motor Control
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connection with the limbic system. Choking, retching, etc., may be accompanied by
tears and a sense of panic and pain.
Masseter muscle spindle afferents synapse with cranial nerve XII premotor
neurons via the trigeminal mesencephalic nucleus (Luo et al. 2006 ) . These synaptic
connections provide some of the proprioceptive mediated jaw–tongue coordination.
Less is known about the proprioceptive feedback from the human tongue during
tongue movement. Anesthesia of the lingual nerve (carrying cranial nerve V afferents)
induces a delay in the corticomotor control of tongue muscle (Halkjaer et al.
2006 ) . It is common knowledge that there are neural network motoneurons, sensory
neurons, and interneurons known as “central pattern generators” that can generate
basic motor patterns for repetitive movement. These networks are particularly useful
for activities such as locomotion that are performed the same way, at the same
rate, many times in a day. Similarly, rhythmic movements of the tongue, driven by
the hypoglossal nuclei, receive inputs from the dorsal medullary reticular column
(DMRC) and the nucleus of the tractus solitarius (NTS). These interconnections are
helpful for repetitive functions such as respiration and chewing (see below) with
sensory feedback from cranial nerves V and IX.
13.5 Functions
13.5.1 Protrusion and Retrusion
The GG muscle originates on the genial tubercle on the inside of the anterior mandible
and inserts on the ventromedial base of the tongue (McClung and Goldberg
2000 ) to pull the tongue toward the mandible, i.e., protrude the tongue. The SG
muscle angles inferiorly from the styloid process of the temporal bone to the ventrolateral
base of the tongue to retract the tongue up and back. Synergistically, the
HG muscle originates on the hyoid bone and runs superiorly to insert on the superolateral
aspect of the base of the tongue (McClung and Goldberg 2000 ) . Downward
movement (depression) of the base of the tongue is performed by the combined
action of the GG and HG muscles. Conversely, the SG and PG lift (elevate) the base
of the tongue.
The functional organization of the intrinsic muscles has been more dif fi cult to
pin down. It is generally accepted that intrinsic muscles shape the tongue to execute
more fi nely controlled movements as well as contribute to protrusion and retrusion.
Protrusion is not a major function of the human tongue, though it is usually the
function tested to evaluate the status of the hypoglossal cranial nerve. A dysfunctional
nerve will result in the GG contracting only on the intact side so that the
tongue appears to “point” to the side of the lesion. It is interesting to study the
mechanics of protrusive activities such as licking and lapping in experimental animals
such as dogs, cats, and rats (Reis et al. 2010 ) . As expected by our own observation,
a dog’s tongue penetrates the water and quickly scoops the liquid into a ventral
“cup” and into the mouth as it closes. In contrast, the cat quickly touches the dorsal