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The spatial distribution and sizes of degenerating hypoglossal neurons were studied following the removal of the genioglossus or hyoglossus muscles or of unilateral glossectomy in common boas with survival times of 7, 14, 21 or 40 days. The spatial organization of this nucleus differs from that of most cranial nerve nuclei in lacking a subnuclear organization: genioglossus and hyoglossus motoneurons are spread evenly throughout the nucleus and intrinsic tongue neurons show only a slight tendency to accumulate in the caudal half of the nucleus. The genioglossus and tongue neurons probably serve both ipsilateral and contralateral muscles with the ipsilateral innervation being largest; the hyoglossus motoneurons probably innervate only ipsilateral muscles. Complementary analyses of the transverse sectional areas of both degenerating and non-degenerating neurons indicates that, although their size ranges overlap, genioglossus motoneurons are small and tongue and hyoglossus motoneurons are progressively larger.

In five young sibling New Zealand white rabbits the muscular complex on the lingual side of the midline of the mandible was severed and a V-shaped piece of bone with the midline muscular attachments was removed from the lower border. In five control rabbits an incision was made down to the lower border but there was no further operative interference. There was no appreciable difference in weight gain between the two groups over a period of 8 weeks. All animals had difficulty in “swallowing” for 2 weeks, and two of the experimental animals gave evidence of such difficulty at 8 weeks. Firm muscular reattachments in the midline of the mandible had occurred in all the experimental animals when examined at the end of the eighth week.

Five cases of untreated fracture and displacement of the bony origins of the genioglossus muscles are reported. Palliative treatment alone yielded excellent results, with no appreciable impairment of tongue function.

Discharge patterns of single motor units were recorded from 5 inframandibular muscles in acutely prepared adult rhesus monkeys and cats. The alterations in interspike interval patterns were studied in genioglossus motor units during respiration, swallowing, laryngeal, and precentral cortex stimulation in animals anesthetized with urethane. Genioglossus motor units were active in both inspiration and swallowing with 87% of the units demonstrating a significant difference in distributions of their first order interspike interval histograms in swallowing as compared to their respiratory pattern. Interaction between several swallows and the prolonged inhibition of respiration (1–7 sec) altered the discharge pattern of a genioglossus unit from its activity during normal respiration. This significant change in the distribution of the interspike intervals of the motor unit in both first order and autocorrelation histograms occurred during the marked increase in intensity and duration of the gross EMG of the genioglossus muscle during the return of respiration. Stimulation of the precentral cortex of the rhesus monkey indicated a third central synaptic influence on the genioglossus motoneurons with a given motor unit discharging in masticatory movements and swallowing. A first order latency histogram demonstrated a fourth synaptic influence with a correlation between the stimulus pulse to the superior laryngeal nerve innervating the laryngeal region and the latency to when a genioglossus motor unit discharged. The latency analysis suggested a vago-hypoglossal reflex from sensory input of the laryngeal region synaptically affecting genioglossus motoneurons by a polysynaptic pathway.

Most authors consider the tongue active in secondary palate closure; some consider it passive; but specific information is wanting. We therefore studied the Holtzman rat foetal tongue by direct observation, histologically, under direct and neural stimulation, and when curarized. The tongue has differentiated into myoblasts (longitudinalis and verticalis) and young fibres (transversus, hyoglossus, genioglossus) four hours prior to closure. At this age it responds to both direct electrical and XII nerve nucleus stimulation. Curarization at high levels neither prevents closure nor abolishes foetal reflexes. The tongue myoneural apparatus is functional at palate closure.

Some of the factors involved in the control of tongue position have been investigated. The electromyographic activity reported here provides evidence that genioglossus, the protrusive muscle of the tongue, is under the control of physiological processes associated with respiration, jaw movements, and swallowing. Additional studies are in progress to define further the neutral mechanisms underlying these effects.

An electromyographic investigation of the activity of the paired genioglossus and geniohyoid muscles of twenty-six human subjects during deglutition revealed a general pattern of muscular activity involving an initial build-up, gradual summation, and tapering of electrical potentials during swallowing of both saliva and water. There is an observable difference in the pattern of swallowing of individuals within a group and among the individual swallows of a single subject. There are longer periods of electrical activity during a saliva swallow than during a water swallow. The type of bolus also seems to affect the pattern of activity in the individual muscles as well as the length of time that they are working. The geniohyoid muscles do not appear to begin their activity with the genioglossus muscles but rather lag behind and they do not appear to be active for as long. Both pairs of muscles appear to remain active during and after the time that the bolus has passed the area of the laryngopharynx. A period of electrical silence occurs prior to the characteristic burst of activity associated with a swallow. This appears to be the result of an active inhibition.

Frequencies of firing of single motor units were investigated primarily in four muscles; genioglossus, geniohyoid, mylohyoid, and anterior belly of the digastric during isometric and isotonic contractions. Isometric contraction was studied as subjects maintained units at regular firing rates by use of feedback provided by auditory and visual displays of the units (motor unit training). Representative results from relatively small samples obtained so far are as follows. Modal firing rates of individual units ranged from about 10 to 40 impulses per second in different units, with most rates between 20 and 30 ips. The latter values can be compared with values of 10–14 ips from brachial biceps. The frequency range within which regular firing can occur appears to be larger in these units than, for example, in biceps. Differences in typical firing rates from muscle to muscle will be considered. Isotonic contraction in speech gestures was accompanied in some units by firing rates of up to eight times their modal...

A study of provoked animals in vivo, of dissection of adult animals and of serial sections of 6 foetuses suggested that the mechanism of drawing the normally intra-nasally situated larynx into the mouth in vocalization is as follows: the joint contraction of the genioglossus, mylohyoid and geniohyoid muscles is suggested as the agent which induces the forward movement of the hyoid. The sternohyoid and sternothyroid, contracting simultaneously with the palato-pharyngeus muscle, could assure the return of the larynx to its original position inside the nasal cavity. A well-developed thyrohyoid muscle would be responsible for the displacement of the larynx as a result of the forces exerted on the hyoid.

Most previous electromyographic studies of tongue muscles have been limited to those extrinsic muscles which are easily accessible: the genioglossus and palatoglossus. Recent experiments have provided access to the styloglossus, among the extrinsic muscles, and to the intrinsic fibers of the tongue. Thus, a fuller picture of muscular synergisms in the positioning and shaping of the tongue is beginning to appear. This study reports findings from three subjects who spoke 10 American English vowels in a labial consonant frame preceded by schwa. All electromyographic activity was recorded from hooked‐wire electrodes inserted into the various muscles by means of hypodermic needles. The results reveal as many differences as similarities between the articulatory strategies of the subjects. It appears that the similar articulatory configurations that underlie the linguistically relevant acoustic structures of the vowels can be achieved by a variety of muscular patterning. [This research was supported by the National Institute of Dental Research.]

Both the traditional vowel triangle and more recent schemes of vowel classification present us with a picture in which the members of at least three pairs of vowels, /i‐I/, /e‐ɛ/, and u‐U/, are said to be differentiated from each other by a feature of tenseness. It has been suggested that differences in tension between the members of a tense‐lax pair of vowels are manifested in the extrinsic muscles of the tongue. In the experiment reported here, electromyographic potentials for the genioglossus muscle were investigated to determine the extent, if any, of tension differences between the members of the pairs of vowels listed above. It was found that the posited differences in tension do exist, although the findings cast some doubt on traditionally proposed height relationships for the front series of vowels, /i‐I‐e‐ɛ/. Electromyographic potentials of muscles associated with jaw opening and tongue backing were investigated to see if the genioglossus data could be reconciled with the usual height ordering of...

Waveforms of single motor units were investigated, primarily in four muscles; the genioglossus, geniohyoid, mylohyoid, and anterior belly of the digastric. Units were sampled at various depths up to 25 mm from concentric bipolar electrodes inserted vertically into the tongue from points inferior to the mandible and near the midline. Units were predominantly of the usual triphasic form although many other types of waveform were observed. Most triphasic potential durations ranged from 5–7 msec, as compared, for example, with 2.5–4 msec from the same muscles with bipolar hooked wire electrodes. Variables determining these waveforms, and differences in waveforms and unit amplitudes from muscle to muscle will be discussed. [This work was supported by grants from the National Science Foundation.]