Jaw reflexes elicited by electrical stimulation of the hard palate of the rat

Immunohistochemical demonstration of nerve terminals in the whole hard palate of rats by use of an antiserum against protein gene product 9.5 (PGP 9.5)

Sensory innervation of the entire hard palate was investigated in the rat using serial sections immunostained for protein gene product 9.5 (PGP 9.5), a neuronal marker. PGP 9.5-immunoreactive nerve endings were widely distributed in the hard palate, but the innervation pattern and density differed among portions. They were numerous at papillary protrusions including the incisal papilla, antemolar/intermolar rugae, and postrugal filiform papillae. Immunoreactive free nerve endings gathered at the summits of the connective tissue papillae, some of them entering deeply into the epithelium. Electron microscopy demonstrated that nerves in the postrugal filiform papillae reached the stratum corneum. The atrial region, possibly the most sensitive in the hard palate, showed unique innervation: its anterior part, adjacent to incisors, developed intraepithelial networks of fine and beaded nerves, whereas its posterior part revealed cone-shaped nerve terminals formed on the connective tissue papillae of the atrial folds which comprised two lines of longitudinal flaps. Taste bud-like corpuscles gathered in the medial walls of the incisal canals and in the "Geschmacksstreifen" (taste stripes) present at the most anterior part of the soft palate. The hard palate of the rat is thus richly innervated, and is characterized by region-specific nerve endings which may be involved in mechano- and chemoreception in the oral cavity.

Reflex changes in the masticatory muscles with load perturbations during chewing hard and soft food

Reflex changes in masticatory muscles were investigated in naturally behaving rabbits while chewing soft food (bread) and hard food (raw rice). To study peripheral control mechanisms of mastication, reflex changes in masticatory muscles were correlated with the jaw movement trajectories. When the hard food was tested, the closing muscle was activated isometrically and the antagonist activity was evident during closure. Under such conditions, the so-called masseteric inhibitory periods (MIPs) and digastric short bursts (DSBs) were found in the closing phase, which was not the case with soft food. The reflex changes in the masticatory muscles were similar to those in an unloading reflex or in a reflex after tooth tap. Precise comparison of the EMG and the movement orbit showed that DSB with MIP was preceded by a trough in the closing velocity which bottomed at 8 +/- 1 ms (mean +/- S.D.: n = 9) before the DSB onset. These results suggest that the DSB with MIP may be a reflex change generated by periodontal mechanoreceptor stimulation after the upper and lower teeth come together with hard food, which have a role in regulatory mechanisms of mastication when the teeth are suddenly loaded by hard food.

Cutaneous oral-motor reflexes of children with normal and disordered speech

Cutaneous reflex responses in the human jaw-closing system to low-threshold, mechanical stimulation were investigated in 30 four- to six-year-old children with normal speech and in seven children with disordered speech. Responses of these children were smaller and less frequently observed than those of a seven- to eight-year-old group in an earlier study, suggesting that cutaneous oral-motor reflexes may develop with the acquisition of speech motor skills. Preliminary observations in a very small sample of speech-impaired children suggest that future work should explore the hypothesis that development of cutaneous oral reflexes is delayed in this group. The results of this and other studies suggest that human oral-motor reflexes undergo a pattern of continuous change throughout the lifespan.

Effect of sensory input from the tongue on jaw movement in normal feeding in the opossum

Opossums were presented with solid and liquid foods. The movements of the jaw and tongue were recorded cineradiographically together with recordings of the EMG activity in muscles opening the jaw and moving the base of the tongue (hyoid). The jaw opening in each cycle was in two stages--01 and 02; 01 had a constant amplitude irrespective of the food ingested. Ingestion of liquid (which involved continuous accumulation of a liquid bolus in the valleculae prior to swallowing) was associated with cycles of oral movement in which 02 was small; tongue retraction was associated with this opening. In contrast, solid and semisolid food ingestion was associated with large angles of jaw opening in 02 that also coincided with the tongue retraction. In this latter case a characteristic pattern of EMG activity, in which all the muscles moving the hyoid were simultaneously active, was added to the pattern seen in lapping; this additional activity had an EMG pattern that was consistent with a jaw opening reflex. The findings contrast with other reports that the jaw opening reflex is suppressed in mastication. Experimentally induced tongue contact with a variety of solid surfaces during lapping (an activity involving accumulation of a liquid bolus in the valleculae) induced neither increased jaw opening nor the additional EMG pattern. However, in situations when there was no bolus in the valleculae, additional jaw opening activity was elicited when the tongue contracted solids intra- or extra-orally. It is suggested that the ability of sensory input, from the anterior tongue, to elicit a jaw opening reflex and to change the type of jaw/tongue cycle was dependent upon the extent of bolus accumulation in the valleculae and therefore indirectly upon the consistency of the food.

A hierarchy of neural control of mastication in the rat

In rats anaesthetized with ketamine, rhythmic jaw-opening and jaw-closing movements were induced by palatal stimulation. The two masseter muscles (jaw-closing) and the four digastric muscles (jaw-opening) were fitted with electrodes, which could be used either for electrical stimulation or for recording electromyographic responses. Electrical stimulation of the masseters in the phase when the digastrics were the contracting muscles, caused responses in the digastrics. The amplitude of these responses was dependent on whether the stimulated masseters were active or not. The responses in digastric persisted when contraction of the masseters during stimulation was prevented by dantrolene sodium but they disappeared when the masseteric nerves were blocked with xylocaine. The responses in digastric are thus reflexes from stimulating afferent fibres in the masseteric nerves. Likewise, electrical stimulation of the four digastrics in the phase when the masseters were contracting, caused responses in the masseters. The amplitude of these responses, however, was independent of the state of activity of the stimulated digastrics. Furthermore, the responses in masseter disappeared when contraction of the digastrics was prevented by dantrolene sodium; but they persisted when the digastric nerves were blocked with xylocaine, provided the digastrics continued to twitch to the electric stimuli. The responses in masseter are thus reflexes in masseter caused by mechanical stretch transmitted from the digastric twitches. In the rhythmic preparation, prevention of contraction of the masseters of digastrics by dantrolene sodium or xylocaine leaves the overall frequency and amplitude of the evoked rhythmic activity unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Reflex responses of the human jaw-closing system depend on the locus of intraoral mechanical stimulation

Innocuous mechanical stimuli were applied to eight sites on the tongue dorsum and palate while subjects used feedback to maintain a constant isometric biting force. Reflex responses of the jaw-closing system were measured as changes in force and in EMGs recorded from right and left masseter muscles. Stimulation at each of the eight sites produced reflex force and EMG responses in most subjects tested. The nature of the reflex responses strongly depended on the site of stimulation. Stimulation of the palate tended to produce suppression of ongoing EMG activity and decreases in background biting force. In contrast, stimulation of sites on the tongue posterior to the tip, most often resulted in excitatory EMG responses and increases in jaw-closing force. Unilateral, early excitatory responses were observed in the right masseter with stimulation of the right side of the tongue. The existence of spatially organized responses of the human jaw-closing system to innocuous intraoral stimulation is not consistent with the view that these cutaneous reflexes are primarily of protective significance.

Ultrastructure of intra-epithelial nerve endings in the hard palate of the rat, Rattus norvegicus

The ultrastructure is described of lanceolate and free nerve endings within the epithelium of the papillae of the intermolar palatal rugae, located either between the basement membranes and the bases of the epithelial cells, or suprabasally in the intercellular spaces of the epithelium. Cytoplasmic processes of epithelial cells invaginated the Schwann cells of the nerve endings; junctions between, or fusion of, the cell membranes of the epithelial cells and the Schwann cells were not found. The neurites of the nervous structures were characterized by numerous mitochondria, clear-cored vesicles and an axoplasmic reticulum. In lanceolate endings, asymmetric membrane densities existed between the neurite and its Schwann cell, the Schwann cell showing signs of pinocytotic activity at all sides of its plasma membrane. The basal lamina of the Schwann cell covering of the nerve endings appeared to be continuous with the basal lamina of the epithelium.

Peripheral influences on the central pattern-rhythm generator for tongue movements in the rat

Rhythmic activity in the tongue and other oral muscles was evoked by mechanical stimulation of the hard palate in ketamine-anaesthetized rats. The relation between neural discharges of single hypoglossal motoneurones and activity in the masseter and anterior digastric muscle, and stimulus parameters was analysed. By varying the stimulus parameters, hypoglossal-motoneurone activity was modulated from phasic reflex activity into rhythmic activity. The tongue and other recorded muscles are likely to be controlled by the same pattern-rhythm generator, when rhythmic activity is induced by palatal stimulation. Exteroceptive oral stimuli can activate the tongue pattern-rhythm generator independently of proprioceptive feedback. Proprioceptive input from oral structures influenced the burst duration of rhythmic hypoglossal motoneurone activity. The cycle duration and the number of discharges/burst, however, was not affected by proprioceptive input. The hypoglossal-motoneurone pool may be influenced by more than one pattern-rhythm generator because the burst characteristics of rhythmically-firing hypoglossal motoneurones depend upon stimulus conditions. These hypoglossal pattern-rhythm generators have possible hierarchic relations because a shift from one burst pattern into another can be evoked by changing stimulus conditions. The pattern-rhythm generators for oral movements may be composed of a neural-pattern generator and a neural-rhythm generator, which can be modulated separately by peripheral inputs.

Phasic and rhythmic responses of the oral musculature to mechanical stimulation of the rat palate

Activity of the anterior and posterior digastric muscles, the intrinsic tongue muscles and the masseter muscles was recorded electromyographically. Muscle-activity patterns depended on the site of stimulation, the strength of the stimulus and its duration. Five types of reflex patterns were distinguished: type A: on-response in the digastric muscles (latency about 7 ms) and in the intrinsic tongue muscles (latency about 15 ms); type B: on-off response in the digastric and intrinsic tongue muscles; type C: on-response followed by long-lasting low-level activity in the digastric and intrinsic tongue muscles; type D: on-off response and long-lasting low-level activity in the digastric and tongue muscles; type E: rhythmic activity in all muscles under investigation preceded by one of the other reflex types. In the type-E response, the frequency of the bursts was about 4.3 Hz; the masseter bursts were not in phase with those of the digastric muscles; the intrinsic tongue muscles showed constant activity with superimposed rhythmic bursts. By increasing the stimulus strength or the stimulus duration, there was a gradual shift from simple, phasic, reflex responses to more complex reflex patterns. This holds for all palatal sites. However, rhythmic activity was most easily elicited in the region of the incisive papilla, less easily in the antemolar region; no rhythmic activity was found in the intermolar region. Thus stimulus duration is a crucial factor in eliciting rhythmic oral activity in the rat.

Merkel cell-neurite complexes in the hard palate of the Wistar rat (Rattus norvegicus)

The complexes were found in the basal layer of the epithelium of the rugae of the ante- and intermolar area of the palate. Asymmetric membrane densities were observed between the Merkel cell and its neurite or the adjacent keratinocytes. The cytoplasm of the Merkel cell was characterized by an accumulation of membrane-bound dense granules at the neurite side of the cell and by a well developed Golgi apparatus at its keratinocyte side.