Jd Vanwilligen

MUSCLE-FIBER TYPES AND MUSCLE-SPINDLES IN THE JAW MUSCULATURE OF THE RAT

The fibre composition and occurrence of muscle spindles was studied in the masticatory, the suprahyoid and the infrahyoid muscles of the rat. Muscle fibres were typed as fast-white, fast-intermediate, fast-red and slow-red according to their ATPase and SDH activity. Fibre type appeared to be closely related to fibre diameter. In most of the muscles, all four fibre types were found. Slow-red fibres were absent in the superficial masseter, the transverse mandibular and the omohyoid muscles; fast-white fibres were absent in the mylohyoid muscle. The masticatory muscles were mainly composed of the three fast-fibre types; the jaw-opener muscles (the anterior digastric, the posterior digastric, the posterior digastric, the stylohyoid and the lateral pterygoid muscle) showed more slow-red fibres. In the masticatory and most of the suprahyoid muscles, the slow-red fibres were restricted to an area with high SDH activity. In the infrahyoid muscles, the fibre types were evenly distributed. Many muscle spindles, often clustered, were found in the masticatory muscles, except in the lateral pterygoid. In most of the suprahyoid muscles, these sensory structures were absent. In the infrahyoid muscles, solitary muscle spindles were found.

PROJECTIONS OF THE PARVOCELLULAR RETICULAR FORMATION TO THE CONTRALATERAL MESENCEPHALIC TRIGEMINAL NUCLEUS IN THE RAT

Projections of the parvocellular reticular nucleus (PCRt) to the contralateral mesencephalic trigeminal nucleus (Me5) were studied in the rat with neurophysiological and neuroanatomical techniques. Three types of responses (classified by latencies) were recorded extracellularly in the Me5 area after electrical stimulation of the PCRt: (1) R1 fast unitary reactions (latency 0.2-0.5 ms) found throughout the entire caudal Me5 area; (2) R2 slower unitary reactions (latency 0.7-1.2 ms) recorded ventral and lateral to the caudal Me5; and (3) R3 compound potentials (latency 1.0-2.5 ms) recorded within the ventrocaudal Me5. Relayed or synaptic fields were not observed. Intracellular recordings of identified cell bodies of periodontal afferents, muscle spindle afferents and interneurones revealed no short-term postsynaptic potentials following PCRt stimulation. In some experiments jaw muscle spindle afferent activity was increased by PCRt stimulation and jaw-opening and jaw-closing reflexes were facilitated in the contralateral musculature. Neuroanatomical experiments, using Phaseolus vulgaris leucoagglutinin as an anterograde tracer, showed direct projections with intensive collateralization of PCRt fibres into the Me5 area. Synaptic contacts of PCRt fibres with primary afferent cell bodies were observed in the ventrocaudal parts of the Me5 only. The electrophysiological results are discussed in relation to the neuroanatomical findings.

Bilateral Brainstem Connections of the Rat Supratrigeminal Region

Efferent and afferent connections of the supratrigeminal region were studied in the rat using iontophoretically delivered horseradish peroxidase and Phaseolus vulgaris leuco-agglutinin. Projections of supratrigeminal efferents were found to the contralateral supratrigeminal region, to the ipsi- and contralateral trigeminal motor nuclei and the medullary reticular formation, and to the ipsilateral facial and hypoglossal motor nuclei. Neurons projecting to the supratrigeminal region were located in the contralateral supratrigeminal nucleus, in the ipsilateral mesencephalic trigeminal nucleus and bilaterally in the medullary reticular formation. This organization is discussed with respect to bilateral oral motor control mechanisms.

IN-TOTO STAINING AND PRESERVATION OF PERIPHERAL NERVOUS-TISSUE

A simple quantitative modification of the in toto staining technique for nervous networks of Sihler is described. The results are demonstrated on the innervation pattern of the hard palate of the rat. Formalin fixed hard palates of rat were macerated and bleached in an aqueous solution of 3% potassium hydroxide with a few drops of 3% hydrogen peroxide added. Thereafter, the specimens were decalcified in Sihlers solution I (1 part glacial acetic acid, 1 part pure glycerin and 6 parts 1% chloral hydrate), the process being controlled by radiography. The specimens were next stained in Sihlers solution II (1 part Ehrlichs hematoxylin, 1 part pure glycerin and 6 parts 1% chloral hydrate). After staining, the non-nervous tissues were destained in Sihlers solution I. Destaining was checked microscopically and was stopped before the finest branches of the nerves began to fade. The specimens were then washed in a weak aqueous solution of lithium carbonate and cleared in increasing concentrations of glycerin. Good visualization of nervous structures and a deep field of observation resulted; orientation of the peripheral nerves with respect to surrounding structures was readily seen and a three-dimensional image of the nervous networks was obtained.

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.

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

Abstract Reflex responses of the oral musculature to electrical stimulation of sensory elements in the hard palate were studied in decerebrate rats. Activity in the suprahyoid muscles, the intrinsic and extrinsic tongue muscles and the jaw-closing muscles was recorded electromyographically; at the same time jaw positions were recorded. Muscle group-activity patterns depended on the place of stimulation and the strength of the stimulus. Stimulation of the antemolar region of the palate evoked:—at threshold stimulation, suprahyoid muscle activity (latency 6–7 ms) which did not result in jaw movement; with stimuli about 1.2–1.5 × threshold, enhancement of suprahyoid muscle activity resulting in a transient jaw opening (latency 15 ms, duration 35 ms) sometimes accompanied by tongue reactions (latency 15 ms, duration 5–10 ms); with stimuli about 1.5–3 × threshold, additional jaw-closing muscle reactions (latencies 5–10 ms and 15–20 ms) sometimes changing the transient jaw opening to a transient jaw closing (latency 15 ms, duration 35 ms). Stimulation of the intermolar region evoked:- at threshold stimulation, two bursts of activity in the intrinsic tongue muscles (latencies 15–20 ms and 40–60 ms) and reactions in the jaw-closing muscles (latency 50–80 ms) resulting in a long-lasting jaw closure; with stimuli about 1.2–1.5 × threshold, a gradual shift from the reactions at threshold to a suprahyoid muscle and jaw-closing muscle response of short latency (6–9 ms), causing fast transient jaw opening followed by prolonged jaw closing; with stimuli about 1.5–3 × threshold, only short latency suprahyoid and jaw-closing muscle reactions. The strongest stimuli at this level abolished the jaw-closing reflex and reinforced the jaw-opening reflex. Stimulation of the intermolar region with two equal stimuli (1.2–1.5 × threshold) and several time intervals showed the following reaction pattern:- At very short intervals (1–7 ms), the jaw-closing reflex was suppressed; intervals larger than 6 ms evoked two bursts in the anterior digastric muscles. At about 12-ms intervals, the second reflex disappeared but reappeared at intervals larger than 30–40 ms, being equal to conditioning value at intervals larger than 60 ms. At stimulus intervals up to 200 ms a second stimulus did not produce a second response of the jaw-closing muscles (long-latency closing reflex).

On the self-perception of jaw positions in man

A longitudinal study shows that, in the short-term, dentate subjects are able to match fairly well the magnitude of jaw separations to any of three types of memorized standards (imagined, verbally imposed and physically imposed) of various dimensions. All subjects showed instability of the comparison mechanism or long-term changes in the engram. About 66 per cent of the matches were performed with the same precision. The relative precision of the matches is almost independent of the size of the standard. The absolute precision of matching of dentate subjects is similar to that of edentulous subjects. In half of the experiments, the subjects showed a match that equals the magnitude of the verbal standard or the real standard. The subjects tend to give more accurate matches when they refer to a verbal standard than when they refer to a real standard, but the long-term instability of matching is much greater when the subjects refer to a verbal standard than when the subjects refer to a real standard.

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.

THE PRECISION OF MEASUREMENT AND THE STABILITY OF THE PREFERRED VERTICAL DIMENSION OF OCCLUSION IN MAN

A longitudinal study on the preferred vertical dimension of occlusion (PVDO) established with edentulous subjects shows that these subjects can be divided into two groups, one in which there is significant instability of PVDO, and a second where the instability of PVDO is not greater than can be expected from the imprecision of the PVDO matches. It is concluded that the subjects in the latter group must have referred to a reasonably fixed standard during their PVDO determinations.

Sensitivity of anterior digastric and intrinsic tongue muscle reflexes to electrical stimulation of various areas in the rat palate.

Abstract The relationship between the sensitivity of various palatal sites to electrical stimulation and threshold reflex activity of the anterior digastric and the intrinsic tongue muscles was studied in decerebrated rats. For the anterior digastric muscle, threshold levels increased in antero-posterior direction; the papilla incisiva and the medial sites on the first antemolar ruga showed the lowest thresholds; medial test spots had lower thresholds than lateral ones. For the intrinsic tongue muscles the papilla incisiva had the lowest thresholds; otherwise thresholds decreased towards postmolar sites; but, in the posterior part of the postmolar region thresholds were markedly raised. Inter-muscle comparison of thresholds showed that in the antemolar region and the most posterior area the digastric had the lowest thresholds. In the intermolar field, no differences between digastric and tongue thresholds were found.