Toshifumi Kumai

Muscle Spindle Distribution in the Levator Veli Palatini Muscle in the Rat

We previously reported that the levator veli palatini muscle (LVP) in the rat is innervated by the glossopharyngeal nerve. The LVP positioned between the mouth and nasopharynx, has important roles in respiration, swallowing, and speech. Muscle spindles, structures scattered through skeletal muscles, appear to function like miniature strain gauges, sensing the degree of tension in the muscle. Muscle spindles were demonstrated in the rats LVP in our neurophysiologic and histologic studies. We think the stretch of LVP modulates the rapid movements of the LVP by the proprioceptive component of the muscle spindle. Our results infer it is important to protect the innervation and the muscle spindles of the LVP from damage during any surgical dissection of the soft palate musculature.

Airflow receptors in the lip and buccal mucosa

Airflow receptor afferents in the oral mucosa responding to changes in intraoral air pressure during blowing were found to be innervated by the infraorbital nerve. They provided one response corresponding to the onset of blowing, a second related to an increase in air pressure, a third corresponding to the cessation of blowing, and a fourth that exhibited little change throughout sustained blowing. Intraoral air pressure in the cavity between the lips and the velopharyngeal portal may be monitored by these receptors, and the data they provide may contribute to the control of phonation.

Role of proprioceptors in the mylohyoid muscle

Afferent discharges of the mylohyoid muscle branch during respiration were studied electrophysiologically in the rat. Afferent discharges from the mylohyoid muscle branch of the mylohyoid nerve were found to be synchronized with respiration. Stretching of the mylohyoid muscle elicited afferent discharges of the mylohyoid muscle branch, suggesting that lengthening of the mylohyoid muscle caused electrical activity in the proprioceptors. When the central cut end of the mylohyoid muscle branch was stimulated electrically, reflex discharges were recorded from the EMG lead at the sternohyoid muscle where it is innervated by the cervical nerve. The latency between the electrical stimulation and the action potential in the sternohyoid muscle was 3-4 ms. Therefore, the mylohyoid muscle branch may transmit information to the sternohyoid muscle regarding the stretching actions of the mylohyoid muscle resulting from movements of the hyoid bone.

Muscle spindles in the mylohyoid muscle of rats

The mylohyoid muscle has several functions in relation to respiration, deglutition, and phonation, but these functions are not fully understood. The interaction of the mylohyoid nerve and muscle in 25 rats was studied by neurophysiologic and histologic methods. Stretching of the muscle elicited electrical responses from the branch of the mylohyoid nerve innervating the mylohyoid muscle, and stretch-sensitive receptors were demonstrated histologically in the mylohyoid muscle. This study indicates that the mylohyoid muscle plays an active role in functions such as swallowing, breathing, and phonation.

Properties of the lingual and LVP branches of the glossopharyngeal nerve

We investigated the effects of various stimuli on the afferent and efferent branches of the glossopharyngeal nerve in the rat soft palate. One of the sensory components, the lingual branch, responded to tactile stimulation, while the LVP branch responded to stretching of the levator veli palatini muscle. We also obtained physiological and morphological evidence of the existence of muscle spindles in the levator veli palatini muscle and showed that tactile stimulation of the contralateral soft palate and stretching of the contralateral LVP modulated discharges from the motor component of the ipsilateral levator veli palatini muscle. Our results suggest that these receptor units with both sensory and motor efferents may be the main determinants of modulation of respiratory movements in the upper airway by the IXth nerve.

Masticatory Function in a Patient with Mandibular Midline Cleft

Bone grafting of a midline cleft is required to allow reasonable masticatory function. However, changes in the rhythm of mastication have not been investigated before and after such bone grafting procedures. We performed bone grafting in a patient with multiple craniofacial anomalies including a midline mandibular cleft. The patient showed a marked improvement in the rhythm of mastication after repair of the midline mandibular defect. Bone grafting proved to be an effective method for securing stable occlusion with better mandibular movement.

Tactile-Evoked Response of Sensory Fibers in Buccal and Submandibular Regions of the Rat

Evoked neural responses to tactile stimulation were recorded electrophysiologically from the mechanoreceptive afferent fibers innervating the buccal and submandibular regions of Wistar rats anesthetized with sodium thiopental. Miniature probes 200 microns in diameter were used, and data analysis was performed on the mechanosensitivity of responses to tactile stimulation in the areas innervated by the mental, mylohyoid, auriculotemporal, and cervical nerves. Mechano-sensitivity of each area showed a characteristic distribution of slowly adapting (SA), rapidly adapting (RA), C-fiber (CF), and hair follicle (HF) units in individual receptive fields. The density of the SA units was high in the areas innervated by the mylohyoid and auriculotemporal nerves. The CF units were concentrated in the small dome in the area of the mylohyoid nerve and the auriculotemporal nerve, as shown by a significant response to the dynamic features of stimulation. Estimation of the current needed for tactile acuity suggests an important role of the SA fibers in the areas innervated by the auriculotemporal, mylohyoid, and cervical nerves.

Isn’t there an inductance factor in the plasma membrane of nerves?

It is established knowledge that the action potential event of nerves is formed by the combination of a phasic inward Na+ current and a following outward K+ current which increases gradually. These changes in current are commonly referred to as conductance changes of channels for Na+ and K+ with time. On the other hand, electric requirements for action potential generation in phenomena such as anode break excitation, hyperpolarizing break stimulation and accommodation, strongly suggest an existence of an inductance factor in the plasma membrane of nerves. In this study, the possibility that the Na+ channel could be simulated by a circuit composed serially of resistance (R), inductance (L), and capacitance (C) was examined using a computer simulation. Electric responses of the RLC circuit (R2/4L2 ≥ 1/LC) to step voltages are as followings: (1) A transient potential is produced on the inductor, (2) the circuit current simulates well the Na+ current manner, and (3) time course of the capacitor potential resembles the K+ current change.

Endplate Potential Oscillation of Jaw Closing Muscles in Natural Chewing

Surface electromyograms (EMGs) in natural gum chewing were recorded in a monopolar manner from paired anterior temporalis and masseter muscles of four subjects. Electrodes were placed on the temple of the former muscle, and on the inferior portion of the latter muscle. The endplate potential (EPP) component was extracted from raw EMGs of the muscles using a digital filter: the frequency of the high-cut digital filter for eliminating action potentials was set to 12.5 Hz or 45 Hz. The EPP component of the EMG burst of each stroke in the natural chewing showed two phases, early negative slow wave and following oscillation, which was same as observed in the clenching task. From FFT analysis, the frequencies of the ipsilateral EPP oscillations were around 30 Hz for both the temporalis and masseter muscles, which was also the same for the clenching task. It is concluded that the contraction of the jaw closing muscles is regulated in an oscillating manner of the EPP even in natural chewing behaviors. This oscillation phenomenon of the EPP gives a useful hint about the mechanism of muscular movements including chewing.