学位研究紹介

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学 位 研 究 紹 介

Muscle spindle discharge patternfrom jaw-closing muscles during
chewing different hardness of foodsin awake rabbits.
異なる硬さの違いによる咀嚼時の閉口筋
筋紡錘の活動様式
Division of Oral Physiology,
Department of Oral Biological Sciences,wire electrodes were implanted bilaterally on massetermuscles. To record jaw movements, a small cylindricalmagnet and a jaw-tracking system consists of twomagnet sensors was fixed in the chin and head,respectively (Fig. 1). Glass coated elgiloy
microelectrode (0.3-0.8 M Ω at 1 kHz) was used torecord the single unit spindle discharge from left
mesencephalic trigeminal nucleus. Rabbits weretrained to accept test foods before recording. Two test
foods with different hardness made of gelatin were

Niigata University

used. A chewing cycle was divided into the fast-closing

Graduate School of Medical and Dental Sciences(FC), slow-closing (SC), and opening (OP) phases

Hossain Mohammad Zakir
新潟大学大学院医歯学総合研究科

口腔生命科学専攻

摂食環境制御学講座

ホッサイン

口腔生理学分野

モハマド

ザャキル

depending on the jaw movements. Ten consecutive
chewing cycles from the early rhythmic chewingperiod were taken for analysis. The mean frequency ofthe discharge for each phase and the area of rectified
EMG of ipsilateral masseter muscle were calculated foreach food.

1. Introduction

3. Results

The information arising from the various orofacial
sensory receptors including muscle spindles modify

Forty muscle spindle afferent units were

the fundamental pattern of chewing induced by thesuccessfully recorded during the chewing of soft andcentral pattern generator (CPG).

hard foods. The mean frequency of discharge was

In most of the previous studies, muscle spindlecalculated for all the units (Fig. 2A). Spindle discharge

discharges were analyzed when the animal was

was significantly higher for hard food than for softchewing one kind of food or lapping liquid. However,food in the FC and SC phases of both ipsilateral andfoods with different physical properties need to becontralateral chewing. Masseter muscle activity wasprocessed during natural chewing. Therefore, our aim

higher during the chewing of hard food than that of

was to analyze the discharge pattern during chewing

soft food and this was the case for both ipsilateral anddifferent hardness of foods to understand thecontralateral chewing (Fig. 2B). We compared therelationship between the spindle discharge and themean spindle discharge between the sides of chewing

hardness of foods.

for the 15 units for which we could obtain the data for

In addition, in previous studies only chewing sided

both-sided chewing (Fig. 2C). In the SC phase ofspindle discharge was investigated. To understand theipsilateral chewing, the discharge was significantly

significance of non-chewing sided spindle information

higher than that of contralateral chewing. Masseter

during natural chewing we analyzed discharges while

muscle activity was higher in ipsilateral chewing than

the animal chewed foods ipsilateral and contralateralthat of the contralateral chewing of hard food (Fig. 2D).

to recorded side of the neuron.

4. Discussion

2. Experimental procedure

The present study showed a clear relationship
Eight male rabbits (Japanese white, 2-3 kg) were

between the spindle discharge and hardness of foods

used. To record the EMG, teflon-coated stainless-steel

during the natural chewing of food. This relationship－ 41 －42

新潟歯学会誌

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：2011

was observed during both ipsi- and contralateralfoods of different hardness in awake rabbits. Brain Reschewing. During ipsilateral chewing, the increase ofBull., 83: 380-6, 2010.

spindle discharge during hard food chewing probably
play a role for facilitating jaw-closing muscle activities
and then increasing chewing force to compensate thehardness of food (so called load compensation),
especially in the SC phase. Indeed, the area under the
integrated EMG of the masseter muscle wassignificantly larger for hard food than for soft food.
Inputs from the muscle spindles facilitate themotoneurons of jaw-closing muscles reflexively duringchewing. The existence of hardness related increase ofspindle discharge on the contralateral side to thechewing suggesting the significance of muscle afferent
information to smooth chewing. The muscle spindle
may play a role for stabilizing the jaw on the nonchewing side and assist in producing the appropriate
force on the chewing side.
A phase dependent difference of spindle discharge
between the chewing sides was also observed. Thissuggests the distinct roles of spindle information on
the chewing and non-chewing sides. On the chewing
side spindle information may be used for generating
chewing force by reflexively increasing jaw-closing
muscle activity. On the non-chewing side the spindle
may give information about the state (i.e. amount,velocity and direction of shortening) of the muscle ofthat side.

Reference
Zakir HM, Kitagawa J, Yamada Y, Kurose M,
Mostafeezur RM, Yamamura K.: Modulation of spindle
discharge from jaw-closing muscles during chewing

Fig. 2 A: Mean spindle discharge of all units. B: The area of
the EMG of the ipsilateral masseter muscle. C: Comparisonof the mean spindle discharge between the sides of chewing.
D: Comparison of the area of EMG between the sides ofchewing.
Fig. 1: Schematic diagram of experimental procedure.
－ 42 －