嚥下運動のメカニズム

Neural mechanisms in swallowing.

I．はじめに

　食物は本能や意志といわれる高次の精神作用によって，消化系の起始部，すなわち口腔に取り込まれてから，消化管腔を輸送される。その間，種々の物理的，化学的，酵素学的作用を受け，次第に細分され，栄養素は吸収され，残渣は肛門から排出される。この内容輸送にさいして消化管は種々の神経性，液素性影響を受け，また局所的要因も加わって，特徴的な運動形態を現出する。嚥下はこのような食物輸送の1つの運動現象であるが，その主演場が，発生学的にみても，また解剖学的にみても，呼吸，咀嚼，発声，吸畷などとの関連が濃い。

　嚥下運動は，ひとたび発生すると，時間的にも，空間的にも，きわめて組織だった経過をもって進行し，口腔内で形成された食塊を咽頭，食道を経て，下部消化管へ送り込む。しかし，気道と交叉する咽頭腔では，輸送の原動力となる嚥下圧の発生にあたって，口腔，鼻腔，喉頭腔，中耳腔などへの開口部を一時的に遮断し，残された最後の開口部，すなわち食道に向かって食塊を移動させるのである。

Food intake occurs in response to systemic deprivation signals interacting with external oro-pharyngeal cues, and is achieved by intricate coordinaition of humoral as well as neuronal net-works extending from the brain stem to various higher brain levels. After entering into the mouth, foods are carried through the whole digestive canals, while receiving various physical, chemical and enzymological actions. The nutritive elements and water contained in the bolus are absorbed on the way of transportation, and the rests are ulti-mately evacuated out of the anus.The propulsive force exerting on the bolus is generated by con-traction of the digestive tract, together with that of the supporting musculature. The pattern of such propulsive movement, however, depending on the locus or division of the tract, i.e. mouth, pharynx, esophagus, stomach and so forth. Swal-lowing is one of these movements which involves the oral, pharyngeal, laryngeal structurecs and the esophgus. The topography and neural control pertaining to swallowing interlock closely with those subserving respiration, sucking, mastication, vocalization, speech and so on. Therefore, a developmental survey of those motor phenomena in the animal kingdom may be pertinent.

At about 10 weeks of gestation, swallowing at first commences in the course of ontogeny, then follows in turn gasping, sneezing, sucking, and later rhythmic respiratory movements. A fetus at about 16 gestational weeks has been ascertained to ingest diurnally 200 to 760 ml of amniotic water. As such, the act of swallowing is efficiently oper-ative before birth, although the relevant central nervous organization is not yet fully matured.