《生理學(xué)》 （三）平滑肌活動(dòng)的控制和調(diào)節(jié)

和平滑肌本身的特性具有多樣性一樣，它們活動(dòng)所受的調(diào)控也是多種多樣的，不象骨骼肌那樣單純。大多數(shù)平滑肌接受神經(jīng)支配，包括來自自主神經(jīng)系統(tǒng)的外來神經(jīng)支配，其中除小動(dòng)脈一般只接受交感系統(tǒng)一種外來神經(jīng)支配外，其他器官的平滑肌通常接受交感和副交感兩種神經(jīng)支配。平滑肌組織、特別是消化管平滑肌肌層中還有內(nèi)在神經(jīng)叢存在，后者接受外來神經(jīng)的影響，但其中還發(fā)現(xiàn)有局部傳入性神經(jīng)元，可以引起各種反射。平滑肌的神經(jīng)-肌接頭有些類似骨骼肌，但不具有后者那樣特殊結(jié)構(gòu)形式。支配平滑肌的外來神經(jīng)纖維在進(jìn)入靶組織時(shí)多次分支，分支上每隔一定距離出現(xiàn)一個(gè)膨大，呈念珠狀，稱為曲張?bào)w，其中含有分泌囊泡，它們在神經(jīng)沖動(dòng)到達(dá)時(shí)可以釋放其中遞質(zhì)或其他神經(jīng)活性物質(zhì)；每個(gè)曲張?bào)w和靶細(xì)胞的距離亦不固定，平均約為80～100nm，這說明由神經(jīng)末梢釋放出來的遞質(zhì)分子要擴(kuò)散較遠(yuǎn)距離才能達(dá)到靶細(xì)胞，而靶細(xì)胞和神經(jīng)末梢的關(guān)系也不可能是固定的；凡是遞質(zhì)分子可以到達(dá)而又具有該遞質(zhì)受體的平滑肌細(xì)胞，都可能接受外來神經(jīng)的影響。

平滑肌細(xì)胞約保持－55～－60mV的靜息電位，產(chǎn)生機(jī)制和骨骼肌類似。單位平滑肌細(xì)胞有產(chǎn)生動(dòng)作電位的能力，而且通過細(xì)胞間通道可使相鄰細(xì)胞也產(chǎn)生動(dòng)作電位。

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