【正文】 s, located at the synapses of nerves with smooth or cardiac muscle, trigger a chain of chemical events referred to as signal transduction. For a cholinergic neuron to receive another impulse, acetylcholine must be released from the receptor to which it has bound. This will only happen if the concentration of acetylcholine in the synaptic cleft is very low. Low synaptic concentrations of acetylcholine can be maintained via a hydrolysis reaction catalyzed by the enzyme acetylcholinesterase. This enzyme hydrolyzes acetylcholine into acetic acid and choline. If acetylcholinesterase activity is inhibited, the synaptic concentration of acetylcholine will remain higher than normal. If this inhibition is irreversible, as in the case of exposure to many nerve gases and some pesticides, sweating, bronchial constriction, convulsions, paralysis, and possibly death can occur. Although irreversible inhibition is dangerous, beneficial effects may be derived from transient (reversible) inhibition. Drugs that inhibit acetylcholinesterase in a reversible manner have been shown to improve memory in some people with Alzheimer39。 in response to activation of this nerve supply, smooth muscle contracts (notably in the gut), the frequency of heart beat is slowed, and glands secrete. Acetylcholine is also an important transmitter at many sites in the brain at nervetonerve synapses. To understand how acetylcholine brings about a variety of effects in different cells it is necessary to understand membrane receptors. In postsynaptic membranes (those of the cells on which the nerve fibres terminate) there are many different sorts of receptors and some are receptors for acetylcholine. These are protein molecules that react specifically with acetylcholine in a reversible fashion. It is the plex of receptor bined with acetylcholine which brings about a biophysical reaction, resulting in the response from the receptive cell. Two major types of acetylcholine receptors exist in the membranes of cells. The type in skeletal muscle is known as ‘nicotinic’。 4 請您刪除一下內容， O(∩ _∩ )O 謝謝?。。?2021年中央電大期末復習考試小抄大全，電大期末考試必備小抄，電大考試必過小抄Acetylcholine is a neurotransmitter released from nerve endings (terminals) in both the peripheral and the central nervous systems. It is synthesized within the nerve terminal from choline, taken up from the tissue fluid into the nerve ending by a specialized transport mechanism. The enzyme necessary for this synthesis is formed in the nerve cell body and passes down the axon to its end, carried in the axoplasmic flow, the slow movement of intracellular substance (cytoplasm). Acetylcholine is stored in the nerve terminal, sequestered in small vesicles awaiting release. When a nerve action potential reaches and invades the nerve terminal, a shower of acetylcholine vesicles is released into the junction (synapse) between the nerve terminal and the ‘effector’ cell which the nerve activates. This may be another nerve cell or a muscle or gland cell. Thus electrical signals are converted to chemical signals, allowing messages to be passed between nerve cells or between nerve cells and nonnerve cells. This process is termed ‘chemical neurotransmission’ and was first demonstrated, for nerves to the heart, by the German pharmacologist Loewi in 1921. Chemical transmission involving acetylcholine is known as ‘cholinergic’. Acetylcholine acts as a transmitter between motor nerves and the fibres of skeletal muscle at all neuromuscular junctions. At this type of synapse, the nerve terminal is closely apposed to the cell membrane of a muscle fibre at the socalled motor end plate. On release, acetylcholine acts almost instantly, to cause a sequence of chemical and physical events (starting with depolarization of the motor endplate) which cause contraction of the muscle fibre. This is exactly what is required for voluntary muscles in which a rapid response to a mand is required. The action of acetylcholine is terminated rapidly, in around 10 milliseconds。 是發(fā)價人在其發(fā)價已經到達收發(fā)價人之后，即在其發(fā)價已經生效之后，將該項發(fā)價取消，從而使發(fā)價的效力歸于消滅。 是銀行依開證申請人的請求，開給受益人的一種保證銀行在滿足信用證要求下承擔付款責任的書面憑證。 是指出租人向承租人提供船舶或者船舶的部分艙位，裝運約定的貨物，從一港運至另一港，由承租人支付約定的費用的合同。實際履行的特點：實際履行是一種違約后的補救方式、實際履行的基本內容是，要求違約方繼續(xù)依據合同規(guī)定作出履行、實際履行可以與違約金、損害賠償和定金責任并用，但不能與解除合同的方式并 用。 115. 預期 違約 ： 是指在合同規(guī)定的履行期限前，某些情況已經顯示出當事人在合同規(guī)定的期限內顯然將很可能不能履行他的大部分重要的合同義務，這一情形被稱為預期違約，預期違約是一種能夠預見到的很有可能的將來違約，是違約事實還沒有實際發(fā)生的違約。 114. 實際違約 ：實際違約，是指：在合同履行期限到來以后，當事人不履行或不完全履行合同義務的行為。 包括所有權擔保和知識產權擔保。 103. 單獨海損 ：單獨海損是指保險標的物由于承保風險所引起的，不屬于共同海損的部分損失。 101. 海上風險 ：海上風險（ Perils of the Sea）又稱海難，是指船 舶或貨物在海上運輸過程中所遇到的自然災害和意外事故。 100. 委付 委付是指保險人同意將受損的保險標的視為推定全損，在補償被保險人全部損失的同時，獲得該受損標的物的所有權。 97. 保險憑證：保險憑證 又稱 小保單 ，指在保險憑證上不印保險條款，實際上是一種簡化的保險單。保險單記載的內容是合同雙方 履行的依據。保險人與被保險人訂立保險合同的正式書面證明。保險人與被保險人或投保人之間訂立保險合同的書面證明。保險利益產生于投保人或被保險人與保險標的之間的經濟聯系 ,它是投保人或被保險人可以向保險公司投保的利益 ,體現了投保人或被保險人對保險標的所具有的法律上承認的利害關系 ,即投保人或被保險人因保險標的遭受風險事故而受損失 ,因保險標的未發(fā)生風險事故而受益。一旦船舶處于滯期狀態(tài)，在計算滯期費時就不再減去周末這樣的除外時間，所以有這樣的說法：一旦滯期，永遠滯期（ Once on demurrage，always on demurrage）。有些合同