【正文】 es of acetylcholine receptors exist in the membranes of cells. The type in skeletal muscle is known as ?nicotinic‘。 and there are some of each type in the brain. These terms are used because nicotine mimics the action of acetylcholine at nicotinic receptors, whereas muscarine, an alkaloid from the mushroom Amanita muscaria, mimics the action of acetylcholine at the muscarinic receptors. Acetylcholine is the neurotransmitter produced by neurons referred to as cholinergic neurons. In the peripheral nervous system acetylcholine plays a role in skeletal muscle movement, as well as in the regulation of smooth muscle and cardiac muscle. In the central nervous system acetylcholine is believed to be involved in learning, memory, and mood. Acetylcholine is synthesized from choline and acetyl coenzyme A through the action of the enzyme choline acetyltransferase and bees packaged into membranebound vesicles . After the arrival of a nerve signal at the termination of an axon, the vesicles fuse with the cell membrane, causing the release of acetylcholine into the synaptic cleft . For the nerve signal to continue, acetylcholine must diffuse to another nearby neuron or muscle cell, where it will bind and activate a receptor protein. There are two main types of cholinergic receptors, nicotinic and muscarinic. Nicotinic receptors are located at synapses between two neurons and at synapses between neurons and skeletal muscle cells. Upon activation a nicotinic receptor acts as a channel for the movement of ions into and out of the neuron, directly resulting in depolarization of the neuron. Muscarinic receptors, 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。 also called action painting and the New York school. It was the first important school in American painting to declare its independence from European styles and to influence the development of art abroad. Arshile Gorky first gave impetus to the movement. His paintings, derived at first from the art of Picasso, Mir243。s turbulent yet elegant abstract paintings, which were created by spattering paint on huge canvases placed on the floor, brought abstract expressionism before a hostile public. Willem de Kooning39。 among other major abstract expressionists were such painters as Clyfford Still, Theodoros Stamos, Adolph Gottlieb, Helen Frankenthaler, Lee Krasner, and Esteban Vicente. Abstract expressionism presented a broad range of stylistic diversity within its largely, though not exclusively, nonrepresentational framework. For example, the expressive violence and activity in paintings by de Kooning or Pollock marked the opposite end of the pole from the simple, quiescent images of Mark Rothko. Basic to most abstract expressionist painting were the attention paid to surface qualities, ., qualities of brushstroke and texture。 the adoption of an approach to space in which all parts of the canvas played an equally vital role in the total work。 the glorification of the act of painting itself as a means of visual munication。 Carreras. Through his exploitation of the nascent phonograph industry, Caruso is also largely responsible for the sweeping interest in opera of the 1910s and ‘20s. And for this, Stanley Jackson wrote in his bookCaruso, he may never be rivaled, for later tenors could not hope to find themselves in a similarly fortuitous position and thus would most certainly ―find it more difficult to win such universal affection as the bubbly, warmhearted little Neapolitan whose voice soared and sobbed from the first wheezy phonographs to bring a new magic into countless lives.‖ Born in Naples, Italy, in 1873, the third of seven children (early sources erroneously state that he was the 18th of 21), Caruso was raised in squalor. His birthplace, according to Jackson, was a―twostoreyed house, flaky with peeling stucco, [acmodating] several families, who shared a solitary coldwater tap on the landing, and like every other dwelling in that locality it lacked indoor sanitation.‖ As a boy, Caruso received very little formal education。 patrons. At the age of ten Caruso began working a variety of menial jobs—mechanic, jute weaver—but his passion for singing often led him back to the streets. Eight years later, an aspiring baritone named Eduardo Missiano heard Caruso singing by a local swimming pool. Impressed, Missiano took Caruso to his voice teacher, Guglielmo Vergine. Vergine on hearing Caruso, pared the tenor‘s voice to ―the wind whistling through the chimney,‖ Michael Scott recounted in The Great Caruso. Although he disliked Caruso‘s Neapolitan caf2