Table of Contents
A In 1937 the great neuroscientist Sir Charles Scott Sherrington of the University of Oxford laid out what would become a classic description of the brain at work. He imagined points of light signaling the activity of nerve cells and their connections. During deep sleep, he proposed, only a few remote parts of the brain would twinkle, giving the organ the appearance of a starry night sky. But at awakening, “it is as if the Milky Way entered upon some cosmic dance,” Sherrington reflected. “Swiftly the head-mass becomes an enchanted loom where millions of flashing shuttles weave a dissolving pattern, always a meaningful pattern though never an abiding one; a shifting harmony of subpatterns.”
B Although Sherrington probably did not realize it at the time, his poetic metaphor contained an important scientific idea: that of the brain revealing its inner workings optically. Understanding how neurons work together to generate thoughts and behavior remains one of the most difficult open problems in all of biology, largely because scientists generally cannot see whole neural circuits in action. The standard approach of probing one or two neurons with electrodes reveals only tiny fragments of a much bigger puzzle, with too many pieces missing to guess the full picture. But if one could watch neurons communicate, one might be able to deduce how brain circuits are laid out and how they function. This alluring notion has inspired neuroscientists to attempt to realize Sherrington’s vision.
C Their efforts have given rise to a nascent field called optogenetics, which combines genetic engineering with optics to study specific cell types. Already investigators have succeeded in visualizing the functions of various groups of neurons. Furthermore, the approach has enabled them to actually control the neurons remotely simply by toggling a light switch. These achievements raise the prospect that optogenetics might one day lay open the brain’s circuitry to neuroscientists and perhaps even help physicians to treat certain medical disorders.
D Enchanting the Loom Attempts to turn Sherrington’s vision into reality began in earnest in the 1970s. Like digital computers, nervous systems run on electricity; neurons encode information in electrical signals, or action potentials. These impulses, which typically involve voltages less than a tenth of those of a single AA battery, induce a nerve cell to release neurotransmitter molecules that then activate or inhibit connected cells in a circuit. In an effort to make these electrical signals visible, Lawrence B. Cohen of Yale University tested a large number of fluorescent dyes for their ability to respond to voltage changes with changes in color or intensity. He found that some dyes indeed had voltagesensitive optical properties. By staining neurons with these dyes, Cohen could observe their activity under a microscope.
E Dyes can also reveal neural firing by reacting not to voltage changes but to the flow of specific charged atoms, or ions. When a neuron generates an action potential, membrane channels open and admit calcium ions into the cell. This calcium influx stimulates the release of neurotransmitters. In 1980 Roger Y. Tsien, now at the University of California, San Diego, began to synthesize dyes that could indicate shifts in calcium concentration by changing how brightly they fluoresced. These optical reporters have proved extraordinarily valuable, opening new windows on information processing in single neurons and small networks.
F Synthetic dyes suffer from a serious drawback, however. Neural tissue is composed of many different cell types. Estimates suggest that the brain of a mouse, for example, houses many hundreds of types of neurons plus numerous kinds of support cells. Because interactions between specific types of neurons form the basis of neural information processing, someone who wants to understand how a particular circuit works must be able to identify and monitor the individual players and pinpoint when they turn on (fire an action potential) and off. But because synthetic dyes stain all cell types indiscriminately, it is generally impossible to trace the optical signals back to specific types of cells.
G Optogenetics emerged from the realization that genetic manipulation might be the key to solving his problem of indiscriminate staining. An individual’s cells all contain the same genes, but hat makes two cells different from each other is that different mixes of genes get turned on or off in them. Neurons that release the neurotransmitter dopamine when they fire, for instance, need the enzymatic machinery for making and packaging dopamine. The genes encoding the protein components of this machinery are thus switched on in dopamine producing (dopaminergic) neurons but stay off in other, non-dopaminergic neurons. In theory, if a biological switch that turned a dopamine-making gene on was linked to a gene encoding a dye and if the switch-and-dye unit were engineered into the cells of an animal, the animal would make the dye only in dopaminergic cells. If researchers could peer into the brains of these creatures (as is indeed possible), they could see dopaminergic cells functioning in virtual isolation from other cell types. Furthermore, they could observe these cells in the intact, living brain. Synthetic dyes cannot perform this type of magic, because their production is not controlled by genetic switches that flip to on exclusively in certain kinds of cells. The trick works only when a dye is encoded by a gene—that is, when the dye is a protein.
H The first demonstrations that genetically encoded a decade ago, from teams led independently by Tsien, Ehud Y. Isacoff of the University of California, Berkeley with James E. Rothman, now at Yale University. In all cases, the gene for the dye was borrowed from a luminescent marine organism, typically a jellyfish that makes the so-called green fluorescent protein .Scientists tweaked the gene so that its protein product could detect and reveal the changes in voltage or calcium that underlie signaling within a cell, as well as the release of neurotransmitters that enable signaling between cells.
Questions 1-5 Do the following statements agree with the information given in Reading Passage 1? In boxes 1-5 on your answer sheet, write
TRUE if the sataement agrees with the information
FALSE if the statement contradicts the information
NOT GIVEN if there is no information on this
1 Sherrington’s imaginary picture triggered scientists’ enthusiasm of discovering how the whole set of neurons operates.
2 A jumped-up domain optogenetic is a pure unexpected accident.
3 Electric tension is one key component to realize the communication between neurons.
4 The variations of voltages is the sole response that the coloration of related neurons could provide when neural discharge takes place.
5 The vital defect synthetic dyes possess is the most challenging obstacle for researchers to overcome .
Questions 6-10 The reading Passage has seven paragraphs A-H. Which paragraph contains the following information? Write the correct letter A-H, in boxes 6-10 on your answer sheet.
6 a sea creature producing light triggered by certain genes
7 first attempts to make a great idea come true
8 the reason to explain the failure of synthetic dyes
9 difficulty in observing how the whole set of neurons works
10 visual indicators to show how information is handled in and between cells in the Brain
Questions 11-13 Complete the following summary of the paragraphs of Reading Passage, using no more than three words from the Reading Passage for each answer. Write your answers in boxes 11-13 on your answer sheet.
Synthesized by enzymatic machinery , 11 ……………….. plays as vehicle for the information flow between cells. Protein is the ingredient of the enzymatic machinery, so first it needs genes in charge of encoding the required protein 12 ……………….. before the neutrontransimitter is produced. This 13 ……………….. can be used to differentiate the dopaminergic neurons from the nondopaminergic counterparts with a premise that the dye is a protein after a transfer process.