第二章人体基本结构概述结构层次细胞组织器官系统个体系统有：消化系统、神经系统、呼吸系统、循环系统、运动系统、内分泌系统、泌尿系统和生殖系统系统(system)由功能上相互联系器官(organ)构成 。如消化系统由口腔、食管、胃、小肠、大肠、肛门以及多种消化腺构成。那么循环系统呢？第一节人体细胞结构与功能细胞是生物体的基本结构与功能单位.高尔基体线粒体细胞质细胞膜细胞核粗面内质网光面内质网中心体纤毛一、细胞的多样性(形态与功能方面均存在巨大差异）血细胞（红细胞、白细胞）扁平上皮细胞平滑肌细胞心肌细胞视杆细胞（模式图）神经元（神经细胞）二、细胞的结构细胞由细胞膜、细胞质和细胞核三部分构成高尔基体线粒体细胞质细胞膜细胞核粗面内质网光面内质网中心体纤毛（一）细胞膜1.细胞膜的结构2.细胞膜也是生物膜的一种。各类细胞器的膜（如内质网膜、内囊体膜等）、质膜和核膜在分子结构上基本相同，它们统称为生物膜。3.生物膜主要由脂质、蛋白质和糖类构成。脂质约占一半，其中以磷脂为主，还有胆固醇和糖脂。非极性尾部非极性尾部极性头部极性头部2. 细胞膜的功能细胞膜是细胞的边界，细胞通过细胞膜与其周围环境进行着复杂的联系。它控制着细胞内外物质的转运，维持细胞内环境的相对稳定。细胞通过细胞膜与外界不断进行物质和信息的交换和传递。（1）物质的跨膜运输 被动运输 passive transport 简单扩散 simple diffusion 水的简单扩散（渗透作用osmosis） 被动运输易化扩散 facilitated diffusion通道蛋白 channel protein 形成亲水性通道载体蛋白 carrier protein 能与被载物结合，有特异性 主动运输 active transport 钠钾泵Na+-K+ pump（动物细胞） 直接消耗ATP 主动运输 质子泵 Proton pump（植物细胞） 直接消耗ATP 主动运输协同运输(继发性主动运输) 间接消耗ATP 胞吞和胞吐作用 endocytosis and exocytosis 生物大分子或颗粒物质的运输物质的跨膜运输 （总结） 被动运输简单扩散 易化扩散 主动运输直接消耗ATP(钠钾泵、质子泵等） 间接消耗ATP协同运输胞吞和胞吐作用 生物大分子或颗粒物质的运输信息跨膜传递是质膜的重要功能。质膜上有各种受体蛋白，能感受外界各种化学信息,将信息传入细胞后，使胞内发生各种生物化学反应和生物学效应。信息传递规律是外源性刺激直接传给膜上受体，经酶的调控产生信号，再激发另一酶的溶性显示出生物学效应。 （2）信息跨膜传递（二）细胞质广义地说,就真核生物而言,在细胞膜的界限以内,除了细胞核以外的其他部分,都属于细胞质。细胞质(cytoplasm) 是由细胞质基质、内膜系统、细胞骨架和包涵物组成。 细胞器 Organelle 细胞膜内是透明粘稠并可流动的细胞质基质(plasma)，细胞器分布在细胞质基质中。 细胞器主要包括：内质网、核糖体、高尔基体、溶酶体、线粒体、质体、微体、液泡、微管、微丝等。有的细胞表面还有鞭毛或纤毛。核被膜 (nuclear lamina)是包在核外的双层膜，外膜可延伸与细胞质中的内质网相连。一些蛋白质和RNA分子可通过核被膜或核被膜上的核孔进入或输出细胞核。染色质 (chromatin)是核中由DNA和蛋白质组成并可被苏木精等染料染色的物质，染色质DNA含有大量基因片段，是生命的遗传物质。核仁(nucleolus)是核中颗粒状结构，富含蛋白质和RNA，核糖体的装配场所。染色质和核仁都被液态的核基质(nuclear plasma)所包围。（三）（三） 细胞核 nucleus, nuclei （四）细胞的增殖（四）细胞的增殖细胞以分裂的方式进行增殖真核细胞的分裂方式有三种，即有丝分裂、无丝分裂和减数分裂细胞增殖是生物体生长、发育、繁殖和遗传的基础一个多细胞生物完全长大以后，仍然需要细胞分裂的过程。这种分裂生成的新细胞可用于替代不断衰老或死亡的细胞，维持细胞的新陈代谢，或者用于生物组织损伤的修复。有分裂能力的细胞，从一次分裂结束到下一次分裂结束所经历的一个完整过程称为一个细胞周期n细胞周期 cell cycle 典型的细胞周期可包括间期interphase和细胞分裂期mitoticphase两部分。间期包括一个（DNA）合成期（S期）及S期前后两个间隙期（G1期，G2期）。细胞分裂期则包括有丝分裂和胞质分裂两个主要过程。有丝分裂是一个连续的过程，根据染色体形态的变化特征可分为前期prophase中期metaphase后期anaphase末期telophasen有丝分裂有丝分裂 mitosis特点：在间期每个染色体复制成两条相同的染色单体，在分裂时有规律地分配到两个子细胞核中。n配子形成与减数分裂由二倍体细胞形成单倍体细胞需要在细胞分裂过程中染色体数目减半，伴随着染色体数目减半的细胞分裂称为减数分裂。第二节基本组织第二节基本组织组织(tissue)为结构相似、功能相关的细胞和细胞间质集合而成。所谓间质是指存在于细胞之间的不具有细胞形态的物质。血浆、组织液、细胞之间的纤维等都是间质。间质不仅是细胞与细胞之间的联系物质，而且是维持细胞生命活动的重要环境。根据组织起源、结构和功能上的特点，人和动物体的组织可归纳为四大类，即：上皮组织、结缔组织、肌肉组织和神经组织。一、上皮组织(epithelial tissue)上皮组织简称上皮，由密集的细胞和少量的细胞间质组成，大部分存在于机体的外表面或衬贴在有腔器官的腔面。细胞排列紧密而规则，细胞间质很少。上皮细胞具有明显的极性，可分为游离面和基底面。游离面因所处的位置和功能不同，常分化出各种特殊的结构，如纤毛和微绒毛等。上皮细胞的基底面附着于基膜，并借助基膜于结缔组织相连。上皮组织中一般没有血管，其营养物质由深层结缔组织的血管供应。上皮组织具有保护、吸收、分泌和排泄等功能。上皮组织主要有两种即被覆上皮和腺上皮（一）被覆上皮被覆上皮广泛分布于机体的外表面或衬在各种管、腔、囊的腔面以及某些器官的表面。功能有保护、分泌、吸收等。被覆上皮单层上皮复层上皮被覆上皮（根据层数）扁平上皮立方上皮柱状上皮（根据上层细胞的形态）1.单层上皮单层上皮由一层细胞组成2.单层扁平上皮单层扁平上皮3.由一层扁平细胞组成。根据分布和功能不同可分为内皮和间皮。4.内皮：分布在心脏、血管、淋巴管的腔面5.间皮：分布在胸膜、腹膜、和心外膜表面单层扁平上皮单层扁平上皮银染单层立方上皮单层立方上皮由一层近似立方形的细胞组成。见肾小管、甲状腺滤泡等处，细胞的游离面常有微绒毛。甲状腺滤泡（示单层立方上皮）A tubule stained to show the pink basement membrane underlying the base of the simple cuboidal epithelium. Stained with periodic acid Schiff reagent (PAS), which stains mucopolysaccharides. 肾小管横切面示立方上皮和基膜单层柱状上皮单层柱状上皮 由一层柱状细胞组成，如被覆于胃肠道、子宫等腔面的上皮。假复层柱状纤毛上皮假复层柱状纤毛上皮细胞高低不一，都排列在同一基底面上，看似多层，实际只有一层。顶端常附有纤毛。分布在呼吸道腔面，具有保护和分泌功能。相关链接组织切片技术相关链接组织切片技术石蜡切片石蜡切片取材固定借助化学药品的作用,使细胞组织的形态保存下来,不使其改变形态和变质。浸洗脱水和透明利于材料的透明和制片（乙醇二甲苯）浸蜡（透蜡）先用二甲苯石蜡混合物，再用石蜡包埋石蜡包埋切片先整修蜡块，通常切片厚度7m左右展片和贴片在热水中进行，水温38-42烤片首选在38-42烤片24-72小时复水、染色、脱水、透明二甲苯乙醇二甲苯乙醇水染液乙醇乙醇二甲苯二甲苯封片贴标轮转式组织切片机石蜡切片操作冰冻切片冰冻切片最突出的优点是能够较完好地保存多种抗原的免疫活性。另外，也用于病理快速制片。 病理医生如何做？病理医生如何做？手术医生切下肿瘤（或其他病变）的一小部分或全部（称为：标本），与病情介绍（病理申请单）一起交给病理科，病理技师将标本制成常规石蜡切片（先把肿瘤浸在石蜡中，用特制的切片机再做成一种薄到透明的膜，然后贴到玻璃片上去染色。病理医生用显微镜观察切片，参考申请单 上描述的患者基本情况，综合分析。然后用书面报告把病理情况提供给临床医生参考（仅仅是参考）。整个过程需要72小时以上2. 复层上皮复层上皮复层扁平上皮复层扁平上皮 由十至数十层细胞构成。浅层细胞呈扁平状，不断角质化脱落。基底细胞可不断分裂，补充衰老的细胞。分布于身体表面，构成皮肤的表皮。角化的复层扁平上皮角化的复层扁平上皮Thickly cornified stratified squamous epithelium. The cells in the bright red layer and in the pale layers above it are completely flattened and dead, and have lost their nuclei.复层移行上皮如膀胱，缩小时有5-6层细胞，膨大时仅有2-3层细胞。右图为缩小时的切片。（二）腺上皮（二）腺上皮凡是以分泌功能为主的上皮称腺上皮。以腺上皮为主要成分的器官称为腺。腺分为外分泌腺和内分泌腺两种，前者有导管，将分泌物排到体外，后者没有导管，分泌物直接进入血液。外分沁腺有：汗腺、皮脂腺、泪腺、臭腺、唾液腺、胃腺、胰腺、小肠腺、大肠腺等。内分泌腺有：甲状腺、脑垂体后叶、肾上腺、性腺、胰岛等内分泌腺(endocrine gland) 由一团具有分泌能力的腺细胞组成外分泌腺(exocrine gland) 由导管和腺泡组成二、结缔组织结缔组织广泛分布于身体各部，种类多，形态多样。如液体状的血液、松软或胶体状的固有结缔组织、固体状的软骨和骨等。结缔组织的特点：是由细胞和大量细胞间质构成。细胞间质分基质和纤维两部分。细胞种类多，无极性，散在于细胞间质中。结缔组织具有支持、连接、营养、保护、防御等功能。结缔结缔组织组织固有结缔固有结缔组织组织软骨软骨组织组织骨组织骨组织血液血液疏松疏松结缔组织结缔组织脂肪脂肪组织组织网状网状组织组织致密致密结缔组织结缔组织（一）疏松结缔组织（一）疏松结缔组织疏松结缔组织在体内分布广泛，可位于器官之间、组织之间、以至细胞之间。疏松结缔组织的特点是细胞种类多，细胞间质中的纤维排列疏松。疏松结缔组织的组成疏松结缔组织的组成 疏松结缔组织巨噬细胞直接识巨噬细胞直接识别和粘附被吞噬别和粘附被吞噬物，如细菌物，如细菌肥大细胞肥大细胞多分布于小血管周多分布于小血管周围。可产生肝素和围。可产生肝素和组织胺，肝素有抗组织胺，肝素有抗凝血作用，组织胺凝血作用，组织胺可使血管扩张，通可使血管扩张，通透性增强透性增强脂肪细胞脂肪细胞（fat cellfat cell）有合成和贮存有合成和贮存脂肪、参与脂脂肪、参与脂质代谢的功能质代谢的功能（二）致密结缔组织（二）致密结缔组织致密结缔组织的最大特点是纤维多而至密，细胞种类和数量少，故以支持和连接作用为主。肌腱、真皮和硬脑膜都是致密结缔组织。（三）脂肪组织（三）脂肪组织由大量聚集的脂肪细胞构成，并被疏松结缔组织分隔成许多脂肪小叶。正常人的脂肪含量，男性约占体重的1020%，女性约占体重的1525%。大都以甘油三酯的形式贮存于脂肪细胞内。脂肪是人体浓缩的能源储备，每克脂肪在体内被完全氧化后，可放出2.2kJ的能量，约相当于相同质量的糖或蛋白质的2倍。脂肪组织具有保温、缓冲、保护、支持等作用。脂肪组织（四）软骨（四）软骨软骨组织由软骨细胞和细胞间质构成。细胞间质呈凝胶固体状，具有一定的坦然硬度和弹性，能承受压力和耐摩擦。软骨可分为：透明软骨透明软骨，分布于关节面、肋软骨、气管环。纤维软骨纤维软骨，分布于椎间盘、耻骨联合等处。弹性软骨弹性软骨，分布于耳廓、会厌等处透明软骨（五）骨骨主要由骨组织、骨膜、骨髓、神经和血管等构成。骨组织是构成骨的主要成分，体内的钙约99%以骨盐形式沉积在骨组织内，是人体最大的钙库。骨组织是由骨细胞和钙化的细胞间质组成的。细细胞有胞有骨原细胞、成骨细胞、骨细胞及破骨细胞骨原细胞、成骨细胞、骨细胞及破骨细胞四四种。种。 长骨的结构长骨由长骨由骨松质、骨密骨松质、骨密质、骨膜、关节软骨质、骨膜、关节软骨及血管、神经及血管、神经等构成等构成长骨的结构（七）血液将在第六章讲述三、肌肉组织（muscle tissue)肌肉组织主要由高度分化的肌细胞构成。肌细胞之间有少量的结缔组织、血管和神经纤维等。肌细胞细长呈纤维状，因此也被称为肌纤维。根据结构和功能肌肉组织可分为骨骼肌、心肌和平滑肌三种（一）骨骼肌 属于横纹肌，骨骼肌纤维是细长圆柱状多核细胞，长度变化范围很大，如镫骨肌纤维长度为1mm，缝匠肌纤维为125mm，而臀大肌为400mm。（二）心肌细胞呈细长圆柱形，有分支，并互相连接成网。细胞核位于细胞中央。心肌细胞相连接处细胞膜特化，凹凸相连，呈阶梯状，称作闰盘。它有利于化学物质的传递和电冲动的快速传导，使心肌成为一个功能性的整体。心肌有自动节律性，属不随意肌。（三）平滑肌平滑肌主要由平滑肌纤维组成，分布于胃肠道、子宫、输尿管和血管壁等处。平滑肌收缩缓慢，属不随意肌。平滑肌纤维呈梭形，无横纹，长约20 200m，直径2 20 m。不同器官的平滑肌纤维长短、粗细不一，妊娠子宫平滑肌可达500 m。（四）神经组织（nerve tissue)神经组织是神经系统的主要组成成分，由神经细胞和神经胶质细胞组成。神经细胞又称神经元，是神经系统中基本的结构和功能单位。神经胶质细胞不参与神经冲动的传导，但对神经元起营养和支持的作用，并参与髓鞘的形成。（一）神经元的结构神经元由胞体和突起组成，突起分轴突和树突。树突和胞体接受其它神经元，通过轴突将信号传至另一与之相联系的神经元。1. 胞体神经元胞体呈不规则的多角形、圆形或锥形等。核大而圆，多位于细胞中央。脊椎动物中最小的神经元胞体直径仅为5 6 m，如小脑的颗粒细胞，大的神经元直径可达25100 m，如脊髓前角运动神经元和大脑皮层的贝茨细胞。2. 突起 树突树突分支多，呈树枝状愈向外周分枝愈细。 轴突轴突每个神经元只有一根粗细均匀的轴突。 轴突长短不一，短者仅数微米，长者可达1米以上。（二）神经元的分类1. 根据神经元的突起数目分类根据神经元的突起数目分类假单极神经元双极神经元多极神经元2. 根据神经元的功能分类根据神经元的功能分类感觉神经元（传入神经元）运动神经元（传出神经元）联络神经元（中间神经元）（三）神经胶质细胞具有突起，但无树突和轴突之分。分布于神经元周围和血管周围，交织成网，构成神经组织的网状支架。神经胶质细胞无产生和传导神经冲动的功能，它们的功能主要是支持、营养和绝缘等。根据神经胶质细胞的形态和功能可分为：星形细胞、少突胶质细胞、小胶质细胞、施旺氏细胞。神经胶质瘤简称胶质瘤，是发生于神经外胚层的肿瘤。神经外胚层发生的肿瘤有两类，一类由间质细胞形成，称为胶质瘤；另一类由实质细胞形成，称神经元肿瘤。由于从病原学与形态学上还不能将这两类肿瘤完全区别，而起源于间质细胞的胶质瘤又比起源于实质细胞的神经元肿瘤常见得多，所以将神经元肿瘤包括有胶质瘤中，统称为胶质瘤。相关链接神经胶质细胞瘤胶质瘤以男性较多见，特别在多形性胶质母细胞瘤、髓母细胞瘤，男性明显多于女性。各型胶质母细胞瘤多见于中年，室管膜瘤多见于儿童及青年，髓母细胞瘤几乎都发生在儿童。胶质瘤的部位与年龄也有一定关系，如大脑星形细胞瘤和胶质母细胞瘤多见于成人，小脑胶质瘤（星形细胞瘤、髓母细胞瘤、室管膜瘤）多见于儿童。胶质瘤的治疗，以手术治疗为主，由于肿瘤呈浸润性生长，与脑组织无明确分界，难以彻底切除，术后进行放射治疗、化学治疗、免疫治疗极为必要。手术治疗的原则是在保存神经功能的前提下尽可能切除肿瘤。 （四）神经纤维神经纤维是神经元的长突起和包在它外表的神经细胞所组成的纤维状结构。根据有无髓鞘可分为有纤维和无髓纤维两种有髓神经纤维无髓神经纤维有髓神经纤维有髓神经纤维 有神经纤维是由神经元的轴突和包裹其周围的髓鞘和神经膜构成。髓鞘和神经膜都有节段性，段与段之间的缩窄部称郎飞氏节（郎飞氏节（node of Ranvier)。轴突的侧支均从郎飞氏节发出。郎飞氏节处无髓鞘包裹，轴突较裸露，适于轴内外离子交换，适于神经冲动的跳跃式传导。 髓鞘是由神经膜细胞的细胞膜反复包卷轴突形成的。无髓神经纤维无髓神经纤维直径较细，每个神经膜细胞包裹数条轴突，但不形成髓鞘，也无郎飞氏节。第三节医学影像技术简介医学（解剖）影像技术极大地促进了医学的发展。据估计在过去的30年里医学的发展成就相当于在此之前全部发展成就的总和，而影像技术在这其中做出了主要贡献。影像技术可以使医生不需要打开病人的身体就能以高清晰度观察到人体内部。尽管很多解剖影像技术都是很新的，但是其概念和技术原型已有较长的历史。 Wilhelm Roentgen 伦琴 (1845-1923) was the first to use X-ray in medicine in 1895 to see inside the body. The ray was called X-ray because no one knew what they were. This extremely short wave electromagnetic radiation moves through the body exposing a photographic plate to form a radiograph. Bones and radiopaque(辐射透不过的） dyes absorb the rays and create underexposed areas that appear white on the photographic film. X-rays have been in common use for many years and have numerous applications. Almost everyone has had a radiograph taken, either to visualize a broken bone or to check for a cavity in a tooth. A major limitation of radiographs, is that they give only a flat, two dimensional(2-D) image of the body, which is a three-dimensional(3-D) structure. 正常胸部X光片UltrasoundUltrasound is the second oldest imaging technique. It was first developed in the early 1950s as an extension of World War II sonar technology and uses high frequency sound waves. The sound waves are emitted from a transmitter-receiver placed on the skin over the area to be scanned. The sound waves strike internal organs and bounce back to the receiver on the skin. Even though the basic technology is fairly old, the most important advance in the field occurred only after it became possible to analyze the reflect sound wave by computer.One of the more recent advances in ultrasound technology is the ability of more powerful computers to analyze change in positions through time and to display those changes as “real time ” movements.Ultrasound is commonly used to evaluate the condition of the fetus during the pregnancy.Computed tomography(CT)断层摄影术断层摄影术The word tomography is derived from the Greek word for “cut”.Computed tomography, or CT, was developed by Godfrey Hounsfield and Allan Cormack, who shared the Nobel Prize in 1979. To accomplish this, an X-ray source is rotated around the head within the plane of the desired cross section. On the other side of the head, in the trajectory (轨道) of the X-ray beam, are sensitive electronic sensors of X-irradiation. The information about the relative radiopacity obtained with different viewing angles is fed to a computer that executes a mathematical algorithm (运算) on the data. The end result is a digital reconstruction of the position and amount of radiopaque material within the plane of the slice. CT的特点是操作简便，对病人来说无痛苦，其密度分辩率高，可直接显示X线平片无法显示的器官和病变，它在发现病变、确定病变的位置、大小、数目方面非常敏感而可靠，而在病理性质的诊断上存在一定的限制。CT与传统X光摄影不同，在CT中使用的X光探测系统比摄影胶片敏感，一般使用气体或晶体探测器，并利用计算机处理探测器所得到的资料。CT的特点在于它能区别差异极小的X光吸收值。与传统X光摄影比较，CT能区分的密度范围多达2000级以上，而传统X光片大约只能区分20级密度。 Magnetic Resonance Imaging (MRI)While still used widely, CT is gradually being replaced by a newer imaging method, magnetic resonance imaging, or MRI. The advantages of MRI are that it yield a much more detailed map of the brain than does CT, it does not require X-irradiation, and images of brain slices can be made in any plane desired. MRI uses information about how hydrogen atoms in the brain respond to perturbation of a strong magnetic field. The electromagnetic signals emitted by the atoms are detected by an array of sensors around the head and fed to a powerful computer that constructs a map of the brain. The information from an MRI scan can be used to build a strikingly detailed images of the whole brain.In the most common form of MRI, the hydrogen atoms are quantifiedfor instance, those in water or in fat in the brain. An important fact of physics is that when a hydrogen atoms is put into a magnetic field, its nucleus can exit in either two states: a high energy state or a low energy state. Because hydrogen atoms are abundant in the brain, there many protons in each state.The key to MRI is making the protons jump from one state to the other. Energy is added to the protons by passing an electromagnetic wave (i. e. a radio signal) through the head while it is positioned between the poles of a large magnet. When the radio signal is set at just the right frequency, the protons in the low-energy state absorb energy from the signal and hop to the high-energy state. The frequency at which the protons absorb energy is called the resonant frequency (hence the name magnetic resonance). When the radio signal is turned off, some of the protons fall back down to the low-energy state, emitting a radio signal of their own at a particular frequency. This signal can be picked up by a radio receiver. The stronger the signal, the more hydrogen atoms between the pole of the magnet.If we used this procedure, we would simply get a measurement of the total amount of hydrogen in the head. However, it is possible to measure hydrogen amount at a fine spatial scale by taking advantage of the fact that the frequency at which protons emit energy is proportional to the size of the magnetic field. In the MRI machine used in hospital, the magnetic fields vary from one side of the magnet to the other. This gives a spatial code to the radio waves emitted by the protons: High-frequency signals come from hydrogen atoms near the strong side of the magnet, and low-frequency signals come from weak side of the magnet. The last step in the MRI process is to orient the gradient of magnet at many different angles relative to the head and measure the amount of hydrogen. It takes 15 min to make all of the measurement for a typical brain scan. A sophisticated computer program is used to make a single image from the measurement, resulting in a picture of the distribution of hydrogen atoms in the head. It is possible to see the effect of demyelinating (脱髓鞘) diseases on white matter in the brain. MRI images also reveal lesions(损害) in the brain, because tumors and inflammation generally increase the amount of extracellular water .PET and fMRI (Functional Brain Imaging)CT and MRI are extremely valuable for detecting structural changes in the living brain, such as brain tumors and brain swelling (肿胀) after a head injury. Nonetheless, much of what goes on in the brainhealthy or diseasedis chemical and electrical in nature and not observable by simple inspection of the brains anatomy. Amazingly, however, even these secrets are beginning to yield to the newest imaging techniques.The two “functional imaging” techniques now in widespread use are positron emission tomography(正电子发射计算机断层扫描), or PET, and functional magnetic resonance imaging, or fMRI. While the technical details differ, both methods detect changes in regional blood flow and metabolism within the brain. The principle is simple. Neurons that are active demand more glucose and oxygen. The brain vasculature (脉管系统) responds to neural activity by directing more blood to the active regions. Thus by detecting changes in blood flow, PET and fMRI reveal the regions of brain that are most active under various circumstance. Until recently,”mind reading” has been beyond the reach of science. With the introduction of PET and f MRI, it is possible to observe and measure changes in brain activity associated with the planning and execute of specific tasks.PET imaging was developed in the 1970s, by two groups of physicists, one at at Washington University, led by M. Ter-pogossian, and the second at UCLA, led by Z. H. Cho. The basic procedure is simple. A radioactive solution containing atoms that emit positrons (positively charged electron) is introduced into the bloodstream. Positrons, emitted wherever the blood goes, interact with the electrons produce photons (光子) of electromagnetic radiation. The location of the positron-emitting atoms are found by detector that pick up the photons. One powerful application of PET is the measurement of metabolic activity in the brain. In a technique developed by Louis Sokoloff and his colleagues at the National Institute of Mental Health, a positron-emitting isotope of fluorine or oxygen is attached to 2-deoxyglucose (2-DG). This radioactive 2-DG is injected into the blood stream, and it travels to the brain. Metabolically active neurons, which normally use glucose, also take up the 2-DG. The 2-DG is phosphorylated by enzymes inside the neuron, and this modification prevents the 2-DG from leaving. Thus, the amount of radioactive 2-DG accumulated in a neuron and the number of positron emissions indicated the level of neuronal metabolic activity.In a typical PET application, a persons head is placed in an apparatus surrounded by detectors. By use of computer algorithms, the photons resulting from positron emissions reaching each of the detectors are recorded. With this information, levels of activity for populations of neurons at various site in the brain can be calculated. Compiling these measurements produce an image of the brain activity pattern. The researcher monitors brain activity while the subject perform a task, such as moving a finger or reading aloud. Different task “light up” different brain areas. To obtain a picture of the activity induced by a particular behavioral or thought task, a subtraction technique is used. Even in the absence of any sensory stimulation, the PET image will contain a great deal of brain activity. To create an image of the brain activity resulting from a specific task, such as a person looking at a picture, this background activity is subtracted out.Although PET imaging has proven to be a valuable technique, it has significant limitations. Because the spatial resolution is only 5-10 mm3, the images show the activity of many thousands of cells. Also, obtaining a single PET brain scan may take one to many minutes. This along with concerns about radiation exposure, limit the number of obtainable scans from one person ina reasonable time. Thus, the work of S. Ogawa at Bell Labs, showing that MRI technique could be used to measure local changes in blood oxygen levels that result from brain activity, was an important advance. The fMRI method takes advantage of the fact that oxyhemoglobin (the oxygenated form of hemoglobin in the blood) has a different magnetic resonance from that of deoxyhemoglobin. More active regions of the brain receive more blood, and this blood donates more of tis oxygen. Functional MRI detects the locations of increased neural activity by measuring the ratio of oxyhemoglobin to deoxyhemoglobin. It has emerged as the method of choice for functional brain imaging because the scans can be made rapidly (50 sec), have good spatial resolution (3 mm3), and are completely noninvasive.