现代神经生物学 Modern Neurobiology

Notes on Modern Neurobiology. Reference: Principles of Neurobiology. Liqun Luo.

This first version draft will be polished.

1 Electrical signals in neurons

• Galvani frog legs experiment

• 恒流源+电容：电压一直上升

  • 

• 水分子的电子云分布——以水合离子的形式在溶液中运动，是极性分子

Ion Movement

• diffusion: Ficks law

  

  • 扩散系数，直径小～ 大
  • ion concentration

• electrical drift: Ohm’s law

  

  • drift flux []
  • mobility []
  • 
  • 电阻率，细胞内外液 $\sim 60\ \Omega\cdot m $

Equilibrium potential

• 电化学平衡：扩散与电迁移达到平衡

• 离子主要分布在膜上？且很少量？

  • 原因：膜薄，正负电荷吸引；膜厚–>电场变弱，需要更多的离子积聚 （）
  • 细胞膜厚度～23 埃
  • 膜是电容

• 平衡电位：能斯特电位

  

  • 推导: 利用玻尔兹曼方程（热平衡状态下～没有热交换）

    

Ion gradient maintenance

Electrogenic pumps. 生电泵：出去的阳离子多，电位更负

• Na/K-ATPase pump
• Ca-ATPase pump
• Na-Ca cotransporter
• K-Cl cotransporter

Movement of ions across biological membrane

• 认为膜上也有离子浓度，有个分配系数 :

• 膜内扩散系数：

• 离子通透性

  

• Goldman-Hodgkin-Katz (GHK) equation

  

神经元是漏电电容器

神经元的等效电路

I–V曲线

• 双脂膜 (lipid bilayer) +离子通道

• 惯例：外向电流——正电荷离开细胞 positive；内向电流——正电荷进入细胞，negative

• RC 并联模型

  • R来源于漏通道
  • 
  • 神经元时间常数：，,
    • 选择刺激时宽非常重要，10ms ～ 100Hz
  • 低通滤波器，容易滤掉高频信号（跟随不上）

• 加入电池：离子通道 （离子泵～俩电池并联）

  

• 平行电导模型：多个离子通道

• 输入电阻

  

• 整流特性 membrane rectification

  • slope & chord conductances

Hodgkin-Huxley Equation

• HH拟合出4次方，其实对应4个亚单位
• 在电流刺激后，Na、K 离子通道的电导是改变的

Propagation of AP in a passive axon

动作电位的轴向传导

极化状态：内负外正

• 轴突模型：电报方程

• 传播速度；轴突粗细
• 跳跃传导
  • Nodes of Ranvier 郎飞结
    • 髓鞘包裹的地方 大、 小—> 大、 小（天才的进化设计）
    • 郎飞结：髓鞘中间的地方，Na、K浓度高
  • 主要local 放电，或主要传到下游

Firing patterns of neurons

pass.

• Q: why delayed firing?

2 Synapse

重点：release probality 怎么来的，quantal size, short term synaptic plasticity

• 不同离子的平衡电位与膜电位

• GABA和甘氨酸产生的突触电位一般会产生超极化

• excitation (~depolarization) & inhibition (~hyperpolarization)

• current-clamp & voltage-clamp

  • 电流钳反映生理状况
    • 生理状况下膜电压的变化
  • 电压钳是比较专业的研究离子通道的方法
    • 离子通道：电压依赖性
    • 电流钳会引起正反馈样的膜电压变化（瀑布式）
    • 补偿电流钳制电压，电流反映特定电压下离子通道的变化
  • depolarization
    • 电流钳和电压得到的电流曲线是相反的？

• membrane potential

  • 静息电位：K离子通道开放
  • 动作电位：Na离子通道开放
  • 突触电位：IPSP，EPSP

• 囊泡释放：Ca离子通道开放

History

• 证明化学突触：Frog heart perfusion 蛙心灌流

• Stephen William Kuffler: studies of neuromuscular junctions (NMJ)

  • Synaptic delay against electrical propagation
  • synaptic fatigue following high-frequency stimulation
  • sensitive to temperature, etc.

• Kaz: Quantal Theory 神经递质释放的量子学说

  • 乙酰胆碱要么不释放，要么释放一个固定的量 (quantum release)
  • release probability , quantal size
    • 突触电位 ~ quantal size

• John Eccles: 研究细胞内刺激引起的突触传递，证明了脊髓（中枢）的突触传递是化学传递，建立了抑制性突触传递的概念

  • Excitatory postsynaptic potential/current (EPSP/EPSC)
  • Inhibitory postsynaptic potential/current (IPSP/IPSC)

• Comparison of NMJ and CNS synapses

  • NMJ: 大突触，一对一突触传递
  • CNS: 主要作用是整合 (integration)，突触比较小

Morphological types of synapses in CNS

• Gray type I: 很厚的突触后膜

  • excitatory, glutamate
  • round vesicle

• type II

  • inhibitory, GABA or Gly

  • flattered vesicle

Short-term synaptic plasticity

• Presynaptic mechanism 衡量突触前囊泡释放的指标

• Examine: paired pulse –> facilitation/depression

• facilitation：low release probability，第一次刺激时Ca离子量少；第二次Ca离子通道进一步开放，Ca离子浓度更高，release probability 提高

• depression: high replease probality，第一次刺激时释放啦大部分囊泡，囊泡需要再循环，第二次刺激产生的突触后电位会变小

• Facilitation–>depression: 药物处理抑制了突触兴奋性

Clearance of extracellular neurotransmitters

• Glu transporter 转运体
  • 谷氨酸是细胞代谢中间产物，细胞内Glu (m mol) 比细胞外 (u mol) 高很多
  • 电梯度和化学梯度都阻碍Glu转运入细胞内
  • 转运方式：共转运
  • 缺氧状态可能会释放出Glu，造成神经损伤
• Astrocyte 星型胶质细胞：The Glutamate-Glutamine Cycle
  • Glu –> Astrocyte –> Glutamine (谷氨酰胺) –> 转运出，被细胞再利用♻️
  • Glu –> Glutamine 不耗能，分解可以产生ATP
  • Glu –> neuron 不耗能，顺梯度共转运？（跟着离子泵进入？）
  • Glu –> Astrocyte （耗能）也是靠转运体，星胶的受体多，很多K离子通道，非兴奋性

Approaches to study exo- and endocytosis neurotransmitters

• Fluorescent imaging (FM–143)
  • 形成囊泡后把 FM–143 洗掉，只剩下包裹在囊泡内的荧光分子
• Capacitance recording
  • 膜电容～细胞膜表面积
  • 通常用在内分泌细胞
• Electrochemical detection
  • 电极上涂上化学物质，记录电化学反应引起的电信号改变

Postsynaptic receptors

• 离子通道型受体（配体门控型通道, ligand gated）
  • 经典电位
• G–蛋白偶联受体
  • “慢”电位，GPCR调控离子通道产生

Electrical synapses: gap–juntction

• Gap-junction的意义： 缓冲作用
• Neuron–glia crossstalk: meadiate by waves mostyly through gap–junction

Molecular mechanisms of neurotransmission

• synaptic vesicle proteins

  • 高度特化，保证极短时间内（1ms）大量囊泡释放

• general fusion mechanism

  • Rab3: docking
  • SNAREs: 囊泡融合的核心分子
  • NSF, SNAP: disassemble the SNAREs complex
  • Munc–18: 保护机制—prevent formation of SNAREs complex

• Unique fusion mechanism

  • synaptotagmin 突触结合蛋白，感应钙离子
  • etc.

• Models of exo- and endocytosis 外吞/内吞作用

  • kiss and stay

  • bulk endocytosis

  • distinct vesicle retrieval pathways in excitatory and inhibitory presynaptic terminals

    • GABAergic—bulk endocytosis
    • Glutamatergic

  • endosome

• 十几年的工作：Presynaptic Endosomal Cathepsin D Regulates the Biogenesis of GABAergic Synaptic Vesicles

3 Trans-membrane Signal Transduction and GPCR in Neurotransmission

跨膜信号转导和GPCR介导的信号转导

Forms of intercellular signaling

细胞间通信方式

• Contact–dependent

  • e.g., Notch signaling pathway

• Paracrine

  • 旁分泌, 信号分子化学扩散

• Synaptic

• Endocrine

  • 内分泌

Components of signaling pathway

• first messages (chemical stimulus)
  • 第一信使：细胞外信号分子
  • Endocrine hormones: GH
  • Neurotransmitters: Ach, GABA
  • Growth factors/cytokines: ILs, IFN, FGF, EGF
• Receptors/ligand
  • GPCR
  • Receptor enzymes: EGFR
  • Ion channels: 5–HT, nAChR
• secondary messages
  • Kinase/phosphatase/adaptors
  • cAMP
  • G-protein
  • 
• signal networking
• responses

G protein

• G protein: Guanine nucleotide-binding proteins

• small G protein: Ras superfamily

  • Ras: 和癌症相关, 是原癌基因 (function: cell proliferation)
  • Ras/MAPK pathway: an essential role for cell physiology

• The GTPase cycle

  
  • (inactive) GDP GTP (active)

GPCR

• 命名问题：GPCR can interact with non-G proteins

• Architecture of GPCRs

  • Heptahelical：跨膜螺旋结构
    • 7TM (大多为7次跨膜，alpha螺旋, serpentine)
  • Glycoproteins 糖蛋白，糖基化

• Physiological roles of GPCR

  • Rhodopsin: first cloned GPCR, visual sense
  • Smell sense: olfactory receptors
  • behavioral and mood regulation: many neurotransmitters’ receptor
  • regulation of immune system activity and inflammation: histamine
  • autonomic nervous system transmission: blood pressure, heart rate, and digestive processes
    • 自主神经系统（英语：autonomic nervous system，缩写为ANS），又称植物神经系统（vegetative nervous system，VNS），与躯体神经系统共同组成脊椎动物的周围神经系统。 所谓“自律(自主)”，是因为未受训练的人无法靠意识控制该部分神经的活动。
  • etc.

• Generalizations of GPCR

  • highly versatile
  • involved in short-term regulation (seconds to hours)
  • mediate slower “modulatory” effects in neurons
    • 调制性反应，好比润滑剂

• Subunits

  • GPCR–>G protein ( Subunits )

  • 

• 

  • PLC–>IP3–>(from endoplasmic reticulum) Ca+DAG—>PKC (蛋白激酶C)

  • 

• Classification

  • Class I—VI
  • 膜外：N端 (–NH)
  • 膜内：C端 (–COOH)

GPCR signaling

• modulation of GPCR/G-protein signaling by accessory proteins

  • GPCR-interacting proteins (GIPs)
  • RGS proteins: regulators of G protein signaling
  • AGS proteins: activator of G protein signaling

• Desensitization (脱敏化, 不再感受膜外信号分子作用) & recyling of GPCR

  Ligand-stimulated phosphorylation of GPCR by associated GRK is probably a general mechanism of desensitization of GPCRs.

  • GRK: G protein receptor kinase 给脱离了 亚基的G蛋白加磷酸基团（打标签）
  • GPCR internalization (内化)/sequestration: arrestin & endocytosis （抑制素结合，引导进行内吞回收，送到溶酶体处理掉或者复敏化）
  • receptor resensitization (复敏化, typically 15–30min)

GPCR functions in neurotransmission

GPCR signaling is finely tuned to optimize signal strength, duration, and location.

Neurotransmitter receptors

• Ligand-gated ion channels （LGICs，配体门控离子通道受体）

  • Nicotinic acetylcholine (nAChR) receptor
  • Ionotropic glutamate receptors
    • NMDA receptor (Ca)
    • AMPA receptor (Na)
  • Glycine receptor (GlyR)
  • GABAA receptor
  • Serotonin receptor (5–HT3)
  • ATP–gated channels (P2X)

• Transporters （转运体）

  • 神经递质回收
  • Glutamate/aspartate transporters (EEATs and VGLUTs)
  • GABA transporters (GATs)
  • Glycine transporters (GlyTs)
  • Monoamine transporters (DAT, NET, SERT, VMAT)
    • DAT: 多巴胺转运体
  • Adenosine transporters (ENTs)
  • Vesicular acetylcholine transporter (VAChT)
    • acetylcholine：乙酰胆碱

• G protein-coupled neurotransmitter receptors

  GPCRs are generally thought to serve neuromodulatory roles in neurotransmission, and achieve their effects much more slowly than LGICs.

  • 神经肽受体大体都是GPCR
  • 调控速度相比离子通道型受体慢，属于 Metabotropic Receptors

Functionality

Neuromodulatory roles in neurotransmission:

1. modulation of gene expression to modulate neuroplasticity

2. presynaptically

3. postsynaptically

• dependence

  • Anatomical locations, Presynaptic/postsynaptic localization
  • Subunits activated

• GPCR–mediated neuromodulation

  • （例）突触前调制：抑制Ca离子通道开放，进而影响囊泡融合

  • （例）突触前调制：抑制K离子通道开放，使得细胞兴奋时间变长，Ca离子更容易进入（Ca离子通道是电压门控，高电压打开），促进囊泡融合

  • （例 直接调制 SNARE） binding to SNAP-25 prevents vesicle fusion

  • interneurons, heteroreceptor

  • GPCR may also be voltage-sensitive

• GPCR function in neuroplasticity

  • Associative Learning: Walters E T, Carew T J, Kandel E R. Associative learning in Aplysia: Evidence for conditioned fear in an invertebrate[J]. Science, 1981, 211(4481): 504-506.

  • Long-term depression (LTD)

    • Parallel fiber, climbing fiber, Purkinje cell

    • metabotropic glutamate receptor –> G-Protein –> DAG (activated)

    • 同时发生：climbing fiber 兴奋，Ca通道打开

    • Ca+DAG–>PKC (activated)

4 Glutamate and GABA receptor

阅读：http://www.bristol.ac.uk/synaptic/receptors/ampar/

Glutamate Receptor

阳离子通道，兴奋性受体，开放后介导膜的去极化

Glutamate

• The major excitatory neurotransmitter in the mammalian CNS, acting through both ligand-gated ion channels (ionotropic 离子型受体) and G-protein-coupled receptors (metabotropic 代谢型受体).
• Glutamate is stored in synaptic vesicles and released by Ca2+ dependent exocytosis.
• 谷氨酸代谢
  • 合成：谷氨酰胺+谷氨酸合成酶
  • 与GABA可以相互转化

1. AMPA receptor

AMPA和NMDA之间的主要区别在于，AMPA受体中仅发生钠和钾的流入 (注: 不绝对，exists Ca permeable AMPA receptors)，而NMDA受体中，除了钠和钾的流入以外，还发生钙的流入。 此外，AMPA受体在核心中不包含镁离子块，而NMDA在核心中包含镁离子块。

• Non-NMDA-type

• 配体是谷氨酸，结合点在胞外

• 四聚体：以来四个亚基，和细胞膜上的一些辅助亚基有互作

• 三次半跨膜

  

• Q/R剪切：决定能否透 Ca

• 胞内有很多调控和修饰位点

• LTP的产生：长时间增强（突触可塑性）

  • 依赖于AMPA受体
  • Glutamates bind to AMPAs and NMDARs, causing AMPAs to open and Na+ flows into the postsynaptic cell, resulting to a depolarization

• AMPA trafficking pathway

• AMPA interacting protein

  • NSF: 促进膜融合（上膜）

• AMPA pharmacology

  • agonists 激动剂
  • antagonists 拮抗剂

2. NMDA receptor

NMDA受体的结合，会导致离子通道非选择性地开启，使阳离子通过，进而使平衡电位改变至接近0mV。NMDA受体进行活化，必须依赖电压，这会导致离子通道会阻挡细胞外的Mg2+与Zn2+离子通过，并允许Na+离子与少量Ca2+离子流入细胞，以及使K +离子流出细胞，并保持电压的依赖性。功能性的NMDA受体必须含有NR1亚单位，多个NR2亚单位与NR1共同形成四聚体或五聚体。NR1是构成离子通道的基本亚单位；NR2是调节亚单位，不同NR2组成的NMDA受体表现出不同的脑内分布与生理学特性。

• NMDA: N-methyl-D-aspartate N-甲基-D-天冬氨酸

• 只有异聚体 (hetero-oligometric protein) 没有同聚体

• Has critical rule in Ca flux

• Subunits

  • NR1: 3个外显子，可形成8个剪切变体

  • NR2：

  • 在发育过程中的数量不同

    • GluN2A和GluN2B在发育过程中的switch

  • 脱敏时间不同

  • 开放概率不同

• Distinction :

  • 需要两个激动剂激活—glutamate and glycine
  • 开放后被 Mg离子 block，得等膜电位去极化到-40mv左右，才能把Mg离子赶出去
    • 和AMPA联动

• Topology

GABA receptor: GABA

aminobutyric，通过氯离子的阴离子通道，介导超极化

• subclasses: GABA, GABA, GABA

• 五聚体, 四次跨膜

• ligand-gated ion channels

• multiple subunits

• 门控特征：持续开闭

• subunits composition differs by locations (synaptic/extrasynaptic)

• there are developmental changes in GABA subunits expression

• shunting inhibition

  • 近胞体侧的GABA激活后分流出传导过来的阳离子（氯离子内流，使阳离子外流），抑制动作电位

• phasic inhibition (突触内) / tonic inhibition (突触外)

• GABA interaction proteins

  • Gephyrin: 抑制性突触特有

• 在发育阶段早期，胞内氯离子多，GABA受体的激活导致氯离子外流，因此会介导神经元的兴奋

  • 氯离子转运体表达的原因

推荐阅读

[1] Variations on an inhibitory theme: phasic and tonic activation of GABAA receptors (Nature reviews,2005)

[2] Regulation of AMPA receptor trafficking and synaptic plasticity (Current Opinion in Neurobiology,2011)

[3] Molecular basis of NMDA receptor functional diversity (European Journal of Neuroscience, 2011)

5 Calcium Signaling

• 胞内钙～0.0001 mMol，胞外钙～1.5mMol，非常敏感

• 细胞内钙离子浓度升高：外钙内流/内钙释放( –> ER –> Ca)

• 动作电位–>电压依赖钙离子通道

• TRP通道：感觉相关通道

• RYR通道

• CAMPKII: CA2+/calmodulin-dependent protein kinase II

  

• CAM: calmodulin target proteins

  • calmodulin：钙调蛋白

• BAPTA/EGTA: 敖合钙离子，BAPRA速率更高

• synaptotagmin：Ca2+ sensor that triggers exocytosis

• Ca2+ clear

• Neuronal Ca2+

• Astrocyte Ca2+

Nuclear calcium signaling

• Ca2+ promotes migration of transcription factors or transcriptional regulators to the nucleus
• DREAMsystem

Ca2+ wave

• Ca 信号的传播
• Ca促进ER释放Ca

Ca2+ indicator

• Bioluminesceny protein
• chemical calcium indicator
  • 亮度和Ca2+浓度有关
• FRET-based sensors
  • Ca2+升高–>CAM构象改变–>CFP能量转移激发YFP
• genetically encoded calcium indicator
  • Fura-2AM
  • GCaMP
    • CAM-M13-GFP
• 值的含义

6 Synaptic Plasticity Learning and Memory

• Postsynaptic density (PSD)
• 突触的量子释放理论

Long-term potentiation (LTP) 长时程增强

• Model for Hebbian theory

  

  • 

  • — learning rate, 描述突触可塑性程度

• LTP in hippocampus

  • early/late phase LTP

• LTP is pathway specific 专一性

• associativity of synaptic LTP

  • 2个弱刺激点联合刺激，可能激发单个强刺激所无法激活的LTP

• reduced failures after LTP

• 激活失败的概率降低，大电流下降，效率提高

• induction of LTP

  • silent synapse model：AMPA and NMDA
  • 大量Ca的进入是激活LTP的初始条件–>NMDA的开放很重要

• Post-synaptic active CaMKII causes an LTP-like effect

• LTP and behavior

  • 水迷宫与空间记忆 Morris maze

• Short-term plasticity

Long-term Depression (LTD)

• various forms of LTD
  • homosynaptic
  • heterosynaptic
  • associative LTD
  • cerebellar LTD

Learning and Memory

• Cell assembly
• H.M. 病人：海马是新记忆产生的起点
• 海马和空间记忆

7 Generation of Neural Network and Regeneration

神经发育

1. How do individual neurons differentiate and connect with their partners?
2. How do groups of neurons wire in a coordinated manner to form a functional circuit, such as a neural map?

Neural tube genesis

The vertebrate nervous system derives from the neural tube, an ectodermal structure. Extrinsic morphogens and intrinsic transcription factors pattern the neural tube differentiation.

• neural plate –> neural fold –> neural tube –> spinal cord

• Bone morphogenetic proteins (BMPs) signaling

  • suppressors of neuronal differentiation

• Wnt signaling

  • Response:
    • cells in anterior express Otx2 –> forebrain, midbrain
    • cells in posterior express Gbx2 –> hindbrain

Neural stem cells

• Ref: Annu. Rev. Neurosci. 32: 149–84 (2009)

• Asymmetric division (进行非对称型分裂)：progenitor cell–> (a) self–renewal (b) neuron/glia

  • neuronal progenitor –> neuron
  • glial progenitor –> astrocyte, oligodendrocyte

• Reason of asymmetric division: the distributino of cell constituents in precusor cells. The protein notch-1 and numb are differentially distributed in the precursor cells of the developing neucortex.

  • vertical cleavage (asymmetric) v.s. horizonatal cleavage (symmetric)

• Notch–delta mediated lateral inhibitation diverfsifies cell fates

Neuron migration

• Radial glial cells

  • neurons migrate along radial glial cells (radial migration)

• Inside–out pattern of neuronal migration

  • 早期产生的神经元位于皮层最内侧
  • 晚期产生的神经元越过早期神经元到达皮层最外侧

• GABAergic neurons（interneurons）: tangential migration (切向迁移)

  • after migration: random walk

• 成年期海马的神经发生是比较稀少的

Morphogens and transcription factors specify spinal cord neuronal fates

The nervous system has a rough blueprint that specifies the numbers and types of neurons at specific locations.

• Ref: New neurons and new memories: how does adult hippocampal neurogenesis affect learning and memory? 10.1038/nrn2822

• 转录因子在神经细胞发育早期高表达

• Dorsal/ventral targeting

• Neuronal growth cone

• Event: crossing the midline 脊髓发育中的重要事件

  • 跨过脊髓中线进行投射

• Neural survival: neurotrophic factor hypothesis 神经营养因子

  • NGF: nerve growth factor
  • BDNF: brain derived neurotrophic factor
  • interacts with a different transmembrane Trk

• Multiple cue-receptor systems guide axons to their final destinaations. 指示轴突到达正确的投射位置

Key steps in brain wiring

Synapse formation & elimination

• 突触修剪：Some neuromuscular synapses are eliminated after birth
• Axon pruning
  • primary axon extension –> interstitial axon branching –> selective axon elimination
• Signaling from surrounding glia

Self–avoidance of axonal and dendritic branches

• Dscam gene control

Repairing the damaged brain

• axons in the periphery regenerate better than those in the central nervous system

• peripheral and central nerves differ in their ability to support axonal regeneration

  • 外周神经系统比中枢神经系统有更好的损伤修复能力

• myelin and glial scar components inhibit regeneration of central axons

• using sources of neuronal cells with reprogramming and transplantation

8 Brain Circuit and Brain Disorders

History

• Localization of function in the brain
  • Broca area
• Golgi: staining, 网状学说
• Cajal: neuron doctrine 神经元学说

Neural Circuit

• What is neural circuit?
• Examples of neural circuit
• Two classes of neurons in neocortex: glutamatergic and GABAergic
  • interneuron diversity
  • three non-overlapping GABAergic interneuron
    • PA,sSA,

Construction

• Feedforward inhibition (FFI, 前馈抑制神经环路)

  • 兴奋性传入，一段时间后抑制性传入主神经元–> narraw time-window

• Feedback inhibition (FBI)

  • 兴奋性传入， 抑制周边兴奋性神经元（侧抑制）

• Disinhibitory: 兴奋性传入，抑制下游抑制性神经元

  • Disinhibitory microcircuit

• 行为学范式：auditory fear conditioning

Neural disorders

pass～

Questions

• how is the inside-out patch used in the in-vitro experiment?

• Prepulse?

• 有没有被验证？

• 电流钳和电压得到的电流曲线是相反的？

• Glu –> Astrocyte 耗能？

  • 让星胶去耗能

• what is the floor plate in spinal cord?

• 什么是受体的同/异聚体？为什么NMDA只有异聚体？

  • Oligomeric proteins may be composed either exclusively of several copies of identical polypeptide chains, in which case they are termed homo-oligomers, or alternatively by at least one copy of different polypeptide chains (hetero-oligomers).

• 什么是IPSC电流？

• What is cAMP?

• VIP neurons, PV neurons

idea

• Cell式的Notes风格
• 有重点地讲
• 关注抽象生物名词
  • e.g., plasticity
• 文献研讨：注意作者调研和课题背景