细胞生理功能

nullnullGeneral Functions of CellsXiaoqun Qin, PhD.,MD.Cellular Basic of Physiologic Functionnull Part Ⅰ: The Functions of Cell MembraneEssential role of cell membrane:Essential role of cell membrane:Action of outpost Exchange of material Accept signal Distinguish mark Programming reactions nullnullCDModel of simple diffusionD = k×△CnullFacilitated diffusion1.Conception: the substances which are insoluble in lipids pass through the cell membrane with it’s grade by the help of membrane proteins. 2.Character: ①with grade; ②specific ;③adjustable . 3.Types: (1)channel mediated diffusion: ①receptor operated; ②voltage gated ; ③mechanic gated. (2)carrier mediated diffusion: specific, saturable,competitive. nullActive transportActive transportCharacter: using ATP; against the grade. Significance: reserving potential; protect cell from swollen. 2 K+3 Na+ATP ADP+PiCharacteristics of the Transport by Na+ pumpCharacteristics of the Transport by Na+ pumpDirectional transport Coupling process ATP is directly required Electrogenic process Importance of the Na+-K+ PumpImportance of the Na+-K+ PumpMaintain high intracellular K+ concentration gradients across the membrane. Control cell volume and phase Maintain normal pH inside cell Develop and Maintain Na+ and K+ concentration gradients across the membrane Electrogenic action influences membrane potential Provides energy for secondary active transportSecondary Active TransportSecondary Active TransportCoupled transport. Energy needed for “uphill” movement obtained from “downhill” transport of Na+. Hydrolysis of ATP by Na+/K+ pump required indirectly to maintain [Na+] gradient.nullSecondary active transportNa+glucoseNa+H+out inout in co-transport counter-transport (symport) (antiport)Co-transporters will move one moiety, e.g. glucose, in the same direction as the Na+.Counter-transporters will move one moiety, e.g. H+, in the opposite direction to the Na+.nullOut side higher potassium and lower glucoseInside lower potassium and higher glucosenullBulk Transport (Endocytosis and Excytosis)Bulk Transport (Endocytosis and Excytosis)Movement of many large molecules, that cannot be transported by carriers. Exocytosis: A process in which some large particles move from inside to outside of the cell by a specialized function of the cell membrane Endocytosis: Exocytosis in reverse. Specific molecules can be taken into the cell because of the interaction of the molecule and protein receptor.nullExocytosis Vesicle containing the secretory protein fuses with plasma membrane, to remove contents from cell. nullEndocytosis Material enters the cell through the plasma membrane within vesicles.nullTypes of EndocytosisPhagocytosis - (“cellular eating”) cell engulfs a particle and packages it with a food vacuole. Pinocytosis – (“cellular drinking”) cell gulps droplets of fluid by forming tiny vesicles. (unspecific) Receptor-Mediated – binding of external molecules to specific receptor proteins in the plasma membrane. (specific)nullExample of Receptor-Mediated Endocytosis in human cellsThe pathway of signal transduction across membrane The pathway of signal transduction across membrane Receptor-gated channel Receptor-GTP binding protein-effecter enzyme system: CA→cAMP; CG →cGMP; PLC →IP3,DG Tyrosine protein kinase (TPK) receptor nullSignal across membrane transduction based on the receptor-G protein-effect enzyme system and induction of the second messages.Part Ⅱ: Cellular BioelectricityPart Ⅱ: Cellular BioelectricityConception about stimulus and excitabilityConception about stimulus and excitabilityExcitation: an action potential occurs after a effective stimulus acting on cell. Excitability: cell’s ability to genesis action potential after being stimulated. Excitable cells: cells which are easy to genesis action potential under stimulus. Essential element of stimulus: intensity ,duration,changing rate.nullPP0T0TIntensity-duration curveThreshold stimulusnullMeasurement of cell bioelectricity:Extracellular record; Microelectrode and Intracellular record; Voltage clamp and patch clampnullnullnullCole and colleagues developed a method for maintaining Vm at any desired voltage level (FBA, Feedback Amplifier) Required monitoring voltage changes, feeding it through an amplifier to drive current into or out of the cell to dynamically maintain the voltage while recording the current required to do soThe voltage clampnullINa=GNa×(Em-ENa)nullnullnullCytoplasmIon channels"Giga-seal"GlassmicroelectrodeSuction1 µmPatch clamp recordingCell Membranenullnullnull100 ms4 pAClosedOpenSingle channel recordnullDescribing words for cell bioelectricity:Trans-membrane potential: including resting potential, polarization, hyperpolarization, depolarization, repolarization Trans-membrane current: inward current, outward current Character of channels: membrane resistance and conductance depend upon the number of open channels. G=1/R (unit: Siemens,S) the voltage-dependent functional states of channels. all or none feature in a single channel opening and integrated behave of all collected channels. nullResting Potential (RP)RP: difference of potentials between both sides of the membrane in resting state.Mechanism of RP formation: (1) unequal distribution of ions between in and out of cell: [Na+]i << [Na+]o; [K+]i >> [K+]o generate ΔCK+, a force to drive K+ flow out of cell; (2) in resting state, only a selective permeability of potassium exists, K+ is driven to flow out but no other kind of ions is allowed to migrate across the membrane, therefore a potential (E K+) generated, which resistance K+ flow out; (2) when ΔCK+ equal to E K+, net of K+ flow become 0, E K+ reaches to a equilibrium potential, that is resting potential.nullIdentification:1. Nernst formula Ek= 59.5 Log [K+ ]o/[K+ ]i (mV) theoretic –87mV，actual –77mV2. Change the concentration of K+ in extracellular fluid3. Using TEA to block potassium channelsnull Action Potential (AP) Action potential is a rapid, reversible, and conductive change of the membrane potential after the cell is stimulated.nullSuccessive Stages: Resting Stage Depolarization stage Repolarization stage After-potential stage(1)(2)(3)(4)nullnull The Hodgkin-Huxley Model of Action Potential Generationnullnull1. Nernst formula ENa= 59.5 Log [Na+]o/[Na+]i (mV) Overshot value = ENa2. Change the concentration of Na＋ in extracellular fluid3. Using tetrodotoxin (TTX) to block Na＋ channel4. Voltage clamp and patch clampcertificationnullTriphasic responsenullModern proof of nature of currentsUse ion selective agentsnullRemoving Na+ from the bathing medium, INa becomes negligible so IK can be measured directly. Subtracting this current from the total current yielded INa. nullVoltage-Dependence of ConductancenullnullHow channel conductances accumulateNext page shows an idealized versionnullnullAll or None character of action potentialThe amplitude of AP do not vary with stimulating intensitydigital signalThe amplitude of AP do not decline during propagatingnullVariation of excitability on different membrane potential level ：absolute refractory period: no any responserelative refractory period: weak responsesubnormal period: hyporesponsesupranormal period: hyperresponse Depend on states of Na channels and distance between Em and threshold potentialnullnullSlide 3 of 28 nullinactivationrecoveryRepolarization to RPdepolarizationautomaticallyResting stateActive stateInactive statenullGenesis and propagation of action potentialthreshold membrane potential the membrane potential which is the lowest level of depolarization to induce a AP and at which the rate of Na channel activating is just counteract the rate of K outward current.nulllocal potential or local responseConception: sub-threshold stimuli—induced a small and short-lived depolarization of membrane potential. * Dependent of stimulating-intensity * Decline----electrotonic propagation * Without refractory period * Summation: temporal and spatial nullPropagation of the Action PotentialPropagation of the Action PotentialnullnullnullFactors that affect the propagationFactors that affect the propagationBioelectric properties of the membrane Velocity and amplitude of membrane depolarizationnull Saltatory ConductionSaltatory ConductionSaltatory ConductionSaltatory ConductionThe pattern of conduction in the myelinated nerve fiber from node to node It is of value for two reasons: very fast conserves energy.nullnullPart Ⅲ Contraction of Muscle Cells Classification of the MuscleSkeletal MuscleCardiac MuscleSmooth MuscleClassification of the MuscleSignal Transmission Through the Neuromuscular JunctionSignal Transmission Through the Neuromuscular JunctionnullSkeletal Muscle InnervationnullIllustration of the Neuromuscular Junction (NMJ)nullnullK+OutsideInsideK+K+K+K+K+K+K+K+K+K+K+Ca2+ induces fusion of vesicles with nerve terminal membrane.ACh is released and diffuses across synaptic cleft.ACh binds to its receptor on the postsynaptic membraneBinding of ACh opens channel pore that is permeable to Na+ and K+.K+Muscle membraneNerve terminalnullEnd Plate Potential (EPP)OutsideInsideMuscle membranePresynaptic terminalMuscle Membrane Voltage (mV)Time (msec)-90 mVVKVNa0ThresholdPresynaptic APThe movement of Na+ and K+ depolarizes muscle membrane potential (EPP)ACh Receptor ChannelsVoltage-gated Na ChannelsInward Rectifier K ChannelsnullMeanwhile ...OutsideInsideACh unbinds from its receptorMuscle membraneso the channel closesNerve terminalACh is hydrolyzed by AChE into Choline and acetateCholine is taken up into nerve terminalCholine resynthesized into ACh and repackaged into vesiclenullNeuromuscular TransmissionProperties of neuromuscular junction 1:1 transmission: A chemical transmission which is designed to assure that every presynaptic action potential results in a postsynaptic one An unidirectional process Has a time delay. 20nm/0.5-1ms Is easily affect by drugs and some factors The NMJ is a site of considerable clinical importanceAnticholinesterase AgentsAnticholinesterase AgentsAnticholinesterase (anti-ChE) agents inhibit acetylcholinesterase （乙酰胆碱酯酶） prolong excitation at the NMJ Uses of anti-ChE agentsUses of anti-ChE agentsClinical applications (Neostigmine, 新斯的明, Physostigmine毒扁豆碱) Insecticides (organophosphate 有机磷酸酯) Nerve gas (e.g. Sarin 沙林，甲氟膦酸异丙酯。一种用作神经性毒气的化学剂))Sarin and Aum Shinrikyo(奥姆真理教) Sarin and Aum Shinrikyo(奥姆真理教) Aum Shinrikyo(奥姆真理教) is a Japanese religious cult obsessed with the apocalypse （启示，天启）. The previously obscure group became infamous in 1995 when some of its members released deadly sarin nerve gas into the Tokyo subway system, killing 12 people and sending more than 5,000 others to hospitals. SarinSarinSarin, which comes in both liquid and gas forms, is a highly toxic and volatile nerve agent developed by Nazi scientists in Germany in the 1930s. Chemical weapons experts say that sarin gas is 500 times more toxic than cyanide （氢化物） gas. nullNMJ DiseasesMyasthenia Gravis （重症肌无力） Autoimmunity to ACh receptor Fewer functional ACh receptors Low “safety factor” for NM transmission Lambert-Eaton syndrome（兰伯特-伊顿综合征 ，癌性肌无力综合征 ） Autoimmunity directed against Ca2＋ channels Reduced ACh release Low “safety factor” for NM transmissionnull箭毒nullMolecular Mechanism of Muscular ContractionThe sliding filament model Muscle shortening is due to movement of the actin filament over the myosin filament Reduces the distance between Z-linesnullThe Sliding Filament Model of Muscle ContractionnullnullThick filaments (myosin)Bundle of myosin proteins shaped like double-headed golf clubs Myosin heads have two binding sites Actin binding site forms cross bridge Nucleotide binding site binds ATP (Myosin ATPase) Hydrolysis of ATP provides energy to generate power strokenullThin filaments (actin)Backbone: two strands of polymerized globular actin – fibrous actin Each actin has myosin binding site Troponin Binds Ca2+; regulates muscle contraction Tropomyosin Lies in groove of actin helix Blocks myosin binding sites in absence of Ca2+nullThick filament: Myosin (head and tail) Thin filament: Actin, Tropomyosin, Troponin (calcium binding site)nullTHE CROSS-BRIDGE CYCLEATPADP + PiAlMA – M l ATPAlMlADPlPiA + M l ADP l PiRelaxed stateCrossbridge energisedCrossbridge attachment Tension develops Crossbridge detachment Ca2+ presentA, Actin; M, MyosinnullCa2+ Controls ContractionCa2+ Controls ContractionCa2+ Channels and Pumps Release of Ca2+ from the SR triggers contraction Reuptake of Ca2+ into SR relaxes muscle So how is calcium released in response to nerve impulses? Answer has come from studies of antagonist molecules that block Ca2+ channel activitynullExcitation-Contraction CouplingDepolarization of motor end plate (excitation) is coupled to muscular contraction Nerve impulse travels along sarcolemma and down T-tubules to cause a release of Ca2+ from SR Ca2+ binds to troponin and causes position change in tropomyosin, exposing active sites on actin Permits strong binding state between actin and myosin and contraction occurs ATP is hydrolyzed and energy goes to myosin head which releases from actinnullTransverse tubules connect plasma membrane of muscle cell to SRnullCa2+ release during Excitation-Contraction couplingAction potential on motor endplate travels down T tubulesnullVoltage -gated Ca2+ channels open, Ca2+ flows out SR into cytoplasm Ca2+ channels close when action potential ends. Active transport pumps continually return Ca2+ to SRCa ATPase (SERCA)Dihydropyridine ReceptorDihydropyridine ReceptorIn t-tubules of heart and skeletal muscle Nifedipine and other DHP-like molecules bind to the "DHP receptor" in t-tubules In heart, DHP receptor is a voltage-gated Ca2+ channel In skeletal muscle, DHP receptor is apparently a voltage-sensing protein and probably undergoes voltage-dependent conformational changes Ryanodine ReceptorRyanodine ReceptorThe "foot structure" in terminal cisternae of SR Foot structure is a Ca2+ channel of unusual design Conformation change or Ca2+ -channel activity of DHP receptor apparently gates the ryanodine receptor, opening and closing Ca2+ channels Many details are yet to be elucidated! Skeletal muscleSkeletal muscleThe AP: moves down the t-tubule voltage change detected by DHP （双氢吡啶） receptors DHP receptor is essentially a voltage-gated Ca channel is communicated to the ryanodine receptor which opens to allow Ca out of SR activates contractionCardiac muscleCardiac muscleThe AP: moves down the t-tubule voltage change detected by DHP receptors (Ca channels) which opens to allow small amount of (trigger) Ca into the fibre Ca binds to ryanodine receptors which open to release a large amount of (activator) Ca (CACR) Thus, calcium, not voltage, appears to trigger Ca release in Cardiac muscle!nullThe manifestation of muscle contraction --------force/tension and shortening The types of muscle contraction ---------isotonic contraction ---------isometric contraction ---------single twitch ---------tetanus Mechanics of Muscle ContractionTension and LoadTension and LoadThe force exerted on an object by a contracting muscle is known as tension. The force exerted on the muscle by an object (usually its weight) is termed load. According to the time of effect exerted by the loads on the muscle contraction the load was divided into two forms, preload and afterload.Types of Contractions Types of Contractions Twitch: a brief mechanical contraction of a single fiber produced by a single action potential at low frequency stimulation is known as single twitch. Tetanus: It means a summation of twitches that occurs at high frequency stimulationnullEffects of Repeated StimulationsFigure 10.15nullnullIsometric Contraction Length of muscle remains constant during contraction. A maximal active tension can be produced but no shortening occur. Isotonic Contraction Length of muscle changes. Tension fairly constant. Involves movement at jointsnullIsotonic and Isometric ContractionsPreloadPreloadPreload is a load on the muscle before muscle contraction. Determines the initial length of the muscle before contraction. Initial length is the length of the muscle fiber before its contraction. It is positively proportional to the preload.nullThe Effect of Sarcomere Length on TensionThe Length – Tension Curve Concept of optimal lengthAfterloadAfterloadAfterload is a load on the muscle after the beginning of muscle contraction. The reverse force that oppose the contractile force caused by muscle contraction. The afterload does not change the initial length of the muscle, But it can prevent muscle from shortening because a part of force developed by contraction is used to overcome the afterload.nullResistance and Speed of ContractionnullnullContractility is identified as an inner property of muscle, which determine the efficacy of muscle contraction.Effects of contractility on mechanics of muscle contractionIts potential acting site including calcium release, calcium sensitivity, crossbridge affinity, activity of ATPase, speed of crossbridge move, heat-work efficiency of ATP,etc.nullnullThank you!See you next time.