12Resting membrane potential（静息电位） A potential difference across the membranes of inactive cells, with the inside of the cell negative relative to the outside of the cellRanging from –10 to –100 mV
13（超射） （复极化） （极化） （超极化） （去极化） Overshoot refers to the development of a charge reversal.（超射）A cell is“polarized”becauseits interioris morenegativethan itsexterior.Repolarization ismovement backtoward theresting potential.（复极化）（极化）Depolarizationoccurswhen ionmovementreduces thechargeimbalance.Hyperpolarization isthe development ofeven more negativecharge inside the cell.（超极化）（去极化）
14electrochemical balance chemical driving forceelectrochemical balanceelectrical driving force
15K+ equilibrium potential (EK) (37oC) The Nernst Equation:K+ equilibrium potential (EK) (37oC)German physical chemist and physicistR=Gas constantT=TemperatureZ=ValenceF=Faraday’s constant（钾离子平衡电位）
16Begin:K+ in Compartment 2,Na+ in Compartment 1;BUT only K+ can move.Ion movement:K+ crosses intoCompartment 1;Na+ stays inCompartment 1.buildup of positive charge in Compartment 1 produces an electrical potential that exactly offsets the K+ chemical concentration gradient.At the potassiumequilibrium potential:
17Begin:K+ in Compartment 2,Na+ in Compartment 1;BUT only Na+ can move.Ion movement:Na+ crosses intoCompartment 2;but K+ stays inCompartment 2.buildup of positive charge in Compartment 2produces an electrical potential that exactlyoffsets the Na+ chemical concentration gradient.At the sodiumequilibrium potential:
18Difference between EK and directly measured resting potential Ek Observed RPMammalian skeletal muscle cell mV -90 mVFrog skeletal muscle cell mV -90 mVSquid giant axon mV -70 mV
19Goldman-Hodgkin-Katz equation The systemic inflammatory response syndrome (SIRS) is a clinical response arising from a nonspecific insult manifested by two or more of the following:Fever or hypothermiaTachycardiaTachypneaLeukocytosis, leukopenia, or a left-shift (increase in immature neutrophilic leukocytes in the blood)Recent evidence indicates that hemostatic changes play a significant role in many SIRS-linked disorders.Bone RC, Balk RA, Cerra FB, et al. Definitions for sepsis and organ failure and guidelines for the use of innovative therapies in sepsis. Chest. 1992;101:Opal SM, Thijs L, Cavaillon JM, et al. Relationships between coagulation and inflammatory processes. Crit Care Med. 2000; 28:S81-2.Goldman-Hodgkin-Katz equation
20Role of Na+-K+ pump: Electrogenic Hyperpolarizing Establishment of resting membrane potential:Na+/K+ pump establishes concentration gradientgenerating a small negative potential; pumpuses up to 40% of the ATP produced by that cell!The systemic inflammatory response syndrome (SIRS) is a clinical response arising from a nonspecific insult manifested by two or more of the following:Fever or hypothermiaTachycardiaTachypneaLeukocytosis, leukopenia, or a left-shift (increase in immature neutrophilic leukocytes in the blood)Recent evidence indicates that hemostatic changes play a significant role in many SIRS-linked disorders.Bone RC, Balk RA, Cerra FB, et al. Definitions for sepsis and organ failure and guidelines for the use of innovative therapies in sepsis. Chest. 1992;101:Opal SM, Thijs L, Cavaillon JM, et al. Relationships between coagulation and inflammatory processes. Crit Care Med. 2000; 28:S81-2.
21Sodium-potassium Pump Click here to play theSodium-potassium PumpFlash Animation
22Origin of the normal resting membrane potential K+ diffusion potentialNa+ diffusionNa+-K+ pump
24Action potential（动作电位） Some of the cells (excitable cells) are capable to rapidly reverse their resting membrane potential from negative resting values to slightly positive values. This transient and rapid change in membrane potential is called an action potential
25Negative after-potential A typical neuron action potentialPositiveafter-potentialNegative after-potentialSpike potential After-potential
27The size of agraded potential(here, gradeddepolarizations)is proportionateto the intensityof the stimulus.
28Graded potentials can be:. EXCITATORY. or. INHIBITORY Graded potentials can be: EXCITATORY or INHIBITORY (action potential (action potentialis more likely) is less likely)The size of a graded potential is proportional to the size of the stimulus.Graded potentials decay as they move over distance.
29Graded potentials decay as they move over distance.
30Local response（局部反应） Not “all-or-none” （全或无） Electrotonic propagation: spreading with decrement（电紧张性扩布）Summation: spatial & temporal（时间与空间总和）
31Threshold Potential（阈电位）: level of depolarization needed to trigger an action potential (most neurons have a threshold at -50 mV)
36The rapid opening of voltage-gated Na+ channels explains the rapid-depolarization phase at thebeginning of the action potential.The slower opening of voltage-gated K+ channelsexplains the repolarization and after hyperpolarizationphases that complete the action potential.
39voltage-gated Na+ channels allows rapid entry of Na+, An action potentialis an “all-or-none”sequence of changesin membrane potential.The rapid opening ofvoltage-gated Na+ channelsallows rapid entry of Na+,moving membrane potentialcloser to the sodiumequilibrium potential (+60 mv)Action potentials resultfrom an all-or-nonesequence of changesin ion permeabilitydue to the operationof voltage-gatedNa+ and K + channels.The slower opening ofvoltage-gated K+ channelsallows K+ exit,moving membrane potentialcloser to the potassiumequilibrium potential (-90 mv)
40Voltage Gated Channels Click here to play theVoltage Gated Channelsand Action PotentialFlash Animation
41Mechanism of the initiation and termination of AP
42Re-establishing Na+ and K+ gradients after AP Na+-K+ pump“Recharging” process
43Properties of action potential (AP) Depolarization must exceed threshold value to trigger APAP is all-or-noneAP propagates without decrement
46Nobel Prize in Physiology or Medicine 1963 How to study ?Nobel Prize in Physiology or Medicine 1963"for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane"Eccles Hodgkin HuxleyVoltage Clamp
48Nobel Prize in Physiology or Medicine 1991 Patch ClampNobel Prize in Physiology or Medicine 1991"for their discoveries concerning the function of single ion channels in cells"Erwin Neher Bert Sakmann
50The Squid and its Giant Nerve Fiber VideoThe Squid and its Giant Nerve Fiber "The Squid and its Giant Nerve Fiber" was filmed in the 1970s at Plymouth Marine Laboratory in England. This is the laboratory where Hodgkin and Huxley conducted experiments on the squid giant axon in the 1940s. Their experiments unraveled the mechanism of the action potential, and led to a Nobel Prize. Long out of print, the film is an historically important record of the voltage-clamp technique as developed by Hodgkin and Huxley, as well as an interesting glimpse at how the experiments were done.
62Factors affecting excitability Resting potentialThresholdChannel state
63The propagation of the action potential from the dendritic to the axon-terminal end is typically one-way because theabsolute refractory period follows along in the “wake”of the moving action potential.
65Action potential Refractory period Depolarization: Activation of voltage-gated Na+ channelRepolarization: Inactivation of Na+ channel, and activation of K+ channelRefractory periodAbsolute refractory periodRelative refractory period
66CasePrimary Hyperkalemic Periodic Paralysis (原发性高血钾性周期性麻痹) A l0-year-old boy has sporadic attacks of muscle paralysis. The patient has four brothers, all of whom have suffered similar symptoms. The onset of these attacks is characterized by pain associated with contractures of the affected muscles. Later in the attack those muscles may become paralyzed and more flaccid. Episodes of pain and contracture frequently occur without subsequent paralysis. Analysis of blood samples taken during an attack indicates that the patient is hyperkalemic. Plasma K+ levels are normal when the patient is not having an attack. Biopsies of the patient's muscle show a significantly diminished level of intracellular K+ (83 mmol/kg wet tissue) compared with control muscle (95 mmol/kg wet tissue). Basal tissue activity of Na+, K+-ATPase is normal. Paralytic attacks are accompanied by diuresis with increased K+ excretion. Electrophysiologic studies of the patient show that during an attack the excitability and conduction times of motor neurons are normal, as is the function of the neuromuscular junction. Microelectrode studies show that during an attack the magnitude of the resting membrane potential of skeletal muscle cells is diminished compared with control muscle fibers. Electromyography shows that early pared with control muscle fibers. Electromyography shows that early in an attack the muscle contractures are associated with spontaneous action potentials in the affected muscle fibers. Later, during the paralytic phase of an attack, muscle cells become electrically inexcitable - the muscle cells do not respond electrically to stimulation of the motor axons that innervate them. A paralytic attack can be relieved by treating the patient with an insulin injection. Long-term administration of the 2-agonist salbutamol dramatically diminishes the occurrence of episodes of both contractures and subsequent paralytic attacks.
67Questionsl. What might account for the patient being hyperkalemic during an attack, while the potassium concentration in his skeletal muscle cells is diminished? What types of alterations of basic cellular processes might underlie this situation? 2. What explains the observation that the magnitude of the resting membrane potentials of the patient's skeletal muscle fibers is diminished during an attack? 3. Does the diminished resting membrane potential have anything to do with the spontaneous action potentials and contractures that occur early in an attack, before paralysis sets in? 4. How might the diminished resting membrane potential contribute to the paralytic phase of an attack, in which muscle cells are electrically inexcitable? 5. How might insulin terminate a paralytic attack? 6. How might long-term administration of salbotamol diminish the occurrence of attacks of contractures and paralysis?
68Answersl. The hyperkalemia with a concomitant decrease in the amount of K+ in muscle cells suggests that the hrperkalemia is caused bf K+ efflux from the cells, but the cause of K+ efflux is not known. Net K+ efflux from muscle cells might occur because of diminished fate of K+ accumulation by the Na+, K+-ATPase, or an increased rate of K+ efflux from the cell, or a combination of both factors. The observation that Na+, K+-ATPase is normal does not completely rule out a malfunction of this protein during an attack. 2. Elevating extracellular K+ and decreasing the intracellular level of K+ would decrease the potassium equilibrium potential and thus decrease the magnitude of the resting membrane potential. 3. The decreased magnitude of the resting membrane potential initially brings the muscle cells closer to threshold for firing an action potential. For this reason, spontaneous, small fluctuations in the resting membrane potential may reach threshold. This results in spontaneous action potentials and contractions of skeletal muscle cells and leads to the contractures experienced by the patient early in an attack. 4. Prolonged depolarization of the muscle cell plasma membrane will lead to voltage inactivation of Na+ channels in the membrane, which will result in the muscle cell's being unable to fire an action potential. This is believed to be the cause of the paralytic phase of an attack and is supported by the observation that during the paralytic phase, the patient's skeletal muscle cells may be electrically inexcitable. 5. Insulin immediately and powerfully promotes the uptake of K+ by cells and the extrusion of Na+ from cells. Administration of insulin thus corrects the hyperkalemia restores cellular K+ levels toward normal, and causes the resting membrane potential for the affected skeletal muscle cells to become closer to the normal resting value. In this way insulin is believed to terminate an attack of contractures or paralysis in these patients. 6. Long-term administration of salbutamol, a β2-agonist, increases the activity of the Na+, K+-ATPase in skeletal muscle cells. In this way, salbutamol administraion leads to increased sequestration of K+ in muscle cells. Apparently this helps to prevent the K+ efflux that underlies episodes of hyperkalemia with resultant contractures that may be followed by periods of paralysis.