0573electrophysiology.ppt
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OUTLINEOUTLINE Resting Membrane PotentialResting Membrane Potential- Definitions and Normal valueDefinitions and Normal value- Ionic basisIonic basis Action PotentialsAction Potentials- DefinitionDefinition- Ionic basis – depolarization, Ionic basis – depolarization,
repolarization, hyperpolarizationrepolarization, hyperpolarization- CharacteristicsCharacteristics Propagation of Action PotentialsPropagation of Action Potentials Local PotentialsLocal Potentials
Membrane Potentials Membrane Potentials caused by Diffusioncaused by Diffusion
K concentration inside > outsideK concentration inside > outside -94 millivolts -94 millivolts diffusion potential of Kdiffusion potential of K Na concentration outside > insideNa concentration outside > inside +61 millivolts +61 millivolts diffusion potential of diffusion potential of
NaNa
Nernst PotentialNernst Potential
The diffusion potential across a The diffusion potential across a membrane that exactly opposes the net membrane that exactly opposes the net diffusion of a particular ion through the diffusion of a particular ion through the membrane membrane
Determined by the ratio of the Determined by the ratio of the concentrations of that specific ion on the concentrations of that specific ion on the two sides of the membranetwo sides of the membrane
EMF = EMF = ±61 log log concentration inside concentration inside concentration concentration outsideoutside
Nernst PotentialNernst Potential
AssumptionsAssumptions The potential in the ECF outside is The potential in the ECF outside is
zerozero The Nernst potential is the potential The Nernst potential is the potential
inside the membraneinside the membrane The sign of the potential is (+) if the The sign of the potential is (+) if the
diffusing ion is negativediffusing ion is negative The sign of the potential is (-) if the The sign of the potential is (-) if the
diffusing ion is positivediffusing ion is positive
Ion ChannelsIon Channels
- Are integral proteins that span the Are integral proteins that span the membrane and when open permit the membrane and when open permit the passage of proteinspassage of proteins
Ion channels are selectiveIon channels are selective Ion channels maybe or open or closedIon channels maybe or open or closed The conductance of a channel is The conductance of a channel is
dependent on the probability that the dependent on the probability that the channel is open or closedchannel is open or closed
Ion ChannelsIon Channels
1. Voltage Gated Channels1. Voltage Gated Channels
- Controlled by changes in membrane - Controlled by changes in membrane potentialpotential
2. Ligand gated channels2. Ligand gated channels- Opened or closed by hormone,Opened or closed by hormone,
neurotransmitters, and second neurotransmitters, and second messengersmessengers
Diffusion PotentialDiffusion Potential
Potential difference generated Potential difference generated across a membrane because of a across a membrane because of a concentration difference of an ionconcentration difference of an ion
Generated only if the membrane is Generated only if the membrane is permeable to the ionpermeable to the ion
Depends on the 1) concentration of Depends on the 1) concentration of ions, 2) permeability of the ions, 2) permeability of the membrane and 3) polarity of the membrane and 3) polarity of the charge of each ioncharge of each ion
Diffusion PotentialDiffusion Potential
Goldman EquationGoldman Equation
> Gives the calculated membrane > Gives the calculated membrane potential on the inside of the potential on the inside of the membranemembrane
> EMF = 61log> EMF = 61log C CNa Na iiPPNaNa+ C+ CK K iiPPKK + C + CCl Cl
ooPPClCl
CCNa Na ooPPNaNa+ C+ CK K ooPPKK + C + CCl Cl
iiPPClCl
Diffusion PotentialDiffusion Potential
Goldman EquationGoldman Equation Na, K, Cl are the most important Na, K, Cl are the most important
ions in the development of the ions in the development of the membrane potentialmembrane potential
The degree of importance of each of The degree of importance of each of the ions in determining the voltage the ions in determining the voltage is proportional to the membrane is proportional to the membrane permeabilitypermeability
Diffusion PotentialDiffusion Potential
Goldman EquationGoldman Equation A positive ion concentration gradient A positive ion concentration gradient
from inside the membrane to the from inside the membrane to the outside causes electronegativity outside causes electronegativity inside the membraneinside the membrane
Na and K channels undergoes rapid Na and K channels undergoes rapid changes during transmission of a changes during transmission of a nerve impulse; Cl channels does not nerve impulse; Cl channels does not change greatly change greatly
Resting Membrane Resting Membrane PotentialPotential
- -90millivolts- -90millivolts Na-K PumpNa-K Pump An An electrogenic pump electrogenic pump - more positive - more positive
charges are pumped to the outside (3 charges are pumped to the outside (3 Na out: 2 K in) causing a negative Na out: 2 K in) causing a negative potential insidepotential inside
An active metabolic processAn active metabolic process Strongly stimulated when excess Na Strongly stimulated when excess Na
accumulate inside the membraneaccumulate inside the membrane
Resting Membrane Resting Membrane PotentialPotential
K-Na leak ChannelsK-Na leak Channels K ions “leak” outsideK ions “leak” outside Channels are 100x more permeable Channels are 100x more permeable
to K to K
Resting Membrane Resting Membrane PotentialPotential
Normal Factors in the Establishment Normal Factors in the Establishment of the Normal Resting Membrane of the Normal Resting Membrane PotentialPotential
K Diffusion PotentialK Diffusion Potential
- K ions inside to outside 35:1- K ions inside to outside 35:1 Na Diffusion through the membraneNa Diffusion through the membrane Na-K PumpNa-K Pump
Voltage Gated Na Voltage Gated Na ChannelsChannels
Activation of Na channelsActivation of Na channels -70 to -50 mV causes -70 to -50 mV causes
conformational change opening the conformational change opening the gategate
Na ions then go inwards, increasing Na ions then go inwards, increasing Na permeability 500 to 5000 foldNa permeability 500 to 5000 fold
Voltage Gated Na Voltage Gated Na ChannelsChannels
Inactivation of Na channelsInactivation of Na channels Slower processSlower process Membrane potential begins to Membrane potential begins to
recover – recover – repolarizationrepolarization Will not reopen until the membrane Will not reopen until the membrane
potential returns to normalpotential returns to normal
Voltage Gated K Voltage Gated K ChannelsChannels
Activation of K ChannelsActivation of K Channels Opens when the membrane potential Opens when the membrane potential
approaches zeroapproaches zero Allows increased diffusion of K Allows increased diffusion of K
outsideoutside Combines with delayed closure of Combines with delayed closure of
Na channels to speed repolarizationNa channels to speed repolarization
Nerve Action PotentialNerve Action Potential
StagesStages Resting StageResting Stage
- Normal polarized state of -90mV- Normal polarized state of -90mV Depolarization StageDepolarization Stage- Increased Na permeability to the Increased Na permeability to the
insideinside- Overshoot phenomenonOvershoot phenomenon Repolarization StageRepolarization Stage
- Na channels close and K channels open- Na channels close and K channels open
Nerve Action PotentialNerve Action Potential
DefinitionsDefinitions1.1. Depolarization – makes the Depolarization – makes the
membrane potential less negativemembrane potential less negative2.2. Hyperpolarization – makes the Hyperpolarization – makes the
membrane potential become more membrane potential become more negativenegative
3.3. Inward Current – flow of positive Inward Current – flow of positive charge into the cellcharge into the cell
4.4. Outward Current – flow of positive Outward Current – flow of positive charge out of the cellcharge out of the cell
Nerve Action PotentialNerve Action Potential
DefinitionsDefinitions
5. Threshold – is the membrane potential 5. Threshold – is the membrane potential in which an action potential is inevitablein which an action potential is inevitable
6. Action Potential – property of excitable 6. Action Potential – property of excitable cells that consist of a rapid cells that consist of a rapid depolarization, followed by depolarization, followed by repolarization. They have stereotypical repolarization. They have stereotypical size and shape, are propagating and are size and shape, are propagating and are all or noneall or none
Events that Cause an Action Events that Cause an Action PotentialPotential
At rest, the conductance for K ions is 50- At rest, the conductance for K ions is 50- 100x greater than Na (K leak channels)100x greater than Na (K leak channels)
At the onset of action potential, Na At the onset of action potential, Na channels (5000x Na conductance) and K channels (5000x Na conductance) and K channels (opening more slowly) become channels (opening more slowly) become activatedactivated
At the end of the action potential, return At the end of the action potential, return of the membrane potential to the of the membrane potential to the negative statenegative state
Role of Other IonsRole of Other Ions AnionsAnions Responsible for the negative charge Responsible for the negative charge
during net deficit of positively charged during net deficit of positively charged ionsions
Ca IonsCa Ions Ca pump - pumps Ca from the interior Ca pump - pumps Ca from the interior
to the exteriorto the exterior Ca- Na channelsCa- Na channels – slow channels; Na – slow channels; Na
and Ca to the interiorand Ca to the interior Hypocalcemia –Hypocalcemia –Na channels become Na channels become
easily activated or highly excitable; easily activated or highly excitable; tetanytetany
Initiation of the Action Initiation of the Action PotentialPotential
Positive Feedback Vicious CyclePositive Feedback Vicious Cycle Threshold for InitiationThreshold for Initiation Na ions entering > K ions exitingNa ions entering > K ions exiting Sudden rise of Sudden rise of 15-30 millivolts15-30 millivolts is is
requiredrequired -65 millivolts, -65 millivolts, threshold for threshold for
stimulation stimulation
Propagation of Action Propagation of Action PotentialPotential
Nerve or Muscle Impulse – Nerve or Muscle Impulse – transmission of depolarization transmission of depolarization process along a nerve or muscle fiberprocess along a nerve or muscle fiber
Has no single directionHas no single direction All or nothing principle All or nothing principle Plateau in action potentialsPlateau in action potentials
1.1. Caused by fast and slow channelsCaused by fast and slow channels
2.2. Voltage gated K channelsVoltage gated K channels
Propagation of Action Propagation of Action PotentialPotential
Occurs by the spread of local Occurs by the spread of local currents to adjacent areas of currents to adjacent areas of membranemembrane
Conduction Velocity is increased by:Conduction Velocity is increased by: Increased fiber sizeIncreased fiber size Myelination – Myelination – saltatory conductionsaltatory conduction
Refractory PeriodsRefractory Periods
Absolute Refractory PeriodAbsolute Refractory Period- Period in which another action Period in which another action
potential cannot be elicited no potential cannot be elicited no matter how large the stimulusmatter how large the stimulus
- Caused by the closed inactivation Caused by the closed inactivation gates of the Na channelsgates of the Na channels
- No action potential can be generated No action potential can be generated until the inactivation gates openuntil the inactivation gates open
Refractory PeriodsRefractory Periods
Relative Refractory PeriodRelative Refractory Period- An action potential can be elicited An action potential can be elicited
during this period only if a larger during this period only if a larger than usual inward current is than usual inward current is providedprovided
- Caused by the higher K conductance Caused by the higher K conductance than at rest, therefore is farther than at rest, therefore is farther from the thresholdfrom the threshold
Refractory PeriodsRefractory Periods
AccommodationAccommodation- The cell membrane is held at a The cell membrane is held at a
depolarized level such that the depolarized level such that the threshold potential is passed without threshold potential is passed without firing an action potentialfiring an action potential
- Seen in Seen in hyperkalemiahyperkalemia