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Membrane Potential

At restAt rest * * A membrane potential at rest is defined A membrane potential at rest is defined

as the potential at which the flow of ions as the potential at which the flow of ions out (mostly Kout (mostly K++) is ) is equalequal to the flow of ions to the flow of ions in (mostly Nain (mostly Na++))

* * To maintain this in a steady state, the To maintain this in a steady state, the cell must utilizecell must utilize pumpspumps

agents most important in membrane potential

•ions electrical Charge

• membrane permeability

• ion concentration in both side membrane

• ion channels and pomp Na-K

• Na K K+ “leak” channelsK+ “leak” channels

ACTION POTENTIALACTION POTENTIAL 1. 1. RESTING PHASERESTING PHASE

- - Voltage gated Na+ channels Voltage gated Na+ channels are in are in resting stateresting state

(CLOSED Activation gate and OPEN inactivation gate i.e. (CLOSED Activation gate and OPEN inactivation gate i.e. NO Na+ NO Na+

passing through)passing through)

- Voltage gated K+ channels are closed- Voltage gated K+ channels are closed

2.2. DEPOLARISATIONDEPOLARISATION

- Membrane depolarizes ( e.g. from an incoming stimulus) to threshold Membrane depolarizes ( e.g. from an incoming stimulus) to threshold

(~ -55mV) and (~ -55mV) and opens Na+ activationopens Na+ activation

gategate Na+ inflowNa+ inflow (down its concentration gradient) (down its concentration gradient)

- Na+ inflow further depolarizes membrane until polarity reverses.Na+ inflow further depolarizes membrane until polarity reverses.

3.3. REPOLARISATION BEGINSREPOLARISATION BEGINS

- At reverse membrane polarity (more depolarization), At reverse membrane polarity (more depolarization), voltage gated K+ voltage gated K+ channels openschannels opens K+ outflowK+ outflow (down its concentration gradient) (down its concentration gradient)

- Na+ inactivation gates closeNa+ inactivation gates close

4. 4. REPOLARISATION CONTINUESREPOLARISATION CONTINUES

- K+ outflowK+ outflow restores RMP. restores RMP.

- Na+ inactivation gates open and K+ gates closes.Na+ inactivation gates open and K+ gates closes.

NOTE : At this stage (4) NOTE : At this stage (4) RMP is restoredRMP is restored, but , but not the electrochemical not the electrochemical gradientgradient..

Calculating the equilibrium Calculating the equilibrium potential of an ion across a potential of an ion across a

membranemembraneWe use the We use the NernstNernst equation to determine equation to determine

this value for each ion (X)this value for each ion (X)::EExx = (RT/zF) log = (RT/zF) logee{X{Xoo/X/Xii}}

– R=Gas constantR=Gas constant T=°KelvinT=°Kelvin– z=valence of ion (X)z=valence of ion (X) F=Faraday’s F=Faraday’s

constantconstant

Reduces to:Reduces to:

EExx =+_ 61 (log =+_ 61 (log1010{X{Xii/X/Xoo})})

Examples of normal concentration values Examples of normal concentration values and equilibrium potentials and equilibrium potentials

Ion outside inside ratio(o/i) EIon outside inside ratio(o/i) Exx (mV)(mV)NaNa++ 145 mM 12 mM 145 mM 12 mM 12 12 +67 +67

KK++ 4 mM 155 mM 4 mM 155 mM 0.026 0.026 -98 -98

ClCl-- 123 mM 123 mM 4.2 mM 4.2 mM 29 29 -90 -90

CaCa++++ 1.5 mM 1.5 mM 1010-7-7 mM 15,000 mM 15,000 +129 +129

(the net effect is a membrane potential of -90mV - how?)(the net effect is a membrane potential of -90mV - how?)

Membrane potentialMembrane potential..

*Membrane potential*Membrane potential is the ‘sum’ of all of is the ‘sum’ of all of the different ions and their concentration the different ions and their concentration differences across the membranedifferences across the membrane

*Generally, we focus on the dominant ions *Generally, we focus on the dominant ions for purposes of simplifying this calculationfor purposes of simplifying this calculation– NaNa++, K, K++, Cl, Cl- - (sometimes calcium is included)(sometimes calcium is included)

Action PotentialsAction Potentials PhasesPhases

– DepolarizationDepolarization Inside plasma membrane Inside plasma membrane

becomes less negativebecomes less negative

– RepolarizationRepolarization Return of resting Return of resting

membrane potentialmembrane potential

All-or-none principleAll-or-none principle– Like camera flash systemLike camera flash system

PropagatePropagate– Spread from one location Spread from one location

to anotherto another FrequencyFrequency

– Number of action potential Number of action potential produced per unit of timeproduced per unit of time

ALL-OR-NONE PRINCIPLEALL-OR-NONE PRINCIPLE

- i.e. Means that each time an AP arises, it always have a i.e. Means that each time an AP arises, it always have a constant constant and maximum strengthand maximum strength and will therefore will be and will therefore will be conducted along conducted along the axonthe axon..

ANALOGY – A long ANALOGY – A long row of dominoesrow of dominoes, once you , once you push the firstpush the first domino, domino, all the dominos along the row will fallall the dominos along the row will fall. So it either falls or . So it either falls or remains standing.remains standing.

Calculating the membrane potentialCalculating the membrane potentialGoldman equationGoldman equation

Vm=+_ 61 logVm=+_ 61 log (X)(X)

XX = P = PKK[K][K]oo +P +PNaNa[Na][Na]oo +P +PClCl[Cl][Cl]ii

PPKK[K][K]ii +P +PNaNa[Na][Na]ii +P +PClCl[Cl][Cl]oo

P permeabilityP permeability

Frequency of Action PotentialsFrequency of Action Potentials

Figure 8-13: Coding for stimulus intensity

Generator PotentialsGenerator Potentials

In response to stimulus, In response to stimulus, sensory nerve endings sensory nerve endings produce a local graded produce a local graded change in membrane change in membrane potential.potential.

Potential changes are Potential changes are called receptor or called receptor or generator potential.generator potential.– Analogous to EPSPs.Analogous to EPSPs.

Phasic response: Generator potential increases with increased stimulus, then as stimulus continues, generator potential size diminishes.

Tonic response: Generator potential proportional to intensity of stimulus.

Figure 10-2

cardiac action potential : Fast

slow

Figure 44.2

dendritddndritereceives information

cell bodycontains nucleus & organelles

axontransmits nerve impulse

axon terminaltransmits to next neuron

synapsejunction between two neurons

Direction of nerve im

pulsedendrite

Cell body

Axon

Axon terminal

synapse

Two types

AbsoluteWhen Na+ channels close, at peak of AP, they do not reopen for a time

RelativeMembrane hyperpolarizedSome Na+ channels still refractory

Refractory Period

ACTION POTENTIALACTION POTENTIAL

PROPERTIES OF ACTION POTENTIALPROPERTIES OF ACTION POTENTIAL;;

1. REFRACTORY PERIODS (Resting periods) 1. REFRACTORY PERIODS (Resting periods)

2. CONDUCTION PATTERN 2. CONDUCTION PATTERN

PROPERTIES OF ACTION POTENTIALPROPERTIES OF ACTION POTENTIAL REFRACTORY PERIOD – is the period ofREFRACTORY PERIOD – is the period of time time that an excitable that an excitable

tissue tissue can not generate another APcan not generate another AP

2 types of refractory period;2 types of refractory period;

1. ABSOLUTE REFRACTORY PERIOD1. ABSOLUTE REFRACTORY PERIOD

2. RELATIVE REFRACTORY PERIOD2. RELATIVE REFRACTORY PERIOD

REFRACTORY PERIODSREFRACTORY PERIODS1. 1. ABSOLUTE REFRACTORY PERIODABSOLUTE REFRACTORY PERIOD

Refers to the time period during which a 2Refers to the time period during which a 2ndnd AP can not be initiated AP can not be initiated even with a very strong stimuluseven with a very strong stimulus

REASONREASON;;

Na+ inactivation gatesNa+ inactivation gates still still closedclosed – they must – they must first return to first return to restingresting statestate before they can get activated. before they can get activated.

REFRACTORY REFRACTORY PERIODPERIOD 2. 2. RELATIVE REFRACTORY PERIODRELATIVE REFRACTORY PERIOD- Refers to the period during which a Refers to the period during which a second APsecond AP can be initiated, but can be initiated, but

only by a only by a stimulus larger than threshold strengthstimulus larger than threshold strength..

REASON:REASON: - Voltage gated k+ channelVoltage gated k+ channel are still are still openopen- The The inactivated Na+ gatesinactivated Na+ gates have returned to their resting state – so have returned to their resting state – so

they are they are ready to be opened againready to be opened again as long as the as long as the stimulus is stimulus is strong enoughstrong enough (i.e. larger than threshold ~55mV) (i.e. larger than threshold ~55mV)

Regulating the APRegulating the AP

Figure 8-12: Refractory periods

که : باورند این بر چون ، ندارد وجود بست بن بزرگ های انسان برای

خواه راهی خواه ند یا راهی یا ، ساخت .ندیافت

پرسند : می خویشتن از همواره و اند مانده راه یک وخم پیچ در بسیاری

گفت . باید بدانها راهیم ادامه از ناتوان چرا ما

. ای دانسته پرمایه را خود که همانجای ای؟ مانده کجا در دانی می

SALTATORY CONDUCTIONSALTATORY CONDUCTIONi.e. Conduction pattern where i.e. Conduction pattern where current leaps from node to nodecurrent leaps from node to node as each as each nodal area depolarizes to threshold.nodal area depolarizes to threshold.REASON:REASON:- When a nerve impulse propagates along a myelinated fiber the When a nerve impulse propagates along a myelinated fiber the

current is carried by the flow of ions through the extracellular fluid current is carried by the flow of ions through the extracellular fluid surrounding the myelin sheath and through the cytosol from one surrounding the myelin sheath and through the cytosol from one

node to the next.node to the next.- But current flows across the membrane only at the nodes.But current flows across the membrane only at the nodes.

SynapseSynapse

• site of communication between two cells

• formed when an axon of a presynaptic cell “connects” with the dendrites of a postsynaptic cell

NERVE CELLNERVE CELL PARTS OF A NERVE CELLPARTS OF A NERVE CELL;;1. CELL BODY;1. CELL BODY; - Contains the nucleus of the nerve cell.- Contains the nucleus of the nerve cell.- Control center of the nerve cell- Control center of the nerve cell2. AXON;2. AXON; - longest cytoplasmic extensionof nerve - longest cytoplasmic extensionof nerve - Conducts impulses away from nerve- Conducts impulses away from nerve3. DENDRITES;3. DENDRITES; - Short cytoplasmic extensions coming off- Short cytoplasmic extensions coming off the nerve cell body.the nerve cell body.- Receives incoming impulses - Receives incoming impulses 4. MYELIN SHEATH4. MYELIN SHEATH- Neuroglia around nerve- Neuroglia around nerve- Provides metabolic, structural support - Provides metabolic, structural support to nerve fiber to nerve fiber