The excitable cell. Resting Membrane Potential and Action Potential (1&2) Dr Sergey Kasparov School of Medical Sciences, Room E9 THIS TOPIC IS ALSO COVERED IN TUTORIALS! Teaching home page: http://www.bristol.ac.uk/phys-pharm/media/teaching/ Why do we call these cells “excitable”? Real action potentials.avi The key point: Bioelectricity is generated by the ions moving across cellular membranes Concentration of selected solutes in intracellular fluid and extracellular fluid in millimols mM 160 140 120 100 Insi de the cell 80 60 In extracellular fluid 40 20 0 1 The Na+/K+ pump outside inside [[mMmM]] + + [[mMmM]] + + + + + + + Na 145 + + 15 + K 4 + + 140 ATPATP ••isis an active ion transporter (ATP‐dependent) ••isis responsible for creating and upholding Na+ and K+ concentration gradients ••isis electrogenic – pumps 3 Na+ versus 2 K+ ions Reminder: Movement of ions is affected by concentration gradient and the electrical field Chemical driving force _ _ + + _ + + Electrical driving force _ + + _ + + + _+ _ + _ + + + + _ + _ + _ + + _ Net flux + + + + _ + _ + + V REMINDER: Diffusion through Ion Channels A leak channel A gated channel Both leak and gated channels allow movement of molecules (mainly inorganic ions) down the electrochemical gradient. So, if the gradient reverses, the ions will flow in the opposite direction. 1. The channels are aqueous pores through the membrane. 2. The channels are usually quite selective, for example some only pass Na+, others K+, still others – Cl- 3. Gated channels may be opened or closed by various factors (for example electrical potential of the membrane). 2 The resting membrane potential (RMP) outsideleaky K+ inside K+ channels only! + + + + + + + + + + + an electric gradient ++ ‐‐‐ builds up! + …is negative inside the cell because positive K+ ions exit, following their concentration gradient Will it ever stop? Chemical driving force _ _ + + _ + + Electrical driving force _ + + _ + + + _+ _ + _ + + + + _ + _ + _ + + _ Net flux + + + + _ + _ + + V The Reversal Potential: Potential of the membrane at which the electrical driving force is exactly equal the chemical driving force and therefore THE NET FLUX of this particular ion is NIL. Reversal potential can be calculated using Nernst equation: 61.5mV Eion = Log10 (Cout /C in) Z 3 What is the logic behind Nernst equation? Logically at equilibrium the chemical and electrical potential energy differences across the membrane are equal but opposite. Chemical energy Electrical energy THIS is what for a given ion: for a given ion: we are after ((VmVm)) [Xi] R *T ln Zx * F* ((ψ -- ψ )) 0 = + Zx * F II O [Xo] R – universal gas constant Zx – valence of an ion ((--1,+11,+1 or T – temperature in Kelvins +2) Xi – concentration inside F – Faradey’s constant Xo – concentration outside ψI-ψo potential difference across membrane Voldemort1.avi [X ] Vm = - R *T ln i = Zx * F [Xo] IMPORTANT: 61.5 is a calculated constant derived from universal gas constant, the temperature (37oC) and Faraday electrical constant and with Log base10, not natural log. For 20oC it is 58.1. For 25oC it is 60. ThisThis is why different textbooks sometimes give you different values (In Vander’s physiophysiologylogy it is 60!). Z – is a valence of an ion. Remember, for a negative ion it will be negative. Using this equation: ENa = 61.5/1 * log (145/15) ≈ 60.5 mV EK = 61.5/1 * log (4/140) ≈ -95 mV This means that in a hypothetical neurone sodium flux through the open channels will tend to bring membrane potential to~+60.5 mV, while potassium flux will bring it to ~ -89 mV. But this could only happen if these ions were allowed to freely flow through the membrane … and they are not! You MUST look through the relevant chapters in the textbooks: E.g.: - Chapter 6 in Vander’s Human Physiology (10th ed) or --ChapterChapter 6 in Boron ––BoulpaepBoulpaepTextbook Be ppprepared to calculate the reversal p otentials of the ions. Also read about Goldman Equation which describes membrane potential when more than one ion is involved: PK[[KKo] + PNa[[NaNao] + PCl[[ClCli] Vmm== 61.5 * Log10 PK[[KKi] + PNa[[NaNai] + PCl[[ClClo] NOTE THAT FOR Cl-Cl- it is the other way round because it is negatively charged 4 The Resting membrane potential 1.The reversal (equilibrium) potential is the potential which the membrane would reach if it was freely permeable to this particular ion, and no other ions. At this potential chemical driving force is exactly equal electrical driving force and the net ionfluxinnil. 2. Intracellular concentration of K+ is neurones is high and Na+ is low relative to the extracellular space 3. The reversal potential for K+ ions in neurones is very negative (~-95 mV) while for Na+ ions positive (~+60 mV). 4. Resting membrane potential is generated mainly by a steady flux of K+ ions through ion channels embedded into the membrane of the neurone. 5. Resting membrane potential is not equal to the K+ reversal (equilibrium) potential because: -K+ channels normally are not fully open -inparallelwithK+ currents there are some Na+ and Ca2+ currents which depolarise themembraneslightly,e.g.makeitmore positive. Na/K ATPasekeeps unequal concentrations of Na+ and K+ across the membrane Na+/K+ ATPase ((ATPase.flvATPase.flv)) Does it create membrane potential directly? If you poison it (or block ATP production in the cell) – what will happen with MP? Test yourself 1. A neurone at rest has a stable RMP of ‐‐70 mV. What is the sum of all ion fluxes? 2. For an RMP of ‐‐75 mV, estimate the relative permeability + for Na ions (if PK+ is 1 and PCl‐ is 0). outside inside [[mMmM]] [[mMmM]] + + + Na 145 + + 15 + + Cl- 115 4 + + + K 4 + + + 140 Ca2+ 1.8 0.0001 A- (organic anions) (to balance) Concentrations for a ‘typical’ neurone (in millimolmillimol)) PNaNa++ ~ 0.03 5 Part-2 The Action Potential Real action potentials Teaching home page: http://www.bristol.ac.uk/phys-pharm/media/teaching/ AP is a brief “all-or-none” depolarisation of the neuronal membrane which, once initiated propagates without decrement. Amplifier 0 mV -60 mV One second Features of the AP – 1: Propagation along the cell’s membrane and along the axon Amplifier Amplifier 1 Amplifier 2 Amplifier 3 Amp 1 Amp 2 Amp 3 Time 6 Features of the AP – 2: “All-on-none” nature In contrast to the “small, sub-threshold” potentials, action potentials have very similar amplitude and shape. Once the threshold has been reached, their amplitude will not increase even if a stronger stimulus is applied. In terms of information transfer they are like logic “ones”, as compared to “zeros”. Glossary of the terms Overshoot Repolarisation Hyperpolarisation Depolarisation Resting membrane potential Ionic basis of AP: Na ions are responsible for depolarisation, K ions are responsible for re-polarisation. Fluxes of both ions are coordinated by voltage-gated ion channels. 7 REMINDER: Diffusion through Ion Channels A leak channel A gated channel Both leak and gated channels allow movement of molecules (mainly inorganic ions) down the electrochemical gradient. So, if the gradient reverses, the ions will flow in the opposite direction. 1. The channels are aqueous pores through the membrane. 2. The channels are usually quite selective, for example some only pass Na+, others K+, still others – Cl- 3. Gated channels may be opened or closed by various factors (for example electrical potential of the membrane). Na+ channels are controlled by the potential of the membrane. Na+ Na+ Voltage sensor and Na+ activation mechanism Na+ Na+ Outside of the cell Aqueous pore Extracellular domain Lipid membrane Intracellular domain narrow selectivity filter “Inactivation gate” Inside the cell (negative) “Positive feedback” is responsible for the very fast dynamics of AP 1. Depolarisation 2. Opening of voltage-gated Na+ channels 3. Sodium currents depolarise membrane further Something has to happen for this process to terminate! 8 At positive potentials voltage-gated Na+ channels become “inactivated” Voltage sensor and Na+ Outside of the cell activation mechanism Na+ Na+ Aqueous pore Extracellular domain Lipid membrane Intracellular domain narrow selectivity filter “Inactivation gate” Na+ Na+ Na+ Na+ current is responsible for the upstroke of the action potential Ion channel activation during an action potential mV 50 REMEMBER: 0 Na+ fluxes depolarise the membrane (EP ~+60), K+ fluxes hyperpolarise the --5050 membrane (EP ~-90) --100100 time K+ channels opening Na+ channels opening Na+ channels inactivate 9 Two factors responsible for AP termination: 1. Inactivation of Na+ channels 2. Delayed activation of K+ channels (delayed rectifiers) Molecular nature of the voltage-gated Na+ and K+ channels A voltage-gated Na channel is formed by association of 4 main (α) subunits 10 In many central neurones AP are initiated at the “initial segment of the axon” 1. Action potentials propagate along the axons. 2. Refractory periods follow APs A) Absolute refractory period (second AP cannot occur under any circumstance) B) Relative refractory period (a stronger-than-normal stimulus may evoke an AP) Absolute Relative mV 50 0 --5050 --100100 time K+ channels opening Na+ channels opening Na+ channels inactivate 11 Saltatory conduction of AP, Schwann cells and oligodendroglia 1. Axons of many types of neurones are “insulated” with multiple layers of myelin. 2. Bare “gaps” between the insulated parts are known as nods of Ranvier. 3. This is known as saltatory conduction. AP “jump” from one node to the next one as electrical field, rather than a wave of Na+ channel openings. 4. This greatly accelerates AP propagation and saves much energy. Saltatory conduction of AP, Schwann cells and oligodendrocytes Schwann cells are responsible for myelinisation in peripheral nerves In the CNS myelin is provided by oligodendrocytes Speed of conductance depends on the degree of myelinisation. “Thick” vs “thin” axons.