Unformatted text preview: Electrical membrane potential 1 Electrical membrane potential
Active transport forms gradients maintenance by ion pumps electrochemical gradients membrane potential Neurons and membrane potential Neuron signals are generated by changes in membrane potential 2 Active transport uses energy to move solutes
active transport needed to: 1. move solutes against their concentration gradient 2. maintain custom environment within cell
www.foundationnews.org/files/1sisyphus.jpg 3 Na+-K+ pump - example of active transport
[Na+] high [K+] low extracellular fluid Na+ Na+ [Na+] low [K+] high Na+ cytoplasm Na+ Na+ Na+ P ATP+ in cell, high [K+], low [Na+] Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ 4 Which side will become more positive?
3 Na+ out
Na+ Na+ Na+ K+ K+ extracellular fluid cytoplasm 2 K+ in 5 Gradients form
OUTSIDE cell + + + + + + + + + + + INSIDE cell - - + + + + + + + + + + + example: 3 Na+ out/ 2 K+ in 6 Gradients form
OUTSIDE cell Na+ Na+ Na+ Na+ Na+ Na+ Na+
[Na+] low INSIDE cell Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+
[Na+] high example: 3 Na+ out 7 Because of active transport:
- Inside the cell is more negative relative to the outside = electrical gradient (voltage) - A concentration gradient of ions forms across the membrane (ex. [Na+]) = chemical gradient Combination of both types of gradients = electrochemical gradient 8 Membrane potential
.... is the voltage difference across a membrane Every cell has a membrane potential as a result of the electrochemical gradient maintained by the cell
http://www.agen.ufl.edu/~chyn/age2062/lect/lect_06/4_21.GIF 9 How is the membrane potential used to `do work'? Coupled transport
- link one substance that is moving down its gradient to another substance that needs to get across membrane 10 &#/'0(&,/-8=(( L0&-/':(8+,%08"(/'*0(5;60"#(<0% ED(,0*%&'850%*=(( )0-/+#(/0'8(&%"(&680(+8"-(/' ( #0>"( &#/'0( &,/-8( &'-( #0'08&,,;&%/-"8= Coupled transport (ED (M+#5 N6+,08"()B#50%*(@/*;(C&D OPD (5+#5 0*%&'850%*(/8(+8"-(<0%(%";B-%&*/0'=(( Q('+#$"%(0< In animal cells, Na+ ions are used to move amino acids and ( 8+,;( &8( ,;06"%&H( ,&+8"( 8"%/0+8( -";B-%&*/0'( <%0# monosaccharides across the membrane ,"'*%&*"-( :6+,08"( &'-( 8&6*8( ;"658( %"8*0%"( 08#0*/, ;"(8&6*8(&'-(:6+,08"(&%"($0*;(*%&'850%*"-(&,%088(*;" 11 Summary
Active transport of ions across a membrane creates and maintains a gradient - electrochemical gradient - chemical and electrical gradient across a membrane Membrane potential is the voltage difference across a membrane, used to do `work' in the cell - Cotransport - Occurs when active transport of a specific solute indirectly drives the active transport of another solute
12 Schwann cell Depolarized region (node of Ranvier) Cell body Nervous system
Nervous systems consist of circuits of neurons and supporting cells Neurons - used as a `model system' to study membrane potential Myelin sheath Axon 13 How do neurons propagate a signal?
By using the electrical potential difference (voltage) across the neuron membrane
Schwann cell Depolarized region (node of Ranvier) Cell body Myelin sheath Axon 14 Resting potential of neurons
= the membrane potential of a neuron that is NOT transmitting signals outside 150mM Na+ 5mM K+ inside 15mM Na+ 150mM K+ 15 LE 48-11 Focus on the movement of
Inner chamber 92 mV Outer chamber 5 mM KCl Inner chamber 15 mM NaCl + K +62 mV Oute cham 150 mM KCl K+ Potassium channel Cl Artificial membrane When electrical Cl gradient balances concentration Na gradient = Sodium channel equilibrium + 150 mM NaCl Membrane selectively permeable to K+ Fig. 48.11a Membrane selectively permeable
16 LE 48-11 Measure difference in charge at equilibrium
Inner chamber 92 mV Outer chamber 5 mM KCl Inner chamber 15 mM NaCl +62 mV Oute cham 150 mM KCl K+ Potassium channel Cl Artificial membrane When electrical Cl gradient balances concentration Na gradient = Sodium channel equilibrium + 150 mM NaCl Membrane selectively permeable to K+ Fig. 48.11a Membrane selectively permeable
17 The Nernst equation
Measure of membrane voltage at equilibrium Eion = 62mV (log [ion]out/[ion]in) a constant
EK = 62mV (log 5mM/150mM) = -92mV "Equilibrium potential [K]outside for K+ ions" in cell [K]inside
the cell Minus sign means that K+ is at equilibrium when the inside of the membrane is 92mV more negative than the outside
18 Focus on the movement of
92 mV Outer chamber 5 mM KCl Cl K+ Cl Na+ Sodium channel Inner chamber 15 mM NaCl +62 mV Outer chamber + Na 150 mM NaCl m Artificial membrane selectively permeable to K+ Nernst eq: ENa = 62mV (log 150mM/15mM) = +62mV
19 Membrane selectively permeable to Na+ How is resting potential established?
CYTOSOL EXTRACELLULAR FLUID [Na+] 15 mM [Na+] 150 mM
[K+] 5 mM [K+] 150 mM -60 to -80 mV At rest, neuron cell has many more K channels open than Na+ channels open
20 Plasma +membrane Resting potential of neurons
- the membrane potential of a neuron that is NOT transmitting signals CYTOSOL EXTR Resting potential of typical mammal cell = -60mV and -80mV [Na+] 15 mM [K+] 150 mM Neither K+ nor Na+ is at equilibrium, and there is a net flow of each ion (a current) across the membrane at rest. Pla me
21 How do neurons signal? Change membrane potential!
HOW? Increase the membrane's permeability to Na+ the membrane potential will: move toward ENa (+62mV) move away from EK (-92mV) http://www.alz.org/brain/images/02a.jpg 22 How do changes in membrane permeability to ions occur?
LE 48-11 Inner chamber 92 mV Ungated channels are always open establish resting potential Oute cham 5 mM KCl 150 mM KCl BUT, Neurons also have gated ion channels, which open or close in Potassium channel response to stimuli. K+ C Artificia membra Membrane selectively permeable 23 Gated ion channels change membrane potential in response to stimuli
Gated ion channels are responsible for generating the signals of the nervous system. http://www.sophion.dk/sophion/Open-close2.jpg 24 Stimuli cause hyper or depolarization of membrane
Some stimuli trigger a hyper (`more')polarization Gated K+ channels open, K+ diffuses out of cell, inside of the membrane becomes more negative. Other stimuli trigger a de (`reduced') polarization Gated Na+ channels open, Na+ diffuses into cell, inside of the membrane becomes less negative. 25 Action potentials
An all-or-none phenomenon Once triggered, its strength is independent of the strength of the triggering stimulus. Action potentials carry information in form of electrical signals, along nerve cells to other nerve cells or to muscle cells 26 Na+ Na+ Na+ Na+ K+ Rising phase of the action potential K+ Falling phase of the action potential +50 Na+ Na+ Membrane potential (mV) Action potential 0 Threshold Resting potential Time 50 K+ Depolarization 100 Na+ Extracellular fluid Potassium channel Activation gates K+ Undershoot Sodium channel K+ Inactivation gate Na+ Na+ Plasma membrane Cytosol Resting state Fig. 48.13
27 Action potential
Na+ Schwann cell K+ Action potential
Cell body Depolarized region (node of Ranvier) Na+ My she K+ Axo K+ Action potential Na+ K+ Figs 48.14 and 48.15
Active transport forms gradients maintenance by ion pumps electrochemical gradients membrane potential Neurons and membrane potential Neuron signals are generated by changes in membrane potential 29 Next Time
Cell Communication http://www.rkm.com.au/imagelibrary/thumbnails/IMMUNE-synapse-150.jpg 30 ...
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