How can you measure membrane potential of a cell?
- A reference electrode placed outside of the cell (zero-volt level)
- Another electrode placed inside the cell : measures a voltage negative compared with outside
What prevents free movement of ions across cell membrane?
Lipid (hydrophobic) cell membrane is a barrier to ion movement and separates ionic environments
How can ions go through cell membrane?
Permeabe pores (ion channels) open and close, depends on transmembrane voltage, presence of activating ligands or mechanical forces
Selectivity change permeability for ions
What is electrochemical equilibrium?
Electrical forces balance diffusional forces
Stable transmembrane potential is achieved
What is equilibirum potential?
Potential at which electrochemical equilibrium is reacged
Potential that prevents diffusion of ion down its conc gradient
What equation is used for calculating equlibrium potential?
-The Nernst Equation
E= RT/zF In X2/X1
R= gas constant T= temp in kelvin
z = charge on ion (-1 Cl- +2 Ca2+)
F = Faradays number- charge per mol of ion
In= natural logorithm (log to base e)
X2= intacellular ion conc X1= extracellular ion conc
In mM what is concentrations of various ions?
Na+ : 150mM extracellular , 10mM intracellular
K+ : 5mM extracellular , 150mM intracellular
Ca2+ : 2mM extracellular , 10^-4 mM intracellular
Cl- : 110mM extracelullar , 5mM intracellular
Why do we use the Goldman-Hodgkin-Katz (GHK) equation?
Ek & Ena are theoretical values
In reality biological membranes are not uniquely selective for an ion
Membranes have mixed and variable permeability to all ions
Typical RMP (Em) is -70mV not =90mV which is Ek
GHK describes membrane potential (Em) more accurately
What is depolarisation?
Membrane potential becomes more positive towards 0 mV
What is repolarisation?
Membrane potential decreases towards resting potential
What is overshoot?
Membrane potential becomes positive (above 0 mV)
What is hyperpolarisation?
Membrane potetial decreases beyond resting potential
What is a graded potential?
They produce the initial change in membrane potential that determines what happens next- initiate or prevent action potentials
Response to external stimulation or neurotransmitters
Decay along axon
When does an action potential occur?
- When a graded potential reaches a threshold for activation of many Na+ channels - leads to ‘all or nothing’ event
- Occur in excitable cells (neurons, muscle cells) and in some endocrine tissue
Are membrane potential changes during action potential due to ion pumps?
No, they are not. They are due to movement of ions through ion channels
What are the 5 phases of the action potential?
- Resting membrane potential
- Depolarising stimulus
- Upstroke
- Repolarisation
- After-hyperpolarisation
(all over within millisecond and half ish)
What happens in phase 1 of Action potential (RMP)?
- Permeability for Pk > Pna
- Membrane potential nearer equilibrium potential for K+ (-90 mV) than for Na+ (+72mV)
What happens in phase 2 of action potential (depolarising stimulus)?
- The stimulus depolarises the membrane potential
- Moves it in posititve direction towards threshold
What happens in phase 3 of action potential (upstroke)?
Starts at threshold potential
Pna increases becauses VGSC open quickly so Na+ enters cell - produces overshoot
Pk increases as VGKCs start to open slowly , K leaves cell, less than Na entering though
Membrane potential moves toward Na+ equilibrium potential
What happens in phase 4 of action potential (repolarisation)?
When peak of action potential achieved :
- Pna decreases VGSC channels close so Na+ entry stops
- Pk increases as more VGKC channels open and remain open
- K+ leaves cell down its electrochemical gradient
- Membrane potential moves toward the K+ equilibrium
What is the absolute refractory period?
At start of repolarisation:
- All sodium channels are open but then undergo CONFORMATIONAL CHANGE called INACTIVATION
- Stops ion flow through sodium channel
- New action potential can’t be triggered despite strong stimulus
- Inactivation removed when cell repolarised completely
What happens in phase 5 of action potential (after-hyperpolarisation)?
- At rest VGKCs still open, K+ leaves cell down electrochemical gradient
- Membrane potential moves closer to K+ equilibrium - some VGKCs then close
- Membrane potenital returns to resting potential
- Enter Relative Refractory Period : some Na+ channels recover from inactivation & open, stronger than normal stimulus needed to trigger action potential
What is meant by ‘all or nothing’ events?
- Once threshold potential reached, AP triggered
- Once triggered full sized action potential occurs- positive feedback
- If graded potential doesn’t reach threshold, it decays along axon
What restores the ion concentration gradients after an AP?
- K+ and Na+ carried across membrane against concentration gradients by different types of ion transporter Na+/K+ ATPase
- 3 Na + out // 2 K+ in
What determines the decay of a site of depolarisation?
1) Diameter of axon : small neurones larger resistance
2) Insulation of neuron : myelinated or not , insulation allows graded potential to decay further along axon (internal& membrane resistance)
How is AP propagated actively down axon?
Stimulatory pulse has depolarised membrane so sodium flows in cell
Depolarisation will decay down adjacent area that is at RMP
If that area reaches threshold potential, sodium channels activated and sodium influx
There is depolarisation and then graded potential decays down new adjacent area etc etc AP propagated
What happens at the Nodes of Ranvier?
- These are gaps in myelin sheath
- Saltatory conduction occurs
- Voltage-gated channels mostly located at nodes
- Makes transmission of AP rapid over long distances
Small diameter non-myelinated axon Vs large diameter myelinated axon :
1m/s Vs 120 m/s
What is passive propagation?
When there is a small voltage change that does not reach threshold and voltage decays down