Membrane Potentials and Action Potentials Flashcards
Voltage
Potential difference
Units: Volts
Generated by ions to produce a charge gradient
Current
Unit: Amps
Movement of ions due to a potential difference
Resistance
Units: Ohms
Barrier that prevents the movement of ions
How to measure membrane potential
A reference rod is placed outside the cell.
This is the zero-volt level
Another electrode is placed inside the cell . It measures a voltage difference that is negative compared with the outside
All cells have a membrane potential
How cell membrane maintains potential
Lipid (hydrophobic) cell membrane is a barrier to ion movement and separates ionic environment.
Cell membrane can selectively change its permeability to specific ions
Permeable pores in the membrane (ion channels) open and close depending on transmembrane voltage, presence of activating ligands or mechanical force
Ion channels
Ion channels can be selective for different types of ions
Movement across the membrane will occur when the concentration of the ion is different on one side of the membrane and ceases upon equilibration
Due to diffusion through a selectively permeable membrane
Nerst Equation
E = [(RT)/(zF)] * ln { [X2]/[X1] }
Concentration of ions
Na+ :
Extracellular 150 nM
Intracellular 10 nM
K+ :
Extracellular 5 nM
Intracellular 150 nM
Ca2+ :
Extracellular 2 nM
Intracellular 10^(-4) nM
Cl- :
Extracellular 110 nM
Intracellular 5 nM
Organic phosphates :
Extracellular 3 nM
Intracellular 130 nM
pH :
Extracellular 7.4
Intracellular 7.1
Osmolarity:
Extracellular = Intracellular = 285 mosmol/L
Why do membrane potentials not rest at E(K) or E(Na)?
E(K) = -90mV
E(Na) = +72mV
Typical E(Membrane) = -70mV
Because there are always some channels open at all times
Calculation for real membrane potential
K+, Na+ and Cl- concentrations all contribute to the real membrane potential
Size of each ion’s contribution is proportional to the real membrane potential
E = -61 log {P(K)[K in] +…} / {P(K)[K out] +…}
Depolarisation
Membrane potential moves towards 0mV
Repolarisation
Membrane potential decreases towards resting potential
Overshoot
When membrane potential becomes more positive
Hyperpolarisation
When membrane potential decreases beyond resting potential
Action potential
Occur in excitable cells (mainly neurons and muscle cells but also in some endocrine tissues)
In neurons they are also known as nerve impulses and allow the transmission of information reliably and quickly over long distances
Play a central role in cell-to-cell communication and can be used to activate intracellular processes
Ionic basis of action potentials
Permeability depends on conformational state of ion channels:
- -Opened by membrane depolarisation
- -Inactivated by sustained depolarisation
- -Closed by membrane hyperpolarisation/repolarisation
When membrane permeability of an ion increases it crosses the membrane down its electrochemical gradient
Movement changes the membrane potential toward the equilibrium potential for that ion
Changes in membrane potential during the action potential are not due to ion pumps
5 phases of the Action Potential
Phase 1. Resting membrane potential
Phase 2. Depolarising stimulus
Phase 3. Upstroke
Phase 4. Repolarisation
Phase 5. After-hyperpolarisation
Phase 1. Resting membrane potential
Permeability for P(K) > P(Na)
Membrane potential nearer equilibrium potential for K+ (-90mV) than that for Na+ (+72mV)
Phase 2. Depolarising stimulus
The stimulus depolarises the membrane potential
Moves it in the positive direction towards the threshold
Phase 3. Upstroke
Starts at threshold potential
Increase in Na+ permeability because voltage-gated Na+ channels open quickly
[Na+ enters the cell down electrochemical gradient]
Followed by increase in K+ permeability as the voltage-gated K+ channels start to open slowly
[K+ leaves the cell down electrochemical gradient]
Less than Na+ entering
Membrane potential moves towards the Na+ equilibrium potential
Phase 4. Repolarisation
P(Na) decreases because the voltage-gated Na+ channels close - Na+ stops entry
P(K) increases as more voltage-gated K+ channels open and remain open
K+ leaves the cell down the electrochemical gradient
Membrane potential moves towards the K+ equilibrium potential
Absolute refractory period
Occurs at the start of repolarisation
Activation gate is open
Inactivation gate is closed
New action potential cannot be triggered even with very strong stimulus
Absolute refractory period continues later in repolarisation
Activation and inactivation gates closed
Phase 5. After-hyperpolarisation
At rest voltage-gated K+ channels are still open
K+ continues to leave the cell down gradient
Membrane potential moves closer to the K+ equilibrium - some VGKC then close
Membrane potential returns to the resting potential
ATPase restores Na+ and K+ conc.
Relative refractory period
Happens after-hyperpolarisation
Inactivation gate is open
Stronger than normal stimulus required to trigger an action potential
Regenerative relationship between P(Na) and E(m)
Once threshold potential reached, action potential triggered
APs are ‘all-or-nothing’ events, once triggered a full-sized AP occurs
Positive feedback causes the cycle of depolarisation, opening of VGSC, increase in P(Na), influx of Na+, to continue
Cycle continues until VGSC inactivate
Following AP there is a absolute refractory period where membrane is unresponsive to threshold depolarisation
Membrane remains in the refractory period until VGSC recover from inactivation
Passive propagation of AP
Only K+ channels open
Internal (or axial) membrane resistance alters propagation distance and velocity
Active propagation of AP
Local current flow depolarizes adjacent region toward threshold
This happens due to passive propagation
The adjacent region is the stimulated to form a new AP
This cycle continues hence propagating the signal down the axon
Nodes of Ranvier
Gaps between myelin sheaths on an axon
Saltatory conduction
Voltage-gated channels are mostly located at nodes
Myelin sheaths insulate the axons preventing leakage of the AP
This means that an action potential generated at one node of Ranvier elicits current that flows passively within the myelinated segment until the next node is reached.
Current flow then generates an action potential in the next Node of Ranvier.
The cycle is repeated along the length of the axon.
Current flows across the neuronal membrane only at the nodes.
This is saltatory propagation, meaning that the action potential jumps from node to node.
Conduction velocity
Both axon diameter and myelination influence conduction velocity
Action potential travels quickly
Small diameter, non-myelinated axons : 1m/s
Large diameter, myelinated axons : 120m/s
Factors affecting conduction velocity
Reduced axon diameter (i.e. regrowth after injury)
Reduced myelination (i.e. multiple sclerosis, diphtheria)
Cold
Anoxia
Compression and drugs (some anaesthetics)
What are the 3 main factors that influence movement of ions across the membrane
Concentration gradient of ions
Charge on ions
Voltage across the membrane
Which ion is important for the upstroke and which is important for the falling portion of the AP?
In which direction do these ions flow
The upstroke mediated largely by Na+ ions moving down their conc. gradient into the cell.
The falling portion dominated by K+ ions going down conc. gradient and therefore exiting the cell
Why is E(K) negative (-70mV) and E(Na) positive (+40mV) when both are positive ions.
There is more K+ inside the cell than outside, tend to flow out of the cell.
More Na+ outside the cell, tend to flow in
Potential to -70mV is needed to attract K+ and stop the net flow outwards while a positive charge of +40mV is needed to repel Na+ from entering the cell
What factors influence the speed of propagation of an action potential along an axon?
Larger axons have lower resistance, so ions move faster - conduction velocity is proportional to the square root of the axon diameter.
There is a linear relationship between conduction velocity and myelin thickness
