membrane and action potentials Flashcards
what is electrochemical equilibrium?
when electrochemical forces (positive charge repelling against more positive ions entering) balance diffusional forces
what is electrochemical potential?
potential where electrochemical equilibrium has been reached
potential that prevents diffusion of the ion down its concentration gradient
what equation is used to calculate electrochemical potential?
Nernst equation
- What do the components in the Nernst Equation stand for?
R = Gas constant
T = Temperature in Kelvin z - charge on ion F = Faraday's number - charge per mol of ion ln = natural logarithm X2 = Intracellular ion concentration X1 - Extracellular ion concentration
what is the Nernst equation?
RT/ zF x ln (X2/X1
what is the extra and intra cellular conc (mM) of
- Na
- K
- Ca
what is the osmolarity extra and intra cellularly?
150, 10
5, 150
2, 10^-2
285 mosmol/l
what does the Nernst equation calculate
what does the GHK equation calculate
the movement of which ion determines the resting potential
Electrochemical potential can be calculated for single ion in ideal (control) system by Nernst equation
Membrane potentials can be estimated in more complex systems (many ions and variable permeability) by the GHK equation
Resting membrane potential due almost entirely to the movement of K+ ions out of the cell
what is a graded potential
Change in membrane potential in response to external stimulation or neurotransmitters
Change in membrane potential is graded in response to the type or strength of stimulation
Graded potentials produce the initial change in membrane potential that determines what happens next – they initiate or prevent action potentials
will decay if it does not reach threshold
when do action potentials occur
Action potentials (AP) occur when a graded potential reaches a threshold for the activation (opening) of many Na+ channels resulting in an “all-or-nothing” event
in what direction do ions move when permeability to that ion increases
what does the change in membrane potential not depend on
When membrane permeability of an ion increases it crosses the membrane down its electrochemical gradient
Movement changes the membrane potential towards the equilibrium potential for that ion
Changes in membrane potential during the action potential are not due to ion pumps
what is phase one of an action potential
Resting membrane potential
permeability for PK > PNa
Membrane potential nearer equilibrium potential for K+ (-90mV) than that for Na+ (+72mV)
what is phase one of an action potential
Resting membrane potential
permeability for PK > PNa
Membrane potential nearer equilibrium potential for K+ (-90mV) than that for Na+ (+72mV)
What is phase two of an action potential
The stimulus depolarises the membrane potential
Moves it in the positive direction towards threshold
what happens during depolarisation
Starts at threshold potential
inc Perm Na because voltage-gated Na+ channels open quickly [Na+ enters the cell down electrochemical gradient]
inc Perm K as the voltage-gated K+ channels start to open slowly [K+ leaves the cell down electrochemical gradient]
but less than Na+ entering
Membrane potential moves toward the Na+ equilibrium potential
what happens during repolarisation
decreased perm Na because the voltage-gated Na+ channels close - Na+ entry stops
increased Perm K as more voltage-gated K+ channels open & remain open
K+ leaves the cell down its electrochemical gradient
Membrane potential moves toward the K+ equilibrium potential
when does the Absolute refractory period occur and what is it?
what happens to the absolute refractory period later on in repolarisation
At the start of repolarisation
Na+ channel activation gate is open, inactivation gate is closed
New action potential cannot be triggered even with very strong stimulus
membrane is unresponsive to threshold depolarisation until voltage gated Na channels recover from inactivation
Absolute refractory period continues
Activation AND Inactivation gates closed
What happens during after- hyperpolarisation?
At rest voltage-gated K+ channels are still open
K+ continues to leave the cell down the electrochemical gradient
Membrane potential moves closer to the K+ equilibrium - some voltage-gated K+ channels then close
Membrane potential returns to the resting potential
When does the Relative refractory period occur and what is it
During after hyperpolarisation
activation gate is open, inactivation gate is closed
stronger than normal stimulus to trigger an action potential
How does positive feedback work during all or nothing action potentials
how are Na channels inactivated
what two types of membrane potentials are there
depolarisation, opens Na+ channels, inc Na perm, Na Influx- causes more depolarisation
By sustained depolarisation
graded and action
what happens during passive propagation
Only resting K+ channels open, when graded potentials decay
myelination of axon and diameter of axon alters propagation distance and velocity
(large diameter myelinated axon- small resistance)
(small diameter unmyelinated- large resistance)
what factors affect conduction velocity
how fast does an AP travel in Small diameter, non-myelinated axons
how fast does an AP travel in Large diameter, myelinated axons
Both axon diameter and myelination influence conduction velocity
Small diameter, non-myelinated axons
1 m/s
Large diameter, myelinated axons
120 m/s
what factors reduce conduction velocity in an axon
Decreases with reduced axon diameter (i.e. re-growth after injury)
reduced myelination (e.g. multiple sclerosis and diphtheria)
cold
anoxia
compression
drugs (some anaesthetics)
What are the three main factors that influence the movement of ions across the membrane?
Concentration of the ion on both sides of the membrane, the charge on the ion and the voltage across the membrane.
Why is the K+ equilibrium potential negative (e.g. -70mV) and the Na+ equilibrium potential positive(e.g. +40mV) when both are positive ions?
More K+ inside the cell than outside so tend to flow out of the cell, while more Na+ outside the cell than in, therefore tend to flow into the cell.
A potential of -70mV is needed to attract K+ and stop net outward flow, 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 diameter 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
