Resting and Action potentials Flashcards
What does flux mean?
the rate of transfer of molecules The number of molecules that cross a unit area per unit of time (number of particles). ie molecules.m−2.s−1
What happens to the net flux in a dynamic equilibrium?
Dynamic equilibrium reached – no net flux
List what causes ion channels to open and close?
Permeable pores in the membrane (ion channels) open and close depending on trans-membrane voltage, presence of activating ligands or mechanical forces.
What equation is used the calculate the equilibrium potential?
Nernst equation
Define equilibrium potential?
The potential at which electrochemical equilibrium has been reached. It is the potential that prevents diffusion of the ion down its concentration gradient
Real membrane potentials (Em) do not rest at EK (–90 mV) or ENa (+72 mV) Typical Em is -70 mV Why?
The pottasium equilibrium potential is when the membrane is uniquely selective for K+. But at rest the membrane isn’t uniquely sleective for pottasium, because there is a small permeability to some other ions. And that is why typically we might achieve a measurement of about -70mV. Whilst at rest, the cell membrane should be completely permeable and uniquely permeable to pottasium but it isnt. There are some finite permeability to some other ions. Membranes tend to have a mixed sodium and pottasium permeability. At rest the pottasium permeability is greater then sodium permeability. There is a small permeability of the membrane to sodium and Ca2+ and a little bit to chloride.
What does the GHK equation describe?
The GHK equation describes the resting membrane potential (Em)
What does the P stand for in the Goldman-Hodgkin-Katz (GHK) equation?
P is permeability or channel open probability
(0 = 100% closed, 1 = 100% open, 0.5 = open 50% of time)
The small i’s and o’s indicate inside and outside the cell.
The size of each ion’s contribution is proportional to how permeable the membrane is to the ion.
What is the origin of the resting membrane potential?
The resting membrane potential isn’t any PUMPS- it keeps the concentration gradient of Na and K as appropriate levels. But it doesn’t actually produce the membrane potential.
The origin of the resting membrane potential is simply pottasium moving from a high concentration inside the cell to a low concentration outside the cell. The origin of the membrane potential is simply that, movemnent of pottasium outside the cell. Eventually it becaomes balanced by the electrical gradient that is set up. All that happens at about -90 mv. But you have a small permeability to sodium at rest, even though that cell is being excited at that point at rest and that account for a small amount of positive charge entering the cell and making it a little bit more positive than i would otherwise be.
Define overshoot, depolarising, repolarising and hyperpolarising.
Negative to more positive- Depolarising
From positive to negative- Repolarising
Move from 0 to more positive membrane potential-Overshoot
More negative then resting potential- hyperpolarising
What is a graded potential?
Graded potentials are changes in membranepotential that vary in size, as opposed to being all-or-none. They arise from the summation of the individual actions of ligand-gated ion channel proteins, and decrease over time and space. They do not typically involve voltage-gated sodium and potassium channels.
Graded potentials must occur to depolarize the neuron to threshold before action potentials can occur.
STUDY THE IMAGE CAREFULLY
Compare the graded potential at the site of the stimulus and at measured 1mm form the stimulus site?
Draw what you would see
Where do graded potentials occur?
Synapse
Sensory receptors
What is the function of action potentials?
Contribute to initiating or preventing action potential
Draw an action potential graph and lable it
State the 3 states of ion channels and state when they are in that state
•Permeability depends on conformational state of ion channels
»Opened by membrane depolarization
»Inactivated by sustained depolarization
»Closed by membrane hyperpolarization/repolarization
What happens During phases 2- Depolarising stimulus?
A stimulus to the nerve has occured and that has caused some opening of some sodium channels.
Small amount of sodium diffuses into the cell, depolarising the cell.
What is the threshold potential?
The threshold potential is the critical level to which a membrane potential must be depolarized to initiate an action potential.
-55mV
What happens after the threshold potential has been met?
P=Permeability
What happens during repolarisation?
Sodium channels become inactivated- Part of the sodium ion channel falls into the pore and blocks the channel
More and more pottasium channels are opening
What is a refractory period?
A period during which a nerve or muscle is incapable of responding to stimulation, esp immediately following a previous stimulation.
Look at the image and study the different types of refractory periods
What happens to the sodium gate during the absolute refractory period?
Inactivation gate is closed.
Sodium channel inactivation- the protein sticks in the pore until the cell repolarises
This means the a new action potential connot be triggered even with a very strong stimulus.
The channel will open and then the ball and chain thing will inactivate it and as the cell repolarises the ball and chain moves away and the sodium channel shuts.
This process gives rise to the refractory period
The refractory periods are due to the inactivation property of voltage-gated sodium channels and the lag of potassium channels in closing. Voltage-gated sodium channels have two gating mechanisms, the activation mechanism that opens the channel with depolarization and the inactivation mechanism that closes the channel with repolarization. While the channel is in the inactive state, it will not open in response to depolarization. The period when the majority of sodium channels remain in the inactive state is the absolute refractory period. After this period, there are enough voltage-activated sodium channels in the closed (active) state to respond to depolarization. However, voltage-gated potassium channels that opened in response to repolarization do not close as quickly as voltage-gated sodium channels; to return to the active closed state. During this time, the extra potassium conductance means that the membrane is at a higher threshold and will require a greater stimulus to cause action potentials to fire. This period is the relative refractory period.
When the ball and chain is in the pore nothing can happen to the sodium ion channel.
Opening then inactivation then as repolarising occurs the inactivation particle falls away and the channel closes.
What happens during the hyperpolarisation phase?
What happens to the sodium gates during the relative refractory period and what happens if you have a stimulus during the relative refractory period?
Draw a grah pshowing how the permeability of Na+ and K+ changes during a action potential?
What is the all or none rule
How does the Membrane escape the refractory state?
Membrane remains in a refractory (unresponsive) state until the voltage-gated Na+ channels recover from inactivation
How is the electrochemical equilibrium restored following an action potential?
- The electrochemical equilibrium is restored following the action potential by K+ and Na+ ions moving through non voltage-gated ion channels
- Some ions are exchanged through pumps but this is a relatively slow process (seconds vs milliseconds)
What causes this positive feedback loop to stop?
Once threshold is reached, the cycle can continue
The cycle stops when the VGSCs are INACTIVATED - closed and voltage insensitive (inactivation gate)
The membrane remains in an unresponsive state until the VGSCs recover from inactivation
What determines the velocity and how far an action potential travels across the nerve?
Diameter of the neuron and myelination affects the speed of the action potential and the distance- less internal( axial) resistance
Myelination prevents loss of charge by acting as an insulator - allows the charge to travel further than with cable transport
How are action potentials propagated along the nueron?
1- Sodium ion channel open allowing sodium ions to diffuse into the neurone
2- Localised increase in concentration of sodium ions inside neurone, the action potential
3-Sodium ions diffuse across the axon/dendron, away from the region of higher concentration
4- Sodium gate, which was initially closed, will open becuase of movement of sodium ions- allowing the action potential to move along the neurone as more sodium ions enter and set up another action potential
The ABSOLUTE REFRACTORY PERIOD - blocking of VGSCs by the inactivation gate means that the section of membrane which is hyperpolarised cannot be depolarised again and the action potential cannot travel in the wrong direction
What is saltatory conduction?
