Membrane Potentials Flashcards
Explain how the membrane potential is established
-Na+/K+ pump -How the difference in concentration occurs
- This is an active transport process - using ATP where it is hydrolyzed to provide the
energy needed to transport sodium-potassium against their concentration gradients
- The pump transports 3 molecules of sodium out the cell and transports 2 molecules
of potassium into the cell
- This process sets up a concentration gradient within the cell
- With this set up gradient potassium ions will want to move from a region of high conc
to a region of low conc
Understand how we can calculate equilibrium potentials and membrane potentials
-Nernst equation Eion= 62mV (log[ion]outside/[ion] inside)
- Used to calculate equilibrium potential of specific ion in a cell
- Equilibrium potential = the membrane potential at which the electrical and chemical
gradients of a specific ion are balanced
- Ek = potassium equilibrium potential
- Ena= sodium equilibrium potential
The goldman equation
- Used to calculate the resting membrane potential of a cell taking into account multiple ion permeabilities - it takes into account the concentration and permeabilities of different ions inside and outside the cell
- Ek = -89.1mV
- Ena = 61.5mV
- Em = -80.1mV - membrane potential
Describe how the action potential is generated
- If a depolarisation shift the membrane potential sufficiently it will result in an action potential
- 2 key cell types this arises in = neuronal cells and cardiac cells ( cardiac myocytes)
Describe the differences in neuronal and cardiac action potentials
Looking at the graph we see the action potentials are different to neuronal action
potentials
- Comparing resting potential neuronal = -70 ish while cardiac = -90ish this tells us the
resting potential is closer to Ek - so we know cardiac myocytes are more permeable to potassium
- We have rapid depolarization but elongated plateau phase and rapid repolarization - because of this the action potential duration is different - neuronal action potential takes roughly 5 ms while cardiac action potential last longer 300ms
- Both action potential duration for neuronal and cardiac are suited for their specific functions
Ion conc in cells
Na+; 15mM inside 150mM outside
K+; 140mM inside 5mM outside
Potassium leak
Potassium moves out of the cell down its concentration gradient
- Potassium can leave the cell through k+ leak channels ( allows potassium to leave
out of the cell ) in the cell membrane
- As both sodium and potassium ions are possibly charged they are going to associate with negatively charged ions on either side of the membrane - there is a lot of chloride on the outside , inside we have large inorganic anions.
- Potassium crosses This leaves behind the negatively charged anions
Electrical gradient
There is a separation of charge across the membrane with a more negative charge inside the cell
- We have a separation of negatively charged ions - this leads to a potential difference - a difference in electrical charge across the membrane
- We have a chemical gradient established by na/k pump - diff concentrations of each ion on either side as well as electrical gradient set up by potassium leak channels
- There is an electrochemical gradient across membrane resulting in a membrane potential
- Membrane potential = voltage ( difference in electrical charge ) across the plasma membrane
- Resting potential - the membrane potential of a cell not sending signals - this exist in all cells in the body - for most cells this figure = -70mV
Equilibrium potentials and membrane potentials graph
The graph shows us that the membrane potential is closer to Ek as the membrane is
more permeable to K+
What would happen if the membrane become more permeable to Na+
- The membrane potential will increase towards equilibrium potential of sodium because if membrane is more permeable to sodium there will be a lot of sodium coming onto the cell bringing in more positive charge resulting in the membrane becoming more positive - membrane potential is lowkey dependent upon the overall charge inside the cell
Depolarisation , repolarisation and hyperpolarisation
Depolarization - When the membrane potential is more positive than the resting potential due to an influx of sodium ions into the cell
- Repolarization - when the membrane potential returns to resting potential after depolarization due to the the movement of potassium out of the cell
- Hyperpolarization - membrane potential is more negative than the resting potential
Ion channels
Changes in the membrane potential occur because cells contain gated ion channels that open or close in response to stimuli - e.g change in voltage of the membrane
Neuronal action potential
Neuron receive inputs, when theres is sufficient depolarisation action potential is
generated and spreads all the way along the axon to the nerve terminals
How does this process happen?
- Cell body receiving inputs from different places (a stimulus)
- The inputs cause membrane depolarization - the membrane depolarisation needs to
be sufficient - meaning it needs to meet the threshold before it can fire off an action
potential
- If threshold is not met we get a failed initiation
- The input causes a large enough depolarization - at this point the sodium ion
channels are open allowing sodium influx into the cell
- Because sodium is positive as it comes into cell membrane becomes depolarized
becoming more positive
- Membrane potential stretches up towards sodium equilibrium potential
- Around +40 the sodium channels inactivate so they close. As they start to close
potassium channels start to open
- There is more potassium inside than out so with open potassium channels the
potassium leaves the cell as it does this the positive charge decreases leaving more
negative charge - this is the repolarization of the cell
- Because of the leaky channels am excessive amount of potassium is able to leave
the cell so the membrane potential is more negative than the resting potential this is hyperpolarization
- Hyperpolarization is important because in this phase it is going to be harder for another action potential to be initiated as we would need to depolarize a lot to initiate an action potential in the hyperbolised state
- Activity of sodium potassium pump returns the membrane potential to resting state
- The membrane potential will not reach Ena because around +40 the channel
inctivates - If we block sodium channels that causes depolarization which causes initiation of
action potential we will not get transmission of a signal - this is a mechanism used in anesthetics - block sodium ions in nerves meaning transmission of pain signals does not occur
How does the signal spread along the nerve
- The action potential spreads along the axon and this is how nerves send electrical
signals
- - Different regions of the nerve get depolarized and the depolarisation spreads along -
depolarization of one section of the nerve causes depolarization is the next section…
( action potential can be described as a wave of depolarization) - As the depolarization spreads we also have repolarization activity in the previous
section - in this phase the nerves will struggle to send any more signals - Once depolarization moves further along we can start to send another signal as we
are back to the resting state of the nerve
What happens when the action potential reaches the synapse?
- At the synapse an action potential will cause neurotransmitter release
- At the synapse the nerve will synapse with a piece of tissue skeletal muscle or another nerve…
- What happens = action potential will move along as it does membrane depolarization occurs at the terminal the membrane depolarisation allows entry of calcium into the nerve terminal
- Calcium allows fusion of vesicles with membrane of the terminal which contain the neurotransmitters into the synaptic cleft
Cardiac action potentials
Signal comes in resulting in depolarization ( point 0)
- Repolarization begins - the inactivation of the sodium channel ( point 1) and potassium ion channels begin to open
- We have plateau phase ( stage 2) - potassium channels are still open but some calcium channels are activated as well which results in this plateau phase - this is not present in neurons - key feature in cardiac action potential because we need calcium to come into the cell as calcium is what initiates the contraction of oir cardiac myocytes
- Stage 3 = repolarization - calcium channels close potassium channels still open
- Stage 4 = resting - ,most sodium and potassium channels are closed
- Action potentials underlie the electrical conduction system in the heart - this controls the rhythm, and synchronicity of the contractions of the heart
- 2 nodes in the heart control the rhythm and synchronicity of the heart contraction
- The electrical impulse is spread along the different chambers of the heart
- The electrical activity spreads along cardiac myocytes in the different chambers of
the heart as this happens we get action potentials being captivated in the different
cells in the different chambers , calcium coming in causing contractions - All of this is controlled so that your heart beats in the correct way as well as the
chambers contracting at the correct time
