Action potentials Flashcards
Describe paracrine cell communication. Name a 3 molecule that uses this type of cell communication
-Acts locally -Neurotransmitters released from nerves - Hormones which act as neurotransmitters (adrenalin)
Describe endocrine cell communication
Hormones released into the blood- have a long lasting effect
Describe how the resting potential is created inside the cell
At rest there is a small potential (voltage) difference. Inside the cell is negative, outside the cell is positive. This is caused by the selective diffusion of ions across the membrane. Sodium and potassium ion channels allow facilitated diffusion of ions along concentration gradient. K+ ions leave cell through leak K+ channels. This causes the cell to become negatively charged as proteins within the cell are negatively charged. Na+ enters cell along concentration gradient. Na+ channels are not as leaky as K+ channels. Less Na+ enters Na+/K+ pump exchanges 3 Na+ out of cell for 2 K+ into cell (against there concentration gradients). This maintains the concentration gradient to enable diffusion to occur.
What are the 3 mechanisms which contribute to the resting potential of the cell?
- passive ion diffusion (leak channels) 2. Active ionic diffusion (ATP is used to transport the K+ out of the cell) 3. Gibbs-Donnan equilibrium effect (
Describe the normal distribution of Na+ and K+ in and out of the cell
Na+ is higher outside of the cell K+ is higher inside of cell
What is the potential (voltage) difference across a normal membrane
Inside is slightly negative Outside is slightly positive
What is Gibbs-Donnan equilibrium (effect)
The concept that proteins inside the cell which are negatively charged are impermeable to the membrane so can not leave. This means there will be asymmetric distribution of permeable charged ions as they move to ensure concentration ratios are equal at equilibrium.
What is the effect of the sodium potassium pump on the resting potential?
exchanges 3 Na+ ions from intracellular space for 2 K+ ions from extracellular space so: 1. makes Na+ concentration high in extracellular space and low in intracellular space 2. makes K+ concentration high in intracellular space and low in extracellular space 3. Result = extracellular space has positive voltage with respect to intracellular space
What is used to measure charge across a membrane?
Voltmeter
How much mV does the Na+/K+ pump contribute to the resting potential?
4 mV
What is the over all mV (volts) of the resting potential?
70mV
What is the Nernest equation?
Assesses the voltage potential across the membrane in the instance of the net diffusion of ions (across the cell due to a concentration gradient) stops.
What does the Nernest equation relate?
the membrane potential to the concentration gradient
Em=61.5 log [C]o _ [C]i For a monovalent cation at 37 degrees What does each part of this equation mean?
61.5- the constant for cl-. [C]o- concentration outside the cell [C]i concentration inside the cell 37 is simply the temperature
Examples to try: [Na+]o = 150 mM [Na+]i = 15 mM Constant is 61.5
log 150/15 = log 10 = 1 ENa+ = 61.5 x 1 = 61.5 mV inside positive
Another example to try [K+]i = 150mM [K+]o = 5mM Constant is 61.5
EK+ = -61.5 x 1.477 = -91mv inside negative
What is the measured mV? It should be the same as Ek+ why is it not?
measured -70mV It is close to EK+ which is -91 mV but not the same because small amounts of Na+ leak across the membrane making the Em less than EK+
What is the relative permeability of K+ and Na+? What does it mean?
P K+ : P Na+ = 50 -100:1 The neuronal plasma membrane is 100 times more permeable to K+ than it is to Na+
What is the difference between the facilitated diffusion leak channels for Na+ and K+?
K+ has more leak channels and the leak channels have a bigger diameter
What happens in excitable nerve and muscle cells?
They undergo rapid changes in voltage across the membrane (action potential) This is caused by the presence of voltage dependant ion channels
What is an action potential? What causes it to occur?
It is the rapid change in voltage across a membrane
It occurs due to voltage dependant ion channels
Draw and label neurone
What is the trigger region for the generation of an action potential?
Axon (Axonal), and especially at axon hillock
Name the two types of voltage dependant ion channels
–Na+ channels
–K+ channels
How many gates are there for Voltage-sensitive Na+ channels? Name them
2 gates- Activation gate
Inactivation gate
What are the features of the activation gate in Voltage-sensitive Na+ channels
–closed in resting state
- Open in response to depolarisation
- Opening very fast <0.1 ms
- Allows about 6000 Na+ ions into cell
What are the features of the inactivation gate in Voltage-sensitive Na+ channels
–open in resting state
- Closes in response to depolarisation
- Closes slow (around 1ms after depolarisation)
What is the consequnce of the inactivation gate of Voltage-sensitive Na+ channels closing slowly?
Na+ is still able to enter during depolarisation before the inactivation gate fully closes
What happens to Voltage-sensitive Na+ channels after repolarisation?
They return to normal
Describe the features of Voltage-sensitive K+ channels
At rest it stays closed.
Delayed opening is triggered at the threshold in responce to deloarisation- (-70mV to 30mV). Opens slightly slower than Na channels close
- They stay open during entire depolarisation
- Increase P K+ compared to > resting P K+
- Produce repolarisation and undershoot (hyperpolarisation)
What is undershoot?
Hyperpolarisation (30mV to -80mV)
Draw and label an action potential graph
What is the threshold for an action potential?
55mV
Describe the stages of an action potential
Stage 1 resting- all gated Na+ and K+ channels are closed (Na+ activation gates closed and inactivation gates open)
Stage 2-Depolarisation- Na+ channels open
Stage 3- repolarisation- Na+ channels inactivating, K+ channels open
Stage 4- Hyperpolarisation- K+ channels remain open, Na+ channels reset
Describe the all or nothining principal of action potentials
–Either depolarization does not reach threshold = therefore no action potential
–or it reaches threshold and elicits an action potential that is always the same
What causes the threshold of an action potential to be reached?
•Threshold for an action potential is reached when the opening voltage-gated sodium channels stimulate other channels to open in a positive feedback loop
What is an absolute refractory period?
– Channels Cannot open regardless of size of the applied stimulus
–Due to inactivated Na+ channels
–1-2 ms
Contributes to all or nothing property
Describe the relative refractory period
–Before closure of K+ channels PK+ is large
–Due to continued outward diffusion of K+
When k+ is low enough during the repolarisation phase, another action potential can be activated if the stimulus is suprathreshold
–Therefore larger stimulus needed to make PNa+> PK+
–Increased threshold
–3-15 ms
•make it possible to encode information by converting the strength of a stimulus into the frequency of action potentials
Label the absolute and relative refractoyr periods on a graph
What is a suprathreshold?
graded potential can produce a higher number of action potentials within a given period
