Action Potentials Flashcards
Contrast Graded potentials from action potentials
Graded potentials are changes in the membrane potential induced by the inputs. They can vary in size and duration depending on the input and even be summed with neighboring graded potentials. Graded potentials can occur anywhere of the neuron but tends to be seen higher at the dendrites, occasionally at the soma and rarely onto the axon terminals. Action potentials can exist only at the hillock and are always the same size and duration.
The membrane potential is determined to be around -60/-70 mV through
A. Summation of the charge difference inside and outside of the cell
B. Difference in charge between the inside and outside of the cell
C. The difference in charge between the soma’s graded potential and potential at the axon hillock
D. The difference in charge between the soma and dendrites graded potential and the potential at the axon hillock
B. Difference in charge between the inside and outside of the cell. Because the membrane is permeable to many ions at once, the RMP is a weighted average of equilibrium of those ions weighted by their permeability.
What happens to all the graded potentials at the axon hillock?
The neurons process these inputs by combining all the graded potentials to determine if they have reached the threshold (~ -50 mV). If the threshold is met an action potential is triggered. If not nothing happens.
Oftentimes, when the postsynaptic cell receives messages from the neighboring neurons or the environment, what is its response often called?
Graded potentials produced at a synapse is called postsynaptic potentials
Contrast postsynaptic potentials from receptor potentials.
Receptor potentials - graded potentials created by a stimuli. Though they can mean the same, this type of graded potential entails a receptor detecting the stimuli from the environment. Whereas postsynaptic potentials are graded potentials created within the postsynaptic cell in response to other neurons or cells. Postsynaptic potentials can be depolarizing or hyperpolarizing
Contrast Depolarization from Hyperpolarization.
Depolarization - the process in which the membrane potential raises in voltage, because the membrane is less polarized therefore less charge separation. Hyperpolarization - the process in which the membrane potential becomes more negative in voltage.
Upon stimulating a neuron in lab, you step away in order to grab the next electrode. When you look back, you notice there was no axon potential generated on the screen because there was no membrane potential changes at the axon hillock. What two important properties must you take into account as to why stimulating the dendrites, each one with enough graded potential does not induce an action potential at the axon trigger zone.
Two important properties of graded potentials: (1) They decay with both distance
and (2) time (therefore the effect is brief and local) Therefore graded potentials have to be close in time and in distance in order for their effects to be summated
The addition of of graded potentials close in time is A Grade potentials B. Action Potentials C. Spatial Summation D. Temporal Summation
D. Temporal summation - is the process in which two graded potentials are additive because they occurred together close in time
True or false: Spatial and Temporal summation at the axon hillock always leads to excitatory action potentials
False. These graded potentials can be inhibitory as well AKA hyperpolarization. This is when you get no change in the membrane potential or a decrease in membrane potential and no axon potential seen
After looking at your set up, you realize that the distance from the dendrite and the axon may be the reason why the degradation of the graded potential occurs so fast. How can you change your setup?
To minimize the degradation, change your site of input, such as stimulation in the soma or at the axon trigger zone. Less distance may lead to decrease decay
Transient Membrane Potentials, also called graded potentials can occur at which of the following locations?
I. Dendrites
II. Soma
III. Axon
A. I and II only
B. II and III only
C. I only
D. I, II, and III
A. I and II. The graded potentials are summed at the axon (the way neurons process information) and fire an action potential if the potential reaches the threshold. Axons is where we find action potential and we don’t think of the axon as where we find graded potentials
Because the resting membrane potential of around -60mV is the difference between the inside and outside membrane, what potential do we hold the outside by convention?
By convention we call the outside as 0 while the inside is called the membrane potential
Name the vital cations of a cell
Vital Cations: K+, Na+, Ca2+
Name the vital anions of a cell
Vital Anions - Cl- and organic anions (many of them) - many of these are proteins with a negative charge
Contrast the two different forces essential in determine the flow of an ion (either positive or negative) in or out of a cell.
- Electrical force - The opposite charge of the cell or the environment drives the ion to go to the side that attracts is.
- Diffusion/chemical Force - higher concentration wants to flow from high [ ] to low [ ].
Working in a chem lab over the summer, you’ve come to observe how a membrane potential is created. Explain the phenomenon of why a K+ ion, would want to leave the cell if the cell has a NEGATIVE charge?
An ion’s travel in and our of a cell is determined by both the electrical and diffusion force. Even though K+ is a positive ion, and should want to stay within the negative cell, the high concentration of the K+ creates a bigger force than the electrical force, driving the K+ out of the cell when it gets and opportunity to leave.
What other ion resembles K+, where the diffusion force is bigger than the electrical force, driving the ion into the unexpected direction?
Cl- Electrical force states that it should want to flow from inside to out, but because chemical force is bigger, Cl- will have the tendency to flow in. These forces tend to be called electrochemical driving forces. Neurons use these forces to perform their functions
Account for how much change in membrane potential organic anions such as proteins made within the cell have.
-5mV change to the membrane. This is not enough for the membrane or cell to function [note: required membrane potential for function is -60 mV]
Account for how much potential Na/K+ pumps have onto the membrane potential.
Contributes about -10mV potential. These pumps are active when the potential is high, therefore it plays a great role in decreasing the membrane potential during and action potential
What force plays the most role in creating the membrane potential of -60mV?
Concentrations of K and Na of high inside and high outside influences the resting membrane potential the most. The leak channels are more permeable to K+ compared to the Na+, leading to a higher concentration of K+ inside.
The equilibrium potential at which K+’s diffusion force is equal to the electrical force is -70mV. However, the potential of the membrane is not that. Why is that the case?
The leak channels allow Na+ to influx in as well. This lowers the membrane potential slightly compared to what we would expect if the cell only have K+ within the cell. Because some Na+ is able to come in, it will influence the RMP causing the RMP to go up to around -60 mV [Remember K+ wants to leave the cell, Na+ wants to enter]
How many K+ has to leave the cell through leak channels does it take to reach this equilibrium (RMP)?
Less than 1% of 1% all the K+ it the cell
Therefore the effect of the cell by K+ leaving is also negligible!!
Contrast the permeability of Na+ by leak channels to K+ by leak channels.
At rest, the permeability of Na is less than K+!!! (it is about ~4% of K+’s permeability)
Contrast the permeability of Cl through leak channels to K+ through leak channels.
RMP has an intermediate permeability to Cl- or around 45% of the permeability of K ions
Cl - doesn’t seem to have much effect on the RMP
True or false: Cl has a role in maintaining the resting membrane potential
Cl - doesn’t seem to have much effect on the RMP due to its low permeability. BUT, the RMP does appear to have a role in the concentration of Cl- within the cell. The RMP creates an electric gradient, driving the Cl- through leaks channels, until its concentration gradient balances out its electrical gradient
What is the purpose of the Cl/K+ symporter?
This is the method used by most neurons to decrease the intracellular [ ] of Cl-. The symporter decrease the Cl- from the soma by harnessing the diffusion force of K+ ions. Therefore K+ and Cl- both leave together as the K+ generates energy to push Cl- out of the cell
Symporter - Same side
What is the effect of the active Cl/K+ symporter?
The Symporter decreases the resting potential of the membrane to around -70mV. This drives some inward of negative value (but this is negligible)
The vital cation Ca2+ is not seen to have a huge concentration within the cell. Why is this the case? What else is utilized to drive them out of the soma?
Na/Ca2+ exchanger - drives Ca out of the Neuron, by harvesting the energy and diffusion energy from Na. Therefore Na freely diffuses, but Ca is actively pumped
Which pathway does Ca2+ tend to lean towards? What is the equilibrium potential of Ca2+ when the electrical force equals the diffusion force?
Ca2+ tends to be attracted to the negative cell soma and because it also has a greater concentration outside the cell, both electrical and diffusion force drives Ca2+ into the cell. The membrane potential when electrical and diffusion force equals one another is +120 mV.
Why does the rate of movement decrease to 0 is an ion such as K+ was allowed to move between the membrane?
Eventually that force (the electrostatic repulsion force) is equal and opposite to the [ ] gradient/force. The net force will become 0 (by Newton’s second law of motion - which states that when the net force is 0, there will be no overall motion
How can we actually calculate what the resting membrane potential is?
Utilize the Nernst Equation V = - (RT)/(ZF) ln([x_in]/[x_out]) to calculate the potential/voltage. R = the gas constant 8.314 J/mol*K T = the temperature in Kelvin Z = the charge of the ion x F = Faraday’s constant, 96,500 C/mol
The gas constant, R, is A. 8.314 J*mol/K B. 8.314 J/mol*K C. 8.314 K*J/mol D 8.314 K*mol/J
B. 8.314 J/mol*K
