Lecture 2 - Neurophysiology Review Flashcards
Describe what happens when light hits our retina to allow us to see. 2 steps
- Photon of light hits rhodopsin, a G-protein coupled receptor heterotrimer (alpha, beta, and gamma subunits) in our retina, it causes an exchange of GDP to GTP at the alpha subunit.
- The activated rhodopsin then influences the phosphodiesterase and
cGMP levels which can influence the opening and closing of voltage gated ion channels, changing the probability of these being open or closed and influencing the
flux of ions and thus influencing action potentials.
What are optogenetics? Purpose?
The creation of protein channels that respond to light
Purpose: potentially to modulate behavior
What is the normal RMP of cells?
-70mV
How do ion channels demonstrate selectivity?
Size/Cross-sectional area of channel opening measured in picosemens
What are the 5 types of ion channels?
- Voltage gated
- Ligand gated
- Pressure/stretch gated
- Non-gated (passive leak)
- Water channel (aquaporins
What diffusion characteristics do ion channels have?
- Passive diffusion
2. Carrier-mediated diffusion
Are ACh receptors ion selective? What are they permeable to?
NOPE
Permeable to K+/Na+/Ca2+
How many ACh need to bind to the receptor to open the ion channel?
2
How many proteins in the ACh receptor?
5
Describe the structure of ion channels.
Contain multiple membrane spanning components
How many membrane spanning components in VG ion channels?
6
What is special about the 4th membrane-spanning domain of VG ion channels? Describe its composition.
It’s the voltage sensor.
It consists of positively charged amino acids (lysine, arginine) every 3rd amino acid in the alpha-helix. This allows us the channel to detect the changes in electric potential across the lipid bilayer.
What is special about the 5th membrane-spanning domain of VG ion channels? Describe its role.
It’s the pore domain, forming the re-entry loop— it serves as the selectivity
filter of the ion channel
Describe how gel eletrophoresis of DNA works. What is the electric field equal to?
Electric field applied over an agarose gel membrane influences DNA migration and separation by size.
That electric field (E) is equal to change in voltage over the change in distance (E=ΔV/Δd).
How is an electric field created across a membrane?
Created by the separation of charge across the membrane
What happens when the voltage is the one at which the VG channel should open?
The charge is detected and causes the alpha-helix (voltage sensor) to rotate, opening the pore and allowing the flux of ions
What does the flux of ions depend on?
The Nernst potential and the membrane potential
What is the Nernst potential?
Potential reached when the cell reaches electrochemical equilibrium— when the flux of an ion due to concentration gradient is equal to the flux of that ion due to the electrical gradient
What is the equation to calculate the Nernst potential?
Vm= (60/Z) . log ([Ion outside]/[Ion inside])
What do conduction velocities depend on? 4 factors
The time and space constants:
- Time constant = Rm . C
- lambda = SR (Rm/Ri)
- Internal resistance (due to diameter of axon)
- Axon membrane capacitance (therefore myelation)
- Membrane resistance
- Activation kinetics of the Na+ channel
Why does it take time for the voltage to attain its max and back to min? How is this measured?
Because of capacitance! Time constant = amount of time it takes to charge and discarge the membrane capacitance
Time constant = Rm . C
Rm = membrane resistance C = capacitance
What does the time constant mean for conduction velocity?
SMALL TIME CONSTANT = FASTER VELOCITY
What does the space constant mean for conduction velocity?
LARGE SPACE CONSTANT = HIGH VELOCITY
What determines how far a current will spread? What is the equation? What does this mean conceptually?
Space constant = distance an AP can travel before it reaches 37% of its initial strength
lambda = SR (Rm/Ri)
Rm = membrane resistance Ri = internal resistance = 1/diameter of the cell
Larger diameter fibers: faster or slower?
Faster
Smaller diameter fibers: faster or slower?
Slower
How does myelination affect membrane resistance?
Increases it
How does myelination affect membrane capacitance?
As the distance between two plates increases, the capacitance decreases. Thus membrane capacitance (Cm) decreases as the membrane gets thicker (due to myelination).
What does membrane capacitance describe?
The membrane’s ability to charge and discharge
How does myelination affect conduction velocity? Why?
It increases it because the decrease in capacitance due to myelination out weighs the increase in membrane
resistance
What determines the placement/clustering of Na+ VG channel at the axon initial segment?
An AIS localization signal (axon initial segment) in the cytoplasmic II-III region (between domains 2 and 3) of Nav1.2
What is the purpose of the clustering of Na+ channels at the Nodes of Ranvier? Why?
Energetically favorable saltatory conduction because
it would be energetically inefficient to try to transport Na+ and K+ across the entire length of an axon
Size of Nodes of Ranvier?
Less than 5 microns
What do all animal VG channel voltage sensors have in common?
High composition of positively charged amino acids at physio concentrations— lysine (K) and arginine (R) every 3rd residue
What is shaker?
VG K+ channel in fruit flies (drosophilia)
Describe a current clamp experiment.
Stimulate with a square pulse of current to measure the voltage of a membrane or action potential
Describe a voltage clamp experiment.
Stimulate with a square pulse of voltage to measure the current.
Describe a patch clamp experiment.
Similar to voltage clamp but measures the current of a single ion channel.
What is the difference between equilibrium potential and reversal potential? When are they equal?
Reversal Potential is determined in a voltage clamp experiment as the potential where the current changes direction and the NET ion flux is 0
Equilibrium Potential is defined by Nernst equation.
When a current is caried by a single ion (i.e Voltage gated Na channels) the Reversal potential is the same as the Equilibrium Potential for that ion
What is the Erev of Na+ under physiological conditions?
+60 mV
What is the Erev of K+ under physiological conditions?
-90 mV
What does it mean for the selectivity of an ion channel when the Erev is 0mV? Eg?
Permeable to both Na+ and K+
Nicotinic receptor to ACh (a little more permeable to K+)
Are selective ion channels only permeable to 1 ion? Eg?
NOPE
Eg: Selective Na+ channel is nearly as permeable to Li+
and even K+ has a small permeability
What is the permeability ratio of Na+ for an Na+ ion channel?
1
What ligand does the NMDA receptor bind? What ion is the channel selective to? What is this important for?
Binds glutamate and is highly permeable to Ca2+ which is import for memory formation and the development of dendritic spines
What does the threshold voltage correspond to?
Voltage at which a cell will fire an action potential = voltage-gated Na+ channels are at equil with leakage K+ channels = critical number of voltage-gated Na+ channels opening to OVERCOME the K+ leakage channels
What is important to note about action potentials?
They are all of nothing events and are all of the same amplitude (except from poison action)
What is the absolute refractory period? What causes the absolute refractory period? When is this?
Time in which no action potential CANNOT be generated
Na+ channels are inactive and need to go back to closed state: when the axon is getting depolarized and hyperpolarized (bell of the curve) the cell is unable to fire another action potential
What is the relative refractory period? What causes the relative refractory period? When is this?
Time during which an action potential can be generated by a STRONGER STIMULUS
The need for the membrane to be repolarized back to RPM: from hyperpolarization to resting potential
What prevents the action potential from propagating in 2 directions?
Na+ channels being in the inactivated state after
What are the 2 types of K+ channels involved in an action potential and what is the role of each?
- Leakage K+ channels: at threshold the voltage-gated Na+ channels are at equilibrium with leakage K+ channels = critical number of voltage-gated Na+ channels opening to OVERCOME the K+ leakage channels
- Voltage-gated K+ channels: responsible for repolarization of the cell
What does the membrane potential approach during the peak of the axonal action potential?
The Na+ equilibrium potential
Is the action potential duration associated with voltage-gated K+ channels? Why or why not?
Yes because the duration of the action potential depends on how quickly repolarization happens
What does the amplitude of the action potential depend on? Why?
Mainly dependent on the number of sodium voltage-gated channels because regardless of the electrochemical gradient, there is a limited amount of sodium that will be able to enter the cell before the potassium voltage-gated channels open.
If outside concentration is SIGNIFICANTLY reduced: amplitude will be lower though
The delay of K+ current activation can also have an impact.
What do we call the speed of propagation of the action potential? Units?
Conduction velocity (m/s)
What is the unidirectional movement of an action potential from the axon hillock to the nerve terminal due to?
BOTH refractory periods of the action potential
Is the potential of the action potential at max depolarization of most cells closer to the equilibrium potential of Na+ or K+? Why?
Na+ because voltage-gated channels are open and permability of Na+ > K+
What are the 3 states of the Na+ VG channels?
- Closed
- Open
- Inactive
When does the Na+ conductance peak during an AP?
Right before the peak of the AP
When does the K+ conductance peak during an AP?
Halfway through repolarization
Negative current: inward or outward?
Inward
Positive current: inward or outward?
Outward
How is an AP different in a cardiac myocyte? Describe the mechanism.
In the myocytes, the action potential needs to last much longer than a neural action potential so we can actually pump blood through our body. This is facilitated by voltage-gated Ca2+ channels who’s conductance is super imposed over K+ conductance, preventing quick depolarization
What is the Ca2+ Erev?
~+135mV
Do all cardio myocyte have an RMP? Explain.
NOPE because some are never at rest— these are the
pacemaker cells in the SA and AV node which APs are due to Ca++ in and K+ out
What is the duration of a cardiac AP?
250 msec
What is the threshold potential of pacemaker cells of the heart?
- 40 mV
What 3 types of drugs target Na+ VG channels?
- Local anesthetics
- Anticonvulsants
- Antiarrhythmic
drugs
What ion channel do ALL electrically excitable tissues have?
Na+ VG channels
How many Na+ VG genes in human genome? Where do these clusters?
9
Cluster on chromosomes 2 and 3
Describe the structure of Na+ VG channels.
4 proteins each containing 6 transmembrane domains (each with a voltage sensor)
What are auxiliary subunits? What are they referred to?
Beta subunits can change the voltage at which the channels activate and the number of ion channels in the membrane
What is the alpha subunit of an ion channel?
The ion pore/channel itself
What is the threshold of the AP?
-55 mV
What is the mechanism of Na+ VG channels inactivation?
Gating mechanism
How are the Na+ VG channel genes designated?
SCN designation
How can drugs target different types of Na+ VG channels?
Na+ VG channels are expressed in different tissues and therefore have different AA sequences and can respond differently to different drugs
What happens when Na+ VG channel inactivation is defective?
Hyperkalemic periodic paralysis, long QT syndrome (impaired cardiac AP), and inherited epilepsy where a mutation causes the incomplete closure of the inactivation gate resulting in an increased level of persistent current which can interrupt other tissues’ APs
How would one be able to diagnose long QT syndrome?
With an EKG
