Neurophysiology 2 Flashcards
Voltage is defined as…
The difference in charge between two points in space
Current is defined as…
Movement of charge
- Typically talking about ions moving across the membrane
Resistance is defined as…
Reduction of movement
Ohm’s law
V=IR
In a completely intact membrane (no channels), resistance is…
Infinite
- because there’s no ion channels so no current
As we add channels to a membrane, what happens to resistance?
Resistance goes down.
- You’re adding more resistors in “parallel”
Capacitance formula (with charge and voltage)
C=q/V
How is capacitance defined?
As the ability to hold charge
Capacitors are composed of…
2 conductive plates separated by an insulator
- The conductive plates allow charges in the plate to move around
- The insulator prevent charges from moving between plates
Describe how capacitors store charge
When circuit is connected to a battery, the positive charges spread out on the plate connected to the positive end of the battery.
- The positives on the other plate are repelled so the other plate becomes more negative.
- If you were to take the battery out, the capacitance would remain.
The maximum amount of charge that a capacitor can hold is…
The voltage of the battery
True or false: the cell membrane itself works like a capacitor
True
- Imagine positive ions spitting out of the end of the current microelectrode (during a positive current pulse)
- the electrode allows for positive charge to build up on one of the conductive plates, while the positive ions are repelled from outside the cell membrane, leaving negative charges outside the membrane
What is Tau (τ)?
The time constant
- The time to reach 63% of max change in voltage
- Due to charging delay
τ formula
τ= RC
- R= membrane resistance
- C= membrane capacitance
What are passive electrical properties?
Passive electrical properties explain how charged particles like ions move around the membrane.
- inherent properties of the neuron’s membrane and cytoplasm that govern how electrical signals spread without involving ion channels that consume energy
- Positive charges build up and disperse. So voltage can be measured at distant sites.
What is the length constant (λ)?
The distance where ΔV is 37% of max (origin, where charge was added)
What is membrane resistance (Rm)?
Membrane resistance (across the membrane)
What is axial resistance (Ri)?
Resistance inside the axon
- i.e. what is getting in the way of charges moving down the axon
What is extracellular resistance?
If we have charged inside the cell moving out of the cell, extracellular resistance describes the things in the way of charges moving out.
λ equation
sqrt(Rm/Ri)
If Rm is low, what is λ?
λ is short because a lot of charge leaks out (remember low Rm means that there’s lots of channels in the membrane)
If Ri is high, what is λ?
λ is short
How does the max voltage of the cell membrane change as a charge moves down an axon?
It decreases
What is the formula for voltage as a function of time?
V(t)=Vmax(1-e^t/tau)
What is the formula for voltage as a function of distance?
V(x)=Vmax(e^-x/λ)
If Rm is high, what is λ?
λ is high (more charges to actually move down the length of the axon)
Describe temporal summation/subtraction
If stimuli are given in quick succession, these stimuli can summate or subtract, making the total membrane potential change either greater or lesser than the stimuli alone.
Describe spatial summation/subtraction
If stimuli are given at the exact same time and are spatially close when administered, these stimuli can summate or subtract, making the total membrane potential change either greater or lesser than one stimuli alone.
Graded potential
Changes in membrane potential that vary in magnitude depending on the strength of the stimulus.
- Graded potentials are local, meaning they decrease in strength as they move away from the stimulus site, and they can summate to reach the threshold needed to trigger an action potential.
If excitatory and inhibitory stimuli are administered at the same time, what happens to membrane potential?
The excitatory and inhibitory stimuli cancel each other out, because the overall charge of the membrane becomes neutral.
Overall, what are the three passive electrical properties that we looked at?
- Time delayed change in membrane potential
- Distance degradation (how charge “degrades” with distance)
- Temporal and spatial summation and subtraction.
What type of electrical properties do dendrites obey?
Passive electrical properties
Describe summation and subtraction in dendrites
Charges enter through channels in the dendrites. These charges build upon each other, then travel down the length of the dendrite towards the cell body (summation), or cancel each other out if oppositely charged and don’t travel down towards the cell body (subtraction).
In general, if there’s enough positive charge to sufficiently change the membrane potential of the cell body…
Information is relayed through the neuron.
What is the axon hillock and what is it responsible for?
The joining point between the cell body and the axon
- High density of voltage gated Na+ channels. If we get enough positive charge such that the cell body becomes depolarized, it may change the membrane potential sufficiently to open these voltage gated Na+ channels.
- If these channels open, the information is passed on (causes an action potential).
Describe the structure of a voltage-gated Na+ channel
4 subunits (aka domains), which each have 6 inter-membrane compartments
The 4th membrane-spanning protein in each subunit of the Na+ (and K+) channel is the…
Describe its function too
Voltage detector of the cell
- When the charge is sufficiently positive from a stimulus, the positive charges on the 4th segment repel each other, thus opening the channel pore and allowing an increased influx of Na+
Describe the structure of a voltage-gated K+ channel
4 identical subunits, each containing 6 membrane-spanning proteins.
- Na+ and K+ channels are very similar in structure.
- 4th segment is still a voltage detector.
What are the two things that contribute to ion channel selectivity?
- Physical size
- Waters of Hydration (hydration shell)
Describe how the hydration shell contributes to ion channel selectivity
- Equally negatively charged and equally spaced amino acid residues are aligned on the inside of the channel
- You can’t just remove water molecules from the ion because this would require energy
- As the K+ moves through the K+ channel, these charged amino acids replace the water molecules (doesn’t require energy)
- The selectivity of these channels has to do with how these amino acid residues are located in the channel.
- The amino acids can’t easily replace the water molecules surrounding Na+ in the hydration shell.
- : If an ion cannot shed its hydration shell efficiently due to the structure of the channel, it will face a high energy barrier to entering the channel. For example, Na⁺ has a smaller ionic radius but a larger hydration shell compared to K⁺. Since sodium ions have a harder time shedding their hydration shell to fit into a potassium channel, they are excluded, ensuring selectivity.
The action potential is an ______ electrical event
Active
How do neurons communicate?
Through action potentials
True or false: action potentials degrade with distance
False
True or false: Action potentials typically move in one direction, unlike passive potential
True
In general, what do action potentials depend on?
Depends on change in membrane potential
- requires passive electrical potentials
On an action potential graph, what is the first bit at -65mV called
Resting membrane potential (R.M.P)
Where is the depolarization on an action potential graph?
The earliest point of the increase
What phase comes after the depolarization phase on an action potential graph?
The rising phase
What is the top of the action potential graph called?
The peak aka the overshoot
What comes after the overshoot on an action potential graph?
The falling phase
What is the final phase of the action potential graph, where it dips below -65 mV?
The undershoot aka the afterhyperpolarization
True or false: an action potential will be bigger if the current stimulus is greater
False, action potentials are simply “all” or “none”
True or false: an action potential will be longer if the current stimulus is longer
False, there would simply be more action potentials, not a “longer” action potential because they’re “all” or “none”
In general, what causes action potentials to move in one direction?
As the action potential moves down an axon, voltage-gated Na+ channels get deactivated which prevent the action potential from moving backwards
Describe the state of K+ leak channels, voltage-gated Na+ channels and voltage-gated K+ channels during resting membrane potential
K+ leak channel: Open
Voltage-gated Na+ channel: Closed
Voltage-gated K+ channel: Closed
Describe the state of K+ leak channels, voltage-gated Na+ channels and voltage-gated K+ channels during depolrization phase
K+ leak channel: Open
Voltage-gated Na+ channel: Closed
- If the membrane depolarizes enough, the voltage-gated Na+ channels open
Voltage-gated K+ channel: Closed
Describe the state of K+ leak channels, voltage-gated Na+ channels and voltage-gated K+ channels during the rising phase
K+ leak channel: Open
Voltage-gated Na+ channel: Open
- And more continue to open as Na+ flows in
Voltage-gated K+ channel: Closed
During the rising phase, when the voltage-gated Na+ channels open, what happens in terms of Na+ flux?
There’s a large influx of Na+ into the cell due to a high driving force and a high emf.
- This is the reason why the membrane potential becomes very positive suddenly
Describe the state of K+ leak channels, voltage-gated Na+ channels and voltage-gated K+ channels during the overshoot
K+ leak channel: open
Voltage-gated Na+ channel: Open
Voltage-gated K+ channel: Closed
Describe the state of K+ leak channels, voltage-gated Na+ channels and voltage-gated K+ channels during the falling phase
K+ leak channel: Open
Voltage-gated Na+ channel: Inactivated (ball on the inside of the cell blocks the pore)
Voltage-gated K+ channel: Open
If Na+ and K+ are both voltage-gated and open due to positive charge, why does K+ open after Na+ closes?
Voltage-gated K+ channel takes longer to open compared to Na+ channel (because K+ needs to change conformation)
Describe the state of K+ leak channels, voltage-gated Na+ channels and voltage-gated K+ channels during the undershoot/afterhyperpolarization phase
K+ leak channel: Open
Voltage-gated Na+ channel: Closed (ball not blocking pore but pore is still closed)
Voltage-gated K+ channel: Open
During the undershoot phase, the permeability of K+ is (greater/smaller) than its permeability at resting membrane potential
Greater
Describe the state of K+ leak channels, voltage-gated Na+ channels and voltage-gated K+ channels during the repolarization phase
K+ leak channel: Open
Voltage-gated Na+ channel: Closed
Voltage-gated K+ channel: Still open, but slowly closing (membrane starts returning to resting membrane potential
Describe how action potentials only propagate in one direction
As action potential propagates down an axon, voltage-gated sodium channels are innactivated (and then eventually close). The voltage-gated sodium channels directly behind an action potential are still innactivated. Voltage-gated sodium channels further down are in their closed state, but charge is too degraded by that point to re-open them.
Absolute refractory period
Period directly after an action potential where it is impossible to generate another action potential
Relative refractory period
Period a little bit after an action potential (after the absolute refractory period) where it’s difficult to get another action potential, but it’s still possible
What are 3 reasons as to why refractory periods exist?
- Inactivation of Na+ channels (remember that Na+ channels move to inactivated state after being open)
- Increased K+ permeability (pushes the membrane closer to K+ equilibrium potential which is negative)
- Decreased membrane resistance (After an action potential, a lot of channels are open like the K+ leak channels, voltage-gated K+ channels, and some Na+ channels so putting charge into the cell would result in very quick charge leakage out of the cell).
How is conduction velocity defined?
Defined as how long it takes for an action potential to move from point A to point B down an axon.
What 3 things affect conduction velocity?
- Axon diameter
- Myelination
- Temperature
How does axon diameter affect conduction velocity?
- Increased diameter= action potential conducts more quickly because an increased diameter decreases axial resistance (Ri)
- Longer length constant (λ)
- Larger diameter allows ions to flow more easily down the middle of the axon.
How does myelination increase conduction velocity?
Due to saltatory conduction
- Myelination increases Rm
- Longer length constant (λ) because more of the charge is going to move farther, since charge can’t leak out as much
- Action potentials happen at the nodes of Ranvier, and then charge quickly moves through myelin sheath
How does temperature affect conduction velocity?
10 degrees C increase in temp= double conduction velocity
- Warmer temperature makes molecular movement increase. Na+ ions move faster so action potential can be generated more quickly.
How are active electrical properties defined?
These involve ion channels that open and close in response to changes in membrane potential (voltage-gated channels), allowing the cell to actively generate and propagate electrical signals.
Faster conduction in axons is associated with a (shorter/longer) time constant tau and a (shorter/longer) length constant
Shorter time constant, longer length constant
