Exam 1 Review (Study in advance of Exams 2, 3, 4, 5!!)

For K+ and Cl- respectively, if Vm > Ex, then there will be an _____ current.

For both, if membrane potential exceeds the equilibrium potential for an ion, there will be an outward current.

The strength of the driving force on an ion in a neuron does not depend on: a) concentration gradient b) electrical gradient c) conductance d) membrane potential e) driving force depends on all the above

a and b) FALSE; driving force is the sum of the electrical and concentration gradients for that specific ion in relation to a biological membrane c) TRUE; conductance, or permeability has no effect on the driving force for an ion (it is not within either formula for driving force) d) FALSE; membrane potential has a huge effect on the strength of a driving force for an ion (DF = Vm - Ex) e) FALSE

What are other names for neurites?

axons and dendrites; all types of neuronal processes (projections from the cell body of a neuron)

What current would you expect from a HERG channel while it is voltage-clamped and held at 0 mV? a) sustained current b) transient current c) no current d) it depends on the concentration of potassium

c) TRUE (no current) HERG channels do not pass current when voltage clamped as they inactivate immediately after stimulation

A neuron is at Vrest. If negative ions flow into the neuron, the neuron would become _________.

hyperpolarized (less likely to fire an action potential)

What can affect the shape of an action potential?

- Na+ and K+ ion concentrations - types of ion channels present on the axon membrane

A beaker with two chambers contains a high concentration of KCl on one side of the selectively permeable barrier and a low concentration on the other. Assume the barrier is only permeable to K+. At equilibrium, what is the direction of driving force for K+?

0 (no driving force for the permeable ion at its equilibrium point)

How many alpha subunits are required to make a functional voltage-gated sodium channel (NaV1.7)?

1

A beaker with two chambers and a selectively permeable barrier contains NaCl. The LEFT side contains a HIGH concentration and the RIGHT side contains a LOW concentration. Assume the barrier is only permeable to Na+. At equilibrium what will the direction of the electrical gradient for Na+ be?

At equilibrium, the direction of the electrical gradient for Na+ will be right to left (from more positive charge to more negative charge). In addition, the concentration gradient for Na+ at equilibrium will be left to right, and at equilibrium for an ion the driving force is zero (so the concentration gradient and equilibrium gradient must be equal and opposite).

Describe the shapes of the curves for driving force (DF), conductance (g), and current (I) for NaV1.7.

DF: driving force shows a straight line relationship increasing directly over time, with an x-intercept at the equilibrium potential for an ion g: conductance will be zero until threshold, where it will increase until it plateaus at a maximum I: current is zero until reaching threshold, where it will depolarize (become negative), then loop up to zero at the equilibrium potential, then hyperpolarize (become positive)

Ion "X" has a valance charge of +1 and current concentrations of 10 mM outside and 100 mM inside. What will happen to the equilibrium potential, when the outside concentration increases ten-fold?

Ex will become 0 mV

A graduate student adds scorpion beta toxin and voltage clamps a voltage gated Na+ channel at -40 mV. Na+ current will be sustained. True or False?

False Scorpion beta toxin lowers the threshold to form an action potential However, Na+ current will be transient (not sustained!)

For NaV1.7, increasing membrane potential 10 fold leads to a 10 fold increase in conductance. True or False?

False As membrane potential increases, conductance will initially increase in a direct relationship, until conductance plateaus to reach a maximum.

The probability that a single KV1.2 potassium channel is open is directly related to the driving force on the channel. True or False?

False K+ channels start to be open (but are delayed) just above threshold, but reach a maximum around 20 mV. The probability of these channels being open is not related to the driving force.

In a voltage clamp experiment, KV4.1 channels can produce a prolonged potassium current above threshold. True or False?

False KV4.1 channels produce a transient (not prolonged!) current above threshold that is inactivated after a short period

Functional K+ selective channels must have 4 alpha subunits, each with 6 or 7 membrane-spanning helices. True or False?

False Some K+ ion channels (such as KIR channels) have different numbers of transmembrane domains. For example, these KIR channels have 4 alpha subunits, but they each have 2 membrane-spanning domains (not 6).

An ion will always move from an area where it is more highly concentrated to an area where it is less concentrated. True or False?

False The direction an ion travels across a biological membrane always depends on the direction of the driving force for that ion. DF = CG + EG

An excitatory neuron projects from the lower spinal cord to the thalamus. By convention, we say that this neuron is afferent from the spinal cord and efferent to the thalamus. True or False?

False (reverse)

HERG K+ channels that are voltage clamped _______ threshold will pass the most current________.

HERG K+ channels that are voltage clamped ___above__ threshold will pass the most current __after the clamp is released__. This is because HERG channels open above threshold, then immediately inactivate. After the voltage clamp is released, the HERG K+ channel will gradually de-inactivate. K+ channels such as the HERG channel are also slow to close, meaning that after the initial de-inactivation, the HERG K+ channel will allow K+ ion flow to hyperpolarize the neuron below threshold.

If Vm < Ex, there will be a: a) flow of negative ions out of the cell b) flow of positive ions out of the cell c) flow of negative ions into the cell d) more than one of the of the above

If membrane potential is lower than equilibrium potential, there will be inward current, meaning either an influx of positive ions into the neuron or an efflux of negative ions out of the neuron. a) TRUE b) FALSE c) FALSE d) FALSE

Describe the difference between an inward current and an outward current.

Inward currents are: - depolarizing - equilibrium potential for the ion is higher than the membrane potential Outward currents are: - hyperpolarizing - membrane potential is higher than the equilibrium potential for an ion

What property of voltage-gated channels causes the undershoot phase of an action potential? - explain the undershoot phase - explain sodium channels versus potassium channels

The undershoot phase occurs during an action potential when a neuron is transiently hyper-polarized. This is caused by the slowed closing of the voltage-gated K+ ion channels that hyper-polarizes the neuron after the depolarization of an action potential. Because these channels are slow to close, they end up hyper-polarizing the neuron past the resting membrane potential (Vrest). This is NOT caused by any property of sodium channels (such as their inactivation), because sodium channels depolarize the neuron when open, and they are inactivated to prohibit Na+ inward current when the membrane potential (Vm) becomes lower than the threshold.

Can there be a driving force without current?

Yes! current depends on conductance, but there can still be a driving force without current

A neuron is voltage clamped and hyperpolarized. The neuron's membrane potential will be _________ and there will be a/n _______ early current. Do not consider capacitive current. a) Positive; inward b) Positive; outward c) Negative; inward d) Negative; outward e) None of the above

a and b) FALSE; when a membrane is hyperpolarized, the Vm becomes negative c and d) FALSE; the neuron is clamped at a hyperpolarized state (below threshold), so no new channels will open e) TRUE

For a given neuron at standard physiological conditions, action potential amplitude depends on... a) intensity of stimulation b) frequency of stimulation c) number of stimulated areas d) more than one of the above e) none of the above

a) FALSE b) FALSE c) FALSE d) FALSE e) TRUE; action potential AMPLITUDE remains uniform in a uniform environment, though intensity of stimulation, frequency of stimulation, and number of stimulated areas may affect the FREQUENCY of action potentials

During the action potential, K+ currents are ______ and ______. a) early and transient b) delayed and transient c) early and sustained d) delayed and sustained

a) FALSE; K+ currents are delayed (Na+ currents are early) b) TRUE; K+ channels are slow to open (delayed) c) FALSE d) FALSE; both Na+ and K+ channels are transient because conductance (g) is voltage-dependent

K+ inward rectifying channels (KIR) are: a) voltage-dependent b) always open c) able to pass equal amounts of current when Vm is 30 mV above or below EK+, but the direction of the current is different d) none of the above

a) FALSE; KIR channels are not voltage-dependent b) TRUE; KIR channels are always open, but the flow of ions (inward) is directed by the driving force c) FALSE; KIR channels can only pass inward current d) FALSE

Which of the following is/are true about K+ inward rectifying (KIR) channels? a) They can pass inward or outward current depending on the driving force b) They are voltage-sensitive c) They have 8 membrane-spanning helices per functional channel d) They have a regulatory subunit

a) FALSE; KIR channels can only pass inward current during a condition of hyper-polarization state of a cell b) FALSE; KIR channels are not voltage-sensitive c) TRUE; KIR channels have 8 membrane-spanning helices per functional channel because they have a total of 4 alpha subunits with 2 membrane-spanning helices/domains per subunit d) FALSE; KIR channels do NOT have a regulatory subunit

Why does the action potential travel away from the stimulus? a) VGNa+ channels automatically close a short time after an action potential b) VGNa+ channels are inactivated after an action potential c) VGK+ channels hyperpolarize the axon after the action potential d) there is insufficient sodium immediately after an action potential to open VGNa+ channels

a) FALSE; VGNa+ channels actually inactivate, not close shortly after an action potential b) TRUE; the inactivation of VGNa+ channels shortly after an action potential is responsible for preventing the action potential from traveling backwards down the axon c) FALSE; hyperpolarization of the axon shortly after an action potential by VGK+ channels can make backwards movement of an action potential more difficult, but it does not prevent it d) FALSE; sodium is still present and ready to pass through the channels after the initial stimulus, but its flow is prohibited when the VGNa+ channels become inactivated shortly after an action potential

Which of the following is true regarding current? a) there can be current without conductance (g) for the ion b) there can be current without voltage c) there must be permeability to have current

a) FALSE; conductance (permeability) is essential to generating a current across a biological membrane b) FALSE; there cannot be current (ion flow) without voltage (difference in electrochemical potential across a biological membrane) c) TRUE; to get a current, a biological membrane must be permeable to the ions that must flow

During an action potential, voltage-gated sodium channels open to allow the passage of Na+. Which of the following is/are true? a) Na+ will have no driving force initially b) Na+ driving force will change over time c) Na+ driving force is a constant force that will not change d) all are false

a) FALSE; driving force for Na+ will occur initially, as it is not reliant on reaching the threshold potential b) TRUE; Na+ driving force will change over time as membrane potential reaches the equilibrium potential for the ion c) FALSE d) FALSE

What increasing depolarization above threshold there is... a) a change in the action potential amplitude b) an increase in the frequency of action potentials c) a longer lasting action potential

a) FALSE; increasing depolarization above threshold has no effect on the shape of the action potential b) TRUE; increasing depolarization above threshold can cause an increase in the frequency of action potentials c) FALSE; the action potential would actually last longer if the voltage-gated Na+ channels took longer to inactivate

Which of the following would increase Vrest? a) increasing inside concentration of Na+ b) increasing inside concentration of K+ c) increasing permeability for Na+ d) increasing temperature

a) FALSE; increasing inside concentration of Na+ would actually decrease Vrest (see Goldman equation which solves for membrane potential [Vm] by relating permeability * [outside concentration / inside concentration] summed for all ions, times RT/F) b) FALSE; increasing inside concentration of K+ would also decrease Vm c) TRUE; increasing permeability for Na+ would increase the resting membrane potential d) FALSE; increasing temperature will decrease Vm

Which of the following best describes the voltage-gated Na+ channels when the cell is at Vrest? a) the ion pore is open and the inactivation domain is blocking current b) the ion pore is open and the inactivation domain is not blocking current c) the ion pore is closed and the inactivation domain is not blocking current d) none of the above answers are correct

a) FALSE; the ion pore is closed at Vrest b) FALSE; the ion pore is closed at Vrest c) TRUE; the ion pore is open at Vrest and the inactivation domain is not blocking current d) FALSE

Which of the following is true about the voltage-gated KV4.1 channels? a) They have 6 alpha subunits per functional channel b) They are mainly selective for potassium but also pass other ions when hyperpolarized c) They exhibit a sustained current above threshold d) Current direction depends on the driving force for potassium

a) FALSE; they have 4 alpha subunits per functional channel, each with 6 transmembrane domains b) FALSE; they are only selective for potassium c) FALSE; they exhibit a transient current above threshold and inactivate shortly after opening d) TRUE; current direction for all voltage-gated ion channels is dependent on the driving force for the ion(s) that channel is selective for (DF = VM - Ex)

At an ion's equilibrium potential, which of the following is true? a) the driving force on the ion is zero b) the driving force is opposite the concentration gradient c) the driving force is opposite the electrical gradient

a) TRUE

Which of the following moves at least one ion WITH their gradient? a) antiporter b) cotransporter c) ion pump

a) TRUE b) TRUE c) FALSE; both ions moved against their gradient

Which of the following channels have 24 transmembrane domains per functional channels? a) voltage-gated K+ channels (KV4.1) b) voltage-gated K+ channels (HERG) c) Ca2+ gated K+ channels

a) TRUE; KV4.1 channels have 4 alpha subunits, each with 6 transmembrane domains b) TRUE; HERG voltage-gated K+ channels have 4 alpha subunits, each with 6 transmembrane domains c) FALSE; Ca2+ gated K+ channels have 4 alpha subunits, each with 7 transmembrane domains (for a total of 28 transmembrane domains per functional channel)

Which toxins could reduce the current through NaV1.7 voltage clamped at -20 mV? a) tetrodotoxin (TTX) b) tetraethylammonium acetate (TEA) c) scorpion beta toxin d) scorpion alpha toxin

a) TRUE; TTX is responsible for the blocking of voltage-gated sodium channels, preventing the amount of sodium that can pass through b) FALSE; TEA is responsible for the blocking of voltage-gated potassium channels and has no effect on sodium channels c) FALSE; scorpion beta toxin reduces the threshold for NaV1.7 channels, which would increase the current that passes through these channels at a voltage clamp of -20 mV d) FALSE: scorpion alpha toxin blocks NaV1.7 channel inactivation, responsible for an increase in current that passes through the channel

Significantly decreasing the sodium concentration outside of a neuron would: a) decrease the equilibrium potential for sodium b) increase the resting membrane potential c) decrease the amplitude of action potentials d) increase the driving force on sodium at resting membrane potential

a) TRUE; decreasing the sodium concentration outside of a neuron would decrease the equilibrium potential for sodium b) FALSE; decreasing the sodium concentration outside of the neuron would not have a large affect on the membrane since permeability for sodium is low (but this would actually lower the resting membrane potential, not increase it) c) TRUE; decreasing the outside sodium concentration would decrease the amplitude of action potentials because the equilibrium potential for sodium controls the amplitude of the action potential d) FALSE; decreasing the equilibrium potential for sodium would actually decrease the driving force on sodium

Which of the following is correct regarding driving force? a) driving force increases with greater difference between Ex and Vm b) there must be an electrical and chemical gradient to have a driving force c) there cannot be a driving force without conductance

a) TRUE; driving force equals the difference between membrane potential and the equilibrium potential for an ion (DF = Vm - Ex) b) FALSE; as long as there is either a concentration gradient or a electrical gradient, there is potential for a driving force (as long as the two do not completely cancel each other out) c) FALSE; both driving force equations do not contain conductance, so driving force does not depend upon conductance

During an action potential, which of the following would INCREASE the AMPLITUDE of the action potential? a) Increase the outside sodium concentration b) Increase the inside sodium concentration c) Increase the outside potassium concentration d) More than one of the above

a) TRUE; increasing the outside sodium concentration would increase the equilibrium potential for sodium, which would in turn increase the amplitude of the action potential b) FALSE; increasing the inside sodium concentration would decrease the equilibrium potential for sodium, in turn decreasing the amplitude of the action potential c) FALSE: changing the outside potassium concentration would have no effect on the amplitude of the action potential since ENa is responsible for the amplitude of the action potential (however, increasing outside [K+] would decrease both EK+ and Vrest) d) FALSE; because b) and c) are false

Which of the following moves ions in the OPPOSITE direction? a) ion pump b) cotransporter c) antiporter

a) TRUE; ion pumps move two ions in opposite directions against their gradients using the energy from ATP hydrolysis b) FALSE; cotransporters send two ions in the same direction across a biological membrane (one against its gradient and one with) c) TRUE; antiporters move two ions in opposite directions (one against its gradient and one with)

What cells myelinate axons in the CNS? a) oligodendrites b) microglia c) astrocytes d) Schwann cells

a) TRUE; oligodendrites are responsible for myelinating axons in the CNS b and c) FALSE; microglia and astrocytes do not myelinate axons d) FALSE; Schwann cells are responsible for myelinating axons in the peripheral nervous system (not the CNS)

What is/are true about 2P K+ leak channels? a) The direction of potassium current depends on the direction of the driving force b) This type of ion channel has no regulatory subunit c) Channel has a different level of activity at different pH levels

a) TRUE; the direction of K+ current within this channel depends on the direction of the membrane's driving force (DF = Vm - Ex) b) TRUE; 2P K+ leak channels have no regulatory subunit c) TRUE; 2P K+ leak channels have different activity levels at different pH levels, with the maximum activity occurring around pH 8

The equilibrium potential for a positive ion is +33 mV. If the membrane potential is -65 mV, when a channel is permeable to this ion opens, it will: a) flow into the cell b) flow out of the cell c) decrease the driving force on that ion

a) TRUE; when the membrane potential is lower than the equilibrium potential for an ion, the driving force will direct an inward current for that ion (which for a positive ion, means influx into the cell) b) FALSE c) TRUE; the driving force will direct inward current, which will drive the membrane potential to reach the equilibrium potential for the ion (which decreases the driving force as the difference between the membrane potential and equilibrium potential becomes smaller)

What will happen to Vm if Na+ concentration inside the cell is increased while at Vrest? a) increases a small amount b) increases a lot c) decreases a small amount d) decreases a small amount

c) decreases a small amount! Increasing the Na+ concentration inside the cell will cause a small DECREASE in the membrane potential at Vrest due to the Goldman equation. This change will also be small because the permeability of a biological membrane for Na+ is low.

At what point during an action potential do KV1.2 channels inactivate? a) when membrane potential reaches a maximum b) when voltage-gated sodium channels inactivate c) when membrane potential is sufficiently hyper-polarized d) never

d) Never! KV1.2 channels never inactivate! They do shut after a period of sufficient hyper-polarization, but they never inactivate. KV4.1 channels do inactivate.

Picture a cell at 19 degrees C where the inside [Ca2+] is increased by 10 fold. How much will the calcium equilibrium potential change? a) increase by 116 mV b) increase by 58 mV c) decrease by 58 mV d) decrease by 29 mV e) increase by 29 mV

d) TRUE See the Nernst equation for calculating equilibrium potential for an ion. Ratio of outside/inside decreases, causing a decrease in equilibrium potential.

Picture a beaker with two chambers and a selectively permeable barrier containing KCl with a high concentration on the RIGHT and a low concentration on the LEFT. Assume the barrier is only permeable to Cl-. At equilibrium, how will the driving force for K+ compare to the driving force at the start?

increased

conductance

measure of the permeability of a charged particle to flow through a biological membrane the greater the conductance (permeability) of an ion for a biological membrane, the greater effect that ion will have on the membrane potential

How to define the reversal potential for an ion?

the membrane potential at which concentration gradient is equal and opposite to the electrical gradient for that ion the membrane potential is equal to the equilibrium potential for that ion THE MEMBRANE POTENTIAL ISN'T NECESSARILY ZERO

Which type of ion channels inactivate?

voltage-gated Na+ ion channels