Voltage Clamps

What is the sodium hypothesis?

rapid influx of Na into cell is responsible for the rising phase and overshoot of an action potential

Who conducted the voltage clamp experiments?

Hodgkin and Huxley

What did Hodgkin and Huxley use for their voltage clamp experiments to evaluate the ionic basis on the action potential?

squid giant axon

What is the function of the squid giant axon?

escape

What is the purpose of the voltage clamp?

does not allow the membrane potential to change (manipulates the axon)

What does the voltage clamp actually evaluate?

which channels open based on current that flows at the particular membrane potential (and how long it takes to open)

What is flux?

movement of charged atoms across neuronal cell membrane

What is the equation for flux?

flux = P_ion * (V_M - E_ion)

What is the equation for current?

I_ion = g_ion * (V_M - E_ion)

What is g_ion?

conductance

What is conductance nearly the same as?

permeability

What is the difference amplifier in the voltage clamp experiment?

puts out a current proportional and opposite to the voltage difference at inputs (if it’s +, it’ll release -)

What does the ammeter in the voltage clamp experiment do?

measures current put out by the difference amplifier

What is command voltage in the voltage clamp experiment?

the voltage chosen by the experimenter

In the voltage clamp, if there is no voltage difference at inputs, there is ______ current output.

no

During inward Na+ current, is the ammeter reading positive or negative current? (at -10mV)

negative

During outward K+ current, is the ammeter reading positive or negative current? (at -10mV)

positive

What is the chemical blocker for sodium?

TTX (tetrodotoxin)

Where is TTX from?

pufferfish

What is the chemical blocker for potassium?

TEA (tetraethylammonium)

What ion channel is open according to the measured current on the graph? (starts at zero, goes up, levels off)

K+ (potassium)

What ion channel is open according to the measured current on the graph? (starts at zero, goes down, goes up, levels off)

Na and K (sodium and potassium)

What ion channel is open according to the measured current on the graph? (starts at zero, goes down, goes back to zero, levels off)

Na+ (sodium)

What is the equation for Ohm’s Law? (using conductance and voltage)

I = g * V

Can voltage and driving force be used interchangeably in the context of Ohm’s Law?

yes

________ at any given time is directly related to the _________ because the driving force is held constant.

Current; conductance (permeability)

If you can measure current, you can compute _________.

conductance

What ion is this conductance graph referencing?

K (potassium)

What ion is this conductance graph referencing?

Na (sodium)

What are the two factors that determine the conduction velocity of the action potential?

current flow down the axon (or dendrite)

current flow across the membrane

Large diameter axons have _____ conduction velocities.

fast

Smaller axons have ______ conduction velocities.

slower

Smallest axons have _________ conduction velocities.

slowest

The spread of current down the axon opens Na+ channels and causes the action potential to __________ along the axon

propagate

What are the two features that determine the degree to which the current spreads down the axon?

internal resistance and membrane resistance

Internal resistance of large diameter axons is _____ so that large amounts of current spread down the axon, even though some current is lost by leakage.

low

Internal resistance of small diameter axons is _______ so that only small amounts of current spread down the axon.

large

What is most current lost by?

leakage

Why does leakage of current occur?

current follows the path of least resistance

What does internal resistance rely on?

area of axon

What does membrane resistance rely on?

number of open channels (varies with the circumference of the axon)

What type of internal/membrane resistance do smaller axons have?

higher internal resistance

higher membrane resistance

What type of internal/membrane resistance do larger axons have?

lower internal resistance

lower membrane resistance

Does increasing area or circumference of an axon decrease resistance more rapid/efficiently? (which one of the two + why)

increasing area because internal resistance changes with r² while membrane resistance is only doubled via 2 pi r

What are the two myelinating cells?

oligodendrocytes and schwann cells

What are in the nodes of ranvier in a myelinated axon?

Na and K channels

The _________ of action potentials along myelinated axons from one node of Ranvier to the next node increases the conduction velocity of action potentials without needing to increase the ________ of an axon.

propagation; diameter

Larger myelinated axons have _________ conduction velocities than smaller myelinated axons.

faster

Myelin disorders affect?

actional potential propagation

Why does loss of myelin cause failed action potentials?

too much current leakage (not enough positive current reaches next node)