Action Potential

Question 1

active signals in nerve cells

Answer 1

• membrane potential is -65 mV
• when stimulated, neg current is hyperpolarization
• positive current is depolarization
• if enough pos current, action potential

Question 2

phases of the action potential

Answer 2

• rising phase-depolarization
• overshoot
• falling-repolarization
• undershoot-refractory period when it’s not possible to have another AP, relative refractory when it just takes more current

Question 3

rising phase

Answer 3

• depolarizing
• sodium in
• reducing external sodium means a smaller and slower AP
• important control is returning soln to normal to make sure you didn’t mess it up

Question 4

voltage mechanisms during AP

Answer 4

• generated by three different voltage sensitive mechanisms:
  1. activation of Na conductance
  2. delayed activation of K conductance
  3. inactivation of Na conductance

-undershoot is due to open voltage gated K channels that gradually close for the membrane potential to return (because they take longer to close and Na are already inactivated)

Question 5

Na gated channel

Answer 5

• open
• inactivating- gate shutting
• inactivated-during repolarization/ K out
• closed

Question 6

K gated channel

Answer 6

• closed and open

- opens later-accounts for undershoot

Question 7

stimulus

Answer 7

depolarizes the membrane potential above threshold

Question 8

activation of Na conductance

Answer 8

1. voltage dependent channel opens and Na ions rush down gradient- INWARD current
2. accumulation of + charge inside and across cell membrane
3. depolarization of the membrane potential

Question 9

delayed activation of K conductance

Answer 9

• after a delay the voltage dependent K channels open
• K rushes out of the cell (outward current)
• reduction of charge inside the cell membrane
• hyperpolarization of membrane potential

Question 10

inactivation of Na conductance

Answer 10

• voltage gated channels transition to non-conductive state as voltage gated K channels being to close
• the cell is refractory an does not fire APs in response to stimulaitno
• inactivation gates open as the channels close

Question 11

absolute refractory period

Answer 11

• Na channel inactivated

- no AP no matter how much stimulus

Question 12

relative refractory period

Answer 12

• can get AP but need more depolarization
• brief hyperpolarized membrane potential due to open resting and voltage gated K channels that ends when voltage K channels close

Question 13

how were changes in Na and K conductance discovered?

Answer 13

• voltage clamp recordings were used to measure the changes in membrane conductance through voltage gated channels
• made the voltage inside cell equal command by a negative feedback loop that prevents voltage from deviatin

Question 14

example of voltage clamp

Answer 14

command signal depolarizes cell to -10mV potential

1. voltage gated Na channels open and current flows in-should depolarize but
2. voltage clamo amplifier removes equal amount of voltage to keep membrane potential at -10
3. amount of current passed by clamp is a measure of the current flowing across the membrane at any given voltage and time (Na flowing at x voltage)

• can vary membrane potentials to measure amount of Na and K moving
• used to interrupt the positive loop between membrane potential depol and opening and closing of channels

Question 15

voltage clamp technique showed

Answer 15

• an early inward Na current
• a late outward K current
• conductance changes with time and membrane potential
• able to develop a model of conductances and predicted properties of AP

Question 16

capacitive current

Answer 16

• supplies charge for the change in charge accumulation across membrane needed to step from holding to new
• allows it to depolarize basically

Question 17

changes in Na and K conductance

Answer 17

• drugs that block Na and K channels selectively block those currents
• tetrodotoxin-TTX- blocks Na conduction- no AP
• tetraethyl-ammonium blocks K conductance- no repolarization

Question 18

AP propagation

Answer 18

-active and passive current flow

Question 19

active flow

Answer 19

is gating of channels and influx of Na

Question 20

passive flow

Answer 20

• depol wave that precedes the AP
• pos ions entering axon via electrode or Na channels move passively down the axon and neutralize the membrane, making it easier to get to threshold
• no passive current through the membrane, only through channels (can leak)
• voltage gated channels become activated further down the axon and the process continues
• refractory period limits freq of fireing

Question 21

AP propagation in myelinated fibers

Answer 21

• wrapping of glial cell membranes around axon
• increases membrane thickness by 100x
• increases insulation and reduces leak of passive flow
• decreases capacitance
• fast passive potentials between nodes of ranvier (decreases capacitance only in that spot)
• generation of AP in the nodes
• saltatory conduction-jumping

Question 22

nodes of ranvier

Answer 22

• gap in myelin
• separated by 1-2 mm
• contain full complement of Na and K channels
• generate APs

Question 23

conduction velocity

Answer 23

• unmyelinated- 0.5-10 m/s

- myelinated- 150 m/s

Question 24

summary

Answer 24

• voltage clamp used to determine Na and K conductance change with time and membrane potential
• APs generated by 4 different voltage sensitive mech- Na conductance, K conductance, inactivation of N, closing of K
• APs reverse the sign of membrane potential at peak (overshoot, gNa up), and then hyperpolarize at undershoot (gK up)- leading to refractory period during which no AP can fire, then harder-relative, undershoot then dissipates- gK down and membrane potential returns to normal