lecture 3

Question 1

What is responsible for the falling phase of the action potential?

Answer 1

Ø Hodgkin and Huxley suspected that it was the delayed outward current carried by K+
Ø They injected the cell with [42K] and measured the outward current
Ø They plotted the results as outward current vs. [42K] efflux and found a high correlation, concluding that K+does carry the outward current

Question 2

Structure of the Voltage-gated Na+ channel

Answer 2

Three subunits: α, β1,and β2

Question 3

Focus on the α-subunit which is highly conserved:

Answer 3

-It has four transmembrane domains, each of which is made up of 6 transmembrane α-helical segments

Question 4

Unlike the K+-channel, the Na+ channel, _________________ modeled by a ball-and chain mechanism, which is a part of the protein, at the mouth of the pore.

Answer 4

inactivates shortly after it is depolarized,

Question 5

At point A, the axon is stimulated by a micro electrode (stimulus depolarized)- this causes _______________________________

Answer 5

a depolarization which opens voltage dependent Na channels at point A

Question 6

Opening of Na+ channels cause an influx of Na+ into thee axon down______________

Answer 6

its concentration gradient

Question 7

Accumulation of Na+ ions at A, repels _____________________

Answer 7

K+ from point a forward point B and C

Question 8

Migration of K charges the membrane which remember is like a capaciitor at point B, _____________________

Answer 8

positive inside, depolarizing. this segment of. the membrane

Question 9

This causes positively charged Na+ ions on the outside of the membrane to be repelled from point B- _______________

Answer 9

they tend to move back toward point A where the concentration is low from the earlier influx of Na+ caused by initial stimulus

Question 10

when the depolarization exceeds threshold at point B, the cycle repeats as

Answer 10

voltage gated Na+ channels start to open in this region

Question 11

Which of the following axons will show faster signal transmission?

• the larger axon with large lamda
• the larger axon with small lamda

Answer 11

The larger axon will since there are a fewer local circuit currents to be generated which will take less time

Question 12

__________ The action potiental is initially generated at the axon hillock near thee soma and travels unidirectionally down the axon

Answer 12

unidirectional flow

Question 13

Why is flow unidirectional

Answer 13

the area of the membrane just traversed by the AP is refractory. The local circuit current excites the region of membrane just ahead of, but not behind the AP

Question 14

The refractory period of the axon is due to

Answer 14

1) Na+ iinactivatipon

2) K+ activation which hyperpolarizes the inside of the cell

Question 15

_____________- the time period following thee AP during which no other AP may be generated independent of the stimulus amplitude

Answer 15

absolute refractory period

Question 16

-________________ time period following the absolute refractory period during which an AP may bee generated by the AP is smaller than usual and the stimulus required to generate the AP is larger than usual

Answer 16

relative refractory period

Question 17

-The AP amplitude is smaller because some sodium channels are still inactivated; the stimulus must be larger ____________________________________

Answer 17

because the cells are hyperpolarized due to the delayed potassium

Question 18

___________________ is responsible for unidirectional flow of ApPs (also for the upper limit of the rate of firing APs in a sensory nerve fiber- we will see this later in the course

Answer 18

refractory period

Question 19

____________________ is in fact a cell produced by Schwann cells or oligodendrocytes

Answer 19

myelin

Question 20

The cells form a high density, high resistance __________ around the axon

Answer 20

lipid sheeth

Question 21

There is a high density. of _____________ at the nodes

Answer 21

voltage dependent Na+ channels

-the. local circuit current passes through the nodes

Question 22

The axon. exhibits saltatory conduction, where the APs appear to jump ___________________

Answer 22

from node to node

Question 23

Each layer of the membrane acts as a capacitor thus layers work like capacitors in series so that the total Cm is lowered

Answer 23

Cm is high at the nodes, which lowers, which allows the Vm to charge to a greater value than threshold

Question 24

Myelin is only found in ______________ but not all __________) axons are myelinated

Answer 24

vertebrates

Question 25

The voltage clamp is a classic _____________________________

Answer 25

electrophysiological technique to measure ion currents across the cell membrane.

Question 26

Under voltage clamp conditions, voltage- gated ion channels open and close as normal, but the voltage clamp apparatus compensates for the changes in the ion current to maintain__________________

Answer 26

a constant membrane potential.

Question 27

The investigator sets a desired holding membrane potential, also referred to as holding voltage or command potential, and the voltage clamp uses negative feedback_____________

Answer 27

to maintain the cell at this desired holding potential.

Question 28

_______________________
Movement of charges onto (and away from) capacitor plates such as the inside and outside of the membrane is referred to as a current flow “through” the capacitor. In electrophysiology it is important to be aware that such currents flow ONLY when the voltage across a capacitor is changing with respect to time (the capacitor is being “charged”).

Answer 28

Capacitive Current

Question 29

Capacitive current (Icap) =

Answer 29

C * dV/dt.

Question 30

The current flow onto a capacitor equals the product of the capacitance and the rate of change of the voltage. If a constant current is injected across a lipid bilayer, the steady current flow (Icap) will produce a constant rate of
change (dV/dt) of the voltage across the capacitance (C) of the membrane, as you just saw in your simulation.

Answer 30

This equation also shows again that whenever the voltage is constant (dV/dT=0), there is no capacitance current.

Question 31

during the voltage clamp experiment

Answer 31

Initial Na+ curent in absent sincère there is no driving force on Na+ when Vm=Ena

Recall Ina= gna (Vm-Ena)

Question 32

The membrane was clamped at 0 mv and the resulting current trace showed an early inward component and delayed outward component. During the depolarization phase of the AP, the clamp provides ___________________ which ______________________

Answer 32

a hyperpolarization current which maintains Vm at a constant level

Question 33

By changing (Na+)o=(Na+)I and stepping 0 mv, the early inward component was eliminated and the time course of Ik alone its result. Subtracting the trace of Ik from the total current gives the course of Ina

Answer 33

using ohms law they were able to calculate
gna=Ina/(vm-ena)
gk=Ik/(Vm-Ek)