Action Potentials

Question 1

The road to understanding action potentials
first stage
1865-1920

Answer 1

-First measured current associated with AP in nerve
- Discovered Na, K, Ca salts must be present in definite proportion
-Proposed excitable cells surrounded by membrane selectively
permeable to K+ ions at rest,Permeability to all other ions increased during excitation

Question 2

Divergent views:
The road to understanding action potentials
first stage-1920

Answer 2

vergent views:
§ Chemical
• Principally chemical signal and AP secondary effect
§ Electrical
• Principally electrical signal and Aps propagate by exciting
membrane in front of the activation

Question 3

The road to understanding action potentials
second stage
1920-1940

Answer 3

-Showed a highly conductive cytoplasm surrounded by low conductance
(capacitive) membrane
-Hodgkin Found a nerve could be affected by an electric signal across a line
of electrical disconnect (cold block)
-squid giant axon showed dramatic decreases in
membrane resistance during AP

Question 4

Hodgkin and Katz Electrical arguments showed

Answer 4

there was an exceedingly thin cell
membrane with ionic permeability that was low at rest but much higher
when active.

Question 5

o Voltage Clamp Technique:

Answer 5

With a voltage clamp, a fixed voltage is enforces across the membrane
and the current required to maintain this is measured.
Two fine wires inside the axon
• One measures Vm
• The other supplies current
Desired membrane potential set by command voltage.
§ Differences between this and Vm drive a current which moves Vm
towards the command voltage
§ Current measure is what is required to hold Vm to the specified constant
value

Question 6

Using the voltage clamp method
§ A step and constant voltage (voltage clamp) means membrane
capacitance current quickly stops flowing and recorded current is just
ionic
• Because capacitive current only flows when there is a change in
membrane potential
• HH found that

Answer 6

early currents reverse direction from inward to
outward at a voltage step around +60mV.
o Suggests that the current is carried by Na
• Late currents are outward for all voltage steps, suggesting that
current is carried by K

Question 7

HH’s goal was to separate Na and K currents
§ By changing the level of extracellular Na and taking difference of
membrane currents, most of the remaining signal is inward Na transient
component
§ They did this by

Answer 7

making extracellular and intracellular concentrations

were equal

Question 8

• Na channels are understood to activate then inactive.

•

Answer 8

Activation is the rapid opening of Na channels during
depolarization.
• Inactivation is the slower process of closing the channels

Question 9

The potassium ion conductance was described as:

Answer 9

gk= gk.n^4
Gk is the peak K+
conductance The membrane conductance for K ions was controlled by
several (4) “particles” that were bound to the membrane

Question 10

sodium conductivity gates eqn

Answer 10

The membrane conductance for K ions was controlled by
several (4) “particles” that were bound to the membrane
one type for activation (m) and
another type for inactivation (h). m^3.h