Action Potential

Question 1

How do action potentials form a neural code?

Answer 1

from frequency and pattern - the AP size and shape never change, but the frequency is the code that changes

Question 2

What is passive current flow?

Answer 2

The decrease in membrane potential change as distance from the stimulation point increases

Question 3

What are the phases of an AP?

Answer 3

Resting phase, Rising phase, overshoot, falling phase, undershoot

Question 4

identify the phases of an AP on the diagram

Answer 4

Question 5

what happens if stimulation remains above the threshold?

Answer 5

APs will continue to be generated - AP firing rate increases as depolarising current increases

Question 6

what is Ohm’s Law for calculating transmembrane ionic current?

Answer 6

Question 7

what is the value of E_K

Answer 7

-80mV

Question 8

What is the value of E_Na

Answer 8

62mV

Question 9

What is the movement of ions if the ionic current is positive?

Answer 9

Positive = current moving outwards

Question 10

How does ion permeability change during an action potential?

Answer 10

During the rising phase, Na+ channels open, Na+ permeability increases. During falling phase, K+ permeability increases

Question 11

What is the impact of changing extracellular [Na+] on action potential and resting membrane potential? (in squid model)

Answer 11

Changing the extracellular [Na+] changes the amplitude of an action potential because the concentration gradient is different (therefore E_Na is different).
It only impacts the rising phase of the AP, not the other phases or the resting membrane potential.

Question 12

What is the total transmembrane current?

Answer 12

total transmembrane current includes ionic currents and the capacitance current (I_c)
I_total = I_ionic + I_c

Question 13

What is Na+ channel inactivation and how does it relate to the refractory period?

Answer 13

Na+ channels have 3 states - resting, activated and inactivated. inactivation occurs when a ball of Mg swings underneath and blocks an activated Na channel. While this Mg is in place, the channel cannot be activated again - this is the absolute refractory period.

Question 14

describe the structure of a voltage-gated Na+ channel

Answer 14

4 transmembrane domains each with 6 segments. a pore loop between S5 and S6 give Na+ specificity. S4 is an alpha-helix hydrophilic segment forced into the lipid bilayer

Question 15

Describe the proposed model for the operation of voltage gated Na+ channels

Answer 15

When the membrane potential changes, the S4 hydrophilic segment is no longer held in the membrane, and pops out, this conformational change opens the channel.

Question 16

What do we think determines the specificity of the Na+ channel?

Answer 16

size - can only accept the size of a partially hydrates Na+ molecule (partially hydrated K+ is too big)

Question 17

Where is the distribution of VGSCs in neurons?

Answer 17

Primarily along the axon, all the way along. high abundance in the axon hillock (site of AP initiation)

Question 18

differentiate between AP variability between neurons

Answer 18

every AP generated in a single neuron will be identical. However, different neurons can generate different size and shape APs

Question 19

What is orthodromic?

Answer 19

AP travels in one direction along axon towards the terminal

Question 20

What mechanism reinforces orthodromic AP propagation?

Answer 20

direction primarily determined by the site of AP initiation. Additionally, behind the AP, VGSCs are in inactive conformation but in front of it they are in resting confirmation ready to be activated.

Question 21

What factors influence conduction velocity?

Answer 21

Axon structure - diameter

Path of positive charge - across the membrane slower than inside the axon (bc of capacitance)

Axon excitability - number of VGSCs

Question 22

What is the primary function of myelin and how does it do this?

Answer 22

increase conduction velocity of action potentials. It allows the AP to travel between nodes of ranvier by reducing capacitive resistance.

Question 23

What cells are responsible for myelination?

Answer 23

Schwann cells in PNS, oligodendrocytes in CNS

Question 24

compare schwann cells and oligodendrocytes

Answer 24

Schwann cells are in the PNS and an individual schwann cell can only do a single segment of myelin.

Oligodendrocytes are in the CNS and they can do many segments of myelin with their multiple arms

Question 25

How long is a typical myelin segment?

Answer 25

1-2nm

Question 26

What is the space between myelin segments and how is the membrane different here?

Answer 26

Node of ranvier. This is where the VG channels will be localised

Question 27

What is saltatory conduction?

Answer 27

when action potentials can skip between nodes of ranvier in a myelinated axon rather than having to generate APs the entire length of the axon