electrical properties of neurons

Question 1

why where giant squid axon initial used to study action potentials

Answer 1

because of their size

Question 2

how was Na+ proved to be responsible for AP?

Answer 2

Na+ proved to cause spike by varying extracellular Na+ concentration.

Question 3

when calculating using only Na+ the nurnst equation predicts:
_____ compare this to the actual recordings
what is the fix?

Answer 3

Nurnst equation predicts Na+ would change membrane potential to +60mV but only reaches +40mV because of K+ leak channels.
Hence the Goldman Hodgkin Catz equation must be used (amalgamation of relevant ionic nurnst eq)

Question 4

how does TTX- Tetrodotoxin function

Answer 4

blocks voltage gated Na+ channels

Question 5

how does TEA-tetraethylammonium function

Answer 5

blocks voltage gated K+ channels from both sides of the membrane

Question 6

Pharmacological studies changing voltage while blocking various ion channels demonstrates:

Answer 6

K+ slower to react and current changes up scaling with voltage
Na+ response more nuanced reaction speed increases with voltage bt begins to decrease opening times with higher voltages.

Question 7

NaV channel structure

Answer 7

4 subunits:
- 3 voltage gated activation gates
- 1 voltage gated inactivation gate
Equations predict structure of ion channels with 4 Alpha subunits resolution of two graphs cretes Vm

Question 8

what is the absolute refactory period proportional to? and what does it regulate

Answer 8

The absolute refractory period is proportional to density of voltage gated K+ channels
Due to inactivation of voltage gated Na+ channels
The absolute refractory period also regulates maximum spike rate

Question 9

what causes the relative refractory period and what are the implications of it?

Answer 9

The relative refractory period is due to partial de-inactivation of voltage-gated Na+ conductances and/or de-activation of voltage-gated K+ conductances
The relative refractory period means that firing frequency varies with stimulus intensity.

Question 10

Patlak and Ortiz demonstrated the function of inactivation gates in NaV channels what was the proposed function

Answer 10

Voltage gated Na+ channels work by a ball and chain mechanism where the time for channels reclosing after voltage gating opens the pore is dependent on the length of the peptide chain connecting the plug to the channel protein.

Question 11

constraints on axons

Answer 11

reliably
at physiologically-relevant speeds
over distances exceeding a metre

Question 12

comment about passive propagation down a leaky axon

Answer 12

If the membrane is leaky current decreases exponentially along the axon
Not only does amplitude drop off with distance but so too does the form of the signal becomes distorted slower peaking

Question 13

what is the range of conduction velocities in the mammalian nervous system

Answer 13

<1 ms-1 to over 100 ms-1.

Question 14

why do axons with larger diameters conduct AP faster

Answer 14

because as diameter increases membrane resistance decreases proportionally to the diameter while axon resistance decreases proportionally to the diameter squared hence the current will take the path of least resistance

Question 15

what effects does mylenation have on electrical properties of an axon

Answer 15

Decreasing membrane capacitance

Increasing membrane resistance

Question 15

what effects does mylenation have on electrical properties of an axon

Answer 15

Decreasing membrane capacitance

Increasing membrane resistance

Question 16

how is myelin formed

Answer 16

Myelin is formed by the fusion of apposed inner leaflets of glial plasma membranes, with no intervening cytoplasm
Schwann cells produce myelin in the peripheral nervous system
Oligodendrocytes produce myelin in the central nervous system

Question 17

Factors affecting salutatory conductance in an axon:

Answer 17

Axon diameter (↑) 
Myelin thickness (↑)
Node length (↓)
Internode length (↑)
Density of voltage-gated Na+ channels at the node (↑)
Decreasing membrane capacitance increases conduction velocity in the internodal regions (~ theta/t)
Increasing membrane resistance enables long internodal regions

Question 18

why does the mammalian brain continue to use unmyelinated axons?

Answer 18

metabolicaly expensive to myelinate every axon
signals that don’t need to be propagated quickly
short distances
axons of diameter <1um as myelenation would slow signal down