Neuronal excitability

Question 1

What is meant by the conductance(g) of Ion Channels?

Answer 1

• equivalent to permeability- conductance easily measured
• Membrane acts as an electrical resistor (R)
• Conductance, g=1/R
• g is directly proportional to the no. of open ion channels , permeability does not have suvh a simple relationship
• Change in conductance for one ion leads to a change in membrane potential

Question 2

What happens to the ion conductances during an action potential?

Answer 2

• Initial stimulus/Depolarisation will open the voltage gated Na⁺channels -increase in g_Na
• Na⁺influx -> further depolarisation-> further increase in g_Na
• E_m approaches E_Na
• Na⁺ channels inactivate - g_Na decreases
• Na⁺ influx stops despite maintained stimulus
• V-gated K⁺ channels open- increase in g_K
  
  • K⁺ efflux-> repolarisation, but increase in g_K is maintained after E_​m reurns to rest
• E_m approaches E_K -> hyperpolarisation
• V- gated K⁺ channels close- g_K decreases
• E_m returns to resting value via leak channels

Question 3

Why does E_m approach E_Na?

Answer 3

During AP, g_Na and Na⁺permeability increases 1000x

at rest

P_Na= P_k/40

During AP

P_Na= 1000x(P_k/40)

= 25P_k

Therefore, using the constant field equation,

E_m=+40mV

Question 4

What is meant by the threshold potential

Answer 4

the threshold potential is E_m where Na⁺ influx from both leak and V-gated ion channels> K⁺ efflux

Question 5

What are the two types of refractory periods?

Answer 5

Question 6

How does propagation of action potential occur in an unmyelinated axon?

Answer 6

• This form of propagation is slow
• The AP have to go through the whole of the axon
• The refractory periods ensure unidirectionality

Question 7

How does propagation of action potential occur in a myelinated axon?

Answer 7

• Saltatory Conduction
  
  • Myelin sheath(spirally wrapped glial cells) increases speed of AP conduction as it prevents conduction along the axon
  • Ap jumps to adjacent nodes of Ranvier
  • This is much faster

Question 8

What is a generator potential. Give an example.

Answer 8

local depolarization of the membrane potential at the end of a sensory neuron in graded response to the strength of a stimulus applied to the associated receptor organ, for example, a pacinian corpuscle