Action Potentials

Question 1

What processes and proteins contribute to the resting potential?

Answer 1

K+ Diffusion down concentration gradient: K+ leak channel
Electrochemical attraction of K+ from large anions in cell
Active transport via Na+/K+ ATPase (K+ into cell, Na+ out)
Insignificant diffusion of Na+ by chance into cell (causing resting potential of ≈ -70 instead of -91mv)
Membrane = highly permeable to K+

Question 2

What ion channels are involved with resting and action potentials?

Answer 2

K+ Leak channel (Simple pore) (K2p) Always open
Na+ / K+ ATPase
Na+ (Voltage operated sodium channel) VOSC
K+ Delayed outward rectifier

Question 3

At rest, what is the membrane permeable and impermeable to and why?

Answer 3

Permeable to K+ (leak channels)
Highly impermeable to Na+ (all voltage channels closed) (very minimal diffusion across leading to -70mv as opposed to -91mv)

Question 4

What is equilibrium potential? in the context of K+

Answer 4

Point when outward flow of K+ due to concentration gradient is balanced by inward flow due to electrical gradient due to large anions within cell (Ek)

Question 5

How is the slow inward leak of Na+ into the cell combatted?

Answer 5

Na+ / K+ ATPase

Question 6

Describe the stages of an action potential

Answer 6

Stimulus received by neuron, neurotransmitter diffuses across synapse and alters membrane permeability
and therefore changes membrane potential

Depolarization:
Voltage gated channels open (if -55mv is reached)
Rapid influx of Na+ into the cell
Reaches approx +40 mv

Repolarization:
This change in membrane potential triggers slow opening of Voltage gated K+ channels such as delayed outward rectifier
This triggers rapid efflux of K+
Resulting in…

Hyperpolarization:
Delayed outward rectifier is slow to open and slow to close
Resting membrane potential is re-achieved by Na+ / K+ ATPase

Refractory period:
Absolute refractory period occurs between depolarisation and repolarisation. No further AP’s can be generated due to sodium channels in inactive state

Relative refractory period is between repolarisation and resting potential, at this time a fresh AP is possible with a larger than normally necessary stimulus

Question 7

What is saltatory conduction?

Answer 7

The leaping propagation of an action potential between Nodes of Ranvier

Question 8

How is conduction velocity improved?

Answer 8

By the presence of myelin sheaths and subsequent Nodes of Ranvier

Question 9

What is the relative refractory period? When does it occur?

Answer 9

It occurs between hyperpolarisation and resting potential

Defined as the stage where a new AP is possible, if a larger than usually necessary stimulus is received by the neuron.