Action potential

What is the meaning of depolarisation, repolarisation + hyperpolarisation ?

	Definition
Depolarization	Decrease in potential  → membrane ↓ -ve
Repolarization	Return to resting potential after depolarization
Hyperpolarization	Increase in potential → membrane ↑ -ve

What is the difference in action potential + postsynaptic potential ? 

	Size of potential	Summation/ cancellation	Relationship
Action potential	Same magnitude everytime ( all/ nothing )	No	Product of postsynaptic potential
Post-synaptic potential	Variable	Yes	Triggers action potential if magnitude large enough ( -50mv )

What  are the different conformations of sodium + potassium channels for passage of ions?

	Activation gate ( in middle )	Inactivation gate	Passage of ion
Sodium + potassium  channels	Closed	Open	No
	Open	Open	Yes
	Open	Closed	No *only for sodium channels

What is the ionic process underlying action potential generation ? 

Event	State of sodium channel	State of potassium channel
Resting	Activation gate: closed  Inactivation gate: open	Activation gate: closed
Triggers → membrane potential > - 50 mV	Activation gate: open   Inactivation gate: open
Na + influx → reached -30mV	Activation gate: closed  Inactivation gate: closed	Activation gate: opens
Na+  enter + K+ leaves cell
Membrane potential falls below -50mV ( threshold potential)	Activation gate: closed  Inactivation gate: open	Activation gate: closed
Repolarisation

How does the action potential spread unidirectionally?

In unmyelinated axons:

1. Action potential depolarise membrane w/ influx of Na+ 

2. Electric current flows to adj part of axons 

3. Adj part is depolarised 

4. Na + channels open 

5. Another action potential is triggered in adj region 

6. And so on down axon

How does refractory periods contribute to unidirectional signal transmission?

The current only flows one direction bc of absolute + relative refractory period 

	Generation of action potential	State of ion channels	Consequence
Absolute refractory period	X	Na+ channels open + inactivated	Unidirectional travel of action potential → the only adj part of axon that is excitable is the part that hasn’t fired an action potential  → the other adjacent part of axon has fired an action potential + in refractory period    Limited firing frequency
Relative refractory period	✓	Some Na + channels closed

What are the factors affecting action potential propagation ?

 

Factor	Explaination
Axon diameter	Thicker axons → faster propagation ( less resistance to current flow )
Temperature	Faster at higher temp
Degree of myelination	Insulation of membrane + speeds up propagation for saltatory conduction → the depolarisation of membrane can spread from one node of Ravier ( site of ion channels ) to another

How action potentials signal information ? 

Strong stimulus: high frequency of firing 

Weak stimulus: low frequency of firing