Lecture 5: Action Potential

What is Action Potential?

What is Depolarization?

What is Hyperpolarization?

What are the different phases of an Action Potential?

1. Resting potential

2. Rising Phase

3. Overshoot

4. Falling phase

5. Undershoot (after hyperpolarization)

What are the characteristics of an Action Potential?

• Threshold for initiation (10 mV depolarization above rest)

• All or Nothing

• Always depolarizing

• Constant amplitude (100 mV)

• Constant duration

• Propagate without decerement

• 2 Part refractory period:

-Absolute (falling phase)

-Relative (undershoot)

What results in Depolarization?

• Positive ion influx

• Negative ion efflux

What results in Hyperpolarization?

• Positive ion efflux

• Negative ion influx

How does Action Potential Generate?

-Requires depolarizing 

• Natural occurrence via excitatory synaptic transmission 

• Artificially via current injection into a neuron with a microelectrode

What does it mean to Propagate without decrement?

What happens when the depolarizing current increases?

• The action potential firing rate increases

If the injected current does not depolarize the membrane?

• No Action Potential will generate

If the injected current depolarizes the membrane beyond threshold?

• Action Potential will generate 

What is the electrical property of Neurons?

• Electrically (excitable) active fire action potentials

What is the electrical property of Gila?

• They are electrically “silent” cells

• Nothing happens

What are the ins and outs of an Action Potential?

• Rising phase: inward socium current 

• Falling phase: outward potassium current 

What is the “Law of Permeability”?

• The membrane potential is always driven toward the E_ion if the ion to which membrane is most permeable

Why do potassium and sodium have different signs even though they are both positive ions?

• The concentration difference between sodium and potassium is what leads to the difference in direction sign although they are both positive ions

What happens with a neuron sends a spike(action potential)?

1. T1, AP threshold for initiation, critical number of voltage-gated Na+ channels open 

2. T2 the voltage gated Na+ channels are inactive

3. T2 The voltage gated K+ channels open 

4. T3 the voltaged gated K+ channels are closing (stopping K+ efflux)

What is the structure of the voltage-gated sodium (Na+) channel?

• transmembrane domains and ion-selective pore 

• Selective: the channel has a size exclusive filter making it permeable only to sodium

What happens during the Action Potential refractory period?

• Na+ channels become reactivated

• Membrane potential is below resting & requires greater depolarization current to fire AP at rest

What are the 3 states of voltage-gated sodium channels?

• Closed 

• Open 

• Inactive 

The Properties of sodium channels?

1. Open with a delay

2. Open for 1 msec

3. Cannot be opened by immediate depolarization 

• Absolute refractory period: channels inactive 

What does TTX do to the Action Potential?

• Blocks Action Potential through selective inhibition of sodium channels

• no changes happen to potassium channels

What is Channelopothy?

• A human genetic disease caused by alterations ot the structure and function of ion channels 

• Generalized epilepsy with febrile seizures

What is the source of TTX?  What does it do?

• The Puffer fish 

• Tetrodotoxin: clogs permeable Na⁺ pore by binding to a specific site outside the channel

What is another channel-blocking toxin?

• Red tide Sacitoxin found in clams, mussels, & shellfish

• Batrachotoxin, found on the skin of the Colombian frog, causes channels to open more negative potentials, opening much longer and messing with information

• Veratridine and aconitine (lilies and buttercups)

What is similar in potassium and sodium channels?

• Open in response to depolarization 

What is different about potassium from sodium channels?

• Gates open after sodium

• serves to reset membrane potential 

• 4 seperate polypeptide subunits join from a pore

What is the Spike-initiation Zone?

• Axon hillock 

• Sensory nerve endings 

Which direction is Orthodromic?

• 1 direction down axon to axon terminal 

Which direction is Antidromic?

• Backward propagation 

What is the conduction length and velocity?

• 10 m/sec 

• 2 msec long

What factors influence conduction velocity?

• Myelin: facilitates current flow 

• Schwann cells PNS 

• Oligodendroglia CNS

-Saltatory conduction of nodes on Ranvier

-Voltage-gated sodium channels conc. at noes

What are other factors that inlfuence conduction velocity?

1. Spread of action potential on the membrane: dependent on axon structure 

2. Path of positive charge: inside axon faster, accross axonal membrane slower

3. Axon excitability: diameter or axon (bigger=faster), # of voltage channels 

4. Myelination