P2: Action potential

Question 1

What is the axon hillock?

What is spatial and temporal summation?

Answer 1

Axon hillock: where the action potential starts and where the axon starts. All the sum of the EPSPs(excitatory potential) and the IPSPS integrate here. And either trigger or not trigger the action potential. It is the processor which “decides” what is going to happen (Summation of the potentials).

Spatial summation: simultaneously: input from multiple presynaptic cells.

Temporal summation: repeated inputs: high frequency of action potentials, PSPs last longer than interval between action potentials  they sum up over time
It can be both together in reality.

Question 2

What are the six phases of an action potential:

Answer 2

0st: Summation of EPSP’s and IPSP’s reach threshold for action potential on Axon Hillock
1st: voltage-gated sodium channels open  sodium floods in. +++inside depolarization
2nd: voltage-gated potassium channels open  potassium leaves the cell bc. of chemical gradient
3rd: Sodium channels close. They become refractory.
4th: Potassium is still moving out the cell because of the electrostatic pressure and diffusion
5th: potassium channels close, sodium channels reset as well
6th: sodium-potassium pump moves 3 sodium out and 2

Question 3

Why is not action potential possible in the absolute refractory phase?

Answer 3

Because the sodium channels reset in the repolarization phase when the potential went down from peak to resting potential. Action potential not possible.
Only when the reset of the Sodium channels is finished an action potential is possible but because of the hyperpolarization (more negative than resting), it is harder to get an action potential (absolute refractory phase).

Question 4

How does the action potential travel along the axon?

Answer 4

by a positive feedback loop supported by myelinated sheets.

Question 5

How is an action potential generated? What is another name for it?

Answer 5

Hutchkin Huxley cycle.
external stimulus depolarizes the Neuron a bit. Some sodium channels open(Na+)–> sodium enters the Neuron–>more depolarization–> you have a cyrcle–>vicious cycle(Teufelskreis) till enough Sodium is inside the Neuron so that an action potential can occur.

Question 6

Why does an action potential not travel backwards?

Answer 6

Because the sodium channels close after an action potential has occured . The sodium channels close at the peak positive point of the action potential and get refractory.