Action Potentials

Ok...I'm just not loving Action Potentials. I've read the textbook, read over my notes AND looked online. It's not sinking in. Can anyone explain this for me? I'm hoping that when I hear it in another way it will click. Right now I feel like I have a brain fart and my 4.0 is going to go down the drain!!

Thanks!!

Diedra

What is it that you are having trouble with?? I may be able to help. Let me know and I'll see what I can do.

Jen

A cell's membrane potential is caused by different electrical charges on the inside and outside regions of the membrane. At rest, the cell membrane is more ngative on the inside and more positive on the outside. A membrane potential is a form of potential energy. A change in in the membrane potential can cause an electrical signal in excitable tissue. (aka, an action potential)

The action potential is a large change in the membrane potential of a neuron from a resting value of about -7-mV to a peak of about +30mV and back to -70mV.

The action potential is generated at the axon hillock, where the are many voltage gated Sodium channels. Here are the steps:

a.. It begins when signals from the dendrites and cell body reach the axon hillock and cause the membrane potential there to become more positive (depolarization).

b. As the axon hillock depolarizes, voltage-gated channels for Sodium open rapidly. Sodium moves down its concentration gradient into the cell. The membrane potential becomes more postive as Sodium moves into the cell.

c.If the stimulus to the axon hillock is great enough, the neuron depolarizes enough to reach a trigger point called threshold, which is -55 mV. At threshold, depolarization opens more voltage gated sodium channels, which causes sodium to flow into the cell, which causes the cell to depolarize further, opening still more voltage gated sodium channels.

A wave of depolarization spreads from area to area of the cell, moving down the axon, depolarizing the area as it goes along. This wave is the nerve impulse.

d. Once the action potential has traveled down the axon, the sodium gates close and the voltage gated potassium channels open, causing potassium to move out of the membrane, following it's concentration gradient. This causes the membrane to repolarize, that is become more negative on the inside compared to the outside. This interrupts the positive feedback loop and ends the rising action potential.

e. In some neurons, the voltage gated potassium channels remain open after the cell has repolarized. Potassium continues to move out of the cell, causing the membrane potential to become more negative than the resting membrane potential (hyperpolarization)Once all the potassium channels are closed, hyperpolarization ends.

Long, but maybe that will help!

Ok...I'm just not loving Action Potentials. !

Diedra

This is a bit lengthy but it is cut down to easy, I hope, to understand lines. I hope it helps you and your studies. Don't let this get to you too much. Good luck.

ACTION POTENTIALS:

Action potentials are nerve impulses by which neurons communicate.

Membrane potential is the difference in electrical charge across the plasma membrane of most cells.

-A cell is polarized if it has a membrane potential across its plasma membrane.

-Resting membrane potential is the difference in charge across the plasma membrane of a resting cell.

Unlike electrical wires, in living tissue, impulses are conducted by a flow of ions (not electrons)

- Flow of ions only occurs in the ion channels of the plasma membrane.

Ion channels -

They allow only specific ions to cross plasma membrane down a concentration gradient.

Two types of ion channels are:

a)Leakage channels - they are always open

- There are more K+ leakage channels than Na+ leakage channels, allowing K+ to leak more

b)Gated channels - they are covered by protein & open & close on command

- Voltage-gated channels open in response to a change in membrane potential

Resting membrane potential -

In neurons, the resting membrane potential is about -70 mV (inside more negative than outside)

-Extracellualr fluid has more Na+ & Cl-.

-Intracellular fluid has K+ and negatively charged proteins & PO4-.

-More K+ leaks out of the cell, while some Na+ leaks in to the cell.

Na+ / K+ pump maintains resting membrane potential by pumping leaked Na+ out and leaked K+ in.

Generation of Action Potentials -

An action potential (impulse) reverses the resting membrane potential of a cell (-70mV to +30mV).

Excitability is the ability of neurons and muscle fibers to convert a stimulus to an action potential.

A stimulus is a physical, chemical, or electrical event that briefly depolarizes a cell membrane.

Threshold is the critical level to which the stimulus must depolarize a cell membrane for an action potential to occur. (Usually, it is about -55mV)

The two phases of action potential are:

1) Depolarizing phase - The inside of the cell becomes positive (from -70mV to +30mV)

2) Re-polarizing phase -Resting membrane potential is restored (from +30mV back to -70mV inside)

-Neurons take 1 millisecond to depolarize & then re-polarize.

STEPS-

• A Stimulus of threshold strength is applied to a neuron
  
• Na++ channels open to let Na++ into the neuron
  
• Depolarization occurs (inside becomes +30mV)
  
• K+ channels open to let K+ out of the neuron
  
• Na+ / K+ pump pushesNa+ out and K+ into the neuron to repolarizing it (inside -70mV)

-After-hyperpolarization stage may occur due to excessive K+ outflow (inside becomes > -70mV).

Local anesthetics (Procaine, lidocaine) block pain by blocking Na++ channels

All-or-None principle -A neuron is fully depolarized or not at all (no half-polarization occurs)

Absolute Refractory Periodis thetime during which another stimulus cannot excite the neuron.

Relative Refractory Period - another supra-threshold stimulus can excite the neuron.

ok...i'm just not loving action potentials. i've read the textbook, read over my notes and looked online. it's not sinking in. can anyone explain this for me? i'm hoping that when i hear it in another way it will click. right now i feel like i have a brain fart and my 4.0 is going to go down the drain!!

thanks!!

diedra

oh yes diedra, i remember this being a killer too. i did end up getting it but doubt i could spit it back without looking at my notes ). i will tell you instead how i studied for it. i looked up as many sites as possible, the book i had has a great website that gives animations, so look for animations on the subject. then i hand wrote out each step. i drew pictures and tried to grasp it, then i explained it to my lab partner, read it learn it, teach it, know it!

action potential here is a link that simplifies the description, once you get the simplest form and know that you can move on to more detailed explanations, you will get it! remember the 3 main steps are depolarization, repolarization, and refractory. try and figure out where the na and k is and which is neg and positive, draw pictures of each phase.

goodluck!

I think I got it! I think I got it! Thank you guys so much!!!! My daughter thought I was nuts for posting to you guys (she still doesn't get it - but I'm the crazy one)!

Thanks again!

Diedra:balloons:

I think I got it! I think I got it! Thank you guys so much!!!! My daughter thought I was nuts for posting to you guys (she still doesn't get it - but I'm the crazy one)!

Thanks again!

Diedra:balloons:

Yay congrats!!