Chapter 1. Action Potentials
1.1 Action Potentials
When external signals stimulate a neuron, the neuron "fires"—it generates an action potential, which is a brief electrical charge that travels down the axon.
How does an action potential move along the axon? The axon can be modeled as a tube with a thin axon membrane and interior of axon.
1.2 Action Potentials
When a neuron is not firing, the axon is in a resting state. The fluid inside the axon is electrically negative, and contains a relatively low concentration of sodium ions.
Resting neurons are not electrically neutral. The intercellular fluid has a negative electrical charge, compared to the external environment - the extracellular fluid. This imbalance results from the presence of large negatively charged proteins inside the neuron. Extracellular fluid contains a relatively higher concentration of positively charged sodium ions, which also adds to the electrical imbalance between the intercellular and extracellular environments.
1.3 Action Potentials
The axon membrane contains channels that, when opened, allow sodium or potassium ions to flow into the axon or out of the axon.
The membrane of a neuron has many channels that allow small electrically charged particles, called ions, to flow in and out of the cell. These channels are proteins that can change from a closed state to an open state to regulate the flow of ions. Some chemicals can stimulate the protein channels to open or close. These chemicals include neurotransmitters, hormones and some psychoactive drugs. Most chemicals that stimulate neuron activity open sodium channels. Because the concentration of sodium is much higher in extracellular fluid, sodium ions rush into the cell, when channels are opened. This movement is produced by the concentration gradient - a natural tendency of particles to flow from an area of high concentration to an area of low concentration. In addition, the positively charged sodium ions rush into the cell as a result of their attraction to the negatively-charged intercellular fluid. This effect is known as electrical gradient.
1.4 Action Potentials
When the action potential arrives at this section of the axon, sodium channels open. Sodium ions flow into the axon, making this section of the axon electrically positive for a brief moment.
As sodium ions enter through channels, the intercellular fluid in that particular area becomes positively charged. This change from a negative to a positive charge triggers the opening of adjacent voltage-sensitive sodium channels, and more sodium ions flow into the cell. This chain reaction of more and more sodium channels opening in sequence transmits the signal, the action potential, along the cell membrane to the axon terminal. Just as the increasing positive charge inside a cell membrane triggers the opening of sodium channels, it also opens a separate type of channel that allows potassium ions to move through the membrane. When potassium channels open, these ions move out of the cell because of the same 2 forces that move sodium ions into the cell initially: first, because there are more potassium ions inside the cell relative to outside the cell, the concentration gradient moves these ions out; second, because inflowing sodium ions have created a brief localized positive charge inside of the cell relative to outside of the cell, potassium ions are attracted to the relatively negative charge outside of the channel by the electrical gradient.
When enough potassium is expelled to turn the local intercellular fluid to a negative charge, voltage sensitive potassium channels then close.
1.5 Action Potentials
After the action potential moves past this section of the axon, ion pumps restore the original concentrations of sodium and potassium, and the axon returns to its resting state.
When the action potential has passed, sodium and potassium channels return to their closed state. However, the concentrations of sodium and potassium ions near those channels are reversed from their normal state: sodium is now in higher concentration inside the cell while the concentration of potassium is lower inside the cell. To bring the ions back into balance, the neuron uses sodium-potassium pumps, which are specialized proteins designed to pump sodium ions out of the cell while, at the same time, pump potassium ions back into the cell. The sodium-potassium pumps rapidly restore a proper ion concentration balance.
1.6 Action Potentials
Researchers can use a tiny electrode to measure the electrical charge at a particular axon location.
Researchers then stimulate the axon to produce an action potential. Note the electrical change in the axon as the action potential sweeps past that location on the axon.
1.7 Explain. Action Potentials
When a neuron is stimulated, it "fires"—sending a neural impulse down its axon to the axon terminals. The neural impulse is actually a brief electrical charge called an action potential. Action potentials are generated by the movement of positively charged ions in and out of the axon. In its resting state, the interior of the axon is electrically negative. As the action potential travels down the axon, each tiny section of the axon opens its sodium channels, allowing positively charged sodium ions to flow into the axon. For a brief moment, that section of the axon becomes electrically positive. But soon potassium channels open, and the positive charge decreases as positive potassium ions flow out of the axon. As the action potential moves further down the axon, the original section of the axon returns to its resting state, as sodium-potassium pumps restore the original concentrations of sodium and potassium ions.