Action Potential Generation
By some explanations, an action potential is presented as a fairly simple process. A nerve cell receives some sort of message from another cell, via chemicals or electric signals, and decides whether or not an action potential will be fired in order to communicate with the next cell(s) in line. In actuality, this is a hugely oversimplified explanation of an action potential. On a molecular basis, quite a bit more takes place.
The process of an action potential begins at a neuron’s dendrites. Dendrites of a neuron can receive neurotransmitters or other chemicals from the presynaptic neuron, provided they have the proper receptors. Receptors are proteins that are capable of transporting specific molecules through the cell membrane that would otherwise be impermeable. If the post synaptic neuron has the proper receptors, the messenger molecule will be taken into the cell. (Nicholls, et al., 2001)
Once the messenger molecule has entered the cell, it can generate either a localized excitatory postsynaptic potential (EPSP), a flow of positive of ions into the cell, or an inhibitory post synaptic potential (IPSP), a flow of negative ions into the cell. Both EPSPs and IPSPs have additive effects on the neuron. EPSPs cause a localized depolarization of the cell. If the cell receives enough EPSPs, the cell will be sufficiently depolarized to fire an action potential. In contrast, the more IPSPs received, the less likely a cell is to fire an action potential (Kalat, 2001).
Depolarizations caused by EPSPs are usually due to a flow of sodium into the neuron. Sodium is highly concentrated outside the cell, and potassium is concentrated inside the cell, but in a much lower amount than the sodium outside. Due the large difference in concentrations of ions, the net charge of the inside of a neuron is negative.
As EPSPs are received, sodium flows into the cell, making the charge inside more positive. Once the membrane potential has reached a point of depolarization known as the threshold, the large majority of voltage-gated sodium channels become activated and sodium flows into the cell rapidly. It should be noted that sodium channels can also open in absence of a threshold voltage change. As each voltage gated channel opens, localized depolarization occurs and the voltage gated channels open in the adjacent area creating a cascade of depolarizations in the cell. The cascading activation of voltage gated sodium channels (and resultant inward flow of sodium) rapidly moves down the axon of the cell. As sodium flows inward, the inside of the cell quickly becomes positively charged with respect to the outside. This is the action potential. (Nicholls, et al., 2001)
The action potential usually results in a release of messenger molecules via a vesicle containing neurotransmitters merging with the cell membrane at the axon terminal. This merger of neurotransmitter-filled vesicles with the cell membrane results in the release of neurotransmitters into the synaptic cleft. These neurotransmitters (or other messenger molecules) are exposed to the dendrites and receptors of the next cell in line and the process of generating an action begins again (Kalat, 2001).
After the firing of an action potential, the cell has a recovery period. During this recovery period, potassium outside the cell flows into the cell rapidly via voltage gated potassium channels. As potassium flows inward, the inside of the cell moves closer and closer toward having a membrane potential more negative than the resting potential (hyperpolarized). Hyperpolarization is due to the large concentration of both sodium and potassium inside the cell and a low concentration of both outside the cell. Gradually the sodium ions are returned to the outside of the cell via the sodium-potassium pump and resting potential is restored. (Kalat, 2001)
An action potential is a fairly complex process when viewed at a molecular level. Messenger chemicals (e.g: neurotransmitters) received at a cell’s dendrites trigger either an EPSP or IPSP which results in localized responses that have an additive effect on the cell. If the cell receives enough EPSPs, the cell will depolarize sufficiently to trigger an action potential. An action potential is the result of a voltage-gated sodium channels opening in a cascading manner. As the sodium channels open, the cell is flooded with sodium ions and potassium ions leave, which causes the cell to have a positive membrane potential. This resultant positive membrane potential opens voltage-gated potassium channels and potassium flows into the cell, causing sodium to leave the cell and the membrane potential to become slightly more negative than resting potential. Eventually the cell is returned to resting potential via molecular pumps channels which restore the proper balance of sodium ions outside the cell and potassium ions inside the cell. Once resting potential is restored, the whole process begins again.
References:
Kalat,
James W. (2001). Biological psychology.
Nicholls, J., Martin, A., Wallace, B., & Fuchs, P. (2001). From neuron to brain.