Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics in 2 minutes or less. In this installment, I will discuss the action potential. The action potential is a momentary reversal of membrane potential that is the basis for electrical signaling within neurons. If you're unfamiliar with membrane potential, you may want to watch my video on membrane potential before watching this video. The resting membrane potential of a neuron is around negative 70 millivolts.
When neurotransmitters bind to receptors on the dendrites of a neuron, the neurotransmitters They can have an effect on the neuron known as depolarization. This means that they make the membrane potential less polarized or cause it to move closer to zero. This chart shows membrane potential on the y-axis and time on the x-axis. When neurotransmitters interacting with receptors causes repeated depolarization of the neuron, eventually the neuron reaches what is known as its threshold membrane potential.
In a neuron with a membrane potential of negative 70 millivolts, this is generally around negative 55 millivolts. When threshold is reached, a large number of sodium channels open, allowing positively charged sodium ions into the cell. This causes massive depolarization of the neuron, as the membrane potential reaches zero and then becomes positive.
This is known as the rising phase of the action potential. The influx of positive ions creates the electrical signal known as the action potential, which then travels down the neuron. Eventually the action potential reaches its peak, sodium channels close and potassium channels open, which allows potassium to flow out of the cell.
This loss of positive potassium ions promotes repolarization, which is known as the falling phase of the action potential. The neuron returns to resting membrane potential, but actually overshoots it, and the cell becomes hyperpolarized. During this phase, known as the refractory period, it is very difficult to cause the neuron to fire again.
Eventually the potassium channels close, and the membrane returns to resting membrane potential ready to be activated again. The signal generated by the action potential travels down the neuron, and can cause release of neurotransmitter at the axon terminals to pass the signal to the next neuron.