Describe action potential and how it allows for signals to be transmitted along neurons.

To answer this question, we first need to understand what a neuron is and what its components are. A neuron is a nerve cell that transmits electrical impulses between cells to continue an action, to end an action, or to start an action. Neurons consist of four working parts. The dendrite(s) are branches that act as impulse receivers. The dendrite(s) are attached to the cell body, or soma. The soma is where the majority of the neuron’s organelles (nucleus, mitochondria, Golgi apparatus, smooth endoplasmic reticulum, etc.) can be found. The axon is a long extension that branches off of the cell body and carries the impulse from the dendrite(s) and cell body to the axon terminal. The direction of the impulse is very important. At the end of the axon is the axon terminal, which serves to send the signal from the neuron, across the synapse (the gap between neurons), and to the adjacent neuron or tissue. Now that this has been established, we may begin to dive deeper into what an action potential is and how it allows neurons to communicate. When a neuron is "resting" it has a resting charge, or resting potential, this is -70 millivolts. This is maintained by protein structures that are embedded in the neurolemma, or neuron membrane. The sodium potassium pump is an active form of transport (requiring ATP) that allows for three sodium (Na) ions to exit the cell and for two potassium (K) ions to enter the cell. This generates a net negative charge as the atomic charge of each ion is +1. Another structure that maintains this membrane potential are ion channels. These are passive (not requiring energy) structures that allow for Na ions to freely flow into the neuron and for K ions to exit the neuron. However, once a stimulus is received by a neuron's dendrites another protein structure (called gated sodium channels) allows for Na ions to enter the cell making it more positive. Given enough channels opening and the cell becoming positive enough, the charge of the neurolemma will reach what is called the "threshold". This is measured at -55mV. At this stage, rapid depolarization (becoming positive) occurs, due to voltage-gated sodium channels opening and flooding the cell with Na ions. At the same time, voltage-gated potassium channels are opening and allowing K ions out of the cell, but this is not significant until later on in the impulse because the voltage-gated potassium channels have a slower response to the threshold being reached. This sudden spike in positivity in part of the membrane is going to send a domino-like effect that causes the membrane next to the depolarized section to also become depolarized. As the wave-like act of depolarization is traveling down the axon towards the axon terminal, repolarization is occurring behind the signal. This is due to the aforementioned voltage gated potassium channels fully responding and letting K ions out of the cell. This is accompanied by the closing of voltage-gated sodium channels, which returns the neurolemma back to the resting potential. However, to prevent signals from being sent back towards this point of origin, a hyperpolarization must take place. This is when the voltage-gated potassium channels are still in the process of closing. This extra negative charge keeps the signal moving in one direction. Once the electrical signal has reached the axon terminal the depolarization will open calcium ion gated channels. With the calcium ions within the axon terminal vesicles containing neurotransmitters will be fused with the membrane and "spit out" the neurotransmitters via exocytosis. These neurotransmitters will follow a chemical gradient along the synapse to reach the neighboring neuron and then either excite or inhibit this action from occurring again.