THE FIRING NEURON

Image of Stained Neuron

NEURON - A neuron is a nerve cell, for this example I will use a bipolar neuron, meaning its processes extend from both ends of the soma.

SOMA - The soma is the cell body of the neuron. It contains all of the organelles, the machinery, which provide all of the life to the cell.

DENDRITES - On one end of the soma, there are branch-like extentions. These extensions serve as the recipients for chemical messages, called neurotransmitters; at the end of each branch there are many receptors for these messages. Once the dendrites receive the messages, they travel into the soma, to the other end where they encounter the axon.

AXON -Instead of being many branches, the axon is one long branch. The messages travel down the axon; the axon terminates in several areas called terminal buttons.

TERMINAL BUTTONS -This is where the neurotransmitter substance gets packaged by organelles for release into the space outside the neuron. The branch of a dendrite from another neuron is on the receiving end of this message.
Now that we have the basic structure of a neuron down, I can talk about some of the internal functions, such as: action potentials, depolarization, axonal transport and synaptic transmission. As with my other entries, these are large topics and cannot possibly be covered completely within this writing. Therefore, I would encourage anyone who reads this and finds something of interest, to continue looking on the internet for further information.
The axon of the neuron has an outside membrane, this membrane has an electrical potential, at resting it is -70mV. Once the chemical messages hit the axon, they change the membrane potential by creating something called an action potential.
An action potential is the result of a chain of events. If the mathematical summation of information is great enough to reach a certain threshold level, then that point on the membrane is allowed to change its charge; if its charge was negative, like on the inside of the membrane, and it changes to positive then the change is called - depolarization. The other way it can happen, is if the negative charge on the inside of the membrane actually increases in negativity, that would be a hyperpolarization. An action potential is the result of a depolarization of the membrane.
The process of an action potential, is the opening and closing of certain ion channels that are located on the membrane. As I said before, if the level of stimulus is enough to reach the level of excitation, approximately -60mV, then that allows for a depolarization of the membrane, opening sodium channels, and creating a large influx of sodium. Because these channels are opened due to changes in the membrane potential, they are called voltage dependent ion channels. This large influx of sodium ions changes the potential of the membrane from its resting state of -70mV to a +50mV. After this occurs, potassium channels also open which counteracts the effects of the sodium. At the peak of the action potential, the sodium channels actually close, but the potassium channels remain open and potassium leaves the cell. As the membrane potential nears its resting state again, it goes through a refractory period where it is temporarily hyperpolarized, here, it is not sensitive to any other stimulus. There are many other processes included in those steps, for one, there are many forces driving these chemicals which accounts for their flow. There are also other considerations about the state of the membrane, does it have a fatty coating around it, called myelin, or not? So this was just a simplified view of the action potential.
This process continues down axon, each area of the membrane depolarizing in turn. If the axon is myelinated, or you could call it insulated, the transport would be a lot faster. Once the message travels to the terminal button, it must be released to go on to a different neuron. At the site of the terminal buttons, the neurotransmitter message is packaged in a membrane container called a vesicle. This vesicle then travels to the actual membrane of the terminal button, waiting for release. The neuron knows that this message has reached that final destination, so when the action potential reaches that terminal button, it opens new channels, called sodium channels. With this new information the vesicles are allowed to fuse with the membrane of the terminal button and release their contents into the interstitial space, or synapse. After the release of its contents, the vesicle remains fused to the membrane and becomes perminantly integrated.
Receptors located on the dendrites of another local neuron will then receive the neurotransmitter message, and the process begins over again.
Once again, there are other processes that take place, like the dissipation of the neurotransmitter, or the deactivation of it. Also the process of negative feedback which tells the pre-synaptic neuron how much neurotransmitter substance is being released and taken up.
Also, a brief note, there are many other kinds of both neurons, synapses and receptors....This is only the beginning.
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