The neuron is the basic functional unit of the nervous system. It is a single cell, capable of recieving, integrating, and transmitting signals. Neurons use a combination of electrical and chemical signals to accomplish this. Electrical signals transmit signals from one end of a single neuron to the other, and chemical signals are used for communication between neurons. Neurons are arranged and constructed in our bodies in a manner which allows them to integrate things like sensory input and memory, resulting in an appropriate reaction by the body based on that input. The nervous system, as most people know, is the main control center for an organism's behavior. But before we get into all of that, let's go over the basic neuron morphology.

Below is a diagram of a generic neuron. In general, neurons vary greatly in their structure, but for the most part, they can be divided into three sections. The dendrites and the axon will be the main focus of our discussion. They are often reffered to as processes, a generic term for wire-like things that stick off of the soma. The soma is the cell body. Some more detail for you:

The Dendrites are traditionally thought of as the parts of the neuron that recieve and integrate inputs. Really, a neuron can recieve inputs on any portion of its surface, but the dendrites are more specalized for such functions. Chemical input from other neurons is converted to electric signals in the dendrites. These electric signals spread in a (generally) passive manner through the dendrites and into the cell body.

Soma The soma acts sort of like the support factory for the neuron. The DNA is located here, in the nucleus, and most of the protein synthesis in the cell occurs here. The soma also contains other organelles essential to the neuron's general function as a living cell. The endoplasmic reticulum (ER) and the Golgi apparatus (Ack! big words!) are where proteins are synthesized and packaged in small membrane bubbles called vesicles. This allows them to be transported to distant regions of the cell, such as the far ends of the neural processes.

The axon is the part of the neuron which has historically recieved the most attention. Its unique membrane characteristics allow it to create an electrical event called the action potential. More on the mechanism of that later. For now, know that the action potential is an electrical signal which actively self-propagates down the axon. This self-propagation allows it to travel very fast, and over very long distances without a significant decrease in magnitude.

          ________dendrites______ ____soma____ ________axon________________
          |                      ||           ||                           |

                 /    \_
             ___/  _    \__        *********                        /=====
                    \     _\______***********                      /
           \    _____\___/       *************=============================
            \__/                 /***********                        \
            ____           _____/  *********                          \
                \___      /                                            \====

Okay, now that you've got a bit of a picture in your head, let's talk about how signals propagate across the length of the neuron. Neurons utilize an electrochemical gradient to create electrical signals. Really, it's not as complicated as it sounds. It all works on ions. The idea is this. The neruon, like all cells, is encapsulated and defined by a semi-permeable membrane. Semi-permeable just means that some things can cross it, and others cannot, and some can cross it only if the cell allows them to. Ions fall into this latter group. They can only cross the membrane though channels, protein structures in the membrane which can be variably selective. At rest, when the cell is just sitting, and not transmitting signals, there are several types of channels active. There are leak channels, which allow the free flow of specific ions. Only one ion can pass through a channel at a time, so these leaks are not very strong, and they are different for different ions (depending on the number of leak channels for that particular ion). The other vitally important type of channel active at rest is the ion pump. These pumps push ions against their electrochemical gradient, and with the leak channels help to maintain steady concentrations of ions inside the neuron.

But wait, you say, what the hell is an electrochemical gradient? Okay, I promise it's not complicated. Ions have electrical charge. Like charges repel, different charges attract. A charge difference between two regions creates an electrical gradient. If one region has more positive charge than another, the positive charges in that region repel each other and will spread into the less positive region, if there is no other force or barrier acting on them. A voltage is a measurement of the electrical driving force of a given electrical gradient. It is also referred to as potential. A chemical gradient is created when there is more of a specific type of substance in one region than another. Assuming these substances are capable of free movement, they will tend to diffuse from regions of higher concentration into areas of lower concentration. Entropy in acton. An electrochemical gradient is just a name for the situation when there exists both an electrical and a chemcial gradient in the same region for a specific ion.

In neurons there are four ions which have major significance to neural function. They are Sodium (Na+), Potassium (K+), Calcium (Ca2+), and Cloride (Cl-). Below is a table listing these ions and some important information about them for typical mammalian neurons.

Ion    Concentration Inside Neuron   Concentration Outside Neuron   Reversal potential 
Na+             18 mM                              145 mM                    +55 mV
K+              135 mM                             3 mM                      -102 mV
Ca2+            100 nM                             1.2 mM                    +125 mV
Cl-             7 mM                               120 mM                    -76 mV

Okay, the concentrations are pretty straightforward. Bigger number means more ions, 1 nM = .001 mM. So you can figure out the direction of the chemical driving force from that. The direction of the electrical driving force for each ion is dependent on the resting potential of the cell. The resting potential is the voltage across the membrane that is set up by the electrochemical gradient of a cell at rest. In typical cells, the resting potential falls between -55 mV and -70 mV. This is a measure of voltage inside the neuron with respect to the outside. Now, I think we need to discuss the reversal potentials. This is what the electrochemical gradient boils down to, and it's the most important thing to remember. The reversal potential for Na+ is +55 mV. This means that if a neuron suddenly becomes more permeable to Na+, its membrane potential will deviate from the resting potential, and move toward +55 mV. Once the membrane potential reaches +55 mV, Na+ will stop flowing. This is because at that voltage, the driving force of the chemcial gradient and the driving force of the electrical gradient are equal in magnitude, but opposite in direction. So they essentially cancel each other out. The reversal potentials are calculated using the Nernst Equation.

Now for the cool stuff.

Signal propagation in neurons Okay, finally. Now that we've got the basics, we can talk about how these electric signals travel through the neuron. Signals in dendrites usually travel passively. This process is known as electrotonic spread and may someday be noded elsewhere (electrotonic spread) Because it is actually more complicated to explain than the action potential, and this is getting pretty long. For know, know that inputs to the dendrites can either raise (depolarize) or lower (hyperpolarize) the membrane voltage in that region. The changes voltage spread though the dendrites, via ionic diffusion, gradually decaying as they head toward the soma. When they contact a spreading voltage signal from a different input, the two signals sum. Thus, in the dendrites, inputs from many sources (as many as on the oder of thousands) can be integrated together. When these signals reach the intersection of the soma and the axon, a region known as the axon hillock, they may generate an action potential. This only occurs if the membrane potential at the axon hillock rises above some threshold voltage. The action potential takes the form of a sudden large increase and subsequent decrease in membrane voltage. Thus, it is often referred to as a "spike." A neuron which is transmitting action potentials is said to be firing.

The action potential is generated by voltage-gated ion channels These are channels that open and close, and are triggered by changes in membrane voltage. The two ions involved in action potentials are Na+ and K+. The sequence of events in action potential generation is as follows:

1. Membrane voltage increases past threshold. Voltage sensitve Na+ channels sense the voltage increase and are triggered to open. They open quickly. Simultaneously, Voltage sensitive K+ channels are triggered to open. These open slowly.

2. Open Na+ channels allow Na+ to flow into the cell, driving the membrane potential upward, toward the reversal potential of Na+. This opens even more Na+ channels, in a demonstration of positive feedback. K+ is beginning to flow out of the cell, but the K+ channels open slowly, so it is not yet significant in relation to the Na+ effects.

3. The action potential reaches it's peak, as two mechanisms begin to take effect. A slow acting, time dependent closing of Na+ channels which is triggered by increased voltage now takes effect. This stops the flow of Na+. The K+ channels are fully open, and K+ outflow pulls the membrane voltage rapidly back toward resting potential, closing K+ channels as it does.

4. When the membrane hits resting potential, all of the K+ channels have closed. The cell sits at resting potential. The Na+ channels go through a time dependant resetting process which prepares them for creation of a new action potential. While the channels are resetting, no more action potentials can occur in that region. This is called the refractory period and limits the rate at which a neuron can fire, and prevents backward propagation of the action potential.

When an action potential occurs in one region, the voltage change creates an action potential in adjacent regions. The action potential does not naturally travel in two directions however, due to the refractory period. So, what results from the above process, is a spike which begins at the axon hillock, and travels the length of the axon. When it reaches the end of the axon, it activates mechanisms which release chemical signals that communicate with other neurons.

So now you've got the basic idea of how a neuron works. Truthfully it can be much more complicated than this. There are other types of voltage sensitive ion channels which have different dynamics and can affect things like the rate and pattern with which neurons fire. Neurophysiology, the study of how neurons behave is nearly a science unto itself.

Back to How your brain works.
Carry on! Electrotonic spread.