Neuron Basics

Neuron Morphology

Figure 1. Neuron Diagram. (Carlson, 1992)

The neuron is the cell that animals use to detect the outside environment, the internal environment of their own bodies, to formulate behavioral responses to those signals, and to control their bodies based on the chosen responses. This is of course a very simplistic definition of what neurons do. But it does cut to the basics. All neurons have a body called a Soma. The Soma contains the nucleus and all of the other organelles that are needed to keep the cell alive and functioning. Neurons also have directionality to them. On one side of the neuron are the dendrites. You can think of this side as being the 'input' side. The dendrites are branching structures which connect with the outputs of other neurons. They typically spread over a wide area in the immediate vicinity of the neuron. This allows the neuron to get inputs from a number of different synapses. The other end is the 'output'. It contains an axon and ends in a number of synapses which usually connect to the dendrites of other neurons or are connected directly to muscles. The axon is usually quite long compared to the rest of the neuron. In fact, you have some neurons with axons that extend the entire length of your body!

Action Potential

The signal output of a neuron can either cause excitation or inhibition in the neuron it is connected to. When a neuron sends an excitatory signal to another neuron, then this signal will be added to all of the other inputs of that neuron. If it exceeds a given threshold then it will cause the target neuron to fire an action potential, if it is below the threshold then no action potential occurs.

An action potential is an electric pulse that travels down the axon until it reaches the synapses, where it then causes the release of neurotransmitters. The synapses are extremely close to the dendrites of the target neuron. This allows the neurotransmitters to diffuse across the intervening space and fit into the receptors that are located on the target neuron. This causes some action to take place in that neuron that will either decrease or increase the membrane potential of the neuron. If it increases the membrane potential (makes it more positive, or depolarizes it.) then it is exciting the neuron, and if it decreases the membrane potential (makes it more negative, or hyper-polarizes it.) then it is inhibiting the neuron. If it causes the membrane potential to pass the firing threshold then it will activate an action potential in the target neuron and send it down its axon. 

Neurons in a resting state normally have a membrane potential around -70mV. This means that the voltage difference between the fluid on the inside of the cell relative to the fluid on the outside of the cell is negative. How is this negative difference maintained? It is done with ions like Na⁺, K⁺, Cl^- , and protein anions. The cell membrane prevents charged particles such as these from freely diffusing into and out of the cell. There are two basics ways that they can get in or out. The first is with passive transport. Basically the cell has a protein in the cell membrane that it can open and close like a water faucet. It is specific for certain kinds of chemicals like these ions. When it opens, then the ions can flow down their gradient from the more concentrated area to the less concentrated area. The other way to get ions in or out of cells is to by active transport. The cell uses some of its own energy to actively pump the chemicals against their gradient. The neuron has a pump that actively pumps three Na⁺ ions out and takes in two K⁺ ions. This means that a net positive charge flows out of the neuron. This is what gives the cell its negative potential. Ions are also what are responsible for the initiation, and transmission of action potentials. When the neurotransmitters from other firing neurons come in contact with their corresponding receptors on the dendrites of the target neuron it causes those receptors to open or close some of the passive ion transports. This allows the ions to flow into the cell and temporarily change the membrane voltage. If the change is big enough then it will cause an action potential to be fired. Figure 3 shows the basics of how ion flow transmits the action potential down the length of the axon.

Figure 3. Ion flow in action potential. (Carlson, 1992)

The action potential travels down the length of the axon as a voltage spike. It does this using the steps outlined above. As a section of the axon undergoes the above process it increases the membrane potential of the neighboring section and causes it to spike. This is like a mini chain reaction that proceeds down the length of the axon until it reaches the synapse. An important thing to keep in mind about the action potential is that it is one way, and all or nothing. The action potential starts at the top of the axon and goes down it. Also, if a neuron fires then the action potential is the same regardless of the amount of excitation received from the inputs. What is important in neurons is the rate of fire. Figure 4 demonstrates this principal. A weak stimulus will cause a lower rate of fire than a strong stimulus. So it is not the amplitude of the action potential that is important, but the number of times a neuron fires for a given time period. However, it has been shown in experiments that the rate of fire of a neuron is directly related to the depolarizing current applied to that neuron. This can be seen in figure 5. This fact will be important later on when the neural model is being explained.

Figure 4. The rate law demonstrates that a stronger stimulus will cause a neuron to fire more often than for a weaker stimulus. (Carlson, 1992)

Figure 5. Experimental data showing the relationship between input current and firing rate of a neuron. (Beer, 1990)