Neuron Morphology
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.
| 1. |
The first step of the action potential is that the Na+ channels
open allowing a flood of sodium ions into the cell. This causes the membrane
potential to become positive. |
| 2. |
At some positive membrane potential the K+ channels open
allowing the potassium ions to flow out of the cell. |
| 3. |
Next the Na+ channels close. This stops inflow of
positive charge. But since the K+ channels are still open it allows
the outflow of positive charge so that the membrane potential plunges. |
| 4. |
When the membrane potential begins reaching its resting state the K+
channels begin to close. |
| 4. |
Now the sodium/potassium pump does it's work and starts transporting sodium out
of the cell, and potassium into the cell so that it is ready for the next
action potential. |
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) |
|
