Dendrites and axons use different kinds of codes to represent excitation and inhibition. Dendrites use a graded response code which can integrate (add up or combine) excitation and inhibition from the many different synapses. Axons use an all- or-nothing impulse code ("digital" code), which is very effective for communicating over long distances.

Excitatory signals cause a decrease in polarization, called depolarization (de = decrease). This means that the membrane polarization moves towards zero. This happens because excitation lets positively charged ions leak through the dendrite's cell membrane into the cell. The added + ions in the cell decrease the negative electrical charge inside the cell.

Figure 1 illustrates graded responses to three levels of excitation.

In our "typical" cell, excitation makes the difference across the cell membrane decrease from the resting potential of -70 mV towards zero mV. The more the excitation, the bigger the depolarization. E1 produces a very weak excitation, which decreases the polarization by only 1 mV to -69 mV. E2 is a bit stronger, decreasing the polarization by 2 mV to -68 mV. E3 is still a bit stronger, decreasing the polarization by 3 mV to -67 mV. (These numbers are for illustration only; different kinds of neurons will have different specific values.)

Inhibitory signals are the opposite of excitatory ones, so they cause an opposite signal: an increase in polarization, called hyperpolarization (hyper = above, more). This means that the membrane polarization moves away from zero. This happens because inhibition lets negatively charged ions into the cell through the membrane of the dendrites. The added - ions in the cell increases the negative electrical charge inside the cell. Figure 2 illustrates graded responses to three levels of inhibition.

In our "typical" cell, inhibition makes the difference across the cell membrane increase from the resting potential of -70 mV further away from zero mV. The more the inhibition, the bigger the hyperpolarization. E1 produces a very weak inhibition, which decreases the polarization by only 1 mV to -71 mV. E2 is a bit stronger, increasing the polarization by 2 mV to -72 mV. E3 is still a bit stronger, increasing the polarization by 3 mV to -73 mV. (These numbers are for illustration only; different kinds of neurons will have different specific values.)

Note: The remaining questions ask you to test your understanding of the ideas with some very simple arithmetic. The only difficult part is trying to figure out just what to add or subtract. Writing out the problem using the information in the question and the preceding text makes setting up what to add or subtract much easier.

Summation is the process by which dendrites combine depolarization (excitation) and hyperpolarization (inhibition), which the synapses from thousands of other neurons produce. Each active synapse contributes a tiny graded effect. These individual tiny graded effects add up (sum) to produce the overall change in polarization that you observe.

Figure 3 is a simplified illustration using three excitatory (E) synapses to stand for the tens of thousands on the average neuron. E1 produces +1 mV depolarization (towards zero). When it is activated the potential changes from -70 mV to -69 mV. E2 produces +2 mV depolarization. When it is activated the potential changes from -70 mV to -68 mV. E3 produces +3 mV depolarization. When it is activated the potential changes from -70 mV to -67 mV.

When E1 and E3 are active at the same time, their depolarization add up:

-70mV +1mV +2mV +3mV = -64mV.

Q3. The dendrites of a neuron receive simultaneous excitation from four sources. They are as follows:       E1 = +2 mV;    E2= +3 mV;    E3 = +3 mV;    E4 = +1 mV. What will the polarization be in response to these four excitations? (Assume resting potential of -70 mV.)
A. -59 mV    B. -61 mV    C. -63 mV    D. -65 mV    E. Not enough information provided
Hint

Inhibition works the same way except that the sign of the graded response is - rather than +; inhibition produces hyperpolarization (further away from zero).

Figure 4 is a simplified illustration using 3 inhibitory (I) synapses. I1 produces -1 mV hyperpolarization (away from zero). When it is activated, the potential changes from -70 mV to -71 mV. I2 produces -2 mV hyperpolarization. When it is activated, the potential changes from -70 mV to -72 mV. I3 produces -3 mV hyperpolarization. When it is activated the potential changes from -70 mV to -73 mV.

When I1 and I3 are active at the same time, their hyperpolarization add up:

-70mV -1mV -3mV = -74mV.

When all three are active at the same time, the net depolarization is:

-70mV - mV - mV - mV = -76mV.

Now suppose E1, E2 and E3 and I2 are activated together, as in Figure 5 above. They summate (add up) in the same way: -70mV +1mV +2mV +3mV -2mV = -66mV. This is the way that the dendritic membrane integrates (combines) excitation and inhibition from synapses from many different neurons.

The combined signal that dendrites produce by summation have to get sent to other neurons to have any effect. The axon of the neuron does this job. It has to communicate this information reliably over distances that can be several feet. To do this, the axon uses a "digital" code. The axon is either off or (briefly) on, as when you turn a light switch on and off. It generates brief pulses of electrical current called action potentials. Action potentials are often described as all-or-nothing impulses (usually impulse, for short), because the axon makes them all (about) the same size. Digital yes/no codes like this are very reliable, as engineers have discovered, because variation in pulse size does not affect reliability. When a neuron is active and sends action potentials down its axon, it is often described as "firing".

Because the all-or-nothing impulses on axons are all the same size, the size of the impulses cannot signal anything. Instead, the axon uses the number of impulses per second (rate) as the code for stimulus intensity. The stronger the excitation is (above a threshold value) the more impulses go down the axon each second. Inhibition has the opposite effect. It slows down the rate of the all-or-nothing impulses or stops them entirely. This rate code is summarized in Figure 6.

Enough depolarization of the dendritic membrane triggers all-or-nothing impulses from the beginning of the axon at its junction with the dendritic membrane. To trigger an impulse, the dendritic membrane must be depolarized enough to reach a threshold. The more the dendritic depolarization exceeds this threshold (goes beyond it towards zero), the higher the rate of firing (the more action potentials per second) on the axon, up to several hundred impulses per second for a few seconds.

You can think of the graded code on the dendrites and the all-or-nothing impulse code on the axon as being like firing an automatic rifle, which fires bullets faster, the tighter you pull the trigger. Pulling the trigger is a graded response: you can pull it gently, very hard or anything in between. The bullets are like all-or-nothing impulses: a bullet fires full force or not at all. (In fact, researchers speak of an axon "firing" all-or-nothing impulses, because they are bullet-like pulses.) On such an automatic rifle, a small pull produces nothing, a slightly stronger pull fires, say, one bullet every 10 seconds; a maximum pull fires, say, 10 bullets per second. The strength of the pull is translated into number of bullets per second. In the same way, the graded depolarization on dendrites is translated into the number of bullets per second.

Here is a specific example of the relation between depolarization on dendrites and all-or-nothing impulses on the axon. Suppose that a neuron has a resting potential of -70 mV and a threshold of -65mV. So if depolarization reached -65mv, the axon "fires", and an action potential goes down the axon. Suppose that for every mV of depolarization above threshold (closer to zero), the axon would fire 5 more impulses per second. A depolarization of +10mv would produce a net depolarization of -60mV; the axon would fire at a rate of 25 impulses/second (5mV above threshold times 5 impulses/second) plus 1 for threshold value for a total of 26.

This neuron receives excitation from 3 sources and inhibition from 2 sources. E1 produces +1 mV depolarization; E2 produces +2 mV; E3 produces +3mV; I1 produces - 1mV; I2 produces -2 mV. Activating E1, E2, and E3 together would produce a depolarization to -64mV, so the axon would fire 5 impulses/second plus 1 for threshold value for a total of 6.

If only E1 and E3 were activated, depolarization would reach -66 mV, so the axon would not fire any impulses, because threshold (-65 mV) was not reached. If E1, E2, E3, and I2 are all activated at the same time, then the net depolarization is again -66 mV and no impulses are triggered.

• A neuron has a resting potential of -67mV,
• It has four possible excitatory inputs: E1 = +1mV; E2 = +2 mV; E3 = +2 mV; and E4 = + 5 mV
• It also has possible two inhibitory inputs: I1 = -2mV; I2 = -4mV.
• The neuron must be depolarized to -61mV to trigger a single impulse on its axon.
• For each mV depolarization above this threshold, the axon fires two more impulses.

How many impulses will be generated for each of the three conditions listed below? [When you calculate the answers, don't forget that at the threshold voltage, the axon triggers one impulse]
Q5A. If E1, 3, & 4 are activated together, the neuron will generate ___ impulses.
    1. zero     2. one     3. two     4. three     5. four     6. five
Q5B. If all four Es and I2 are activated together, the neuron will generate ___ impulses.
    1. zero     2. one     3. two     4. three     5. four     6. five
Q5C. If all four Es and both Is are activated, the neuron will generate ___ impulses.
    1. zero     2. one     3. two     4. three     5. four     6. five
Hint

The correct answers are: Q1: C; Q2: D; Q3: B; Q4: D; Q5A: 6; Q5B: 1; Q5C: 1

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