As information flows through the nervous system, it passes from one neuron to the next via a synapse. Recall that the first neuron is called the presynaptic neuron and the second neuron receiving the information is called the postsynaptic neuron.
But synapses do not just transfer information, they also are the key element in processing neuronal information. Synapses are a site of neuronal decision making. Thus, each presynaptic action potential arriving at a synapse does not trigger one corresponding action potential in the postsynaptic neuron.
Multiple presynaptic action potentials are almost always required for a postsynaptic action potential. (As discussed at the end of this page, the multiple action potentials can arrive either from one presynaptic neuron or from multiple presynaptic neurons converging on the same postsynaptic neuron.) Therefore, the sequence of action potentials in the postsynaptic neuron is rarely identical to the sequence of action potentials in the presynaptic neuron.
As a first step, we want to look at what happens at one single synapse. But bear in mind that each individual neuron receives input at hundreds, or even thousands, of synapses.
Refer to the figure to the right and observe two possible types of postsynaptic receptors in the postsynaptic membrane. The first is a ligand gated ion channel which is opened directly by the neurotransmitter. The resulting flow of ions produces a fast postsynaptic potential. This type of postsynaptic potentials only last a few milliseconds. It is the main type we will be discussing.
But observe the second possibility shown. Here the neurotransmitter binds to a seven transmembrane domain receptor, which in turn activates a trimeric G protein. The subunits can skate under the membrane to open ion channels. These postsynaptic potentials last roughly a second and are called slow postsynaptic potentials. These are the type that adrenergic and muscarinic receptors in the autonomic nervous system produce. For example, it is via slow postsynaptic potentials of this sort that sympathetic and parasympathetic neurons speed and slow your heart rate. Also, slow postsynaptic potentials are found in areas of the brain that control levels of alertness, sleep and emotions. Note also that the trimeric G protein subunits potentially could also activate enzymes, such as adenylyl cyclase, that produce further intracellular actions.
Another type of slow postsynaptic receptor, which is not shown in the figure, is the NMDA receptor. It works by a completely different mechanism. It is a ligand gated ion channel. But a small amount of glutamate will not open the channel. This is because normally a Mg++ ion acts as a cork and blocks the channel. The Mg++ only leaves with persistent depolarization of the postsynaptic neuron. Then glutamate will open the channel, and a postsynaptic potential occurs. Thus the signal only is passed across the synapse if the postsynaptic neuron has been persistently depolarized by other postsynaptic receptors. This type of receptor is found, for example, in areas important for memory. The important ion moving through this channel is Ca++.
Let's now look at how the membrane potential changes in the postsynaptic neuron as neurotransmitter is released from the presynaptic terminal. The electrical recording below shows the actual membrane potential in the postsynaptic neuron, as would be measured using a micropipet inserted into the cell body. The dashed line shows the level of the threshold in the postsynaptic neuron.
As the recording begins, you will first see occasional, tiny random potential changes. These are due to single vesicles of neurotransmitter being released spontaneously. The vesicles are so primed for release, that an occasional one will release its neurotransmitter without any depolarization. These show you the effect of just one vesicle of neurotransmitter.
As the recording continues, observe what happens as action potentials arrive in the presynaptic terminal.
Watch the schematic drawing of the presynaptic and postsynaptic neurons as the recording moves along. What does the color yellow indicate? Answer
Only one postsynaptic action potential occurs rather late in the recording. But what happens in the postsynaptic neuron as a result of the earlier releases of neurotransmitter from the presynaptic terminal? Answer
When a single action potential arrives at the presynaptic terminal, after a delay of a little less than a millisecond, a number of vesicles are released almost simultaneously. Since we are assuming this is an excitatory synapse, the resulting depolarization is termed an excitatory postsynaptic potential (EPSP).
Next observe what happens at this synapse when a pair of action potentials arrive. Notice the second postsynaptic potential is a little larger than the first. This is because a few more vesicles of neurotransmitter were released as a result of the second action potential. This illustrates the fact that the amount of neurotransmitter released is not necessarily fixed. Here the occurrence of the first action potential caused more to be released by the second. (This is called "facilitation" or "potentiation". If less is released, the term is "depression".) Some synapses show effects of this type; others do not. There are other phenomena, too, that can modify the number of vesicles of neurotransmitter released.
Next, observe the result of a burst of four presynaptic action potentials. The resulting EPSPs pile on top of one another, adding up to a larger depolarization. This is called summation. But still the depolarization does not reach threshold, and no action potentials occur.
Finally, a burst of five presynaptic action potentials does result in a postsynaptic depolarization that surpasses threshold, and an action potential occurs. Note that a "decision" has been made here. (Of course, only a small portion of the action potential is visible in this figure. The peak of the action potential would be way above the level visible in this figure (roughly four times higher than the membrane potential changes visible here).
Now observed the figure below and DRAW ON A PIECE OF PAPER what you think the expected response would be if somehow the voltage gated Na+ channels in the postsynaptic membrane are blocked. (But the voltage gated Na+ channels in the presynaptic membrane are assumed to remain functional.)
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In the schematic example above, for clarity only one presynaptic terminal is shown on the postsynaptic cell. The example shows summation of postsynaptic potentials because they occur close together in time. In reality, hundreds or even thousands of presynaptic terminals connect with a postsynaptic neuron. The postsynaptic potentials caused by all these presynaptic terminals add together and this is called summation too. But in this case the summation of postsynaptic potentials is spatial, in addition to the summation over time, as shown above. An action potential then occurs if the summation of postsynaptic potentials, for whatever reason, depolarizes the postsynaptic cell past threshold.
In the figure below a slightly more realistic picture of a neuron is shown, with seven presynaptic terminals shown. Some are excitatory and some are inhibitory. But as described above, in an actual neuron there would be hundreds or thousands of presynaptic terminals.