You began to learn about synapses back in the section on muscle... the neuromuscular junction is just one particular synapse. Here you'll gain some more information about synapses, because neurons can talk to other neurons through synapses.
Here's a figure that I took from a web page in another language, and changed the labels into English. In this figure you see a neuron that receives information through synapses from other neurons. The neuron that receives the information is the postsynaptic neuron. Some of the synapses onto this neuron are onto its dendrites. Other synapses onto this neuron are onto its cell body. This one neuron can receive thousands of synapses! The neurons that synapse onto this one neuron are presynaptic neurons.
The presynaptic neurons end at their axonal terminals in swellings called either end knobs or boutons (or even "buttons"). These boutons contain many vesicles filled with chemical neurotransmitter. The one neurotransmitter that you have already learned about at the NMJ is acetylcholine.
You have learned that when the action potential reaches the end of the presynaptic axon, it enters the axonal terminals (or boutons) and causes neurotransmitter to be released. This neurotransmitter affects the postsynaptic membrane after binding to postsynaptic neurotransmitter receptors. I will now give you a few more steps to this process, and I hope a better understanding of it, too.
Here are the steps required for information to cross a synapse:
Try to use this figure to walk you through these steps... they are much easier to understand if you find them in the picture. (this one was also taken from that foreign web page and then modified)1. action potential invades presynaptic bouton, causing the whole bouton to be depolarized
2. depolarization triggers the opening of calcium voltage-gated channels
3. calcium ions flood in (they are in higher concentration in the extracellular solution)
4. calcium ions interact with the synaptic vesicles, allowing for their exocytosis
5. neurotransmitter spills out into the synaptic cleft (exocytosis released it)
6. neurotransmitter diffuses across the synaptic cleft
7. neurotransmitter interacts with receptors at the postsynaptic membrane
8. receptors tend to be ligand-gated channels, so they open when neurotransmitter binds
9. ions flow through the ligand-gated channels, causing a change in the postsynaptic membrane potential
Most of these points were ones you learned already as you studied the NMJ. You shouldn't have to memorize each step number; I only numbered them to help guide you through. The only new ideas are about calcium entering and the ligand-gated channel that is also the postsynaptic receptor. (Also, the last step listed above, #9, is the subject for another web page, on postsynaptic membrane potentials.) So, let's just consider the calcium and ligand-gated channel topics for now...
Keep in mind that the presynaptic and postsynaptic neurons are separate entities. We cannot simply send an electrical signal from one directly into the other. Synapses are breaks between cells, or spaces, where chemicals (neurotransmitters) are used to relate the electrical information from one neuron to the other.
How do we transfer the presynaptic electrical information into chemical information? And then how do we transfer the chemical information back into a postsynaptic electrical event? Let's answer these two questions briefly, and then in more detail:
How do we transfer the presynaptic electrical information into chemical information?
Simply-- use the electrical information to open voltage-gated channels... then allow a signalling molecule, calcium ions, to pass through those channels.
When the action potential invades the axonal terminal, that has to get converted into a chemical signal-- the release of neurotransmitter. How is that electrical information translated into chemical information? We did it before in muscle... remember? We had to translate the action potential on the sarcolemma to chemical movement (sliding filaments), and there we were able to use calcium ions as our intermediate. Here, in the synapse, we do the same.
The action potential depolarizes the entire presynaptic terminal. That depolarization opens special voltage-gated calcium ion channels. Once open, calcium diffuses along its concentration gradient (into the cell). Within the cell, it interacts with the synaptic vesicles containing neurotransmitter. It is the interaction of calcium ions with the synaptic vesicles that enables exocytosis (before the calcium entered, the vesicles were held firmly in place). Once exocytosis occurs, the neurotransmitter can cross the synaptic cleft.
How do we transfer the chemical information back into a postsynaptic electrical event?
All you have to do is include a ligand-gated channel that opens in response to the chemical signal... once it opens, ions can flow through and begin a new electrical event.
Most neurotransmitter receptors are more than just receptors-- instead, they are ligand-gated channels. Their ligand is the neurotransmitter. So once the neurotransmitter binds to the receptor, the ligand has bound to the channel and the channel can open.
If the channel that opens allows sodium ions to cross, that will cause a new depolarization at the postsynaptic membrane. This is the topic for the web page on postsynaptic potentials.
For some more information on this topic, as well as the others from this week (and resting potentials from last week), head on over to this powerpoint presentation on the web... to just look at the information on the synapse, scroll down the slide list until you come to the first slide entitled "The Synapse." This is slide #35 (of 49 total slides). Slide #36 has the figure below on it. This is the same stuff I talked about, but just done slightly differently.
There are many, many choices for neurotransmitters, and also there is more information for me to give you on postsynaptic potentials. Therefore, those two topics are treated in separate web pages.
© 2011 STCC Foundation Press