Cellular communication

- [Voiceover] To help us understand how cell to cell communication works, I want you to think back to your good old elementary school days. In particular, I want you to think about the things you'd have to do in order to secretly talk to your friends during class. Here you are sitting in your classroom and you desperately want to tell your friend here something so you write a little note and you pass it to them. >From your hand directly to their hand. They see the note but no one else sees it, it's just for them. But let's say you want to tell your friend across the room something. I don't know about you but when I was in elementary school I used to write notes on pieces of paper, fold it up into a paper airplane, and I had pretty good aim back then so I'd throw my paper airplane over to my friend across some distance and they'd get my message. And again, they'd be the only one who got the message, no one else got my little paper airplane message. If I wanted to tell a few friends something, I might call a little huddle and actually say a few things just to this group of friends here. My voice would cross this small distance between us. Now, let's kick things up a notch. I was, kind of, a rebel in elementary school and sometimes I'd go to the secretary's desk and take over the intercom to say funny things to my friends, maybe to tell them to meet me at the flagpole or on the playground at recess or something. This intercom message that I'd send out, that would go to everyone, it would be broadcasted to the whole school. Those who wanted to come meet me would do that and those who wouldn't, wouldn't. We can think of cells as little people that do really similar things because you might not always think about it but it's really, really important that cells are able to talk to each other. Evolutionarily, cells being able to communicate with each other are a major reason why we're as complex as we are, as human beings. I'll give you examples of when cells might talk to each other as we go along here. How do cells pass notes to each? How do they directly communicate with each other? One way is by, actual, direct contact. Cells have lots of protein stuck into their plasma membranes here that serve a lot of functions. The most important one is for communication. Let's look at a macrophage here, this is a type of white blood cell that's a part of your immune system. When these macrophages see a foreign invader, maybe a little bacteria or a virus, they can ingest it. They can ingest it then they break it down and then they display a little piece of it which is now called an antigen, on their surface. They show it off on their surface with one of these cell surface proteins here. This one in particular is called an MHC II protein. Now, this little antigen here has become the note that they want to pass on, this antigen is the message. Another white blood cell, maybe a helper T cell might come along and then grab hold of this antigen here with one of its cell membrane proteins, in this case a T cell receptor. Just by doing this, the macrophage here managed to pass a message onto the helper T cell here. Now, based on which antigen this is, the T cell can decide whether to start a full-blown immune response. Maybe it'll go off and ring more alarm bells by activating other antibody cells which are called B cells or not, maybe they'll just do nothing. It just depends on what type of note this is. When cells directly touch to communicate, sort of, unsurprisingly this is called direct cell-cell communication or just direct binding. Now, what about our other methods of communication? Cells can also communicate over short distances. This is our paper airplane here. For example, let's look at two neurons. They're in close approximation but one end of a neuron doesn't quite touch the start of the next neuron here, there's a little gap there called the synaptic cleft. What neurons do is they release little signals called neurotransmitters to communicate with each other. Neurotransmitters get released from the end of this neuron and they'll diffuse across this little distance here until they bind onto one of the dendrites of this next neuron. That effectively passes the message on from this neuron to this next neuron. The paper airplane is thrown from here to here. This is called neural communication. Over here, just like calling a huddle, one cell can talk to a small group of cells locally as well. For example, just underneath our skin, let's say the skin inside our nose, we have these immune cells called mast cells. They're really important in mediating allergic reactions, that's why I purposely picked the nose here. Let's say you're allergic to certain pollens and one day in the spring you find yourself walking through the park. Inevitably, you'd breathe in some pollen and then the pollen would go on to attach to these antibodies stuck to our mast cells. What happens as a response to this is that the mast cells release little chemical signals called histamine. Histamine acts as our short-range message. It travels around to cells in the area then let them know that an allergen has been found and lets them know to start preparing for an allergic reaction to take place. This type of communication is called paracrine signaling, paracrine, meaning nearby. Finally, the intercom take over. How do our cells talk to huge groups of cells at once? They do that by endocrine signaling. For example, cells in our Pituitary gland in our brain make a lot of the important hormones in our bodies. Let's say that they're making Growth hormone, GH, to send around to all the cells in the body. Sending his hormone is their form of communication here. They'll create the Growth hormone inside their cell bodies and they'll release them into the bloodstream. Now, our Growth hormone can travel through the bloodstream and get to, literally, any place in the body. Every cell of your body has the opportunity to get this Growth hormone message. It doesn't mean that every cell will, necessarily, respond to the message. Some cells just don't have the right proteins to bind certain hormones, but either way, you can see that this is a long distance form of communication.