in layers two and three of the human cortex, where we have our pyramidal cells and where many neurobiologists believe the complexity that’s going on there is really what makes us uniquely human or makes our brain a uniquely human brain. And so what they discovered is dendritic action potentials that are occurring there, not the typical local potentials that we would expect to see along dendrites and the soma in any part of the nervous system. These are dendritic action potentials.

(02:44):
And so, yeah, they are going to continue to travel and get to the axon we know. But an interesting thing about them is that, unlike those other action potentials that we normally think of, they are graded. And what that means is that they’re not always going to get to a certain peak the way we normally think of an action potential. They could have a lower amplitude or a higher amplitude, depending on the pathway and the circumstances and so on. So that makes it kind of weird. But that’s not the only thing that makes these weird. Number one, they’re in a dendrite and they’re an action potential. Number two, they can be graded. But secondly or thirdly. Boy, I can’t even count. I don’t think my dendritic action potentials are working. Thirdly, what triggers them, the mechanism behind them, maybe is a better way to say that, is the influx of calcium ions. Normally when we’re looking at an axon, what’s the mechanism? It’s the influx of sodium ions that causes that shift in voltage, that becomes the peak of the action potential.

(04:03):
But in dendritic action potentials, what is happening is calcium channels are opening, and calcium influx is what triggers the creation of the instance of a dendritic action potential. And so that’s different. I mean, that’s not highly unusual. We see calcium causing shifts in membrane voltage in muscle cells, for example. So yeah, that’s something that happens in nature, but we didn’t expect it to happen here and under these particular circumstances. So all of that is kind of weird. But the importance of it is something that’s very profound, I think. And that is, it enables processing, decision making at a more complex level and in a simpler or at least a different way than we expected. And so this might be part of the key to understanding the uniqueness of the human brain. Now, who knows, we might end up finding this in other brains somewhere else, but then that would make them complex and in this unique category, I guess, as well.

(05:19):
So how does it do that? How does it fit in and change that computation? Well, first of all, we start with this idea or this model of looking at the brain as if it were a computer. And of course, it’s not a computer. And like any model, that falls apart at certain levels. But it’s a widely used model right now. When we have nerve signals coming to a neuron, they’re going to be coming in by way of synapses on the dendrites and on the soma. That’s where we’re getting signals from other neurons. And those signals, they might poop out before they get to the axon. And if so, then that’s the decision that we get. We shut it off. The switch gets turned off.

(06:08):
And that’s partly how computers work with their little transistors and so on too. They’re like little gates that acted like a gate that turned off the signal. If the signal, coming into the dendrite gets to the axon and triggers those voltage-gated channels there, then we’ll have an action potential. And that will get to the distal end of that axon, and now that’s the switch being turned on. It’s a gateway that allowed the signal to get through. When we use the lingo of computation, we normally think of axons as being able to send AND messages where signals coming in from the dendrite and soma, they are summated and maybe they work together, and they can send the signal forward. That is if signal X and signal Y are sent. Then together, that’s enough to trigger those voltage-gated channels, and the signal gets sent forward.

(07:10):
Or we could have a situation where we have multiple connections to dendrites. And that’s certainly going to happen in these pyramidal cells in the cortex, layers two and three of the cortex. And so we get something called an OR message possible. That’s where if we get a signal from one pathway, then that’s going to be enough to send the signal all the way through to get to the beginning of the axon and trigger that action potential and send it through. But we could also get a sufficient signal from a different pathway. And when that comes in through the dendrite and gets to the beginning of the axon, if that’s sufficient enough, that’s going to trigger the voltage-gated channels. And yes, we’re going to get a signal.

(08:01):
So a shorthand way of saying that is, if we get a signal from X, then the message gets sent along. If we get a message from Y, the message gets sent along. So it’s X or Y. So we can set up situations here. We can set up pathways where you need both X and Y, and then the signal will get sent. Or you could set up a situation where either X or Y could send the signal forward. So that gives us quite a bit of flexibility in how that network is going to work and what kinds of signaling, what kinds of decisions are made, what kind of computations are made.

(08:43):
But in complex computer design, there are other kinds of signal processing that can occur. And one of those is called the X-OR gateway. The X stands for exclusive. So what that means is it’s an exclusive-OR option. So remember or is X or Y, either one is going to trigger it. Exclusive-OR means that X or Y, but the possibility of turning off X or turning off Y. So we’re going to need another kind of signal that’s happening along with the main signal. And that signal is going to tamper or modify whether this signal or that signal, it really can get the message through.

(09:40):
So I realize I’m oversimplifying this, but the idea is that those kinds of computations can happen in the nervous system. We can see them happening in the nervous system, but only if there is a network of neurons that are connected with each other and taking on those various aspects. In other words, that separate signal that is needed is happening in another neuron, and that’s what’s allowing either message X or message Y from getting through. Well, here we just have one pyramidal neuron that’s having its usual typical potentials. Alongside that are these dendritic action potentials, and they’re acting as the modifier. So what we’re saying is, using just single cells, we can perform the complex functions that are associated typically with a network of neurons. So how do you pack more neurons into a smaller space? Well, you arrange it so you don’t need all those neurons. So you have neurons that have these weird dendritic action potentials that allow them to act like multiple neurons, at least in terms of their processing power.

(11:03):
So looking at neurons and networks of neurons as if they were a computer processing system is a good idea in general. But there are neurobiologists who get down to this level of figuring out the ons and offs and modifications of these switches and so on. And what they’re telling us by the discovery of this dendritic action potential and the observations they made of them is that, wow, there are some cells that can do some things that other cells can’t, and that makes them work in a much more complex way than usual. So circling back to what we tell our students, number one, promise me you’re not going to tell your students about this. But if they ask, if the discussion goes down that road, now after having read this research, you’ll be able to say, “Well, yeah, there are exceptions to these basic principles that I’ve been giving you, but they’re in very special cells that do very special things, and possibly exist only in humans.”

Transducer Model of the Brain

Kevin Patton (12:16):
The computer is often used as a model for the human nervous system, particularly the brain and its complex functioning. I think it’s important for us to understand that that model is a recent model. In the early days of neurobiology when we were first beginning to understand that neurons have a role in the nervous system and what that role is and how the nervous system works, in those early days, we often used a telegraph system as a model for the nervous system. If you’re not familiar with a telegraph system, it’s simply a wire stretched across a long distance. And by creating an electrical signal in that wire, you can produce a tap or some other kind of signal at the other end. So basically, you switch it on, switch it on, switch it on, or switch on, off, on, off.

(13:12):
And a guy named Morse came up with a whole code, the Morse code, where you could send telegrams, that is a telegraphic message across the wire where it was just a series of clicks, short clicks and long clicks, and a certain combination can be interpreted at the other end. And so that was used as a model for sending signals in the nervous system. And then as technology got a little more complex and we started building telephone systems, we start using that as the model for the human nervous system. And then as time went by, we realized, well, it’s more complex even than that. And oh, look, we have computers. That’s a good model. And so there are very many aspects of computer function that we’re seeing that there are similar or analogous things happening in the human brain.

(14:06):
But as I pointed out way back in episode 112 when I was talking about the idea of using models and codes for things in A&P to help us understand those things. And really, models are used a lot in science to help scientists understand what it is they’re looking at and form new quests, new questions that is, on what to look at next and fill out and expand those discoveries. So models are analogies that help us understand things. They help us learn things. They help us discover new things. But they’re just models, they’re not the concepts themselves. They’re not the discoveries themselves. So it’s important to always be open to setting aside older models and using new models, or maybe even having several different models that we use to look at several different aspects of whatever it is we’re trying to learn. So there’s that part of it.

(15:08):
But I want to introduce to you a new model of the human nervous system that is probably so wacky, it’s going to turn out to not be anything close to reality, but it kind of gives us the idea that there are other ways of thinking about how the nervous system works. And this comes from an essay written by the psychologist, Robert Epstein, who’s kind of known for stepping outside of the mainstream occasionally and helping us understand things in a different way and think about things in a different way. And here he is not exactly proposing this new model, but setting it out there as something to just hold onto and think about, and not actually apply to the human brain right now, but don’t disregard it because it could lead us down a pathway that we otherwise wouldn’t have gone down if we’re sticking so closely to that computer model.

(16:08):
And what model is it that he’s putting out there? He calls it the transducer model. Now we know what transduction is, right? We talk about that core concept a lot in human anatomy and physiology. We talk about how a chemical signal, for example, a neurotransmitter is transduced to become a signal on the other side of the synapse. In other words, a neurotransmitter crosses the synapse, it hits a receptor, signal transduction occurs in that process of that receptor reaction, and it produces a reaction in the postsynaptic cell. We know that hormones are involved in signal transduction with their target cells and produce some effect in the target cell. And we see sensory transduction happen all the time. We see where different wavelengths of light and frequencies of light, they’re transduced into information that eventually gets to our brain and is interpreted in a certain way. Sounds are transduced by the auditory functions that begin in our ear. We have tastes and smells and stress and other kinds of sensory stimuli that are transduced and become part of our sensory awareness somewhere along the line.

(17:29):
So what Epstein is saying is, what if we thought of the brain as a transducer rather than a computer, a two-way transducer where it’s sending information out and getting information back. So one sort of way that he imagines this is to think of our brain as sort of like a mobile phone where there’s all kinds of things I can do with my mobile phone. I can get all kinds of stored information. And some of that information is actually stored in my phone, but a lot of information I would need to get from outside the phone. I would need to tap into a database somewhere on the internet, for example, and search for that information and find it, and then apply it in whatever way I’m going to apply it.

(18:22):
And he’s saying, maybe we do that. Maybe we send information out of our brain and then bring information back into our brain. And maybe not everything is stored in our brain. Maybe some things are stored in our brain. And you know, neuroscience has yet to explain exactly how all of our memories are stored, the exact mechanisms where they’re stored, where they’re stored, how they’re stored. Now, a lot of it’s been worked out, but not nearly enough to really understand the process. So he’s saying, well, what if we can’t understand the process because we’re limiting ourselves to looking in the brain?

(18:59):
In other words, if we take a mobile phone and look at it, we can’t really explain everything because not all that information resides there. Not all of the processing that our search engine is doing on the internet, that’s not all happening in our phone. A lot of that’s happening somewhere else on a server somewhere. And so the question then is, where are the servers that our brains are tapping into and sending information to and getting information from? And well, that’s a big question, isn’t it? That’s why we’re not adopting this model, are we? No, because we don’t know where that is, or if such a thing exists, such a mechanism exists.

(19:41):
And he throws out there, I don’t know if he really sees it as a serious possibility or just the idea that science has been discovering things beyond our conscious awareness. And he throws out there this idea of other dimensions, which physicists, many physicists at least, will tell you absolutely do exist. There are these other dimensions, maybe parallel dimensions, maybe there’s several possibilities, some of which apparently have been shown to exist in some way or shape or form. Can we completely eliminate the idea that maybe there’s a way that our brain is tapping into that? Maybe there’s some kind of little quantum thing going on where we’re dumping memories into another dimension, and then when we need them, we pull them back out of that other dimension. Maybe some computations or processing or searching or some other kind of algorithm is happening in that other dimension, or a parallel universe, or some sort of shared or networked consciousness, or who knows?

(20:50):
We don’t have to know that part of it yet, not to simply start asking different questions about the function of the brain that go beyond the current ways of thinking. Maybe that’s part of why we biologically have had such a hard time pinning down some of the complexities of human consciousness and the evolution of languages and things like that. So he is sort of just throwing that out there as, what if we think about the brain as a transducer rather than solely as a computer? And I’m thinking that is wacky. I’m not ready to adopt it yet. I bet you aren’t either. But I don’t know. It’s just fun to think about, isn’t it?

Chemical Signals & Signal Transduction

Kevin Patton (21:43):
Chemical signaling and transduction of those chemical signals are core concepts in human anatomy and physiology that show up time and again throughout many different topics in our course. For example, in the nervous system, when we’re talking about chemical synapses, of course, there’s a neurotransmitter that when it hits the postsynaptic membrane, it needs to be transduced. And then we get an effect, we get a result of that signal, we get a reaction to that signal. The same thing happens with hormones. When they hit their target cell, that signal has to be transduced and then we get a change in that target cell. And we see that in the immune system, where various cytokines are sending signals to various target cells, immune system cells, for example. And that signal needs to be transduced. So we see all kinds of signal transduction occurring in various different ways, shapes and forms throughout the human body.

(22:53):
And as with any of the core concepts, these are concepts that we need to help our students understand are repeating, that they do occur in many different areas of the body. So when they show up, we need to find ways to help them recognize those core concepts as concepts they’ve seen before, as mechanisms that they’ve seen operating before, but in maybe a very different context. And by doing that, they’re going to understand both contexts, the story that’s unfolding in both contexts much more deeply than they would have if we keep them separate.

(23:35):
So that’s, I think, part of our role as instructors, is to be connectors, to help the students connect those core concepts. So I think that should be a goal of ours. But also, we need to make sure that our students know that that’s their goal too, is to connect those concepts, is to start looking for those connections themselves. And I’ve talked about various specific strategies such as running concept lists, for example, and concept mapping, that students can help themselves do that and see those connections. And we can help our students do that by modeling those strategies ourselves in our teaching and in our discussions and in our advice that we give students in how to study and how to learn anatomy and physiology.

(24:30):
I also want to point out, at this moment, that transition that we make from nervous system function to endocrine function in our courses, and I realize that not everyone teaches the topics in exactly the same sequence. There’s sort of a common sequence that many of us use, but a lot of us modify it from time to time. So not everyone does it this way. But in many A&P courses, a discussion of the nervous system and learning about the nervous system is then immediately followed by endocrine concepts and understanding endocrine hormonal signaling.

(25:10):
And one of the benefits of that is that we can take this signaling process in one context that is the nervous system, and transport many of those core concepts right over to the discussion of endocrine regulation. And that transition period where we’re just beginning that discussion of the endocrine system, having so recently explored the nervous system, that’s a good place to stop and talk about connecting the core concepts across topics, and use this as an example of, look at nerve signaling, look at hormonal signaling. There are some differences, but there are some similarities too. They’re basically the same concepts, but they’re going to play out in different ways and in different contexts between the two. And it’s not just the concept of signaling, but the structures that are used in signaling. There’s some similarity too.

(26:17):
And let’s start there, with structures. Something that I always point out to my students is there are some, what we call endocrine structures that are producing chemical signals that are hormones. I mean, they literally are hormones because they’re acting like a hormone. And that’s our definition. It’s a functional definition of a hormone, is a secreting molecule, secreted into the bloodstream to have an effect outside of the tissue that produced it. When we are looking at the neurohypophysis and we’re looking at the adrenal medulla, and we look at the pineal gland, and I know the other pronunciation is pineal, but I like pineal because it reminds me of a pine cone, and that’s what it’s named after.

(27:03):
So all of those glands are really, in a way, modified neurons. They’re secreting what would otherwise be considered a neurotransmitter, except that there’s no synapse there. There’s no postsynaptic cell right there. So they diffuse into the bloodstream, and then they find that postsynaptic cell, which is not really postsynaptic because it’s not part of a synapse, it’s the target cell. So that’s what replaces the postsynaptic cell, is the target cell. But it’s still signal transduction and still works the same way of that other chemical signal transduction mechanisms work. At least in general, it works the same way. So we can see that there is a connection right away between those two systems. Conceptually, at least, there’s a connection because some glands really are just modified presynaptic neurons.

(28:04):
But let’s go to the functional side of things because there, I think, we can even more clearly see the connection, the conceptual connection between nervous system regulation and endocrine regulation. And by doing that, I think our students can more easily identify the different kinds of regulation and how the nervous system and endocrine system are complimentary systems in terms of regulation. One similarity we can point out is in the nervous system, it’s all about regulating effectors. And one of the general goals is to help maintain homeostasis. And in the endocrine function, we see that it’s regulation of effectors to maintain homeostasis, it’s the same. So they have the same overall function. I mean, we’re oversimplifying the function, of course, but we’re at the beginning of the story, so we want to do that.

(29:05):
Basically what we’re doing is regulating effectors throughout the body. Which one is it, endocrine or nervous system doing that? It’s both of them doing that, but they’re going to be doing it in different ways. When we look at the endocrine system, we see that, well, there are regulatory feedback loops that operate, yeah. And we can call those endocrine reflexes if we want, those feedback loops. And then we look at the nervous system and we see, yeah, there are regulatory feedback loops that operate there too. And we might call those nerve reflexes when those occur. So yeah, there’s some similarity there too, at least in the way the information can be processed in both those systems. And then we look at the endocrine system. It has effector tissues, those we call either endocrine effectors or target cells. Virtually all tissues are target cells of one or more hormones.

(30:03):
Then we look at the nervous system and we see, well, there’s all kinds of nervous effectors, and they include mostly muscles and glandular tissue, but we know there’s some others like adipose tissue and so on, but they need to be connected to the system. So the endocrine system, these endocrine effectors and these target cells throughout the body and the nervous system, well, you got to be hooked up to a synapse. You got to be in the right place at the right time. So we’re going to see postsynaptic cells being sort of the target cells of the endocrine system or equivalent to the target cells in the endocrine system. So there’s a similarity there too.

(30:51):
I always use a table, by the way, that compares the two, and we kind of go through that as a group, and I sort of unveil the different rows of the table as we go through so we can kind of predict and discuss them as we go through, so that it’s not just reading through some facts, but actually figuring out the facts. And then like, oh, yes, we were right. Sort of like in Jeopardy, oh, I guessed right. Another thing that we can look at is the chemical messenger itself. In both places you have a chemical messenger, right? In the endocrine system, the chemical messengers are called hormones. And in the nervous system, the chemical messengers are called neurotransmitters. So in that way, they’re alike. And we have, in both systems, cells that secrete the chemical messenger. So those would be either the glandular epithelial cells or the neurosecretory cells, if we’re talking about the endocrine system. And if we’re talking about the nervous system, those are the neurons, the presynaptic neurons.

(31:53):
And then here’s a big difference between the two. If we look at the distance traveled by that chemical messenger and the method of travel of that chemical messenger, now we see some big differences. In the endocrine system, it’s a long distance that’s traveled by the hormone, and the method is by way of the circulating blood. The nervous system on the other hand, has a short distance for that chemical messenger to travel. That neurotransmitter goes across a microscopic synapse. And so that’s diffusion and it’s a short distance. So there’s a contrast there between the two.

(32:36):
And then we look at, well, where are the receptors located in the effector cell? Well, in the endocrine system, on the plasma membrane or within the cell is where we’re going to be finding those receptors. And it’s pretty similar in the nervous system. It’s just on the plasma membrane, we’re not going to be receiving the neurotransmitters inside the postsynaptic cell, even though that’s an option in the endocrine system. So again, we’re back to really pretty similar scenarios or strategies or mechanisms that are operating there. And then we look at the characteristics of regulatory effects. And in the nervous system, the students would’ve learned that the effects usually appear rapidly, but they’re short-lived. Unless we keep them going, they don’t last very long. And now that’s in general, of course.

(33:34):
And then we look at the endocrine system and we say, well, it takes longer for them to appear because they have to get into the blood supply, they have to circulate around, finally hit their target cell. So it just physically is a longer distance. So yeah, it’s going to take longer for the regulatory effects to appear, but because the hormones kind of linger for a while and they don’t all hit the target cells right away, and they kind of go back around again and come back. And yeah, there might still be some left in the blood there, they’re prolonging that effect.

(34:07):
Now, within that group, when we look at hormones, we see that some produce effects sooner than others, some last longer than others. So yeah, there’s a range of activity within that. But when you’re just looking at a very simple level and comparing endocrine versus nervous, we can say that endocrine is slow, but long-lasting. And nervous system effects, they appear rapidly, and they’re short-lived. That’s just kind of an approach. It may or may not fit into the way you do things in your course, but I think that’s a good opportunity for us to not only make that transition from nervous system to endocrine, but in doing so, look at some of these core concepts of chemical signaling and signal transduction and how they operate similarly in the two different systems, and how they also sometimes play out differently in the two different systems.

Staying Connected

Kevin Patton (35:09):
Well, let’s see. In episode 139, I put on my thinking cap and shared some ideas about how we can help our students understand the core concepts of chemical signaling and signal transduction in different contexts. And before that, I briefly described the transducer model of brain function, a wacky new idea that psychologist, Robert Epstein, thinks may help us work out where we’re stuck in understanding how the nervous system works, and especially the brain, in those complex ways that are uniquely human. And we started this episode with a new discovery and nerve signaling in the brain called dendritic action potentials that involve the calcium mechanism instead of the sodium mechanism, and may help explain the complexity of human cortical function.

(36:13):
Now, as always, I have links for you for all these topics. If you don’t see links in your podcast player, go to the episode page at theAPprofessor.org/139. And while you’re there, you can claim your digital credential for listening to this episode. And don’t forget to call in. Please call in with your anecdotes, tips, and questions at the podcast hotline. That’s 1-833-LION-DEN, or 1-833-546-6336. Or send a recording or a written message to podcast@theAPprofessor.org. I’ll see you down the road.

Aileen Park (37:06):
The A&P Professor is hosted by Dr. Kevin Patton, an award-winning professor and textbook author in human anatomy and physiology.

Kevin Patton (37:20):
Listening to this episode may cause permanent changes in the brain.