Nervous system signals and their implications, con.

Paul Grobstein's picture

Welcome to the on-line forum associated with the Biology 202 at Bryn Mawr College. Its a way to keep conversations going between course meetings, and to do so in a way that makes our conversations available to other who may in turn have interesting thoughts to contribute to them. You're welcome to post here any thoughts that have arisen during the course this week (and to respond to thoughts others have posted).

Some issues worth further exploring this week are the implications of thinking of the nervous system, and behavior, in terms of action potentials, receptor and generator potentials, and synaptic potentials.  How are we doing on the battery metaphor?  What does this help (or not help) to make better sense of?  What does it seem difficult to account for this way? What new questions does it open up?  

Margaux Kearney's picture

Brain as a computer? a machine?

Last class, Pr.Grobstein left us with a question on whether or not the brain could be compared to a computer. During class we discussed that this was not a good analogy as neurons are not wires. Instead, it is better to state that neurons are in fact the computers as they take complex inputs, analyze them (determines whether or not threshold is reached) and create inputs (action potentials). However, after doing some research online, I came about this site that compares the brain to a computer: http://faculty.washington.edu/chudler/bvc.html. I was perfectly content viewing neurons as the "computer" until I read this site. I can also see how the brain can be compared to a computer as well. What are the summary of observations that show that the brain could be compared to the internet, for example? It's not possible to have the brain and neurons compared to the same analogy.

jwong's picture

The idea of the brain as a

The idea of the brain as a battery is an understandable theory but one which I still have difficulty wanting to fully grasp. It IS because this is not the way one normally views the nervous system, which makes it harder to accept, perhaps because it seems a rather one dimensional view, regardless of emotion or other outside effects. Something as tangible and impermeable to change as my image of the battery is not an effective parallel to the natural, inconsistent ability of the brain to adapt and change. A more effective comparison is to the brain as a computer; while both have their faults, they are able to adapt to changes when certain functions backfire or shut down—there are backup sources of power and focus, the brain consisting of a complex function of neurons and neural connections. In comparison to the battery, the brain is able to reconnect to different areas if it is initially unable to determine a connection.

Jessica Krueger's picture

The Battery Analogy, and why it doesn't hurt my feelings...

While I agree that rehearsing a concept that was tricky enough the first time around twice (or even three times) can be frustrating and confusing, I kind of respect an appreciate what Professor Grobstein was doing here.

The NBS concentrators-forum last week included questions about how we thought researchers choose their topics and form their questions. This got me thinking about why researchers don't chose the topics they don't. It's very easy to remain within the limits of your biases, and even moreso to allow the backgroud truths provided by your language and culture go unquestioned. The Bryn Mawr College Intro Bio culture supports only one "truth" or account of how an action potential travels down an axon. It would be very easy to assume that this event can only be thought of in those terms, which would significantly limit the questions you could approach the topic with or the language you could use to describe it.

In breaking the uniform perception of "this is how neurons work," Professor Grobstein is providing a new approach and asking us to think in a manner to which we are not accustomed. I can relate from personal experience that an invaluable lab skill is the ability to generate your own new approaches to scenarios you thought you understood: thinking about the problem in a manner to which your are not accustomed. By providing two accounts of the same event, he is also reminding us of the loopy, story-telling nature of science. Many of the hard and fast "laws" of science are simply ways of describing what was observed; the formula for velocity is just one way to describe an object moving through space. Just becuase one account was taught first doesn't mean that it's explanatory power is somehow better (or worse). Also, language is imperfect and limited, so the more ways a concept is related, the more likely one is to understand the nuances of the process.

Paul Grobstein's picture

My last word on batteries ... for now

Thanks for your thoughts on this. They reminded me of my own experience "learning" about action potentials (and the related membrane phenomena). Yep, "rehearsing a concept that was tricky enough the first time around twice (or even three times) can be frustrating ....". On the flip side, I didn't in fact "get it" the first time, or the second, or, for that matter, the third. And, deep down inside, I knew I didn't get it.

That feeling of not quite getting it didn't begin to go away until I had heard at least four different versions of the story, and started trying to tell my own. That one drew from commonalities and complementarities in the ones I'd heard but also from thinking about what didn't work (for me) in each of them and about why different people tell stories in different ways.

I am, of course, still working on this particular story. I'll tell it differently in the future because of what I've learned from peoples' reactions to it this time around, both frustrations ("its not a REAL battery") and insights ("it stores energy") that hadn't occurred to me. Yep, "it would be very easy to assume that this event [or any other] can only be thought of" in only one way. But it turns out to be a lot more fun to look for new way of thinking about things. Maybe that's better "science" as well?

Lyndsey C's picture

lots of questions, as usual!

i am surprised to read that my peers are still so concerned with the battery analogy mentioned in class to represent the relationship of an impulse traveling through a medium, or the action potential flowing through an axon. i dont have much of a background in biology so this idea is very comprehensive for me. what im not so clear about is the concept of a resting potential, which we havent covered yet but i am interested to learn more about. resting potential seems to contradict the idea that ions are constantly in motion, and i wonder if it simply means that the sodium potassium pumps are inhibitted so that permeabillity is halted? i also still have my question from earlier which asked why the propogation simulation extendedin both directions when the middle node is pressed. i was under the impression that messages only propogate down the axon?

i also wonder about inputs which are obstructed during the action potential. what consequences arise when inputs are not propogated? is the output different? is there no ouput? what sorts of outside factors can influence the propogation of an action potential or make it less sufficient?

i noticed a lot of people are interested in myelin so to keep the conversation going i had a quick question about problems in myelination and what this could mean for the input-output connection. what happens when there is too much myelin? what happens when there is too little? (i think epilespy is an outcome of decreased amounts of myelination, causing information to be crossed among neurons causing seizures). can the myelin be punctured since it is simply a thin lipid layer? what happens if the myelin does not cover the whole axon?

EB Ver Hoeve's picture

Making the connection

For me, the problem comes in the translation. It seems that other classes tend to examine the initial interaction or the final reaction, but no one has dared to explain the complex processes in between. For instance, Intro Bio freshman were introduced to the concept of the action potential, setting up the solid groundwork for more upper level bio and chem. courses. Perhaps because of this, I never really had a hard time understanding the battery analogy because it had already been “put together” for me. Similarly, Intro to Psychology examined how to interpret and explain social interactions and human behavior. Perhaps because of this, I was very intrigued by our mind vs. brain discussions. However, I feel like with this class (a cross between the biology and the psychology), we have the time and we need to make the commitment to actually discuss the translation. Of course, we have yet to examine the observations that show how a specific action potential causes a specific behavioral change, but I am intrigued and excited for us to make that step. I am also genuinely curious about how the brain/CNS/PNS’s translations are resulting in social behavior. I am especially interested in the boundaries of these translations and the capabilities of the action potentials. So, I am ready to accept the battery metaphor as a foundation prior to moving on to discussions where we will begin to make the real connections.
I.W.'s picture

Why do we need it?

I agree that the battery analogy works better when removed from the literal embodiment of a Duracell battery, but I still don’t think that it is a particularly useful analogy. To begin with the word “battery”, whether we want it to or not, will always be associated with the little metal things that power stuff.  The word itself is just far too loaded.  If a literal battery isn’t what is being discussed then that shouldn’t be what we call it.

Regardless of the wording I think that my main problem with using this analogy is that we don’t need it. The analogy would be useful if we were just learning about the nervous system for the first time, but we aren’t.  We all should have a basic understanding of what an action potential from intro bio.  I find that trying to force myself to understand this analogy just makes me lose sight of what I already know.  I already knew that there was a charge across the membrane, and that is all the battery tells us. Maybe there is some deeper meaning that has not been revealed, but as of right now I just don’t see why we need the battery. 

eambash's picture

Questions of scale

Reading over other's comments, like those by llamprou, it seems as if a lot of us want some way to fit the battery analogy (and more detailed explanations of action potentials and voltage gradients) into the other, equally comprehensive systems and metaphors we've discussed. I too am slightly confused as to how the little details fit into the images. At the same time, though, I think information about how action potentials work has actually helped clarify for me the reason we keep talking about input/output boxes, not just inputs and outputs.

Maybe all this talk of voltage and action is a way of talking about how the input translates to the output -- what happens in between, in the box. We spent so much time a few weeks back just talking about what kinds of arrows might connect one box to another, whether things have to have clear-cut causes or effects, how a big box relates to a little one, how the outside of the body relates to the inside. Gradient change helps me, to an extent, to see an input as something different than just one tiny button that one presses in order to see an output. An entire system is affected when one little element of it changes. The slash in "input/output" describes that entire gradient -- the way an entire stream of molecules interacts and affects things around it.

To the extent that it was confusing for me initially to talk about the nervous system as a series of inputs and outputs because the words "input" and "output" seemed too clear-cut or narrowly defined, I find it extremely useful to talk about changes of whole groups of charged particles. Similarly, I appreciate the idea that every battery is different; that backs up the idea that every input/output box is separate and can function independently -- either inputs and outputs independently of one another or the whole box independently of every other box.

I'm still sort of confused about how each individual battery functions. When we talk about all the different things that can propagate a change (or alter a battery), like diffusion or membrane changes, what's the scale we're talking about? If one thing starts to change, does the process of change, or propagation of it, continue once it gets to the end of the axon? Does change in one battery, such as in one particular axonal membrane, necessarily lead to change in the next axon (the next neuron)? Also, if we're talking about an action potential as the process of changing to a separate battery and then back again, are we saying that there is always a default battery that the axon starts with? Would that default be the same for every battery? I would assume not, but I'm curious as to how we can define the process as always reverting back to its starting point, if the starting point seems to fluctuate as much as the midpoint.

gflaherty's picture

Because we all love that myelin...

I was interested in the role the myelin plays in quickening the pace at which an impulse moves through a neuron.  Because the rate at which signals move through the body seems to be an important factor in determining the overall robustness of a nervous system, I decided to look into the myelin a little bit more.

I found that demyelination is the underlying disorder for such diseases as multiple sclerosis. While researching online, I came across this website http://www.mult-sclerosis.org/demyelination.html.  In this website, it states that a demyelinated axon can transmit nerve impulses ten times slower than a myelinated axon.  In some cases, demyelinated axons can become re-myelinated.  However, due to the nature of demyelination and the tendency for scarring along the axon, the re-myelinated axons transmit signals at a slower pace than a normal myelinated axon. In extreme cases, demyelination also leads to degeneration of the axon.  So, it can be concluded that demyelination, although not always, can be an irreversible event with detrimental results.  Of course, there is a lot of information out there about demyelination and its ties to MS and other neurological disorders.

Madina G.'s picture

What's a right and wrong perception?

I agree largely with both Nelly and Nana on the idea that perceptions are only as good as our words. Nana raises a perticularly valid point that the occurence of evolution encompasses perception and therefore we should indeed expect a variety of this in our population, therefore who's to say that any particular perception is by definition defective? What makes one perception right and another wrong when it could just be that our ability to characterize something is limited by our vocabulary.

So, putting that aside lets evaluate this from another perspective then. What if there really is a right and wrong perception? How far can we go and say that someone may not have the ability to process the "right" perception? What about criminals? Is it their upbringing and environment that causes them to commit crimes or can they take up the same argument and say that there is no true correct perception and so who's to say that what they did was wrong? Of course there are some things in this world that are just wrong, and one can not argue that the reason you call it wrong is because you have a different perception of what to classify as wrong. Then there's the question of where to draw this line. When can we say that there IS a difference between right and wrong and when can we say that its purely based on our own personal perception? One could say that this would be the conscience, but since this differs in each individual how do we account for the "correct" conscience like the "correct" perception? I'm in no way attempting to justify the actions of criminals as I do believe that there clearly is a line between right and wrong although I just thought I'd raise some of these questions.

Sophie F's picture

Behavior and meaning

At the end of Thursday’s class, Professor Grobstein said, in conclusion to our discussion, “Behavior happens unless it is inhibited.” This really struck me because it seems since the age of Freudian psychoanalysis, which long-reigned as the pinnacle of behavioral understanding, there is an impetus, socially, medically, culturally, to describe behavior based upon cause and effect. Why do things happen? What “drives” people to behave in certain ways? Freudian theory would have it that there are two “causes” of behavior, sexual impulses and aggressive impulses ( http://www.rpi.edu/~verwyc/FREUDOH.html ). There must be more to behavior than two driving impulses. But, if behavior is rooted in passive current flow and the random movement of ions, there is something that impels us all to “be” and to “do” that is altered by our inhibition of that behavior. In such a paradigm, behaviors we consider aberrant are so much more easily reconciled given that perhaps certain dysfunctional neuronal mechanisms do not allow certain aspects of behavior to be inhibited by certain people. Superficially, one might think that “meaning” is stripped from behavior if viewed as random and the product of passive current flow. However, behavior (and anything else, really) has only the meaning or significance that we ascribe to it.

As Erich Fromm wrote in Man for Himself, “Man must accept the responsibility for himself and the fact that only by using his own powers can he give meaning to his life. But meaning does not imply certainty; indeed, the quest for certainty blocks the search for meaning Uncertainty is the very condition to impel man to unfold his powers. If he faces the truth without panic he will recognize that there is no meaning to life except the meaning man gives to life by the unfolding of his powers…”

cheffernan's picture

After Thursday’s class, I

After Thursday’s class, I understand the battery analogy much better. I think what helped me to understand it better was once what Prof. Grobstein finally explained what he meant by a battery (as a difference in voltage across a membrane), the rest of the analogy was quick to pick up. I think when the battery metaphor was initially proposed, I looked at it in the same way that a battery (like in a flashlight) worked, and I struggled trying to understand that because there is a rather steady charge maintained within a membrane while the membrane is at rest. For the battery I assumed that there was a positive end that connected to the negative end, and that was how the action potential was carried down the axon.

I agree with Zoe’s comment about how this fits in exactly with what we started with of the input output boxes. It feels as if we have abandoned what we discussed the first weeks of class. I am interested to see how these two analogies will be tied together. I would like to see this before we go into discussing various diseases and their effect on the nervous system.
Emily Alspector's picture

lots of questions

I thought mcrepeau's comments on Tourette Syndrome was another interesting direction to take this conversation. I hope in class we can begin to apply what we've learned with our battery theory and action potentials in helping to explain things that are more commonplace. This will only help further our understandings of what a "battery" is, and I agree with Evan as well that it is difficult to think of a battery as not something that is in our remote controls but as a machine itself. But seeing the whole picture, I think the battery example is definitely up to par in explaining action potentials (Although I'm not sure we covered where the "first pulse" comes from; what are sensory transducers? and how do they become stimulated?). Also, I'd like to talk more about the "leaky region of the membrane", and why leakages allow for signals to start and end in the middle of the system. Moreover, what is "the middle" of the system?

I would also be interesting in talking more about multiple sclerosis and loss of myelin.What are the mechanics involved in replacing lost myelin? Why does MS get worse over time?

After reading "Inverting the Relationship", I find I'm still puzzled as to where "meaning" comes in to all this. To me, "meaning" implies volition, and I don't think our neurons are aware of the pulses and depolarization occurring, much less the motor response that is the eventual outcome. But then, that doesn't necessarily mean the occurrences are "random," because there is a system and a pattern. I'm not sure these two things are opposites, or perhaps even comparable. The opposite of random would be planned, or something synonymous, and the opposite of meaning would be pointless. So can something be both random and have meaning? Or planned and pointless? I think so..

Caitlin Jeschke's picture

MS

Here is a website I found with some basic information about Multiple Sclerosis, including the 4 most common courses that the disease usually takes:

http://www.nationalmssociety.org/site/PageServer?pagename=HOM_ABOUT_what_is_ms

Since MS is thought to be an autoimmune disease, I was surprised to learn that the replacement of myelin does seem to occur in the majority of patients (about 85% experience periods of relapse and remission).  Each demyelination can cause scarring of nerve tissue, which would explain why, even though myelin can be replaced, symptoms of MS tend to get worse over time.

I would also like to talk more about neuron structure (i.e. "what is 'the middle' of the system?") and the process by which action potentials are transmitted from one neuron to the next. 

Additionally, we have not yet discussed ion channels other than the "voltage-gated" type; I remember discussing stretch-gated ion channels in intro bio, and I think that these channels, which are responsible for some reflexes, could help to clear up some of the confusion about "voluntary" v. "involuntary" actions.

anonstudent01's picture

Autism and Input-Independant Action Potentials

This is the topic for my webpaper but I thought it related to well to what we discussed on Thursday. My 3 year old cousin Katie was adopted and spent the first 5 months of her life in an orphanage in rural China. She was diagnosed with autism about six months after my uncle and aunt picked her up, and after communication with the orphanage to try and assess the level of sensory stimulation she received they discovered that Katie had been (like so many other children in these situations) left in a crib in the same room without any toys and was only picked up to have her diaper changed. She had looked at the same walls and ceiling everyday, heard the same sounds and had the same physical contact with her caregivers (essentially no sensory variation, no chances to make summaries of observations etc).

So though we concluded that an action potential can start and end in the middle of a nervous system, I am wondering how that much activity is possible when there have been so few inputs. Katie's inputs were severely limited and thus her development was hindered, so how can the "leaky regions of membrane" idea work in this case when there are so few- if any- sources of stimuli for the brain to work with? Or are these input-independent action potentials the reason why my cousin survived for her first five months?

llamprou's picture

I am also in agreement with

I am also in agreement with Zoe and Evan. It was exceedingly difficult for me to view battery as anything more than a metal cylinder used to make fairly trivial devices such alarm clocks, and calculaters work. I also viewed batteries as something extremely temporary, human beings are replacing them all the time, it is very difficult to do that with even the most trivial 'things' found in the human body. It is also fairly more confusing for me because simultaneously to this class I am embarking on a parallel journey through Intro Bio, and although we just finished understanding action potentials, voltage gated channels and concentration gradients, the word 'battery' was not mentioned once. Interestingly enough however as far back as I could remember my father used the word battery in describing my body, for example he would say "...get to bed Lisa if you don't you will not have enough time to recharge the battery before tomorrow", or "...you need to give your battery time to recharge". Is sleep how humans recharge their batteries? Why do batteries stop functioning?

Finally I completely agree with Zoe in the fact that I am beginning to lose sight of the model we began with. I have almost forgotten why we began comparing action potentials to batteries in the first place. I must also say that I do not have a full understanding of the battery metaphore as of yet, but class on Thursday did enhance my understanding a little bit. I wonder if the battery metaphore could be replaced by something that is a little more human, or is the point of the battery model to remind us that we in fact are just a bag of chemicals and the notion of what was human was invented by the most powerful chemicals in the bag, those residing in our brains?

 

mkhilji's picture

Brain as a computer

This has been a topic that I have considered before and thought about it more from this discussion about the brain as a battery. As Professor Grobstein had thrown out, what about the concept of the brain as a computer. I was searching the internet and came across a website(http://faculty.washington.edu/chudler/bvc.html) that is meant for little kids and discusses the similarities and differences between a machine and the brain. I think we are forgetting that metaphors do not mean that two things that are being compared are necessarily the same. Also it is important to remember as many have mentioned above that in order to understand the battery metaphor we would have to redefine what a battery is.

Going over the similarities between the machine and a computer made me realize that the battery metaphor if explained simply can also apply. For example the brain uses chemical signals to transmit information—which can be related to the electrical signals used through the wires in a computer.

Another similarity is that both a computer and the brain transmit information. A computer is binary as it uses switches or commands that are either on or off. Looking at the brain, the neurons are either on or off by either firing an action potential or not firing an action potential. The difference is that neurons are more than binary, as the “excitability” of a neuron is always changing as the neuron is constantly getting information from other cells through synaptic contacts. Also as pointed out in previous weeks that information or inputs across a synapse does not always result in an action potential (output).

Mahvish Qureshi's picture

Battery

By the end of class on Thursday, I found the comparison between the nerovous system and a battery to be a much more acceptable theory. I think that the difficulty in relating the two, is as evan said above, that it is not hte way we usually think of hte nervous system. It is hard to imagine something tangible like a battery holding hte same basic concept as this untangiable nervous system with its complex array of neurons etc.

Another reason I think that may have contributed to the diffuculty in accepting this metaphor is that in a majority of science classes the nervous system is taught in a fact oriented manner, rather than a more philosophical and simplistic manner.

I think that overall the batter does a good job at explaining hte action in the nervous system

Anna G.'s picture

Okay...

Okay...so I get the idea of the battery is stored energy across a synapse...but...I'm still wondering...why is this a more conducive way to look at it? As an alternative way I can accept, but does it open doors for better understanding action potentials?

 

I like that Angel brought up the fact of outputs that appear to have no input, because I too was on the side that believed that it was there, just separated in time. The fact that random motion in ions can cause depolarization which causes action potentials is interesting, but I have to stick by my gut which tells me that there has been previous input which has modified a structure so that it has a tendency to fire an AP.

 

While not doubting that at times there is random movement that may cause an AP, I think that it is regulated (although I'm not sure how) so that the action potentials that are "supposed to happen" actually propagate.

 

This leads me to the place where I am confused. Action potentials have a purpose. No one would doubt that. Yet, we have also seen that they can be/are purposeless, merely obeying the laws of physics. The line between the two, where intent emerges is what I'd like to understand more of.

 

I just can't get my head around the fact that...I'm sitting here, thinking this...causing action potentials to occur, but...without directing those action potentials. How does the first neuron know to depolarize and send its message?

 

 

Paul Grobstein's picture

do action potentials have a "purpose"?

Maybe it will help to take this apart a bit? Maybe there isn't any "supposed to happen" at the level of action potentials? Its not "at times" that "random movement" underlies an action potential. It always does, since the battery wouldn't exist without random movement of ions across a semi-permeable membrane. THEN the question is when does a permeability change occur. And the answer is ... sometimes because of an input (we'll get to this) and other times just because of a leaky membrane, random movement, and passive current flow.

What's interesting about all this is not that it denies the existence of "purpose" but rather that it says that "purpose" must be accounted for in terms not of neurons and action potentials but rather of assemblies of neurons and action potentials. Moreover, it says that randomness is not antithetical to purpose or meaning but must instead be the underpinning from which purpose or meaning comes into existence (see Evolution/Science: Inverting the Relationship Between Randomness and Meaning).

Yep, "thinking ... causing action potentials to occur." Which implies what about "thinking"? Is it before action potentials, after action potentials, or .... ?

Anna G.'s picture

But...how?

Exactly, I don't believe that action potentials work through any system of purpose or intent, but rather, at some level, our I-function intent can impose it's will on them and cause them to fire. Neurons also have a certain pre-destined form and behavior, evolutionarily and embryonically, which gives them their own characteristics...which can be dubbed "how they are supposed to act."

 

The fact that all action potentials are underlined with random diffusion of particles I can see, but isn't it true that for action potentials to be utilized, a signal comes down which changes the voltage? This doesn’t always happen sporadically, but can have a reason, such as us smelling a certain smell or hearing a sound.

 

I think the missing piece in my understanding is that I don't understand the significant/frequency of simple random depolarization without input that causes action potentials. Do those synapses end up no where? Or does that cause thoughts/behavior that "we" the I-function, didn't mean to cause? How does the I-function send signals to the rest of the brain, and how does the rest of the brain respond? This can't all be random, but I would assume there has to be some sort of pattern.

 

Purpose emerges out of this random movement and behavior, but…how? What are the intermediates that allow us to process random movement and utilize it so that we can act how we do?

 
Jackie Marano's picture

Patterns in Leaking and Firing?

Anna, you bring up a very interesting point. So far, we have discussed that action potentials that fire 'with purpose' from an identified input and those that are fired 'without purpose' from an unidentified input (such as a leaky membrane), are two key factors that influence behavior. I too, was wondering about where randomness and reason fits in.

For example, although the emotions, behavioral patterns, desires, and interests differ from person to person on a very broad basis, why is it that the grand majority of us can recognize the smell of popcorn, or that we all withdraw our hands from a hot object, or that so many of us are bothered by the sound of nails scratching a chalkboard? Which sorts of outputs are a result of leaky membranes, and which are the result of non-leaky membranes? Or, one could think about it the other way; which sorts of inputs are a result of leaky membranes, and which are the result of non-leaky membranes? Do leaky and non-leaky membranes correlate with common and uncommon behaviors in any special way? How could we even go about measuring/quantifying in the first place?

Zoe Fuller-Young's picture

Big Picture?

I agree with Evan's point that it is difficult to communicate what we know and have seen as a battery to what we know and have seen (under a microscope of course) of axons. For me it has been difficult to transform a battery from a small piece of metal, that DIES, to a more abstract definition of positive and negative fields, permeability, concentration gradients, random movement, and CONSTANT movement. Margaux's questions are similar to those that am still puzzling over. How do we recharge our batteries, and why does it seem that there are so many possibilities (myelin) for damage to our action potentials? What, if any, is the influence of excercise on action potentials and recharging?

I do not think that I can say I agree or disagree with the battery metphor, but more that I understand it more or less as we move through the reasoning. I have come to understand it more, but now I find myself forgetting what an action potential is doing. I've become lost in trying to understand the metaphor and have lost sight of the "bigger picture." Therefore, on the one hand the battery analogy has helped me understand an action potential, but my understanding of the system, that is our family of input output boxes, has not progressed.

evanstiegel's picture

battery metaphor

I like the battery metaphor.  I think the reason why a lot of us reject the metaphor is because we have been thinking of batteries as those metal-coated things we but in flashlights.  Then we think, "Wow, it makes no sense that there are a bunch of batteries (in the metal-coated thing sense)throughout our body."  But, if we think of a battery more abstractly as available stored energy in a cell (cell not in the biological sense but in the electrochemical sense), then it is a viable metaphor.  Similarly, it is difficult to think of our arm as a machine because we typically think of machines as those large metal contraptions in factories.  However, if we look a machine more abstractly and see that it is simply a device that modifies force(i.e. less input force over a greater distance will result in a larger output force over a smaller distance) then our arm as a machine is also a viable metaphor. 
Paul Grobstein's picture

metaphors in biology

Yep, arm as machine a "viable metaphor". How about brain as a machine? Or brain as a computer? or brain as a nested set of semi-autonomous input/output boxes? or .... ?
jchung01@brynmawr.edu's picture

batteries???

The fact that the human body is a battery is still a confusing topic for me.  I understand the point that there is a voltage gradient that triggers certain movements and inputs/outputs like a battery, but I think to isolate the brain like a battery is too much of a forced analogy. 

It makes sense that the voltage gradients trigger a current of action potentials throughout a chain of neurons, however, wouldn't this make more sense as the brain being more of an electric circuit than a battery?  The actual energy source isn't from the brain itself, it is from our digestive system....

I hope I am not missing a huge point here...

Angel Desai's picture

Leaky Cables

After class on Thursday and reading some of the above posts, I realize how infinitely precious having a complete, working system/battery really is. If, as mcrepeau ponders,something as seemingly insignificant as a leaky cable really is a potential source of circuitry failures, it amazes me that so many batteries function without malfunctioning. Thinking about the complexity of the nervous system in such a way is a little bit daunting. In some ways, the more observations that are presented in class and through the thoughts of others on the forum, the more I am amazed that things manage to function constantly at all or conversely, the less confident I feel that that the system will continue to function as it has in the past.

On another note, the conclusion we came to at the end of class on Thursday concerning outputs without input finally makes sense to me. When we imagined this scenario a couple of weeks ago, I was on the side of those who believed that perhaps no input really is an input which would mean that its corresponding output was an output with an input (sorry for that garbled mess...I hope this makes sense!) However, we only considered this situation in respect to leaky regions of the membrane (which would cause the membrane to generate an action potential in the absence of input.) I wonder if there are other possibilities that could point to output without input!

Margaux Kearney's picture

Battery cont'd

After Thursday's class, I can definitely relate more to the battery analogy. I understand that the change in permeability is due to the change in protein (change in shape). If the membrane has a less negative field, sodium permeability occurs and depolarizes the membrane and so forth. I can also see how an action potential can be generated in the middle of an axon and how passive current flow goes in both directions. The only thing that prevents me from fulling "embracing" the battery analogy is the fact that I see a battery as an object that needs to be constantly changed. For example, if your flashlight doesn't work, you change the batteries.  What happens to the batteries that are in our body? Are they recharged after every action potential or are these special batteries that never run out of charge? If these batteries need to be "recharged", what is responsible?

Paul Grobstein's picture

rechargeable batteries

That's what the pump is there for. In practice, very few ions move during an action potential. For this reason, there is a quite small effect of action potentials on the concentration gradients that in turn (with selective permeability) constitute the battery. With no new energy supply (a "dead" worm or whatever) you can get lots of action potentials before the concentration gradients are reduced. But yes, over time the concentrations gradients will "run down". Life includes continually re-establishing the concentration gradients/recharging the batteries.
heather's picture

analogies

as much as i like the battery idea and where it's going, i am wary of the use of analogies in general. if we try to explain biological phenomena by tying them to easily relatable situations, we risk oversimplification and incomplete models. i guess i'd just rather tackle the reality of neurological workings.

mcrepeau's picture

Tourette Syndrome and Premonitory Sensory Phenomenon

Like epilepsy, Tourette Syndrome is another interesting phenomenon that involves the random generation of outputs from the nervous system without any understandable input to the nervous system and thus would seem to fit our model of the internal generation of action potentials and through them visible behavior. Tourette syndrome involves the production of both physical (motor) and verbal (phonic) "tics" that occur intermittently and unpredictably out of a background of normal motor activity" 1 having the appearance of "normal behaviors gone wrong."2 However, the cause for Tourette syndrome is largely unknown (i.e. the exact gene that pertains to the susceptibility of the phenomenon has still yet to be identified), although it does tend to be an inheritable trait. The cause of the "tics" themselves (excessive blinking, twitching, bodily jerking, coprolalia--the utterance of social unacceptable words or comments---etc.) are believed to be related to particular cable dysfunctions (i.e. axonal operations) in the thalamus, basal ganglia and the frontal cortex of the brain, in which "models implicate failures in circuits connecting the brain's cortex and subcortex"3. Could these circuit failures possibly be related to leaky circuits i.e. leaky cables?

What's more is that the state of these circuitry failures, these potentially leaky cables, are very plastic and mercurial. The very nature of Tourette’s is that the symptoms are almost always entirely sporadic and unpredictable, often occurring in irregular bouts with great stretches of time taking place in between episodes. Also, in most cases, Tourette’s is a phenomenon that tends to decrease in both number and severity of episodic occurrence as a person ages and often almost disappears entirely by the time a patient reaches adulthood. If we are to suggest that the circuitry failures underlining the visible symptoms of Tourette’s (i.e. the tics) is related to sodium leaky axons than what accounts for the change in the relative leakiness of these cables (i.e. how can they change their state of leakiness and if relative leakiness is plastic is leakiness then not part of the overall design of the nervous system...is Tourette’s just some kind of unregulated phenomenon of inappropriate leakiness?).

 

Also, in relation to Nelly's questions about voluntary behavior versus involuntary neurological phenomenon (the control of our own action potentials) Tourette syndrome also deals with "premonitory sensory phenomenon", in which Tourette's tics are understood as a semi-voluntary or "unvoluntary" (v. involuntary) 3 action, i.e. the "I-function" is conscious of the onset of their implementation and can suppress them to a certain degree the same way we anticipate a sneeze or an itch and, to a limited extent, can exercise some control over these actions. I am curious to known where this idea of the "unvoluntary" action, of the tic that is at once a seemingly random neurological generation beyond our perception, as well as, a semi-cognitive action, falls into our spectrum of "conscious v. unconscious" inputs and outputs, "random action potentials v. coordinated behavior".

 

 

 

1. The Tourette Syndrome Classification Study Group. "Definitions and classification of tic disorders". Arch Neurol. 1993 Oct;50(10):1013–16. PMID 8215958 Archived April 26, 2006.

 

2. Dure LS 4th, DeWolfe J. "Treatment of tics". Adv Neurol. 2006;99:191-96. PMID 16536366

 

3. "Tourette Syndrome". Wikipedia the Free Online Encyclopedia. Wikipedia Foundation. <http://en.wikipedia.org/wiki/Tourette_syndrome>.

Caitlin Jeschke's picture

Changing Pathways?

Your ideas about the seemingly plastic nature of the circuits responsible for producing Tourette episodes remind me of some questions that I posted in last week's forum.  Here is a portion of that post:

"...in class, we touched on the possibility that neuron connections are capable of changing, and that such a change would cause a change in behavior.  Are "random firing" types of disorders caused by changes in neuron connections?  Why is it that some pathways seem to be more prone to change than others?"

It seems to me that the ability of many Tourette patients to better control their episodes as they get older could be an example of new neuron pathways being developed.  I too am having trouble thinking of a way in which an axon that allows sodium ions to leak across its membrane could fix itself.  However, it is possible for the nervous system to counteract these "faulty signals" caused by the random leaking/passive flow of positive charge.  For example, a second signal could be sent via a functioning pathway to cancel out the effects of the first.  This way, the structure of the leaky axons themselves would not need to change.

How might the I-function play a role in all of this?  From what I have experienced, I think that the I-function is one of the areas of the nervous that are most plastic (ex: thought pathways are constantly modified, whereas pathways controlling physical reflexes do not seem to change at all). If, as you suggest, the I-function is aware of the output generated by "faulty signals" in people with Tourette Syndrome (the behavioral tics), then perhaps the I-function is responsible for generating the "opposing" signals to prevent this output.  It seems to me that many symptoms of Tourette Syndrome are behaviors that only the I-function would "want" to stop.  (For example, there is no physical reflex that stops a person from making socially unacceptable comments).  Pathways to stop these behaviors would take some time to develop, which could explain why children have less control over their episodes.  Furthermore, a person is constantly developing thoughts as to which behaviors are socially acceptable and which are not as he/she grows older, and these new thoughts could contribute to an older individual's greater control over the effects of the disorder.

Just some thoughts :)

Simone Shane's picture

Another suggestion?

This is coming from absolutely no founding, but maybe repairing cells cover the leaky areas with myelin?
Caroline Feldman's picture

Sodium channels and epilepsy

Class was interesting on Thursday. We got pretty far in our “battery” discussion. The topic that struck me the most fascinating dealt with leaky regions of the membrane. I explored this on the internet and found an article titled “Leaky Sodium Channel Makes Brain Prone to Seizures”. Christoph Lossin and his team in the Division of Genetic Medicine, Center for Molecular Neurosciences at Vanderbilt constructed sodium channels that are either normal or that contain mutant genetic material from people with GEFS+. What they found was a persistent "leak" of sodium through the mutant channels increasing excitability of the neurons and making them more likely to fire, causing a seizure. According to Gregory L. Barkley, MD, chair-elect of the Epilepsy Foundation's professional advisory board, "This is an exciting demonstration of a mechanism of epilepsy in brain cells. It is important confirming evidence showing how one inherited defect in sodium channels favors abnormal firing patterns in the brain and increases the likelihood of epilepsy. I just thought this was an important finding that maybe we could discuss in class next week.
http://www.epilepsyfoundation.org/epilepsyusa/holden.cfm
Nelly Khaselev's picture

Leaky Regions may cause epilepsy

Yes, I can see how leaky regions may cause seizures...its totally understandable if a part of every cell or almost...has an area that is permeable to sodium regardless of voltage or concentration that action potential would occur randomly and unpredictably causing seizures! However what I don’t understand is how normal people without epilepsy DO NOT just get seizures all the time? Is there something that counter acts the random action potentials produced by the leaky regions?
Simone Shane's picture

More on Leaky Regions

Have we established that typically functioning people have leaky regions? I'm thinking yes. Maybe the reason we don't have seizures is because these regions are sending messages for things we need to always be doing no matter what the stimulus is? For example, breathing and other subconsious bodily processes controlled by the brain? I'd really like to find out where these leaky regions are and what they control and what individual differences might mean for behavior
ptong's picture

Callosotomy vs. New View

I'm actually writing my paper on epilepsy. I think this article is interesting because it has a different approach. My paper mentions a lot about the effects of callosotomy (medical procedure of separating the right and left hemisphere of the brain). To me, I think this research implies a possible cure for epilepsy that does not require surgery. I would be very interesting if in the past 5 years they have found any more evidence or leads to reducing epilepsy non-surgically.
Nelly Khaselev's picture

I dont get the punch line!

On Thursday’s class, we answered all our questions about 'the batter'. What is a battery? How does a battery occur? Change? Propagate? And more...and at the end we generally agreed we are happy with 'the battery' theory to explain action potentials and in return behavior. We discovered how an action potential can start and stop on its own, via leaky regions on the membrane (where the membrane is highly permeably to sodium ions regardless of voltage) and a threshold. So if action potentials can start with no stimulus, then how do we still have controlled behavior – at least somewhat controlled? I just don’t get it… Leaky regions produce unpredictable action potential. However, i'd like to say we do have some predictable behavior. I just can't seem to rap my head around this....
Molly Pieri's picture

So, what *is* it like to be a bat??

Just some thoughts on the significance of perception...

In class on Tuesday, I was asked, as a dyslexic person "what does a page of text look like to you?" since then,I have been struggling to think of a way I could explain my experience as a dyslexic person to you non-dyslexics out there. This is made pretty difficult by the fact that I have nothing to compare by perceptions with- I have never not had dyslexia. That is the answer I gave in class, but looking back, I feel that it is a little bit of a cop-out to just revert to personal relativity. I might as well have answered Paul's question by asking "Well, what does a page of text look like to you?". If there were no way to convey personal perceptions, how would any sort of exchange ever take place between two individuals? It is in the name of this goal that I'm going to try, now, to explain to you what it's like for me to be dyslexic.

In first grade, I was told that in order to tell my left from my right, I could hold out my hands, palms away from me, fingers up and thumbs held horizontally. My forefinger and thumb of my left hand, my teacher told me, would make an L. From that day on until I was about 10 years old, I was convinced I had two left hands. (For the record, I have anatomically normal hands, I just couldn't see the difference between an 'L' and a backwards 'L'.) This story illustrates nicely the difficulty I have distinguishing left from right. It's not just that I don't know which hand is which, but I have a much deeper confusion about this sort of directionality. I really don't see a difference between letters written forwards or backwards, left to right or right to left. (this made b's and d's particularly difficult when I was younger.) I learned tricks to help me get by in elementary school. For instance, I now write with my paper held horizontally so that my actual notes are written in horizontal lines (relative to me)... apparently I don't have the same problem distinguishing up from down as I do with left and right (although, now p's and b's pose the same problem as b's and d's used to).

I read an article in the New York Times several years ago (“A Boy, a Mother, And a Rare Map of Autism’s World”; By Sandra Blakeslee; New York Times; 19 November, 2002... http://query.nytimes.com/gst/fullpage.html?sec=health&res=9404EFDA1130F93AA25752C1A9649C8B63 ) in which an autistic boy named Tito, who cannot speak or socially interacting with others by any normative standard, writes very eloquently about his experiencing of the world as an autistic individual. The article explains that when Tito was 4, he was looking at a cloud when he heard someone talking about bananas. It took him years to realize that bananas and clouds were different. Reading this, I recognize something of my own struggle to illuminate to you how I experience the world. I can give you anecdotes and specifics, but I cannot seem to find a way to impart to you a complete understanding of me reality as a dyslexic person any more than Tito can let me know, really know, what it's like to not be aware of my own body, or distinguish the difference between bananas and clouds. It seems that we are all ultimately mired in our own perceptions of the world, because outside of our perceptions, how are we to conceive of reality?

Okay... I think I've said more than I originally meant to. And I never worked in the subject line. It's referring to an essay by a philosopher named Thomas Nagel entitled "What is it like to be a Bat?". It's a pretty good piece- at least, I had fun reading it. If you would like to take a look, almost all of it is available on google-scholar (link below), the gist of his point is that even though he can imagine several possibilities of a bat's reality, at the end of the day he concludes that he's not a bat, so he doesn't know the answer to the question he has posed. I haven't read any counter-positions to this point, but if any body had any, I'd love to take a look at them...

((((What is it like to be a bat:

http://64.233.179.104/scholar?hl=en&lr=&client=firefox-a&q=cache:7sElIgW9i4QJ:www.clarku.edu/students/philosophyclub/docs/nagel.pdf+

It's a little easier to read if you download it as a pdf... click the link that will appear right at the top of the page in the little google-y box for the pdf version. ))))

Nelly Khaselev's picture

thanks

First I’d like to say I am glad you tried to explain to us “what a text looks like to you” – thanks! Ever since I moved to America, every English teacher I had, had referred me to get tested for dyslexia as well; and every time the test reveals I just have poor spelling skills. I inverted letters, or skipped some if I was not being extremely careful, the word ‘experience’ was and is still never spelled right. Thank god for spell check on word! I read most of the article “what is it like being a bat” and found it very interesting. The idea that our perceptions are only as good as our words and language is important to understand. There is always that feeling you want to convey, but just can’t express in words even after you hastily search through the thesaurus! Building on this, Thomas Nagel claims man cannot know what is it like to be a bat because he is not a bat himself and his perception of what it is like being a bat is only as good as an essay or data could reveal. This is due to being human, and it is our crime of subjectivity that prevents us from really knowing what it is like to be bat. But of course we cannot escape from being human, and with that we cannot escape our curiosity to try to get as close as we can to know what it is like being a bat – or dyslexic for that matter.
nasabere's picture

Perceptions are only as good as our words

"The idea that our perceptions are only as good as our words and language is important to understand."

Nelly, this is such a profound statement; it illuminates the very essence of the human perceptual experience in a way that I hadn't quite processed until now. I want to reflect on this by sharing my own story. I have these pajama shorts that I really like to wear in the springtime. My friends and I always have arguments about its color; I am certain that they're black, others insist that they're navy blue, and still another is sure that they're a dark purple. Assuming that my perception was somehow defective, I eventually conceded and gave into the theory of the majority-- that the shorts were navy blue. To this day, every time I wear something that I think is black they'll ask something like “what color are your pants?" and I'll give an absurd answer like "tickle-me-pink;" we'll all laugh, but I don't answer because I am really unsure of what answer to give and I hate being in the wrong. I find that this is not limited to just my pajama shorts, but to several instances in which the question "what color is ‘x’?" is asked. But what is defective perception anyway? We are constantly evolving, thus it would follow that perception too should evolve and that with its evolution should come a large degree of variation in the way we see and understand our world.

So back to the shorts—I wonder, do my friends and I really see different colors when we look at them? Is our dispute simply due to different understandings of said colors, or am I/you/all of us truly lacking the vocabulary to better describe what we see?

Jessica Varney's picture

I'm so relieved!

It's not just me then!
Our '94 Pontiac Trans Sport is purple. At least, I think it is. The rest of the world tells me that it's blue. You can decide for yourself (though I will admit that in this photograph, it does look bluer to me than usual).

I have had a similar conversation with my high school calculus teacher, who was red-green color blind. He knew that there was a color, "green," and he could identify what he knew was called "green" - but he didn't see this lush color green, and it was hard for him to tell that it wasn't "red." (The wikipedia page for color blindness is actually really interesting, if this is your kind of thing.) I think a possible explanation for why we label different colors is not due to vocabulary but to a slight difference in the way that our brains interpret the signal, in this case wavelength of light, they receive. The "blue" box and the "purple" box are pretty similar, as are in the black and navy blue... in my opinion. I've already admitted that the world is against my judgment of the color of our van, so I may have a bias.

To briefly touch on a conversation happening at the other end of this week's forum, I do want to say that I'm definitely a lot more comfortable with the action potentials as batteries analogy after Thursday's class. Like many of my classmates have already said, once we clarified what we meant by "battery" from the literal battery (small, powers my flashlight, usually associated with an Energizer bunny) that we experience every day, things started to click.

Since what we're dealing with is so abstract, I become confused when I try to relate ideas from class with phenomena that I've experienced. Would an output without an input be something like a muscle twitch, or is it something else? Do we have any idea how often an input occurs without an output or vice versa? It'd be helpful for me if we were to start relating examples to concepts.

Skye Harmony's picture

re: color blindness

In that picture I think it looks greenish-blue! But our neighbor used to have a car that I considered purple and my dad swore was blue, so I know how you feel.

My high school psychology teacher told us her brother is red-green color blind, and he describes the color of peanut butter as green. I don't know if that means he perceives things other people tell him are green in the same way as he perceives the actual color of peanut butter, or what. But I think it's a very interesting idea that we all perceive color slightly differently. I wonder if any of it is cultural; in semantics class we talked about how different languages have different color scales- one language would have two words for slightly different colors, but another language would have only one word that applied to both colors. I wonder if this just shows the struggle of different people to apply labels to concepts that are not actually easily distinguished.

I would also really appreciate examples of what we talk about in class. It seems that we either talk about vague stuff like mind vs. brain without using scientific evidence, or we go into detail about the nervous system, but don't apply it to the questions we have raised when making our model of the nervous system.

Paul Grobstein's picture

on being a bat, or a dyslexic, or an individual human being?

Yep, my thanks too to Molly, both for the effort to explain what things "look like" to her, and for connecting that issue to Nagel's "What it Like to be a Bat?" Maybe the "crime of subjectivity" not only prevents us from knowing what it is to be a bat, and prevents  someone who is dyslexic from knowing what is to be non-dyslexic (and vice versa), but in fact prevents any of us from knowing what it is to be anyone else? And maybe that's .... a good thing rather than a bad one?
randomness