Nervous system signals and their implications

I'm rather glad to be discussing the idea of the action potential and the ways that signals and responses actually interact.

I've had some trouble picturing the nervous system in terms of analogies (since, as a humanities major, I tend to start from details and move towards an image, not the other way around). However, when I compare this analogy to all the other things we've been talking about and the other images we've looked at -- the leech nervous system's independent responses, the anatomy of the brain, the parts of a neuron -- I do find it quite useful. I have been wanting a way to account for the fact that there seems to be a gradient of responses and levels of response. Sometimes reactions to stimuli don't seem that great, and sometimes things happen without any stimuli but seem much more obvious externally.

I appreciate rdelacruz's concept of a line of batteries connected to one another. To me, it's very helpful to picture multiple batteries, since, even with the I-function, I still tend to picture neurons as independent input/output boxes and to forget to look at the box that boxes them ALL in. At any rate, what I want most right now is a clearer explanation of how or why signals can start from inside the nervous system. Since all the neurons are connected to some degree, can we not still say that the outside (stimuli) has some effect?

I'm so perplexed; my confusion lies in the notion of zero input yielding outputs. It would seem that an action potential, which ultimately yields some response, can only by initiated by a depolarization in the axon membrane. Maybe I’m missing something major here, but shouldn’t it follow that something—perhaps some input—must be present in order to generate a change in membrane permeability, and thus a subsequent output? I guess what I’m really asking is this; when, where, and how does the whole process start if such a phenomenon as “zero input” exists? I’m having a difficult time understanding how a membrane at resting potential can spontaneously alter its permeability and generate action potentials without any sort of input. For this reason, I’m inclined to think of nervous system communication as a series of ever-perpetuating action potentials, where at no instance can an action potential arise without some sort of stimulus—even when it appears that “zero input” yields an output, an input may indeed present.

Someone please enlighten me...

Like most of my other classmates, I also sturggled with trying to understand action potentials as a battery and not thinking of them in terms of electricity. Zoe made a good point that batteries provide electricity so it is hard to not associate the two, but my struggle in understanding comes from my inability to forget what I have previously learned in other courses. It makes sense considering how Professor Grobstein asked us to question all truth, to question what we already learned as truth as well.

I understand trying to relate such a complicated system as the nervous system to something with which we are all familiar with, in this case the battery and previously with the Input/Output Box Model. While at the same time, I still find it hard to try to form a model with the same processes. For example the movement of ions, it is hard to replicate the electromagnetic-concentration gradient established along the axon. I am interested to see how the battery self stimulating will be related to the "I function" and look forward to finding this out on Tuesday.

I’ve thought about this a lot and read the thoughtful comments of the people who posted before me. I am still having some trouble conceptualizing the way in which this battery-operated system works.

So, back to the idea of random movement of ions… It makes sense that random movement occurs, based upon Professor Grobstein’s explanation of ion concentration within and outside a neuron being a statistical phenomenon. Extrapolating the idea of randomness from the microcosm of signal transduction to the effect of that random movement, namely outputs without inputs, for me begs a question: what sorts of outputs result from this random movement? Are they visible (to an observer) behavioral outputs? Are they autonomic processes of which we are not aware that mediate physiological processes? If no two brains are alike, and each brain capable of myriad cables connected to boxes, connected to other boxes… are some organisms more susceptible or likely to experience outputs without inputs? Is there some sort of “potential” energy stored that initiates the turning “on” of the battery?

The battery analogy, I feel, is both great at explaining particular aspects of action potentials yet lacking in other areas. There's no better way to imagine the electrical gradient across the membrane that provides the means by which a depolarizing effect can take place, than to picture a battery, the voltage source in a complete circuit. This is all very good for the purpose of simplifying the mechanism by which action potentials work in order to understand it more clearly, however it is difficult to refer to this model to account for other properties belonging to action potential propagation. One of the most notable factors in this process is myelin, a component of the neuron required for rapid propagation and is required for effective conduction of electrical signals. I don't recall an analogous insulating feature in the battery analogy that makes the effective propagation of action potentials possible, although I may be wrong.

On a different note, I found the hearing thunder and seeing lightening concept Prof. Grobstein brought up really intriguing although I would like to hear more clarification on that topic. Can the idea of proper wiring, that is the correct connection of input and output boxes when all of the action potentials throughout the body are the same, explain some disorders such as dyslexia, where the input results in a different output from those who suffer from the disorder? If so, since this can be remediated to a certain extent via therapy and training, is this an actual rewiring of the input-output connection?

These are just some thoughts from Tuesday’s class, in no particular arrangement:

The idea that the mind does not have complete control over the body: Why does the mind need to feel like it is the center?

Why do we say that what we think takes precedence over the sensations that we feel?

Do we keep thinking above physical sensations because it helps to separate us from other animals (because we cannot enquire of them whether they are conscious or not, although if consciousness comes from the neo-cortex, then we are no longer separated from all other animals)?  Do we need to be separate from other animals so that we can survive (that is, so that we don’t over think moral implications of killing them for food…)?

 

 

I found it very interesting that when asked whether they would prefer to have a conscience (as opposed to not having a conscience) most of the class raised their hands.  I would be interested in hearing some reasons why.  My thoughts on the matter are that it doesn’t matter either way.  If you have a conscience, you might miss it if you knew that you had to live without it; on the other hand, if you don’t know what you are missing…  I guess that the way that I have expressed my view here illuminates an assumption I seem to have: that having a conscience is somehow better than not having one.  I think perhaps this is because I associate having a conscience or consciousness with meaning, and so gives life a significance, which perhaps is only necessary for those organisms with an “I-function” (if the “I-function” truly serves the purpose that we discussed in class associated with conscience); I assume that it is not a concern to those without because it never comes up in thinking. 

Where topographic organization fits into this discussion?

 

If sensation is the product of a pattern of outputs, but this pattern is the result of several identitcal action potentials traveling along cables at a measurable (and relatively slow) rate of speed, would topographic organization affect how this information is perceived?

Olfaction is considered one of the most "direct" senses in that it does not pass through the thalamus during processing (1). Yet we humans seem to dedicate so little attention to olfaction that we don't even have a very profound or insightful vocabulary to describe scents, (2) with one text even describing the sense of smell as "much less essential than vision or audition..." (3). Our primary sensations are routed through the thalamus, meaning it takes more cabling, and conceivably more time, to perceive what we see or what we hear. What does this mean for our experience of the world? Why do the things we see and smell coincide? Why don't we smell well in advance of seeing things in front of us?

1. Olfaction and the Brain, Brewer Castle & Pantelis

http://books.google.com/books?id=UM_A_FDe8lkC&printsec=frontcover&source=gbs_summary_r#PPA70,M1

"Encoding of olfactory stimuli in relation to semantic processing and affect."

2. Physiology of Behavior, Carlson

Chapter 7 Audition, the Body Senses, and the Chemical Senses: Olfaction

Pg 238

3. Olfactory Pathways and the Limbic System

www.neuroanatomy.wisc.edu/coursebook/neuro3(2).pdf

The first rule of thermodynamics is that "energy can neither be created nor destroyed", but is instead continuously reallocated, to the ground, to the air, from one organism to another, from one atom to another; all life, it would thus seem, is one giant game of pass the potato, the "god-machine" a giant perpetual motion device of which all materials of existence are apart. This is true of all energy whether chemical, or electrical. We have spent a good deal of time discussing what an Action Potential is like and how one can manipulate its propagation, and pondering how one identical signal can produce an infinite number of outcomes. In class we have discussed how an action potential can be "passed" on from one battery to another, but we have not yet discussed where the action potential (the very first one in the line of propagation), in terms of the energy that it is, comes from. What initiates the first action potential in an axon (changes in membrane potential due to the presence of neurotransmitters?--what of the very first axon to generate the action potential---how do the sensory neurons in the eye for example generate an stimulus induced action potential---what in the stimulus triggers the depolarization of the cell?), where does this source of energy come from. It must already be there somewhere in the cell and in the body, from the food we eat to the cells perhaps, but in other instances, such as the beginning of embryonic development the initial depolarization that sets the whole process of creating you (the first spark of energy taken in--the first change in cellular potential) does not hail from a dietary source, or even from a nervous system, but from the depolarization and change in membrane potential of an egg cell by a sperm cell---are we then, when philosophizing in terms of action potential = energy, somehow continuously propagating these initial potential changes throughout our lives, are we then (since the catalysis for these energy changes came from our parents) perpetuating an action potential of some kind from our parents, from our ancestors?

While at first the analogy of an action potential to a battery, was confusing and a bit of a strange concept for me. I have learnt about action potentials, etc. prevously from other bio classes and have used batteries attached to a voltmeter to measure the voltage between two terminals, I have never thought of the action potential as a battery.

However as Thursday's class progressed I began to understand the connection between a battery and an action potential much more. I think it is amazing how similar the two methods of transferring energy are so similar.

I think that thinking of the nervous system and behavior with these new terms in our vocabulary it is a bit more plausible to explain the phenomenon of an input with out a visible output. If the signal does not generate a strong enough action potential then there will not be a stimulation throughout the nervous system to generate a response/output. Similar to this a battery can die out if the concentration at an annode or cathode becomes to high and inhibits the chemical reactions from happening. If the chemical reaction is inhibited there is no response and if the action potential s not generated strongly there is not out put.

 

Like Jackie, I found the idea of action potentials being like a battery interesting. While the idea makes sense to me, I don't necessarily understand the need for the analogy, and am looking forward to seeing where this goes.

 

If we accept this analogy, we have to look at what this means in respect to the whole system. If the system is simply a flow “like" an electric circuit, we don't have a way yet to explain how the action potentials are propagated, and how each neuron speaks to the next.

 

I really liked Jackie's "proof" for the battery idea, by discussing the Intro Bio lab. I didn't think about it that way, but it's true. Using a naked nerve cord that was "dead" but physiologically active still, is similar to a battery that you can just remove. This way of utilizing the analogy shows that the signals, arguably one of the lowest levels of work in brain, are nothing more than a mechanical/electrical system.

 

I think that an important thing to remember when were talking about action potentials all being the same is that sensory organs are made up of specialized cells. So that while action potentials may all be the same, the cells that are receiving them take that input and translate them to the i-function in different ways.

 

I found our Thursday discussion about action potentials and batteries to be very interesting. What we discussed on the scientific level complements most of what I learned in Intro Bio, but the approach is certainly different. While we certainly covered concentration gradients and the transfer of a message via electric actions between sodium and potassium ions in Intro Bio, we never related such a critical biological process to the function of a battery. I decided to do some further searching to see if there was additional information about action potentials behaving as batteries, and this is what I found:

Wikipedia: "Cell membranes that contain ion channels can be modeled as RC circuits to better understand the propagation of action potentials in biological membranes. In such a circuit, the resistor represents the membrane's ion channels, while the capacitor models the insulating lipid membrane. Variable resistors are used for voltage-gated ion channels, as their resistance changes with voltage. A fixed resistor represents the potassium leak channels that maintain the membrane's resting potential. The sodium and potassium gradients across the membrane are modeled as voltage sources (batteries)"

I think this further details the relationship between action potentials and batteries, and thus strengthens our argument. Additionally, a major contributor to the validity of our battery model is, in my opinion, the fact that action potentials can still be produced/measured for some amount of time AFTER the system/organism has died. Clear evidence of this is that students in BMC Intro Bio lab collected action potential data from nerve cords completely cut/removed from earthworms (that died after this 'nerve surgery'). Perhaps the best analogy I can think of is that even when the circuit within a flashlight/battery-powered device ceases to work (for reasons other than dead batteries), one can STILL remove the batteries and use their potential energy to power another device!