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An ongoing conversation on brain and behavior, associated with Biology 202, spring, 1998, at Bryn Mawr College. Student responses to weekly lecture/discussions and topics.

TOPIC 6 (a choice):

The "I-function" reared its head at the beginning of the course and again as we were talked about the output side of the nervous system. Discuss the extent to which this now appears to be a necessary concept for understanding the nervous system and a useful concept for understanding behavior.

Negative feedback loops were discussed as both an important concept in understanding motor control and as a useful concept for thinking about behavior more generally. Discuss the basic ideas involved in this concept and its usefulness for understanding behavior generally.

Name: Dena Bodian
Subject: Heisenberg
Date: Mon Mar 16 20:13:11 EST 1998
Thanks to Daniel's comments, I was thinking about Heisenberg: how the observed changes with the observer and how this relates back to the study of crayfish nervous systems, which we remove to observe the organism's most basic responses. However, how are we to be sure that in the process, we aren't somehow altering the experiment? I am reminded of a discussion I once had with Amber about her series of frog experiments: she was trying to divine how a specific brain alteration changed the way the frog saw and reacted (if at all) to visual stimulus. But frog brains are tiny: one imprecise movement, and the experiment goes awry. Also -- animals, like people, are all slightly varied. Things affect organisms differently. Can we really be so sure (in studying a few organisms, probably genetically altered -- both because lab animals are farmed, and so as to understand its background) what makes a particular species "click"? One last question: in observing the choices an isolated nervous system makes, how do we know that the actions we see weren't chosen solely because we have disabled the organism's ability to do anything else?
Name: Donald Ball
Subject: Bidirectionality
Date: Mon Mar 16 21:10:11 EST 1998
I wanted to take a step back and make a few comments on the concept of bidirectionality discussed in class a few weeks ago. This is a particularly useful concept for explaining the feedback nature of the nervous system and the potential influences on the development of human behavior. Reality to the nervous system is a composite of sensory input, central pattern generators and corollary discharge signals. When an action is taken in reponse to something in the outside world that action, or output, was not only generated by the nervous system but the output itself is also used by the nervous system as self-correcting input.

Take the following situation: I lift my arm to ward off the blow of an attacker. The flow chart of activity that courses through the nervous system to make this happen and the consequence of that action to the nervous system is incredibly complex. A simple model based on what we have discussed in class would be, sensory input (visual, tactile etc.) suggests that moving the arm to defend the self is imminent, the central pattern generators in the spinal cord initiate the cascade of motor neuron activity that accompanies this action, the corollary discharge signals associated with this action record that this action is occuring and relays the information in a constant feedback loop about arm position and activity to the brain. In this way the nervous system keeps constant tabs on its activity and allows for a change in activity based on new sensory input. The "self" learns to be more adept at a particular activity through repetition because the activity of the nervous system is recorded in a large data base known as the brain (CDS). This past reference allows for the improvement of the bodies response, and perhaps, in the efficiency with which that activity takes place.

I know that this is more or less a reiteration of the basic principles mentioned in class but I wanted to underscore how useful this concept of bidirectionality is in explaining behavior, learned or otherwise. At this point in the semester it seems almost intuitive that the nervous system should work based on this feedback model of operations but I can't say that the subtleties of the operating system and what they allow to happen (learning) were so obvious to me at first. As a teacher and a coach I find this to be pretty cool stuff....

Name: Julia Johnson
Username: jfjohnso
Subject: Eyes, Ears, the Brain and I
Date: Thu Mar 19 14:28:25 EST 1998

I have been thinking recently about the relationship between seeing and hearing and the role of the I-function in both. In particular, I have been mulling over the often cited example of people who become blind and who develop an increased capacity for hearing and discerning different sounds. I am assuming here that these changes in auditory capability are gradual and develop either as sight is gradually lost or over time after a blindness-causing incident. The jury is still out over here at my camp as to whether these changes in ability are more largely due to feedback loops over which the person has no conscious control or whether there is a large degree of learning (and I-function participation) that goes into being able to hear a truck from a car.

Looking at eyesight and hearing as "involuntary" feedback loops, we can say that our individual abilities to hear different decibel level sounds is due to our personal hearing set points. There is also, perhaps, a set point for seeing that is determined by the physical formation of the eye. When that set point is not met at the onset of blindness, the set point for hearing is raised (or the inhibition placed on greatly enhanced hearing ability is removed). Due to some coordinated circuitry (that I am not 100% positive exists), as one goes down, the other is allowed to be raised. As sensory input is cut from the eyes, more is allowed to be used through the ears. Bringing the corollary discharge signals into the picture jazzes it up a little. These are programmed to receive sensory information, some of which are due to sight. When this is cut off due to blindness, perhaps there is a certain disorientation and mismatch between what input is expected and what actually comes in. The nervous system needs enough sensory input to get around and so its reaction might be to simply jack up the auditory set point. Is this cockamamy (sp?)?

There must, however, be major individuals differences in the capabilities of the blind for hearing. This brings in the role of the I-function. Perhaps enhanced hearing can take place with none of the above and is dur more particularly to conscious control and learning. If a blind person knows that she needs to be able to hear a car from a truck, can she gradually pull more sensitive sensory information through her ears? (That was obviously not meant literally.) Like the person with the spastically flexed arm, is the blind woman's I-function helping to cause an increased auditory capacity. Is this less cockamamy (sp?)? At least it makes sense to me.

Name: Pernar
Username: lpernar
Subject: Feedback
Date: Sun Mar 22 14:26:28 EST 1998
A negative feedback loop in its essence is responsible for turning down or up output, depending on which way the loop is 'wired' to respond to levels of output, neurotranmitters, hormones, input, or the other aspects that may be mediated by a negative feedback loop. It was discussed in class in numerous instances how negative feedback lops are importantly involved in mediating behavior and such. Essentially, the concpet of the negative feedback loop was a rosy one. So when I came across a negative feedback loop with an intersting (negative) slant, I felt, it may be of interest.

This concerns a mechanism by which Alzheimer's Disease is thought to be potentially mediated. The hippocampus has several important functions. For the purposes of this, only its involvement in memory and its mediating role in the inhibition of glucocorticoid release shall be considered. The hippocampus, as just stated, inhibits the release of glucocorticoids via a negative feedback system. So, if receptors in the hippocampus detect a certain levels of glucocorticoids in the body, they will signal to turn of glucocorticoid release. Interestingly enough, glucocoriticoids, if impacting on a susceptible hippocampus, destroy hippocampal cells. Hence, less cells are available to detect and signal the need for turn off of glucocortcoid levels, and less cells are available for memory functioning. The more cells get destroyed in the hippocampus, the more glucocorticoids are relaeased to destroy cells in it, and the more memory is impacted. The problems faced in finding a mechanism to slow down, let a lone a cure for, Alzheimer's are evident. Apparently, 'brain gymanstics', i.e. acumulating memory and excercicng the mind are the best bets against Alzheimer's, as this behavior may lead to an increase of hippocampal cells; at least reasearch so suggests.

This information is not directly relevant to the course or the discussion of negavtive feedback loops, it jut struck me as informative and felt that it was worth sharing.

Name: jeremy hirst
Subject: I function
Date: Sun Mar 22 17:06:20 EST 1998
The I function, as I understand it now, encompasses what I think of as "me". My feelings come from me, come from this I fct. My actions come from this I fct. My behavior comes from this I fct too. Or do they?

This week's lectures got me thinking about how my behaviors may come from regions beyond this I fct. The example I found especially thought provoking was the rigid type of paralyis accomplanying the destroyed motor cortex. This action of breaking the motor cortex seemed to correlate with a breaking of the function of the I fct. People became incapable of personally moving their arms. Yet they could still exhibit a behavior, an action I suppose is more explicit, when a ball was thrown toward them.

The interesting part of this to me is trying to decipher where this behavior originates from. It seems certain that since the folks can not move their own arm when they want, absence of a ball, it must come from a space outside the I fct. Or are there areas of the I fct that do not rest within our conciousness?

Voluntary and involuntary behaviors can be used to describe which behaviors come from the I fct and which come from a place outside the I fct.

So, if raising ones arm used to be voluntary and now, after destroying the motor cortex it becomes involuntary it seems possible to remove places from the I fct. This sounds like it is in contradicition with the thought that the I fct is me. Because, the arm is still them, they just do not have control over it any longer. I think a better definition of the I fct is needed for a full exploration into the origin of voluntary behaviors.

What I don't like is that it is possible to remove areas of the I fct. Is this demonstrated in disorders which bring about memory loss? Memories are certainly a major part of who I am, and the removal of these seems like it would be resulting in a removal of a portion of the I fct.

Name: Rachel Mosher
Subject: sleep
Date: Sun Mar 22 18:05:08 EST 1998
I was thinking about the changing pattern of sensory activity in people who become blind (from Julia Johnson’s comment in Eyes, Ears, the Brain and I- she notes that there is often an increased capacity for hearing and discerning different sounds after blindness occurs) when I stumbled across an interesting question. What happens to sensory input when you are sleeping? Does the body still process the same stimulai to the same degree as it does when the body is awake? Does it respond the same way? Does it go around the brain through a different route that does not include consciousness (because obviously we do not think about what we are sensing when we are asleep)?

The hypothesis that I would comeup with before being told what is going on is as follows. I would assume that the body goes into a sort of default sleep mode. There are certain senses that need to be processed in order to maintain the body’s comfort. For instance, the body must be aware of how hot/cold the surrounding temperature is, if there is anything crawling on it, if there is anyone shaking it, or if there is any loud noise or bright light that might signal an emergency- and the body is endanger if it does not respond. So, I am thinking that perhaps the body has a sleep mode where it does take in input but if there is no unusual senses, it doesn’t bother to process much and the senses do not enter consciousness in anyway. (Everyone probably has a certain ‘set point’ for the degree as to which things become unusual by the way) However, if the body receives stimulai that need to be responded to right away (such as a fire alarm) the input is processed and at some point the processing awakes the consciousness and the senses are realized. With minor changing stimulai, such as a tickle in the throat, it is quite possible that this input is processed and dealt with, (with a cough) without waking up consciousness. It must be an action associated with frequently used pattern of neurons for it not to need to wake up the self.

I am now interested to know what neurobiologists think is going on with sensory process during sleep!

Name: Vera
Username: vbarkas
Subject: Question one
Date: Sun Mar 22 20:48:04 EST 1998
The question of the existence of the "I function" is somewhat disturbing. When we question if it exists at all we are really questioning whether we have conscious control over our actions or whether we are simply a budle of neurons firing in an infinite number of patterns such that we believe we have contol over our behavior. it also questions the nature of the brain. Is the brain just a network of densly organized neurons or is it organized in particular way such that we can use our brain cells in a different way taht we use our motor cells that function in other parts of our body. The concept of the "I function is also useful when we finally acknowedge for the moment that it does exist. Without the I function for example we could not explain how parapalegic "knows" the he/she didn't control movement in their foot when it was pintched. Therefore we can find the I function both useful and frustrating. If we assume that the "I function" exists and that it is located in the brain we are confronted with a new slew of questions to ponder. What is the nature of its existence, how does it develop, how can it be studied, understood or controled, how do the neurons function differently? When we want to study other neurons of the body we can study them individually because we can separate them from the rest of the body and study how other neurons affect them in a controled experiment. But how do we go about studying the I function. we can only do so by "external" experimentation (for lack of a better word) on a living person. But then of course, when we ask them what they "know" or what the "know" they did, we are really asking for the I function's intetrpretation of the action that occured. We will never so how that translates into signals or a certain pattern of nerve communication. But to assert that it does exist to some extent explains how we can try to control our behavior. It helps us understand what makes human's intellectual and not cats. It is important to understand how it works so that we can have a greater understanding of human behavior because the two elements are very connected. Why do humans cry when they are insulted, or more interestingly, why do some cry and other's don't when told the same insult? I believe that the I function is indeed the "person" part of the human body or the "mind" in other words the part of the nervous system that makes us human and inables us to ponder the neverending questions about life. It seems to be the only part os the nevous system that is difficult to purely understand scientifically. But the need to control our behavior human emotions and undersatnd why we think in certain ways and what makes all human minds different is what keeps us from giving up tha quest. By admitting that the I function does exist also means we agree that complicated forms of behavior can not be explained simplyby saying that neurons fire in a certain pattern. Which makes our quest to understand behavior much more difficult as well.
Name: Ruchi Rohatgi
Username: rrohatgi
Subject: I function
Date: Mon Mar 23 19:11:39 EST 1998
In class we discussed that there are sensory inputs that affect motor outputs and certain motor outputs that affect inputs in a bidircetional system. Along the way, there may also be certain connections between the I function and motor outputs. What does this now mean? It means that some behaviors can be voluntary and under active control by "the person" while other behaviors cannot be based on whether those important connections are made. I think that now the concept of the I function is beginning to fit into the picture better than before where it seemed to be out of the scene- a box totally separate from the rest of the nervous system. However, are there individuals who can actively control certain involunatry behaviors? Are the I function connections based solely on genetic factors? If not, I am wondering what else specifies their connections and if certain connections can just arise some time during one's life? However, it is beginning to become difficult to see behavior as a dichotomy between that which is controlled by the I function and that which is carried out by the nervous system without the knowledge by the I function. The process by which a behavior is completed-the special pattern of motor activity- is the same regardless of the fact that one is actually aware of it. The only difference in autonomous behavior then, is just a connection of neurons...Also, there seems to be more evidence that the I function is spread throughout the entire system. There must be some room for error or mutation-the I function is such an important part of awareness and sense of self. I can see that in paralysis, individuals cannot acctively control certain behaviors and thus there may be damage to areas where signals cannot be effectively relayed to certain neurons including the I function connections. However, could the reverse ever be true-that individuals can have connections that allow them to control normal involunatry behaviors? Would that still be considered a part of the I function?

How does the process of learning new things tie into the I function- since learning is in fact increasing a awareness, is the I function somehow increasing? Maybe not, but I am still a little confused on this issue.

Name: Libby O'Hare
Subject: negative feedback
Date: Mon Mar 23 19:59:06 EST 1998
One intriguing idea that came out of our discusion of negative feedback loops is that these systems provide a means of monitering the bi-directionality found in the nervous system. Every bi-directional relationship within the nervous system is tempered with a negative feedback loop which allows for behavior to have a purpose and for behavior to reflect modifying objectives. The end result is stabilized, consistent behavior. Negative feedback systems are a useful concept for understanding behavior in that while often they are involved in the regulation of hormone levels, body temperatures etc., they can also be used as an analogy for understanding behvaior at a larger more overt level.

The basic characteristics of a feedback system involve maintaing a established set point (this becomes established through experience) by either continuing a pattern of activity in the nervous system (if levels are below the set point), or terminating a pattern of activity in the nervous system (if levels are below the set point). To understand how this definition applies to overt behavior, consider the following situation. Most people attempt to behave in a consistent way. These "ways" are what might be a basis for one's personality. This consistent behavior might be a set point. In other words, if you experience some devastating news which causes you to behave in a manner that you are not accustomed to (i.e. experiencing a lot of sadness), then you nervous system might modify behavioral objectives to bring you back to a stable consistent type of behavior that is your usual manner. Now that I reread what I've just written, it doesn't seem to make a whole lot of sense. But I'll leave it , with the explanation that basically I believe that negative feedback loops are involved at every level of behavior. I don't know why there is this sentence fragment at the end of my text. Please ignore it. call a person lot of emotion and sadness) then your

Name: Emma
Subject: Who's in control?
Date: Mon Mar 23 20:29:34 EST 1998
As we have been discussing this I-function over the course of the semester, I have developed in my little head a sense of it being the un-name-able "it"--the soul, the sense of being, the sense of self, the consciousness. This nice little box seems to contain the "me-ness" of me, my emotions, my wishes, my self-determinization, my morals--all the things that just can't be 100% pegged down to corollary discharges and patterns of activity across networks of neurons. In my mind's eye, I almost imagine the I-fct as being this nice little executive person sitting in an office being consulted by all the different parts of the brain and the nervous system.

I don't think that one can say that the I-fct can be considered a "vital" part of our nervous systems; simply pointing to the fact that our bodies behave perfectly well when the I-function doesn't seem to be functioning, as in the case of our friend with spastic paralysis who can still catch a beach-ball like everyone else. It almost seems that in these cases--of paralysis, etc--the I-function has stopped being consulted by certain parts of the body or the nervous system. Something has occured that arrests the voluntary aspect of the I-fct, so all that remains are the involuntary behaviors that we seem to have no control over ourselves.

In this way, we can divide our behavior into those which are influenced by the I-fct and those that are not. In a weird philosphical sense, one might almost say that we do not actually own our own bodies, we--our sense of self, our I-functions that make us who we are--are merely little voyagers being carried along and hosting for a while these corporal bodies that are capable, for the most part, of functioning perfectly well without us. Would a perfectly intact human being missing an I-fct still be alive and function with involuntary behaviors? I think so. I think it is necessary to understand the role of the I-fct as being the co-habitor of the nervous system instead of the controller, because there are simply things that the I-fct doesn't, can't, and certainly doesn't need to control.

To say a few words about negative feedback loops, this is the nice little system in our nervous systems which accounts for the regulation and 'up-keep' of our systems. Feedback loops help to keep everything stablized by maintaining what we have talked about as a 'set-point' which generally stays the same, but can shift according to environmental and circumstantial changes. The set-point acts like a little mental tab against which the nervous system can check itself and make changes according to how large a gap there is between the tab and the actual body.

Name: Kristin Chimes Bresnan
Username: kchimes
Subject: The I-Function Revisited
Date: Mon Mar 23 21:55:16 EST 1998

I have to report that I now do in fact actually believe both that brain = behavior and that there is an “I-function” box in the brain.

The ifxn box is necessary, it seems to me, to be the repository of what we would normally call “will”. It is not about the descriptive “I”, the one to which we ascribe blue eyes or a fondness for bananas or sleepiness. In other words, only one of the “I”’s in the following sentences refer to the ifxn box: the “I” in “I am so sleepy.” does not; the “I” in “I am not going to sleep now.” does. The former is descriptive, the latter is an expression of will. As an expression of will, the ifxn box also gets defined into existence via its conflict with other boxes. The decision to stay awake despite sleepiness is one example; the failure to stay awake despite the decision to stay awake is another. In both cases, will (my ifxn) is evident because it is in conflict with some other part of myself (not ifxn). Thus, the ifxn box is not the sum total of who I am or the spot in which “my splendid self” resides. It is the box within my nervous system which is responsible for generating (and implementing?) willful action.

This begins to seem like a necessary concept when we discuss the paralyzed person who says “I can’t move my arm” or the paraplegic who says “I can’t feel you pinching my foot, but I can see you, you big jerk”. In the case of the paralyzed person, the statement “I can’t” indicates that the ifxn box is intact, but that there is a breakdown somewhere in the pathway between the ifxn and the implementation. Support for this line of thinking is also found in the research that shows that when someone with nerve damage tries to do something that they can’t do (arm movement in the case of our hypothetical paralyzed person) the corresponding brain activity is in a location which is not the same location associated with actual movement. Trying to move your hand and actually moving your hand are in different locations in the brain. It would be difficult to characterize this discrepancy without some idea similar to the ifxn box.

In the case of the paraplegic, the ifxn box seems even more useful because is shows us that one input pathway is still open (sight) but that another is not (eg. feeling below the waist), and that the ifxn is capable of generating a willful response based on the remaining functional pathways (berating the pincher if not withdrawing the foot).

I would be interested to know if my understanding of the ifxn box is the one intended. It seems likely that the ifxn box might also include a certain perceptiveness, although I am not sure where those lines might be drawn. The banana, for instance. The discussion in class about set points and voluntary vs. involuntary behavior led me to believe that my ifxn is not involved in the following chain of events:

1. I notice that I’m hungry.

2. I notice that there’s a banana in the house.

3. I go get the banana and eat it.

Or rather, my ifxn is only involved to the extent that it is willing to go along with the satisfaction of my hungers. If, however, I decide for whatever willful, headstrong and ultimately foolish reason not to satisfy my hunger, then my ifxn box shows itself in its attempt to restrain the behavior of eating the banana.

It occurs to me that the existence of the ifxn box must certainly correlate with the existence of psychology as a discipline.

Name: Bonnie Kimmel
Subject: I function
Date: Mon Mar 23 22:00:36 EST 1998
We have now modeled behavior into two categories, voluntary and involuntary, the former occurring with activity in the I function and the latter without. To me, it seems to clearly be the case that the I function is a continually and constantly changing entity greatly affected by experience. But on what is the initial blueprint based? This returns to the age-old debate of genes vs. culture, how much of who we are is present at day one, how malleable that is, and what behaviors/attitudes/beliefs are allowed to happen/not inhibited. No hard answers, but an interesting query that keeps popping up nonetheless. We have already set the stage with the example of the paraplegic to consider the fact that the nervous system can act without the involvement of the I function. We have largely isolated this to a mind/body separation of sorts. But I also wonder about how this can all occur completely within the ambit of the mind. What of psychiatric conditions, or the use of the insanity defense, that render an individual supposedly unaware of or not the cause of their actions. How could one show this and what would the internal circuitry look like? Can the I function be aware and observe, but not participate? Does what I'm writing even make sense?

As a random aside, I was also wondering about the condition (I forget the name of it) in which people claim to feel colors and taste shapes. If anyone knows anything about it (PG, perhaps?) I am curious what is understood about this.

Name: Alicia
Subject: I-function and hallucinations
Date: Mon Mar 23 22:23:30 EST 1998
What I have been wondering about is the relationship between the I-Function and hallucinations. Through studying the notor cortex, we have learned that action through the nervous system can be caused without you being "aware" of it, or without the I-fct being involved. We also have learned that these actions can take place with the involvement of the I-fct and some actions have been delared either voluntary or involuntary and some may be either/or, depending on the circumstances. Some questions that arose in my head were how this can relate to hallucinations. Obviously, when you see something that is not really there, you are not "aware" that it isn't really there. I assume that the I-function is probably not involved in specifically causing one to hallucinate. However, I wonder if the I-fct plays any role in constructing what exactly is seen. For instane, is someone able to hallucinate soemthing they've never seen before? I am quite curious about how the brain, I-fct, and personal experience and memory relate to hallucinations. I imagine that the I-fct probably plays some sort of role in "creating" the vision one has when they hallucinate. I guess this all plays into the issue of what exactly is meant by seeing as well. I cannot offer any answers to these questions, but I found them interesting and wanted to share. If anyone can offer any explanations, or further thoughts, they are most welcome.
Name: Christy Taylor
Subject: Negative Feedback
Date: Mon Mar 23 22:25:32 EST 1998
This past week's discussions in class were very interesting to me because my first short term paper was on Huntington's Disease and negative feedback is an integral part in understanding the causes of this devastating disease.

According to our textbook, Foundations of Neurobiology by Fred Delcomyn, Huntington's Diasese is "caused by the death of inhibitory GABAergic neurons in the striatum." Because these inhibitory neurons do not function correctly, they are not able to inhibit certain types of motor movement and without this inhibition, sufferers of Huntington's Disease display chorea, or unwanted and uncontrollable movements.

I also assume that this malfunction of negative feedback in the brain is also believed to cause Parkinson's Disease.

I think it is so interesting that negative feedback loops play such an important and integral part in the body's internal communication system. It plays such an important role in so many things from the regulation of heartbeats, to the control of unwanted movements, to the regulation of body temperature.

Before this class, I had never really thought of behavior as the INHIBITION of unwanted firings of neurons! A very interesting and thought-provoking idea!

Name: Karen
Subject: I function
Date: Mon Mar 23 22:28:56 EST 1998
One aspect of lecture struck me in particular this week. Assuming that there is an I function (whether or not it is a box), it is interesting to me that in some cases the I function makes the final decision but in other cases the I function is overruled.

One example of this is car sickness. Why can't the I function say- "Listen CD, I am in a car and I am traveling 65mph without moving my legs." How come the corollary discharge can contradict the I function to such a degree- that a person feels sick. Perhaps I feel as though the nervous system should be "smarter." Or that the I function should carry more weight.

But why do I want the I function to be the end all and be all of the decision making process of the nervous system? I believe that for myself the I function is a convienent way to explain brain = behavior without losing sense of self. But in class, over the past week or so it seems like the I function is begining to lose some of its power and mystique.

I say that the I function is less powerful because it does not have the final decison. As mentioned in one of the previous essays, the I function is not necessary for the nervous system to work (involuntary motion)and the I function can be overruled(car sickness, paraplegic kicking his leg, person with spastic paralysis blocking a beach ball). It seems as though more and more actions that are normally attributed to the I function ( someone says "I did that") can be accomplished without it.

The mystique is starting to fade because instead of the I function existing within the brain or maybe above the brain, I am getting the idea that it is limited by the brain. Through experiments, it was determined that the I function can't exist in certain areas (the basal ganglia). Doesn't it seem like the 'I FUNCTION' (pretend this is in boldface) should be everywhere in the brain? It seems that the I function can't make the final decisions or develop its own mechanisms but instead use the tools that are already around it. Perhaps with the example of the blind person enhancing their hearing, the I function works to divert the sensory input to an area that can better process it. But then, the I function becomes a 'manager' in the brain- not the embodiment of self.

Name: Rachel Kaplan
Subject: stress
Date: Mon Mar 23 23:08:11 EST 1998
Negative feedback loops are integral aspects of human functioning. In humans, we are used to talking about these loops as involuntary, internal processes triggered internally or externally. Bodily temperature, for example, is stabilized around a set point, usually 98.6 * F. When the environment is very cold, the body automatically performs behaviors such as shivering in order to create heat to maintain the set point. Similar compensatory mechanisms are employed all the time in many different situations in the human body.

Voluntary, negative feedback (certain biofeedback methods) also play a large role in human behavior. Through biofeedback a person can actually learn how to affect her autonomic nervous system; the system which controls involuntary responses. Learning biofeedback methods may be a lifesaver for someone prone to stress. The “fight or flight” response of the sympathetic nervous system can lead to hypertension, a weakened immune system, and other problems if triggered too frequently. Unfortunately, the hypothalamus cannot differentiate between actual danger and stress-inducing thoughts; therefore, stress hormones and adrenaline are released in both situations (URL: These hormones cause constriction of the smooth muscles of the arterioles and blood vessels which leads to an increase in blood pressure (URL: The stress response may be tempered by biofeedback methods aimed at restoring the resting state of the individual, the “rest and rumination” condition which exists when the parasympathetic nervous system is in control. Techniques such as “progressive relaxation and autogenic training” may be used to lower blood pressure (URL: Therefore, a person may stabilize her condition by consciously affecting her autonomic nervous system through internal and/or external cues.

Name: Anne Frederickson
Subject: Negative Feedback
Date: Mon Mar 23 23:29:22 EST 1998
I am constantly fascinated by the idea of negative feedback loops, partly because it seems like a rather backward way for the body to control things. I think most people consider behavior to be the initiation of some action, thought, whatever. However, if we consider negative feedback loops and their effects on behavior, much of what we "do" is not really initiated behavior but alterations of behavior. The behavior is already there. However, somewhere in the nervous system, a signal has indicated that the behavior is sufficient or needs modification. The NS must have a separate pathway for these signals to travel along to the appropriate region of the NS. It must then have another pathway in which to inhibit the behavior that is already occuring. This may be a totally different neural pathway or simply a different neurotransmitter. This is amazing to me. That is at least 3 different pathways to initiate and control a single behavior. I tried the think of a way that the NS could have been organized to control behavior, without being a negative feedback loop but didn't come up with much. Perhaps the I-function could be considered a way to by-pass the negative feedback loop. If neg. feedback is an automatic process, then the I-funtion would be the way in which our NS could control that process. The I-function provides yet a fourth pathway for controlling behavior, yet another way for the NS to alter already existing behavior. The motor cortex would be an example of the I-function controlling the negative feedback loop of movement by inhibiting the signal. However, there could presumably be other ways for the I-function to control behavior, either through control of the feedback or some other way. I am curious if there are ways in which the I-function limits or prevents the feedback from reaching the appropriate region of the NS.
Name: Akino Irene Yamashita
Subject: setpoints and negative feedback
Date: Tue Mar 24 01:18:50 EST 1998
I found the concept of setpoints very interesting and useful in our understanding of how the body maintains homeostasis. For example when the body is exposed to an environment that would tend to raise or lower body temperature from the normal setpoint of the body it exhibits behaviors intended to counteract that external change by shivering if cold or sweating if hot.

The explanation of why a person with a fever can have chills also seemed to show how subjective our experience of sensory input is. For not only does the body exhibit involuntary behavior like shivering at high temperatures when the setpoint is also raised, the person feels s/he is cold, and behaves accordingly by crawling under many blankets for example. It also helped explain something which I've noticed in my family all my life. Me and my father "feel cold" at much higher temperatures than my mother and sister, who "feel hot" at lower temperatures than me an my father. I notice that often my body temperature is a bit higher than 98.6. So perhaps I have a higher setpoint for body temperature than my mother or sister, and thus feel cold at higher temperatures because they represent deviations from my setpoint.

I also found interesting the discussion about the motor cortex and spastic paralysis. It seems much, if not most, of the NS is based on the principle of inhibition of impulses and behaviors than active causation. It was also interesting how motor development in children is mostly the refinement of gross motor skills and not the development of completely different movements the child can perform.

Perhaps the I-function also functions more by inhibiting CPGs and other nerve impulses to suppress certain behaviors than thinking up and executing completely new ones.

Name: Jonathan Ball
Subject: The "I function"??
Date: Tue Mar 24 01:25:53 EST 1998
The notion of the “I function” seems to be a valuable concept for understanding behavior. It allows researchers to explain instances of behavior that are not easily, or at all, explained in terms of the current understanding of behavior. But the benefits of using an “I function” do not come without consequences. These consequences while not necessarily being weighty enough to completely discourage the use of the “I function” are certainly worth considering.

One of the major consequences of the ease with which the “I function” helps understand behavior is that it could discourage other explanations. This is especially problematic because the idea of an “I function” is so appealing to many humans, because it still bears some resemblance, though one cloaked in science, to the soul. This consideration may seem trivial, but many discoveries in science have been delayed or lost because of humanities resistance to changes in the way it perceives the world. As scientists it is our goal to transcend the restraints placed on knowledge by past information and as Prof. Grobstein put it continue to try and be “less wrong”. What I mean to say is that the “I function” has some very compelling aspects even to a cynic such as myself, but we must be careful that we don’t just accept it because it fells right. We must continue to examine its function and be ready to send it the way of the flat earth and the geo-centric universe when or if the time comes along.

A question perhaps related to the usefulness of the “I function”, or perhaps not, is if the “I function” is organic where does it exist, and more importantly when did it develop? We feel comfortable allowing humans to have an “I function” but many people would undoubtedly feel uncomfortably with ascribing one to a fruit fly. The science of comparative psychology has demonstrated the evolution of many behaviors from the “lower” organisms to “higher” ones including many behaviors thought to be solely human. For example Piagetain psychologist have often used the ability to solve a problem of the sort; Jill is smarter than Jane and Sue is smarter than Jane is Sue smarter than Jill to mark the entrance into a new stage of cognitive development. The ability to utilize the logical resigning necessary to solve this problem was once thought to be a uniquely human ability, or primate ability, but it has been recently demonstrated in pigeons. Although it is much more difficult for pigeons to solve this problem than for humans, it still demonstrates an evolutionary continuum of logical behavior. So if we hold that the “I function” is like any other behavior, then it is we may need to say that other organism develop have a partial or lesser develop “I function”. This can be a troubling assertion because if the “I function” provides humans with our sense of selves then we would have to that lower animals either also have a sense of self or a partial sense of self. I don’t think these assertions make the “I function” a bad concept, in fact a continuum of sense of self makes sense on from evolutionary standpoint, but if we accept the “I function” we need to be more careful about the sanctity which we as humans place on self.

The use of an “I function” maybe a valuable tool, if we use it for know as a sort of constant in the equation of behavior, but we must eventually examine the true nature and necessity of this constant. If we fail to do this and begin to accept the “I function” on faith then we have done nothing with science but created a new religion with an organic soul.

Name: moriah
Subject: a story about sleep
Date: Tue Mar 24 01:47:47 EST 1998
Rachel's ideas about sleep brought up an interesting (and only tangentially related) memory for me . . .

Two summers ago I was a day-camp counselor at a Philadelphia rec center and one of the field trips the city sponsored for us was to "Fire Camp," a day at the fire-fighters' academy where the kids learn about fire safety, the dangers of drugs, and all the other rhetoric large school systems force on kids. BUT one of the fire-fighter/instructors piqued my interest: when explaining the need for smoke detectors, he talked about the fact that you would never wake up because you smelled smoke. Though you feel, to some extent, all your other senses, you cannot smell while you sleep.

While the lecture went on, my mind wandered and I started to think about the fact that I couldn't remember ever having a dream that involved smell.

I think Rachel must be right that there are certain threshhold amounts of, for example, noise, that will disturb your sleep, but I wonder why an ire sense seems to be excluded. And I also wonder how these threshhold amounts change throughout your life - the way a baby can sleep through a noisy party or the way the parent of a newborn will suddenly wake so easily.

What is the relation between the behavior we perform when we are awake and unconscious?

Name: Doug
Username: dholt
Subject: feedback
Date: Tue Mar 24 09:43:38 EST 1998
There has been some interesting discussion as to the set points of temperature control of the body. I am curious as to the mechanism of control of the set points. It would appear to me that there is "anticipatory cooling" by some mechanism that seems to sense that the environment that the body is in is much colder than the body even though the body may not have dropped any degrees in temperature. As if the body is preempting cooling rather than reacting to it. This is definitely noticable when you walk outside without coat on in the winter, even if it is for just a few minutes. Personally, this I am affected in this manners whenever I enter into water that is reasonably cool. In one of my previous jobs, I was required to spend a lot of time in the ocean off of san diego where the temperature of the water rarely got above 56 degrees. After a while, each time my body enters the ocean, i get the same response, not matter how warm the ocean water is. I ended up wearing a 7mm wet suit even when diving in hawaii! Will this set point remain where it is or can it be reset?

But my real curiousity on temperature control is how much can the body override the set points by means of "will power". By this I am not referring to walking around outside without a coat in winter, but doing it and not being "cold". Teachers always used to tell us in class that if we were cold to think of hot things: hot chocolate, the beach, sitting in the sun.. etc and this would warm us up.. to some extent they were correct. Whey would this occur even though no physical changes happened to the body during that time period?

Moving on to another track. The inhibitory control of hte neocortex is required for voluntary motor movements. Would this explain the movements of the chicken which had its head removed? With the head gone, the brain would not be able to exert inhibition over the central pattern generators in the spinal cord? In a similar manner, how does alcohol affect the inhibitory pathway.. I have heard that it inhibits parts of teh brain that would normally create inhibitive behavior.

One final note.. in a recent lecture, I heard that by severing the central connections between the left and right halves of the brain (to stop extreme cases of epilepsy), there is no left-right communication and this can be done with some eye testing. But, even without communication between the two halves, emotional responses can be triggered in one half which then seem to bypass the severance and alert the other half. Do emotions travel in different pathways and in a different manner than "thoughts" do?

Name: jenny
Subject: Negative Feedback
Date: Tue Mar 24 09:50:36 EST 1998
The discussion in class last week sparked my interest in negative feedback loops. Negative feedback loops regulate body temperature, body weight, heartbeat, hormones, etc. These loops stabilize behavior due to the bidirectionality of input and output pathways. When our body temperature or weight fluctuates, the internal setpoints in the body change causing a change in behavior. For example, if someone begins to starve their body of food, there is a noticeable change in behavior. The person becomes irritable, tired, faint, and weak. After a while the body becomes accustomed to the new internal setpoints, and it becomes easier for the person to starve their bodies because the consequences of the act are less inhibiting. The same is true when you have a fever and your body temperature rises. Once again the internal setpoint is changed and you behave in a manner which is consistent with having a fever. Anyway, the environment causes changes in the functions regulated by the negative feedback loops and as a result our behavior changes due to different internal setpoints. This portrays how the our behavior is affected by outside forces in the environment.
Name: Elizabeth
Username: ewindsor
Subject: negative feedback
Date: Tue Mar 24 10:10:04 EST 1998
Negative feedback loops are important within our bodies on several different levels. Many hormones in our bodies work within the context of a set point and negative feedback loops. The I-function is unaware of this activity. Then, as we discussed in class, there are the set points and negative feedback loops involved with maintaining temperature etc. The I-function may be aware of this, by observing that we sweat in response to warm weather and shiver in response to cold weather, but it does not control it.

Another level of negative feedback loops within us deals with behavior that the I-function is directly involved in regulating. Human "drives" can be demonstrated as negative feedback loops. In class we talked about boredom. If you are bored you go do something fun, but eventually you get tired and stop doing the activity. If I spent my whole life just sleeping and watching television I would be bored and unhappy with life. But after working all day it feels really relaxing to watch some television and take a nap. The I-function plays a big role in this type of negative feedback loop. If I chose to, and if it was financially possible, I could spend my whole life hanging out around my house. Or I could choose to spend my whole life working. The I-function has considerable power in this area of behavior. Yet this type of behavior does seem to be regulated by the same system of punishment/reward that other, more basic, areas of behavior are regulated by. If my body needs food but I put off eating I will feel increasingly hungry (punishment). When I finally do eat I will feel increased pleasure (reward). This makes sense because eating is vital. But in the work/rest scenario this type of punishment/reward system is also in place. If I spend most of my day working, I will get more pleasure out of relaxing around the house than if I hadn't worked much at all that day. The I-function can be involved in this behavior because it si not vital, though we seem to function best if we follow the balance of work and rest the body seems to prefer. Too much work and we have breakdowns, too much rest and we get bored.

Name: Rob Miller
Subject: Color
Date: Tue Mar 24 10:48:08 EST 1998
I was beginning to think about vision and sight in relation to what we talked about last Thursday in class and I found myself pondering the nature of color in our mind's eye. What is color and how do we perceive it. It dawned on me that color can only be described by someone using other colors and words that relate to specific colors. For instance, I can describe green as a mixture of blue and yellow, but I cannot actually describe what green looks like to a person who has never had sight? Does this mean that every person may perceive a signal color differently? Does the color I associate with green actually look like what I call purple to someone else? When I look at a tree and call the leaves green, it's only because I learned when I was young that the particular shade of color on leaves was called green. Someone else may look at the same tree and see another color, but has always identified that color as green. How can we know for sure what colors are if we only can identify in terms of other colors?

This type of argument can be made for other perceptions as well in the brain. When only one sense is used to describe or identify something, there may be differences between how different brains perceive these things. Tastes and smells can also only be described in terms of other tastes and smells which are known. Could these possible differences in human senses be responsible for the preferences people have for different colors, fragrances and tastes.

There could be no way of ever distinguishing what different people are observing with their senses, because each persons sensory neuron have different connections and patterns. I not sure yet what part the I function plays in sensory pathways in the brain, but my guess is that there is plenty of correlation between sensory input and I function.

Name: Chris Lord
Subject: Negative Feedback Loops and what they don't explain.
Date: Tue Mar 24 11:03:11 EST 1998
In class we talked about how we are always trying to return to a state of homeostasis and we use negative feedback loops to help us return to that state. This poses an interesting question about the I-function and individual differences. I'm sure that there is a lot of room for individual differences, but still, why is it that one person's homeostasis is another's excitement or boredom? We have all felt this when hanging out with our friends. Either we are not exciting enough, or not boring enough and we run into problems with deciding on activities to pursue. Our internal negative feedback loops are causing us to seek and perform certain behaviors, whether they be internally caused and performed or externally caused and performed, to bring the body back to it's acceptable realm of activation. Now the I-function probably plays a big role in the selection of the activities, but still to just say that it's all I-function is, I think, to find an easy way out. Why would someone sky dive in order to bring their bodies back up to where it wants to be? This is a life threatening activity. There is a percentage of sky divers who don't come back after a jump. Most human beings would not do something this extreme to bring their levels back up. It would be past the point of their threshold. Assuming we experience the needs the same way, why do they not manifest themselves in more similar ways in the human person? If we don't experience these needs the same way, then why the difference? We essentially have the same basic parts, and this is a very basic part of human life.
Name: Miriam Kulkarni
Username: mkulkarn
Date: Tue Mar 24 11:08:58 EST 1998
Yesterday, I had a conversation with a friend of mine who is a computer science major and is taking Computational Models of Biological Systems. (I think that's the name of the course). She was reading an article about neural networks, which to the best of my understanding are computer models of the brain.

Neural networks are being developed to perform complex tasks once thought to be only possible for humans, such as pattern recogintion. Right now, however (according to my friend) computers are not very good at recognizing complex patterns such as speach, or faces. However, some computer scientists believe that computers will be developed that are as good at these tasks as humans are.

The idea of neural networks intigues me. Why is it that conventional computers, which can process data at incredible rates, are incapable of pattern recognition? Why is it that humans, which are much worse at data processing than computers are so good at pattern recognition?

I had never before really considered how difficult a task recognizing a face or understanding speech is. These types of tasks are so simple for humans that they do not even require thought. But when you consider these tasks from the perspective of trying to design a computer to perform them, they become monumental. For example, how humans sort out which details of speech are relevant, understanding the words and ignoring details such as pronunciation or tone of voice that vary from person to person?

Name: Zach Hettinger
Subject: defractionation
Date: Tue Mar 24 11:14:44 EST 1998
I was really intrigued in class this past Thursday when we started discussing the development of children’s motor control. After looking at my notes and the forum, I made a connection between what Julia Johnson had said about the increased awareness in those that lost individual sensory systems and the idea of defractionating motor control to achieve a greater level of control. What had originally caught my attention in class was the concept of children having to defractionate groups or clusters of motor neurons in order to gain fine motor control. The example of a child only moving all their fingers in unison until they could only move one at a time was a very vivid image. So is it then possible to assume that most of the hardware is already there at an early age, but that we just have to learn how to use it? That a child has the potential to move all their fingers individually, that they then find a need to do so, and so they then separate the movement in order to perform the desired action. Can this understanding also be applied to the senses? For instances, that we are all born with the ability to have very fine hearing, but that at some point in our development we stop defractionating our ability to hear, that we unconsciously decide that we can hear enough and put more attention to visual and tactile cues. Yet in the event of an accident that leaves an individual without sight they then begin to depend more heavily on their hearing and start to develop it past the point where they stopped as a child. Is it then just a matter of training your each to differentiate between sounds, just as a child must differentiate between fingers? This would then make a blind person’s increased hearing more of a realization of their potential instead of creating something that was never there before. Maybe it’s too much of a stretch to apply a theory on developing motor control to the development of sensory input, but the connections do seem possible.
Name: Anneliese
Subject: hallucinations
Date: Tue Mar 24 11:26:29 EST 1998
Just wanted to throw in a bit of info I found on hallucinations (a topic I hope to be writing my next paper on). As it turns out, we're just about at the right point in the course to be talking about them, since they are, in a sense, sensory experiences. As I understand it, hallucinations are sensory experiences that occur in the absence of, or, without being caused by external stimuli. Your brain accesses areas involved in memory and images and gives them priority over the actual sensory stimuli it is receiving from the environment. So is the I function involved? Although I, like everybody else, am as yet in no position to answer this conclusively, I'd like to speculate a bit. It seems to me that the I function has connections with most or all areas of the nervous system, some of which are more prominent than others. These connections, as stated by many others, are not essential for the body to function, i.e., our bodies can in many instances exist quite well despite breaksdowns in communication. Yet when these connections are intact, when we do have the ability to create voluntary actions, it seems we are better off, more complete, more "fit". It was established in class a few weeks ago that the reality I perceive is different from that which another person experiences, and it seems to me that this would not only produce a unique set of memories and mental images of the world, but also influence what is experienced during hallucination. I think it is very possible that one might, during a hallucination, see or experience something one has never seen before--think of nightmares and bizarre dreams you've had. But maybe these supposedly new things are really constructions of patches torn from other things we have experienced, sewn together into a new, but somehow familiar, experience. I guess I'm left with a similar question as Alicia...are we limited to what we've experienced when it comes to creating things such as hallucinations, or are our brains capable of constructing entirely new images? That's all for now; sorry, I'm a bit under the weather and find that my brain isn't working as well as I would like.
Name: Fumiko Konno
Subject: the I-function
Date: Tue Mar 24 18:07:59 EST 1998
We are able to explain why animals behave differently under different circumstances, under the same stimulus. However, this does not fully explain our behavior as a whole. This is because of the I-function. If will or choice, that we think we each have, is the I-function, then it is necessary that we cover this material for the better understanding of the entire nervous system and our behavior. Study of the I-function is beneficial to the understanding of the nervous system because we think that we have a choice and a sense of self and that these ultimately lead to many of our actions. To know how this system works in relation to the rest of the nervous system is important.
Name: anonymous
Subject: Phantom limbs
Date: Tue Mar 24 23:30:07 EST 1998
I recently read an article in Discover that was about what we were discussing in class. It said that people are born with a neural map encoded genetically and this results in ghost limbs because the neural map does not match the actual neural information. But what happens when someone injures themselves? If you mangle a body part in an accident and change the shape and the neural pathways, then that would result in a difference between the genetic map and the actual one. That should cause pain because there is a difference. But I don't think that happens, so the alternative is that the genetic map can be altered through life experiences. As your body changes, likewise your map changes. Could this be why all amputee victims do not experience the ghost limb phenomenon? Possibly the ghost limb victims have problems adjusting their maps. In the article the people born lacking limbs seemed to only experience the phenomenon infrequently and during specific times.

Also, how do amputee victims compare with paraplegics on this level? Both are not receiving information from their limbs (paraplegic has a break in the cord at the neck, and amputee at the base of where the limb should be). Although the amputee still has all of the cell bodies in the spine which can get signals to the brain. If the ghost limb occurs when there is a difference, then there is a difference between the signals and the neural map in both cases. The paraplegic sends no signals and the amputee may send odd signals. Why is it that paraplegics do not experience ghost limbs or do they?
Name: Eric Odessey
Subject: blind spot
Date: Wed Mar 25 09:09:44 EST 1998
A new blind spot test occurred to me in class yesterday, as the dot I drew on the right side of the page blipped out of existence. We know about examples of the brain filling in for the blind spot with colors and even continuous shapes, such as poles. I was wondering, however, just how clever the brain is. I tried the blind-spot experiment on the back of everyone’s head in the classroom to see if the brain would fill in all of the distinct shapes that my eye wasn’t seeing. The only conclusion I to which I came was that I’m not very good at determining fine details of an object if I’m not looking at it. I know that there was no black hole in my vision, so my brain must have stuck something in there. I just can’t tell whether it was repeating shapes it saw around the blind spot, or if it actually remembered what was there from before. It would astound me to no end if the latter were true, because that would imply that the brain has a running memory of all of its surroundings (at least in recent history). Therefore, I suspect that it just repeats the pattern, so you actually see a few heads more than once (have no fear, this also greatly impresses me). I was hoping someone might have some insight into this question.
Name: Elaine de Castro
Date: Wed Mar 25 10:34:29 EST 1998
A little unrelated, perhaps, but a thought that occured to me in class during the set-point-temperature discussion was - how do those people walk on burning, flaming coals without pulling away in pain? Besides that, do they sweat profusely while doing it? I suspect this has more to do with desensitization than internal set point for temperature - but it was a thought. I guess it also relates to I-function, the whole "mind-over-matter" idea that you can withstand any pain if only you use more of your brain.

Another thought, and this goes way back to the birds not really "needing" to learn to fly - do human babies really need to "learn" to walk? (I wonder who would have the guts to do experiments on this, though). Tie that in with, do children "learn" to move one finger at a time, or does this come with time, and if so, how much time? Does the acquisition of this skill vary drastically per child, as per their attempts at movements, or come with a certain age and physical development? I'm sure we've all seen those 3-year-old piano-playing prodigies running around. Did they need to be taught, prodded by their parents to touch the piano until their fingers could gain control, or did they just develop those neural capabilities early on? (Well, that they can play the piano at such a young age would probably signify more is going on in there besides finger control anyway).

As a side note - if anyone who reads this goes to Swarthmore on occasion and would like to make some money and eat chocolate, get in contact with the visual perception lab - they're running lots of fun experiments and can always use participants!

Name: ruth czarnecki
Subject: reaction
Date: Wed Mar 25 19:52:53 EST 1998
Because I couldn't think of any worthwhile to say, I have decided to respond to the previous person who posted, Elaine. As far as the hot coal walkers go, they coat their feet in a petroleum based product and they also have huge callouses, therefore they can't feel the heat. Movie producers use the same technique a lot. For instance they will cover a person's hand in petroleum jelly and then coat it with gasoline and light it and the hand doesn't get burnt. Why does this work? Petroleum based products are combustible, not flammable, and because they are not in contact with the heat long enough, they don't reach the proper temperature so they can ignite. I wouldn't try this at home though. :)

About the babies walking, I read a long time ago that babies are born "knowing how to walk". They have the neural patterns of walking stored somewhere in their nervous system. It is however the muscle development that needs to come before they can walk. It isn't that they need to learn, it is that they need to be strong enough.

About child prodigies, I think that is very interesting. How do they know how to play a piano the first time they sit at one? I doubt it has anything to do with the parents, though. My guess is that they have highly tuned audio/visual learning skills. A photographic memory wouldn't hurt either. But I wonder how that can be explained neurobiologically.

Name: Daniel Casasanto
Subject: Weekly Essay #6
Date: Wed Mar 25 20:58:50 EST 1998
Excitation and Inhibition in Classical Chemical Synaptic Transmission

The purpose of this essay is to examine classical chemical synaptic transmission, to contrast it with electrical synaptic transmission, and to distinguish it from neuromodulatory transmission. The intended topic of the essay was the neural basis of decerebrate rigidity, which I hoped would provide a useful model of excitation and inhibition in the nervous system. However, upon reading relevant articles I recognized my need first to review some fundamentals.

Chemical synaptic transmission differs significantly from electrical synaptic transmission. The chemical synapse uses a neurotransmitter intermediary between the presynaptic and postsynaptic neurons, whereas the electrical synapse transfers current directly via gap junctions. Chemical synapses are polarized; information flows one way, from presynaptic neuron to postsynaptic neuron. Electrical synapses are not polarized. They allow the reciprocal flow of information. Chemical transmission has a somewhat less predictable outcome than electrical transmission. Its outcome depends upon the properties of the receptor proteins that bind specific neurotransmitters to the postsynaptic neuron, and upon the influence of any cells other than the presynaptic neuron that communicate with the postsynaptic neuron. Chemical synaptic transmission takes 0.5 to 1.0 milliseconds longer than electrical synaptic transmission. Nevertheless, it is frequently mandated because the response that it elicits from the postsynaptic cell is variable.

A chemical synaptic transmission can be classified as either excitatory, inhibitory, or neuromodulatory, depending on its effect on the target cell. Excitatory postsynaptic transmission results in a flow of ions through the postsynaptic cell's membrane, which causes a membrane depolarization called an excitatory postsynaptic potential (epsp). An epsp increases the membrane’s positive charge relative to its resting potential, thus increasing the likelihood that an action potential will be generated by the target cell. Inhibitory postsynaptic transmission results in an inhibitory postsynaptic potential (ipsp). This flow of ions causes hyperpolarization of the postsynaptic cell's membrane, increasing the negative charge inside the target cell, and decreasing the likelihood of an action potential. Neuromodulatory transmission is a type of chemical synaptic transmission that differs from the aforementioned "classical" types of transmission, both in mechanism and in function. Neuromodulatory transmission can be either excitatory or inhibitory. Neuromodulation, which regulates the target cell's response to subsequent input, is slow relative to classical synaptic transmission, and its effects on the postsynaptic cell persist longer than those of classical electrical or chemical synaptic transmission. The details of neuromodulation will be omitted from this essay.

There are two types of classical chemical synaptic inhibition: postsynaptic inhibition and presynaptic inhibition. These are distinguishable in important ways. Postsynaptic inhibition involves direct synaptic contact between the inhibitory neuron and the cell whose action is inhibited. The ipsp results from either the influx of Cl- ions or the efflux of K+ ions, depending upon the ion specificity of the protein channels activated by the inhibitory neurotransmitter. Typically, one postsynaptic soma will synapse with both an excitatory and an inhibitory neuron, which may fire simultaneously. When this happens, the epsp causes protein channels to open, allowing positively charged ions to enter the cell. Meanwhile, the ipsp opens channels that allow positively charged ions to exit the cell, and may also open channels that allow negatively charged ions to enter. The net effect is the mitigation, if not neutralization, of the excitatory neuron's influence on the probability of an action potential in the postsynaptic cell.

In presynaptic inhibition, the inhibitory neuron does not synapse directly on the neuron that it inhibits. Instead, the inhibitor synapses on the terminal of an excitatory neuron, which then synapses on the inhibited postsynaptic neuron. The inhibitor acts indirectly, by reducing the effectiveness of the intervening excitatory neuron. This can be accomplished via several different mechanisms, all of which reduce Ca2+ influx at the excitatory presynaptic terminal, which suppresses excitatory neurotransmitter output from the terminal, thereby diminishing the epsp. In postsynaptic inhibition, the coactivated excitatory and inhibitory neurons modulate postsynaptic potential by "competing" to raise or lower the positive ion concentration within the inhibited cell. Conversely, in presynaptic inhibition, postsynaptic potential is modulated by the inhibitory neuron's impairment of chemical processes within the excitatory neuron. The functionality of the target neuron is not impaired. The postsynaptically inhibited cell responds to input poorly during the instant between hyperpolarization and the return to resting potential. In contrast, the presynaptically inhibited cell can receive input uninterruptedly. Presynaptic inhibition enables a polysynaptic cell to receive input from certain sources, while input from other sources is blocked. This is of particular importance biologically, as it allows the organism to respond selectively to one stimulus, while remaining sensitive to other stimuli.

Communication among individual cells is essential to neurobiology. Understanding the interplay of excitatory and inhibitory signals in synaptic transmission is necessary in order to trace the neural pathways responsible for behavior.

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