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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.
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.
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....
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.
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.
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.
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!
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.
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
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.
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.
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.
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!
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.
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: http://www.ne-mindbody.com/streskey.html). These hormones cause constriction of the smooth muscles of the arterioles and blood vessels which leads to an increase in blood pressure (URL: http://www.ne-mindbody.com/streskey.html). 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: http://www.ne-mindbody.com/streskey.html). Therefore, a person may stabilize her condition by consciously affecting her autonomic nervous system through internal and/or external cues.
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.
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.
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?
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?
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.
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.
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?
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!
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.
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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