Three Kinds of Potentials
Remember that an ion is an atom with too many or too few electrons, and so they carry an electrical charge. Normally there are more negatively-charged ions inside the neuron than outside. This creates the resting potential, which is the “default” membrane potential. A neuron with a membrane at resting potential is like a compressed spring that has not yet been released. For a tutorial with a quiz see this site.
Whereas the resting potential is a persisting “default” condition, the action potential is momentary. It’s a travelling collapse and restoration of the resting membrane potential, when so many sodium ions enter the neuron that its inner negativity becomes more positive. The action potential moves along the axon, taking in sodium ions and expelling potassium ions as it moves. The ions move up down through the membrane like pistons, while the action potential moves forward like a car. Try this cartoon for the story on resting and action potentials. You can see the resting potential and the action potential in a slower motion at this site.
In many of our axons the action potential leaps along the axon in a pattern called saltatory conduction. It jumps from one node of Ranvier to the next, much faster than an unmyelinated axon could support.
How does the action potential keep from moving in reverse? It has to do with the refractory period. You can read about the refractory period, described almost halfway down this lecture (this is another good resource in addition to the textbook). Once the action potential begins its transit down the axon, the neuron is in the refractory period. During the refractory period it is impossible or difficult to get the neuron to fire. There are two stages of the refractory period. The first stage is the absolute refractory period, in which the neuron will not fire again no matter how strong the incoming messages may be. The second stage, the relative refractory period, immediately follows while the neuron gradually gets back to its resting state, the resting potential. In this period, the neuron could fire, but only if the incoming signal is much stronger than normal. The refractory period is also the reason for the rate law. Intensity of stimulation is not reflected in the strength of the action potential, because the action potential is always the same speed and same voltage. Instead intensity is reflected in the rate of firing of a neuron: the higher the intensity, the faster the neuron fires. The higher the intensity of the stimulus, the more likely the neuron will fire during refractory period – in this way the neuron can fire again more quickly, because it can fire before it gets to the resting state.
PostSynaptic Potential (PSP).
A PSP begins at a synapse, where a neurotransmitter causes the resting potential of the receiving neuron to decrease (depolarization) or increase (hyperpolarization). Depolarization is excitatory, so a depolarizing PSP is called an excitatory postsynaptic potential or EPSP. Similarly, a hyperpolarizing PSP is called an inhibitory postsynaptic potential or ISPS. Click on "Communication" here , for a short review.
How do postsynaptic potentials differ from the action potential? Postsynaptic potentials propagate, but not as fast or as far as action potentials. They are tiny waves of depolarization or hyperpolarization that move across the membrane of dendrites and the soma to trigger (by depolarization) or inhibit (by hyperpolarization) action potentials within the same neuron. Here’s another cartoon for PSPs.
Here are the questions (please answer all 3 questions -- 2 to 3 sentences for each question should be enough):
1. In a resting potential, sodium ions are always available in the extracellular fluid but they rarely pass into the neuron. In your own words, why can’t they get in?
2. In an action potential, sodium ions suddenly gain access to the inside of a neuron. Can you explain how this happens in your own words?
3. There are several mechanisms by which a neuron can be kept from firing. In your own words, describe at least one way that a neuron can be kept from firing an action potential. If someone already described one of these ways, see if you can describe a different way.
2.Drugs, transmitters, agonists, antagonists, metabotropic receptors, ioniotropic receptors, transporters, lipids, catchecholamines, peptides, transporters, etc., etc., -- I am still amazed at how complex we are at the cellular level – how do we even function? As you get familiar with how neurotransmitters and drugs work in the synapse, it will be easier to understand the later chapters when specific neurotransmitters are discussed for certain behaviors or disorders. For example, how cocaine works in our brain is something that you’ll understand better once you get a grip on this week’s material.
To review more general information on neurotransmitters, see the Neuroscience for Kids site: Neuroscience For Kids - neurotransmitters
For a better understanding of how various neurotransmitters affect postsynaptic neurons, try this interactive lessons here.
There are many ways that drugs can work as an agonist or antagonist for a given neurotransmitter, many which are depicted here: ACT_04_1_Drugs
Review of Receptors
Understanding how neurotransmitters and receptors work helps us understand how drugs act in the brain. Neurotransmitters from terminal buttons on the presynaptic membrane of one neuron can bind to receptor molecules in the post-synaptic membrane of the next neuron (on a dendrite or cell body). The binding of a neurotransmitter to the receptor causes ion channels to open, when then triggers an EPSP or IPSP. The simpler method of opening ion channels are the neurotransmitter-dependent ion channel, also known as ligand-gated channels (ligand is another name for neurotransmitter). These are generally known as ionotropic receptors. The other type of receptor is much more complicated: the metabotropic, or g-protein-coupled, receptors.
Here’s an analogy I like from John Gustafson, another professor at UMUC: Imagine you are a transmitter molecule that wants to open a door for ions to enter into a house. If your hand fits the doorknob (receptor), the door opens and ions can enter. That’s how an ionotropic receptor works. Metabotropic receptors are a variation on the ligand-gated theme. After the transmitter binds to a receptor, a mechanism is set in motion inside the neuron to open a channel for ions at another place nearby. When the transmitter binds with a metabotropic receptor it's as if you, the transmitter, had to ring a doorbell rather than grasping a doorknob. That sets in motion a mechanism inside to bring someone to open the door, which is located to one side of the doorbell, so the ions can troop in.
Drug studies need to control for placebo effects to determine the true effect of a drug. Some researchers, like Tor Wager, believe the plain old placebo effect is worth studying. It certainly has made a mess of some antidepressant studies.
Here is the question:
For this discussion, think about a drug that you or someone you know might be taking, or any drug that you want to know more about, or a recreational drug of interest to you, and do a little research on it to understand better how it works. (Don’t worry if someone else is already discussing your chosen drug, there’s plenty to learn about each of them.)
To start, make sure that you read the NOBA textbook “Psychopharmacology” chapter. Then, you might want to explore the links provided at the end of the chapter, the links I provide above, and/or do your own web search (make sure that your sources are credible).
In your thread, give us a summary on what you’ve learned about how the drug works at the level of the neuron and synapse, while pulling in some concepts from this week’s material. To get you started, think about these questions: Is the drug an agonist or an antagonist? What neurotransmitters does it affect? What is it’s effect on the body and behavior? What are the side effects? What is the drug’s mechanism of action on the neuron? What are some considerations for prescribing this drug to children or the elderly?
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||08/28/2015 12:00 am