Thursday, December 23, 2004 - How neurons work


Blogger Mister Spark said...

Neural impulses, synaptic transmission, and drug action : A 3 part series

Part I : Membrane potentials and action potentials

Part II : Synaptic transmission, some neurophysiology

Part III: Action of selected drugs

How does cocaine work? How about THC? Amphetamines? X? Ketamine? How does Alzheimer's cause memory loss? Why do people with Parkinson's get the shakes? What is pain? What are the endorphins, and how do they work? Some of these questions have been worked out, some are still inder investigation, and some are the topic of heated debate in the scientific community. The mechanism of drug action has historically been a controversial field, filled with misconceptions in the eyes of the public as theories change with the introduction of new data, new mechanisms, and even new molecules. Here, we will begin to explore some of the concepts behind these mechanisms so they can be presented and discussed. In future installments, we will explore the potential mechanisms of these phenomena, and what these mechanisms may mean for the future.

This is the first of a three part series. I'm starting out with the topics listed above, and may move into diseases later on. My apologies if the first few installments are a bit below your knowledge base, but I want to make sure that everyone is on the same page. In the future, I may write a bit more on pathophysiology of the CNS (Alzheimer's, Parkinson's) and concepts of pain, reflexes, and somatic sensation.

The Neuron

The neuron is the functional unit of our nervous system. Neurons only make up a tiny fraction of the cells found in the central nervous system, but are the reason for the existence of all the cells around them. The astrocytes, ependymal cells, microglia, Schwann cells and oligodendrocytes (collectively known as the glia) exist only to support, protect, and serve the neurons. Neurons form the basis of thought, emotion, motion, sensation and every interaction that we have with or environment, and even our own bodies. They comprise our communication network, our sensory network, our processing center, and memory. But how do they work? You have probably heard that they are electrical in nature, but it's not the same thing as copper wire and solder. They are a lot more complicated, and require an introduction of a few concepts from cell biology. In the first installment of a three part series, I will talk about some of these basic concepts. From there, we can move on to a discussion of how signals get transmitted in the brain, and how certain drugs may affect that signaling.

The anatomy of a neuron

The classic neuron consists of three main parts : the soma, or cell body, the axon, and the dendrites. The soma is similar to the main cell body of any other cell in the body, except for the presence of synapses, which are discussed in the next edition. The cell body supports the rest of the cell, manufacturing proteins and peptides, shunting energy sources and waste products, and providing a genetic control center (the nucleus). Primary cell functions take place in the nucleus. The dendrites are are fingerlike projections that provide the input for the neuron. These receive input from other neurons, or in the case of sensory neurons, from the outside world. They can transmit this information in the form of electrical impulses to he cell body for transmission along the axon. A given neuron will have a very large number of dendrites, from dozens to thousands. The axon is a long, single, bifurcating projection from the neuron. This is the output. Only one axon leaves a neuron, but nearly all bifurcate into multiple treelike telodendria, which can also number in the thousands. Each telodendrite will terminate at an effector cell of another neuron. These connections are commonly in the form of a synapse, a point at which the two structures come exceptionally close, but do not actually touch. This synapse may be with an effector cell, such as a muscle or a gland, or with another neuron. In the brain, each neuron will receive input from, on average, 2000 synaptic endings. For an estimated 10^11neurons in the human brain, this means 200 trillion connections. By raw complexity, this outpaces the Pentium 4 processor by about six orders of magnitude.

The membrane potential

We are all familiar with diffusion, I hope. And we can all recall the high school biology class where we talked about semipermeable membranes, and the establishment of a concentration gradient. Well, every cell in out body has a concentration gradient set up of several different ions. The important ones for this discussion are sodium (Na), chloride (Cl), potassium (K), and calcium (Ca). Chloride is negatively charged. The rest are positive. Throughout our bodies, the levels of Na, Cl, and Ca are higher outside the cells than in. Potassium is higher inside. We are all aware of the tendency of these ions to move down their concentration gradient, or their chemical gradient, but many of us are probably not familiar with the idea of an electrical gradient. These ions are charged, and therefore set up an electrical potential across the membrane, which is determined by the small region of space directly on either side of the membrane. This thin region is known as the dipole layer. Ions outside this layer do not influence the membrane potential. By convention, the extracellular space is referenced as zero. Relative to that, the inside of a neuron is about -90mV when resting. This can change very easily, however, by altering the permeability of the membrane to certain ions. By regulating the flow of ions macros the membrane, you can transiently affect the voltage across the membrane. These changes are commonly referred to as hypopolarization (a decrease in the magnitude of the membrane potential), hyperpolarization (an increase in the magnitude of the membrane potential) and depolarization, which will be discussed in the next section. Remember: hypopolarization will actually be an increase in the potential. The resting potential is negative, so it must increase (become more positive) to approach zero.

So, how is this done? There are specific proteins in the membranes of cells called ion channels. These are controlled, or gated, in different ways. Some are opened by interaction with another molecule, either from outside or inside the cell. These are called ligand gated ion channels. Others are opened by mechanical forces. Many sensory neurons use these mechanically gated ion channels. Still others open in response to changes in the membrane potential, and are referred to as voltage gated ion channels. Recall the relative concentrations of ions. If a cell has a ligand gated sodium channel, and it is opened, positively charged sodium will flow into the cell, altering the electrochemical makeup of the dipole layer in that region, making it more positive. This will be a hypopolarization, or, in the case of a neuron, an excitatory potential. Alternatively, if a ligand gated chloride channel is opened, it will allow an influx of negatively charged chloride into the cell. This will result in a hyperpolarization of the membrane. To change things around a bit, imagine that a potassium channel opens. This would result in efflux of positively charged potassium from the cell, again hyper polarizing the cell.

Action Potentials

Remember the voltage gated ion channels? Most of these are sodium and potassium channels, and are found scattered along the axons of neurons. These typically open when the membrane potential reaches about -65mV, but this threshold can be modulated by the action of some neurotransmitters. When a local region of the membrane reaches or crosses this threshold, the voltage gated sodium channels in that region open rapidly. The resulting influx of sodium completely depolarizes the membrane, and the potential enters the positive domain. Some ions will diffuse along the membrane, altering the voltage in the neighboring regions. More voltage gated ion channels open in this region, depolarizing that region as well. This causes a cascade all the way down the axon.

Voltage gated sodium channels have two gates, one on the extracellular face and another on the intracellular face. In the resting state, the outer gate is closed and the inner gate is open. Upon crossing the threshold, the outer gate opens very rapidly, and the inner gate begins to close, but comparatively slowly. As a result, the gate remains open for only a short period of time, without the need for an additional outside force to act on the channel. The closing of the sodium channels prevents further influx of sodium, and the sodium that has flowed in begins to diffuse away from the membrane, deeper into the cell. Once the ions have left the dipole layer, they no longer influence the membrane potential and the potential can return to resting. Additionally, the same voltage that opens sodium channels stimulates a delayed opening of potassium channels. These do not fully open until the sodium channels have already begun to close, so they do not affect the membrane potential during this phase. Once they do open, they allow the efflux of potassium, which contributes to the reestablishment of the resting potential. Diffusion of sodium out of the dipole layer begins a slow repolarization, and the opening of the potassium channels initiates a rapid repolarization. The result is a sudden increase, then reestablishment of the membrane potential in one area, and this wave of depolarization propagates down the axon to the synapse.

One last feature of the sodium channels : the observant reader may have noticed that this should result in a continual series of action potentials, as that sodium from the next segment will diffuse in both directions. Sodium ions flowing backwards from the second segment to the first should raise the membrane potential there back to the threshold, reactivating the voltage gated sodium channels and repeating the depolarization. This, however, does not happen. Once a voltage gated sodium channel has been activated, it is no longer in its resting state. It now sits with its inner gate closed and its outer gate open, and it must be retooled. Anterograde propagation is prevented by the necessity of the channel to retool; it can only do so at the resting potential of -90mV. By the time the membrane returns to this potential, the action potential has propagated well beyond the range at which its sodium ions could affect the now responsive sodium channels.

So how is the membrane depolarized in the first place? This discussion begins with the assumption that a region of the membrane has crossed the -65mV threshold. How does this region of the membrane get to that potential? This is the function of the synapse : to control the membrane potential in a local region of the neuron via regulation of ion channels. This is the receptor potential, and we will discuss this in the next installment of the series.

4:17 PM  
Blogger Mister Spark said...

So far, we have covered the propagation of an action potential, but not its source nor its effects. To address these, we will examine the example of the sympathetic ganglia. The sympathetic ganglia (or sympathetic chains, or paraveretebral ganglia) are conglomerations of lumps of neuron cell bodies, connected by conduits of axons, all of which run along each side of the spinal cord. These are responsible for coordinating, propagating, and assisting in the control of the sympathetic branch of the autonomic nervous system. Now, for this discussion, it's not so important what the sympathetic nervous system does. We're just interested in the structures. The autonomic system is different from some of the other nerves you are used to, in that normally they form at least one synapse outside the central nervous system before hitting whatever their target may be. A signal originating in the spinal cord will intersect a neuron in the ganglion, and the signal must jump from the spinal neuron to the neuron in the ganglion. In order to do this, it must cross the synapse.

A synapse is a tiny gap between two neurons, or a neuron and its target tissue. The action potential cannot cross this gap, so the electrical impulse is transformed into a chemical signal that carries the impulse along to the next cell to have some effect. The effects can be quite varied, from acting as a simple relay of the signal to altering the metabolic state of the downstream cell, or altering the regulation of its genes. The simplest of these is an excitatory postsynaptic potential, which we will examine here.

OK, back to the ganglion. In the simplest case, a neuron will extend its axon from the spinal cord and form a synapse with a cell body in the ganglion. Consider an action potential traveling down this axon. It will propagate along, the depolarization of the membrane causing a cascade of opening and closing voltage gated ion channels down the axon until it reaches the bulbous structure at the end, the terminal bouton (aka, terminal knob). Here, the makeup of the plasma membrane changes. Mixed among the voltage gated sodium channels are various other ion channels, such as a voltage gated calcium channel. As you would expect, this channel behaves in a manner similar to the voltage gated sodium channel. It opens when the membrane is depolarized and allows calcium to flow into the cell. The terminal bouton has another feature important for our discussion — It is full of vesicles. These are little membrane bound bags stored in the bouton. In each "bag" is a mixture of neurotransmitters, small molecules or peptides that will act on the postsynaptic cell in some way. In our preganglionic sympathetic neuron, these vesicles are filled predominantly with acetylcholine.

As the wave of depolarization washes over the terminal bouton, these channels open, and the concentration of calcium inside the bouton rises. This calcium influx starts a cascade. First, it binds to an small protein inside the cell, called calmodulin (CaM). When bound to calcium, CaM changes its shape, and can then bind to other proteins in the cell that act as enzymes, altering the structure and activating other proteins involved in the scaffolding support matrix of the cell. All these work together to start moving some of the vesicles towards the end of the bouton, the part facing the small gap, the synaptic cleft, between the bouton and the next cell. Once the membrane around the vesicle comes in contact with the plasma membrane, another cascade starts and the two membranes fuse, dumping the contents of the vesicle into the gap. Normally, only one vesicle will make it all the way to the membrane. The rest stop before they get there. Sometimes, if there is a lot of stimulation, two or three may fuse.

Once in the gap, the acetylcholine (ACh) is faced with a gauntlet to run. The cleft is full of an enzyme called acetylcholinesterase, bound to the membranes and floating in the cleft. This is an enzyme capable of breaking down acetylcholine into a pair of inactive products that are then reabsorbed by the neuron that just dumped it out (this is reuptake). The ACh that does make it across can then bind to a protein on the postsynaptic cell. In our scenario, this is a nicotinic cholinergic receptor (nACHhR), a new type of ion channel — a ligand gated ion channel. Instead of being opened by a change in voltage, these are opened by the presence of a small molecule that specifically binds to it, known as a ligand. In our scenario, ACh is the ligand. These proteins are among a group of proteins called receptors. Receptors are not restrcted to neurons, nor are they always ion channels, but they all cause changes in a cell in response to a ligand.

When ACh binds to this receptor, it opens and allows an influx of sodium, pushing the cell towards depolarization. If enough of these channels are stimulated at once, the membrane potential will cross the threshold and a new action potential will start. This is an example of the simplest case — an excitatory postsynaptic potential.

But, as is said frequently in biological sciences, it's never really that simple.

The changes in voltage instigated by the opening of one channel are transient, restricted in space, and small in magnitude. They will quickly dissipate, and the membrane will return to its resting potential. These also only affect a small area of the plasma membrane; the drop in polarization is over only a very small area. Therefore, these signals must be summated both spatially and temporally. There must be enough signals received in a small enough area, and over a short enough period of time to force the membrane potential across the threshold. And even that is simplifying matters a bit.

Each neuron has a large number of synapses, of wide variety. On average, each central nervous system neuron receives input from about 2000 synapses that are spread out over the cell body, dendrites, axon and bouton. Not all of these are excitatory, nor are they all postsynaptic. Some synapses express GABA or glycine receptors, which are ligand gated chloride channels on the cell. Chloride has a negative charge, and is concentrated outside the cell. Opening a chloride channel will hyperpolarize a cell due to the influx of negative charge, which will make the cell harder to depolarize for a short time. This is an inhibitory potential, and can be post or presynaptic (normally on or near the bouton).

Presynaptic signals can hyperpolarize the terminal bouton, effectively quenching the inbound action potential. Other presynaptic inhibitory potentials inhibit the calcium channels on the bouton, interrupting the signal before it is passed to CaM. Other neurotransmitters are modulatory, and affect the metabolism of the target cell. These modulatory neurotransmitters don't normally act through ion channels, and have longer lasting effects. These can alter gene regulation, cause a reduction or increase in the number of excitatory or inhibitory receptors, affect scaffolding proteins, affect the milieu of neurotransmitters in that cell's bouton, reduce the sensitivity of CaM to calcium (this is a very recent discovery), change the localization of the receptors, or a host of other effects. In our example, the target neuron in the ganglion will receive input from other spinal cord neurons and from neurons in other parts of the paraveretebral chain. This input will be summated, and the signal will be sent out, or it will not.

Our ganglionic neuron must summate all of these inputs, inhibitory, excitatory, and modulatory before transmitting a signal. In the end, this signal is binary. It is a decision whether the neuron will fire, or it will not. Whether and action potential will propagate down the axon, or not. In our example, the downstream effects are immediate. A blood vessel will constrict, or a bronchiole will dilate, or a sphincter in the GI tract will close. In the central nervous system, complexity becomes further complexity. Here, the individual signal summated from thousands will itself become one more signal among thousands to be summated at the next neuron.

4:17 PM  

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