Download Understanding the Transmission of Nerve Impulses

Understanding the Transmission of Nerve Impulses
Nerve impulses have a domino effect. Each neuron receives an impulse and must pass it on to the next neuron and make sure the correct impulse continues on its path. Through a chain of chemical events, the dendrites (part of a neuron) pick up an impulse that's shuttled through the axon and transmitted to the next neuron. The entire impulse Remember that when the neuron was polarized, the outside of the membrane was positive, and the inside of the membrane was negative. Well, after more positive ions go charging inside the membrane, the inside becomes positive, as well; polarization is removed and the threshold is reached. passes through a neuron in about seven milliseconds — faster than a Each neuron has a threshold level — the point at which there's no lightning strike. Here's what happens in just six easy steps: holding back. After the stimulus goes above the threshold level, more 1
gated ion channels open and allow more Na+ inside the cell. This Polarization of the neuron's membrane: Sodium is on the outside, and potassium is on the inside. causes complete depolarization of the neuron and an action potential is created. In this state, the neuron continues to open Na+ channels all Cell membranes surround neurons just as any other cell in the along the membrane. When this occurs, it's an all‐or‐none body has a membrane. When a neuron is not stimulated — it's just phenomenon. "All‐or‐none" means that if a stimulus doesn't exceed sitting with no impulse to carry or transmit — its membrane is the threshold level and cause all the gates to open, no action potential polarized. Not paralyzed. Polarized. Being polarized means that the results; however, after the threshold is crossed, there's no turning electrical charge on the outside of the membrane is positive while the back: Complete depolarization occurs and the stimulus will be electrical charge on the inside of the membrane is negative. The transmitted. outside of the cell contains excess sodium ions (Na+); the inside of the cell contains excess potassium ions (K+). (Ions are atoms of an element with a positive or negative charge.) You're probably wondering: How can the charge inside the cell be negative if the cell contains positive ions? Good question. The answer is that in addition to the K+, negatively charged protein and nucleic acid molecules also inhabit the cell; therefore, the inside is negative as compared to the outside. Then, if cell membranes allow ions to cross, how does the Na+ stay outside and the K+ stay inside? If this thought crossed your mind, you deserve a huge gold star! The answer is that the Na+ and K+ do, in fact, move back and forth across the membrane. However, Mother Nature thought of everything. There are Na+/K+ pumps on the membrane that pump the Na+ back outside and the K+ back inside. The charge of an ion inhibits membrane permeability (that is, makes it difficult for other things to cross the membrane). 2
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Resting potential gives the neuron a break. When the neuron is inactive and polarized, it's said to be at its resting potential. It remains this way until a stimulus comes along. When an impulse travels down an axon covered by a myelin sheath, the impulse must move between the uninsulated gaps called nodes of Ranvier that exist between each Schwann cell. 4
Repolarization: Potassium ions move outside, and sodium ions stay inside the membrane. After the inside of the cell becomes flooded with Na+, the gated ion channels on the inside of the membrane open to allow the K+ to move to the outside of the membrane. With K+ moving to the outside, the membrane's repolarization restores electrical balance, although it's opposite of the initial polarized membrane that had Na+ on the outside and K+ on the inside. Just after the K+ gates open, the Na+ gates close; otherwise, the membrane couldn't repolarize. 5
Hyperpolarization: More potassium ions are on the outside than there are sodium ions on the inside. When the K+ gates finally close, the neuron has slightly more K+ on the outside than it has Na+ on the inside. This causes the membrane potential to drop slightly lower than the resting potential, and the membrane is said to be hyperpolarized because it has a greater potential. (Because the membrane's potential is lower, it has more room to "grow."). This period doesn't last long, though (well, Action potential: Sodium ions move inside the membrane. none of these steps take long!). After the impulse has traveled through When a stimulus reaches a resting neuron, the gated ion the neuron, the action potential is over, and the cell membrane channels on the resting neuron's membrane open suddenly returns to normal (that is, the resting potential).
and allow the Na+ that was on the outside of the membrane to go rushing into the cell. As this happens, the neuron goes from being polarized to being depolarized. brain, but in other parts of the body, impulses are carried across 6 synapses as the following chemical changes occur: The refractory period is when the Na+ and K+ are returned to At the end of the axon from which the impulse is coming, the their original sides: Na+ on the outside and K+ on the inside. While the membrane depolarizes, gated ion channels open, and calcium ions neuron is busy returning everything to normal, it doesn't respond to (Ca2+) are allowed to enter the cell. Refractory period puts everything back to normal: Potassium returns inside, sodium returns outside. 1.
any incoming stimuli. It's kind of like letting your answering machine pick up the phone call that makes your phone ring just as you walk in 2.
Calcium gates open. Releasing a neurotransmitter. the door with your hands full. After the Na+/K+ pumps return the ions When the calcium ions rush in, a chemical called a neurotransmitter is to their rightful side of the neuron's cell membrane, the neuron is back released into the synapse. to its normal polarized state and stays in the resting potential until another impulse comes along. 3.
The neurotransmitter binds with receptors on the neuron. The chemical that serves as the neurotransmitter moves across the The following figure shows transmission of an impulse. synapse and binds to proteins on the neuron membrane that's about to receive the impulse. The proteins serve as the receptors, and different proteins serve as receptors for different neurotransmitters — that is, neurotransmitters have specific receptors. 4.
Excitation or inhibition of the membrane occurs. Whether excitation or inhibition occurs depends on what chemical served as the neurotransmitter and the result that it had. For example, if the neurotransmitter causes the Na+ channels to open, the neuron membrane becomes depolarized, and the impulse is carried through that neuron. If the K+ channels open, the neuron membrane becomes hyperpolarized, and inhibition occurs. The impulse is stopped dead if an action potential cannot be generated. If you're wondering what happens to the neurotransmitter after it binds to the receptor, you're really getting good at this anatomy and Transmission of a nerve impulse: Resting potential and action potential. physiology stuff. Here's the story: After the neurotransmitter produces its effect, whether it's excitation or inhibition, the receptor releases it and the neurotransmitter goes back into the synapse. In the synapse, Like the gaps between the Schwann cells on an insulated axon, a gap the cell "recycles" the degraded neurotransmitter. The chemicals go called a synapse or synaptic cleft separates the axon of one neuron and back into the membrane so that during the next impulse, when the the dendrites of the next neuron. Neurons don't touch. The signal synaptic vesicles bind to the membrane, the complete must traverse the synapse to continue on its path through the nervous neurotransmitter can again be released. system. Electrical conduction carries an impulse across synapses in the http://www.dummies.com/how‐to/content/understanding‐the‐transmission‐of‐nerve‐impulses.html & http://faculty.washington.edu/chudler/ap.html Lights, Camera, Action Potential http://faculty.washington.edu/chudler/ap.html T
his page describes how neurons work. I hope this explanation does not get too complicated, but it is important to understand how neurons do what they do. There are many details, but go slow and look at the figures. Action Potential Much of what we know about how neurons work comes from experiments on the giant axon of the squid. This giant axon extends from the head to the tail of the squid and is used to move the squid's tail. How giant is this axon? It can be up to 1 mm in diameter ‐ easy to see with the naked eye. Neurons send messages electrochemically. This means that chemicals cause an electrical signal. Chemicals in the body are "electrically‐
charged" ‐‐ when they have an electrical charge, they are called ions. The important ions in the nervous system are sodium and potassium (both have 1 positive charge, +), calcium (has 2 positive charges, ++) and chloride (has a negative charge, ‐). There are also some negatively charged protein molecules. It is also important to remember that nerve cells are surrounded by a membrane that allows some ions to pass through and blocks the passage of other ions. This type of membrane is called semi‐permeable. The resting potential tells about what happens when a neuron is at rest. An action potential occurs when a neuron sends information down an axon, away from the cell body. Neuroscientists use other words, such as a "spike" or an "impulse" for the action potential. The action potential is an explosion of electrical activity that is created by a depolarizing current. This means that some event (a stimulus) causes the resting potential to move toward 0 mV. When the depolarization reaches about ‐55 mV a neuron will fire an action potential. This is the threshold. If the neuron does not reach this critical threshold level, then no action potential will fire. Also, when the threshold level is reached, an action potential of a fixed sized will always fire...for any given neuron, the size of the action potential is always the same. There are no big or small action potentials in one nerve cell ‐ all action potentials are the same size. Therefore, the neuron either does not reach the threshold or a full action potential is fired ‐ this is the "ALL OR NONE" principle. Resting Membrane Potential W
hen a neuron is not sending a signal, it is "at rest." When a neuron is at rest, the inside of the neuron is negative relative to the outside. Although the concentrations of the different ions attempt to balance out on both sides of the membrane, they cannot because the cell membrane allows only some ions to pass through channels (ion channels). At rest, potassium ions (K+) can cross through the membrane easily. Also at rest, chloride ions (Cl‐)and sodium ions (Na+) have a more difficult time crossing. The negatively charged protein molecules (A‐) inside the neuron cannot cross the membrane. In addition to these selective ion channels, there is a pump that uses energy to move three sodium ions out of the neuron for every two potassium ions it puts in. Finally, when all these forces balance out, and the difference in the voltage between the inside and outside of the neuron is measured, you have the resting potential. The resting membrane potential of a neuron is about ‐70 mV (mV=millivolt) ‐ this means that the inside of the neuron is 70 mV less than the outside. At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron. Action potentials are caused when different ions cross the neuron membrane. A stimulus first causes sodium channels to open. Because there are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron. Remember, sodium has a positive charge, so the neuron becomes more positive and becomes depolarized. It takes longer for potassium channels to open. When they do open, potassium rushes out of the cell, reversing the depolarization. Also at about this time, sodium channels start to close. This causes the action potential to go back toward ‐70 mV (a repolarization). The action potential actually goes past ‐70 mV (a hyperpolarization) because the potassium channels stay open a bit too long. Gradually, the ion concentrations go back to resting levels and the cell returns to ‐70 mV. And there you have it...the Action Potential http://www.dummies.com/how‐to/content/understanding‐the‐transmission‐of‐nerve‐impulses.html & http://faculty.washington.edu/chudler/ap.html