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Understanding the Transmission of Impulses

Remember that when the was polarized, the outside of the Nerve impulses have a domino effect. Each neuron receives an impulse membrane was positive, and the inside of the membrane was and must pass it on to the next neuron and make sure the correct negative. Well, after more positive go charging inside the impulse continues on its path. Through a chain of chemical events, the membrane, the inside becomes positive, as well; polarization is (part of a neuron) pick up an impulse that's shuttled through removed and the threshold is reached. the and transmitted to the next neuron. The entire impulse passes through a neuron in about seven milliseconds — faster than a Each neuron has a threshold level — the point at which there's no . Here's what happens in just six easy steps: holding back. After the stimulus goes above the threshold level, more gated channels open and allow more Na+ inside the . This Polarization of the neuron's membrane: is on the causes complete of the neuron and an action potential outside, and is on the inside. is created. In this state, the neuron continues to open Na+ channels all 1Cell membranes surround 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 When an impulse travels down an axon covered by a sheath, cell contains excess potassium ions (K+). (Ions are atoms of an element the impulse must move between the uninsulated gaps called nodes of with a positive or negative charge.) Ranvier that exist between each . You're probably wondering: How can the charge inside the cell be : Potassium ions move outside, and sodium ions negative if the cell contains positive ions? Good question. The answer stay inside the membrane. is that in addition to the K+, negatively charged protein and nucleic acid molecules also inhabit the cell; therefore, the inside is negative as 4After the inside of the cell becomes flooded with Na+, the compared to the outside. 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 Then, if cell membranes allow ions to cross, how does the Na+ stay outside, the membrane's repolarization restores electrical balance, outside and the K+ stay inside? If this thought crossed your mind, you although it's opposite of the initial polarized membrane that had Na+ deserve a huge gold star! The answer is that the Na+ and K+ do, in fact, on the outside and K+ on the inside. Just after the K+ gates open, the move back and forth across the membrane. However, Mother Nature Na+ gates close; otherwise, the membrane couldn't repolarize. 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 Hyperpolarization: More potassium ions are on the outside an ion inhibits membrane permeability (that is, makes it difficult for than there are sodium ions on the inside. other things to cross the membrane). 5When the K+ gates finally close, the neuron has slightly more gives the neuron a break. K+ on the outside than it has Na+ on the inside. This causes the to drop slightly lower than the resting potential, When the neuron is inactive and polarized, it's said to be at and the membrane is said to be hyperpolarized because it has a its resting potential. It remains this way until a stimulus greater potential. (Because the membrane's potential is lower, it has 2comes along. 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 channels on the resting neuron's membrane open suddenly returns to normal (that is, the resting potential). 3and 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. , but in other parts of the body, impulses are carried across as the following chemical changes occur: Refractory period puts everything back to normal: Potassium returns inside, sodium returns outside. 1. gates open. 6 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. any incoming stimuli. It's kind of like letting your answering machine 2. Releasing a . pick up the phone call that makes your phone ring just as you walk in 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 . to its normal polarized state and stays in the resting potential until 3. The neurotransmitter binds with receptors on the neuron. another impulse comes along. 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 — 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 stuff. Here's the story: After the neurotransmitter produces Transmission of a nerve impulse: Resting potential and action its effect, whether it's excitation or inhibition, the receptor releases it potential. 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

his page describes how neurons work. I hope this explanation does Tnot 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 . This giant axon extends The resting potential tells about what happens when a neuron is at from the head to the tail of the squid and is used to move the squid's rest. An action potential occurs when a neuron sends information tail. How giant is this axon? It can be up to 1 mm in diameter ‐ easy to down an axon, away from the cell body. use other see with the naked eye. 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) Neurons send messages electrochemically. This means that chemicals causes the resting potential to move toward 0 mV. When the cause an electrical signal. Chemicals in the body are "electrically‐ depolarization reaches about ‐55 mV a neuron will fire an action charged" ‐‐ when they have an electrical charge, they are potential. This is the threshold. If the neuron does not reach this called ions. The important ions in the are sodium and critical threshold level, then no action potential will fire. Also, when potassium (both have 1 positive charge, +), calcium (has 2 positive the threshold level is reached, an action potential of a fixed sized will charges, ++) and chloride (has a negative charge, ‐). There are also always fire...for any given neuron, the size of the action potential is some negatively charged protein molecules. It is also important to always the same. There are no big or small action potentials in one remember that nerve cells are surrounded by a membrane that allows nerve cell ‐ all action potentials are the same size. Therefore, the some ions to pass through and blocks the passage of other ions. This neuron either does not reach the threshold or a full action potential type of membrane is called semi‐permeable. is fired ‐ this is the "ALL OR NONE" principle.

Resting Membrane Potential

hen a neuron is not Wsending 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 Action potentials are caused when different ions cross the neuron pass through channels (ion membrane. A stimulus first causes sodium channels to open. Because channels). At rest, there are many more sodium ions on the outside, and the inside of the potassium ions (K+) can cross neuron is negative relative to the outside, sodium ions rush into the through the membrane easily. Also at rest, chloride ions (Cl‐)and neuron. Remember, sodium has a positive charge, so the neuron sodium ions (Na+) have a more difficult time crossing. The negatively becomes more positive and becomes depolarized. It takes longer for charged protein molecules (A‐) inside the neuron cannot cross the potassium channels to open. When they do open, potassium rushes membrane. In addition to these selective ion channels, there is out of the cell, reversing the depolarization. Also at about this time, a pump that uses energy to move three sodium ions out of the neuron sodium channels start to close. This causes the action potential to go for every two potassium ions it puts in. Finally, when all these forces back toward ‐70 mV (a repolarization). The action potential actually balance out, and the difference in the between the inside and goes past ‐70 mV (a hyperpolarization) because the potassium outside of the neuron is measured, you have the resting potential. The channels stay open a bit too long. Gradually, the ion concentrations go resting membrane potential of a neuron is about ‐70 mV (mV=millivolt) back to resting levels and the cell returns to ‐70 mV. ‐ 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 And there you have it...the Action Potential neuron and more potassium ions inside that neuron.

http://www.dummies.com/how‐to/content/understanding‐the‐transmission‐of‐nerve‐impulses.html & http://faculty.washington.edu/chudler/ap.html