Action Potentials Read this: The sodium and potassium gates in the nerve cell axon are voltage-gated. This means they open and close in response to changes in transmembrane potential. When the nerve cell is depolarized to -55mV at the trigger zone, it has reached threshold. When the cell reaches threshold, many sodium gates on the axon open. These sodium gates open due to changes in transmembrane potential; they are voltage-gated. When the sodium gates open, sodium rushes into the cell. 1) What effect will the opening of many sodium gates have on transmembrane potential? a. Depolarization – the transmembrane potential will become less negative b. Hyperpolarization – the transmembrane potential will become more negative 2) Imagine that many voltage-gated sodium gates open and many, many sodium ions rush into the axon of the cell. The charge on the axon near the trigger zone is now +35 mV. Is the inside of the cell now more positive or more negative than the outside? By the time the cell reaches +35 mV, the change in transmembrane potential has caused the potassium gates to open. Consider the potassium ions inside the cell. Consider the chemical and electrical gradients. 3) Where is the chemical concentration of potassium higher? a. Inside the cell b. Outside of the cell 4) Based on the chemical gradient, which way would you expect potassium to move? a. Into the cell b. Out of the cell 5) The part of the axon near the trigger zone is now +35 mV (positive inside). Based on the electrical gradient, which way would you expect potassium to move? a. Into the cell b. Out of the cell 6) If a cell has a charge of +35 mV and many potassium ions leave the cell, which of the following is a possible charge on the cell? a. +35 mV b. +25 mV c. +45 mV
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When many voltage-gated sodium gates open, sodium ions rush into the axon of the cell, making the inside of the cell more positive. So, the transmembrane potential will become less negative, which is called depolarization. Show more…
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The input end of a human nerve cell is connected to an output end by a long, thin, cylindrical axon. A signal at the input end is caused by a stretch sensor, a temperature sensor, contact with another cell or nerve, or some other stimulus. At the output end, the nerve signal can stimulate a muscle cell to perform a function (to contract, provide information to the brain, etc.) The axon of a so-called unmyelinated human nerve cell has a radius of 5x10^-6 m and a membrane that is 6x10^-9 m thick. The membrane has a resistivity of about 1.6x10^7 Ω.m. The fluid inside the axon has resistivity of about 0.5 Ω.m. The membrane wall has proteins that pump three sodium ions (Na+) out of the axon for each two potassium ions (K+) pumped into the axon. In the resting axon, the concentration of these ions results in a net positive charge on the outside of the membrane compared to negative charge on the inside. Because of the unequal charge distribution, there is a -70 mV potential inside compared to outside the axon. When an external source stimulates the input end of the nerve cell so the potential inside reaches about -50 mV, gates or channels in the membrane walls near that input open and sodium ions rush into the axon. This stimulates neighboring gates to swing open and sodium ions rush into the axon farther along. This disturbance quickly travels along the axon - a nerve impulse. The potential across the inside of the membrane changes in 0.5 ms from -70 mV to +30 mV relative to the outside. Immediately after this depolarization, potassium ion gates open and positively charged potassium ions rush out of the axon, repolarizing the axon. Sodium and potassium ion pumps then return the axon and its membrane to their original configuration. Part A: The charge density on the axon membrane walls (positive outside and negative inside) is 1.0 x 10^10 C/m^2. Suppose the membrane of a 0.9-m-long axon discharged completely in 0.02 s. Find the electric current across the membrane wall of one such hypothetical axon discharge. Express your answer to one significant figure and include the appropriate units. Value _____________ Units
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1. The value of the resting membrane potential depends on: ____________________________________________________________________ ____________________________________________________________________ 2. In a resting neuron, the inside of the cell has a more (circle or highlight one): negative / positive charge compared to the outside of the cell. 3. True or False (circle or highlight one): If you hold a voltmeter outside a neuron, it will read -70 mV if the cell is at rest. 4. Sodium has a higher concentration in the _______________________________________ fluid. 5. Potassium diffusion into and out of the neuron is affected by both the chemical concentrations and the electrical gradient. This is called an ___________________________________________________. 6. The resting membrane potential of a typical neuron is: ___________________________. 7. The concentration (amount) of sodium and potassium influences the value of the resting potential. What is the other factor that influences this value? __________________________________________________________________________________ 8. At a resting membrane potential of -70 mV, the chemical gradient driving potassium out of the cell is slightly ____________________________ than the electrical gradient pulling it back in. 9. The chemical gradient pushes sodium (circle or highlight one) into / out of the cell. The electrical gradient pushes sodium (circle or highlight one) into / out of the cell. 10. True or False (circle or highlight one): Even when the neuron is at rest, sodium constantly leaks in and potassium constantly leaks out.
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An action potential is a rapid and substantial depolarization of the neuron's membrane. It usually lasts only about 1 ms. Typically, the membrane potential changes from the RMP of -70 mV to a value of +30 mV and then rapidly returns to its resting value. How does this marked change in membrane potential occur? Using the following activity, test your knowledge of the way action potentials are generated. Indicate the order of events during the generation of an action potential by placing the descriptions of the events in the correct relative order. Each step must be completely typed out for full credit. Students cannot just place the numbers in correct order. 1. K+ moves from cell to extracellular fluid. 2. Depolarizing stimulus is received. 3. Membrane depolarizes to threshold. Voltage-gated Na+ channels open and Na+ enters cell. Voltage-gated K+ channels begin to open slowly. 4. K+ channels remain open and additional K+ leaves cell, hyperpolarizing it. 5. Membrane potential is in resting state. 6. Na+ channels close and slower K+ channels open. 7. As K+ channels close, some K+ enters cell through leak channels. 8. Rapid Na+ entry depolarizers cell. 9. Cell returns to resting ion permeability and resting membrane potential.
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