New Chapt 2- resting pot. pdf(2)

Neuroscience, Fourth Edition

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10 CHAPTER 2 THE RESTING POTENTIAL In the last chapter, the idea that nervous system is a huge electrical system designed for communication was introduced. Neurons act by conducting electrical pulses called action potentials along their axons, and one neuron communicates with another neuron by releasing chemical neurotransmitters when an action potential invades the axon’s terminal. The effect of the release of neurotransmitter is that the chemical binds to receptors on the postsynaptic neuron and opens pores in its receptors. The opening of pores then allows positive charges to enter the cell, and if the interior of the postsynaptic cell becomes sufficiently positive, an action potential is generated at the axon hillock of the postsynaptic cell. The action potential then propagates down the axon and the whole cycle is repeated. But exactly what an action potential is and how it is created was not described. Indeed, all of the electrical features and electrical terms, which are fundamental for the operation of neurons, were used in very vague ways and were not explained. In this chapter, the electrical nature of cells will be explained in greater detail. Subsequent chapters will explain how the electrical features described here are used to evoke highly specialized electrical signals, action potentials , which are unique to nerve and muscle cells. Cells act as a battery because their membranes separate positive and negative charges One feature of all living cells is that they act as a battery. This is true not only of nerve and muscle, but all living cells. A battery is simply a device that separates positive and negative charges. In cells, the separation is achieved such that the inside of the cell has a slightly larger number of negatively charged molecules than the outside of the cell. The separation of charges is due to two major things. The First is that there is an imbalance of various ions on the inside compared to the outside of the cell. These include sodium (Na + ), potassium (K + ), chloride (Cl - ) , and calcium (Ca ++ ) ions. The concentration of these ions is different in the cytoplasm than it is in the extracellular fluids in which the cells are bathed. Of particular importance is that the cytoplasm, or inside of the cell, has a higher concentration of K + (indicated by [K + ] i ) than does the extracellular fluid that bathes the outside of the cell (indicated by [K + ] o ). Conversely, the inside of the cell has a lower concentration of Na + (indicated by [Na + ] i ) than does the extracellular fluid (indicated by [Na + ] o ). The Second is that the cell membrane is semipermeable. Semipermeable simply means that the membrane has pores or channels that allow only certain molecules to pass while preventing others from crossing from one side to the other. The semipermeable membranes of all cells allow K + ions to pass through easily, but act as barriers for the passage of Na + , Cl - , and Ca ++ ions. Thus, K+ can readily go from the inside to the outside or in the opposite direction through the
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New Chapt 2- resting pot. pdf(2) - CHAPTER 2 THE RESTING...

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