Human Anatomy & Physiology (9th Edition)

Published by Pearson
ISBN 10: 0321743261
ISBN 13: 978-0-32174-326-8

Chapter 11 - Fundamentals of the Nervous System and Nervous Tissue - Review Questions - Page 426: 17

Answer

A healthy human cell is bounded by a selectively permeable membrane within which are the cytoplasm, and organelles of various kinds The cytoplasm( cytosol) inside hplasma membrane i snormally has a lower voltage difference of -40mV to -80mV with respect to the outside of the cell. The voltage difference between two l ocations can be expressed by saying that tere is a polar difference between the two location so objects or that one is polarized with respect to the Other. In thecase of the cell ctoplasm and the extracellular fuid we say the cytoplasm is polarized with respectto te ECF or structores external to the cell membrane. This situation is not static. It is maintaiened by several electrochemical devices . The voltage difference is due to charged ions and the accompanying differences in concentrations of relevant ions (K+, Na+, Cl-) in and out of the cell. Solutes flow down their concentration gradients and positive charged particles flow toward negatively charged particles. These forces operate naturally to abolish the polarized state, Therefore for the the polarized cell state to continue t exist there must be forces working constantly to maintain it. Some of these forces are: 1. The semipermeable cell membrane 2. Ion channels and transporter proteins. 3. Membrane pumps. Na+/K= ATPASE 4.Charged particles

Work Step by Step

The polarized state of muscle cells, neurons and other cells is represented by a cross plasma membrane difference of 60-80mV (-70mV). The main ions responsible or generating and maintaining this state of polarization are cations (K,Na, Mg Ca) and anions (Cl-, HCO3-, COO-). The inside of the cell is negative with respect to the outside :--the concentration of Na+ is higher on the inside and there are more K ions inside. Also, there are certain ions and small protein molecules that cannot cross the bilipid layer of the plasma membrane without special assistance of carriers or transporters. As is to be expected the K+ and the Na+ ions are constantly trying to flow passively down thier concentration gradients--Na to leak out of the cel,l and K to leak in .Ions also tend to move naturally to an area of opposite charge. If these tendencies were not opposed, the cross-membrane voltage difference and the polarized state would vanish. Devices that maintain the polarized state: 1. The selectively permeable cell membrane allows certain solutes to pass by diffusion, but blocks others. Further, among those allowed in, some pass with greater ease than others. Thus K=+ions diffuse out much more easily than Na ions can leak in o the cytosol. This is probably the most important factor in maintaining the polarized state. 2. Anions. Certain anions they help to maintain the negative voltage difference between cytosol and ECF 3.Ion channels also ply a role in maintaining the polarized state Some ion channels, the non-gated, kind are always open. Gated ion channels close or open in response to chemical, electrical, or pressurechanges. 4. Membrane pumps or transporters are trans membrane proteins in the plasma membrane that can move chemical species across the membrane-- in or out of the cell. Charged particles. The charged particles K=, Na+ , Ca++ Mg++, Cl-, HCO3- are the species that carry the electrical charges and are responsible for the polarized state, However the relative difference between the Na+ concentration and the K+ concentration-- in and out of the cell-- is the major factor in keeping the -70mV difference. At rest the plasma membrane is 20 times more permeable to potassium than to sodium ions. 4. The Na/K+ATPASE pump is of paramount importance in restoring the polarized state after a disturbance by some event like an action potential
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