Posted by: Thixia | September 4, 2008

Diagram: How Nerves Work 4 of 6


Ion Channels


Because ions are charged and water-soluble, they must move through small tunnels or channels (specialized proteins) that span the cell membrane’s lipid bilayer. Each channel is specific for only one type of ion. There are specific channels for sodium ions, potassium ions, calcium ions and chloride ions. These channels make the cell membrane selectively permeable to various ions and other substances (like glucose). The selective permeability of the cell membrane allows the inside to have a different composition than the outside.



© Photographer: Eraxion | Agency: Dreamstime


For the purposes of nerve signals, we are interested in the following­ characteristics:


·    The outside fluid is rich in sodium, a concentration about 10 times higher than the inside fluid

·    The inside fluid is rich in potassium, a concentration about 20 times higher inside the cell than outside.

·    There are large negatively charged proteins inside the cell that are too big to move across the membrane. They give the inside of the cell a negative electrical charge compared to the outside. The charge is about 70 to 80 millivolts (mV) — 1 mV is 1/1000th of a volt. For comparison, the charge in your house is about 120 V, about 1.2 million times more.

·    The cell membrane is slightly “leaky” to sodium and potassium ions, so a sodium-potassium pump is located in the membrane. This pump uses energy (ATP) to pump sodium ions from the inside to the outside and potassium ions from the outside to the inside.

·    Because sodium and potassium ions are positively charged, they carry tiny electrical currents when they move across the membrane. If sufficient numbers move across the membrane, you can measure the electrical currents.



Nerve Growth and Regeneration


When nerves grow, they secrete a substance called nerve growth factor (NGF). NGF attracts other nerves nearby to grow and establish connections. When peripheral nerves become severed, surgeons can place the severed ends near each other and hold them in place. The injured nerve ends will stimulate the growth of axons within the nerves and establish appropriate connections. Scientists don’t entirely understand this process.


For unknown reasons, nerve regeneration appears most often in the peripheral and autonomic nervous systems but seems limited within the central nervous system. However, some regeneration must be able to occur in the central nervous system because some spinal cord and head trauma injuries show some degree of recovery.



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How Stuff Works.


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